Posted by: oikosasa | November 21, 2014

Same looks, different behavior

At first sight, these nematodes all look the same. Nevertheless, they each belong to a different species. Such cryptic species- species that morphologically look the same but show genetic divergence- are more different than we first might think. Previous research already showed that they have different environmental preferences and competitive abilities. In our paper, “Active dispersal is differentially affected by inter- and intraspecific competition in closely related nematode species”, we show that differences in active dispersal behavior occur: in addition to differences in time until first dispersal, the triggers for dispersal also differ between the species. One of the species is most triggered by interspecific competition, two others by competition with conspecifics, and the fourth one is a time-dependent disperser, with fast dispersal regardless of inter- or intraspecific interactions.

These differences in dispersal behavior may be important to explain the coexistence of these species. According to Darwin’s classical competition theory, we can expect that very similar species will not co-occur because competition will be too high. Differences in dispersal behavior may lead to postponed or avoided competition, rendering temporal coexistence possible in a patchy habitat.

The authors through Nele de Meester

Posted by: oikosasa | November 20, 2014

Predation and transmission of direct life-cycle parasites

Find out what role predation plays in the transfer of less complex parasites in the Early View paper “The underrated importance of predation in transmission ecology of direct lifecycle parasites” by Giovanni Strona. Below is his short summary of the study:


Predation is the primary route for transmission in parasites having complex life cycles. However, despite being one of the strongest evolutionary forces, little is known about its role in the ecology and evolution of simple life cycle parasites (that is parasites that spend all of their life on a single host).

Monogeneans are one of the most abundant group of fish parasites, and are peculiar in that they do not use more than one host during their whole life. Being well investigated, they constitute a good benchmark to explore if predation has some relevance for parasites when not directly involved in transmission from one host to another. For this, I used a large dataset and different approaches to test whether predators and preys share more monogenean parasites than one would expect from their geographical distribution, habitat preference and phylogenetic relationships. It turned out that preys and predators do share more monogenean parasites than expected.


The observed overlap degree was much higher at the genus level than at the species one. This suggests that predation may play an important role in promoting monogenean host range expansion. In addition, a good proportion of considered prey-predator pairs showed a significantly high parasite overlap at the species level. This last result promotes some intriguing hypotheses. In particular it may indicate a tendency of some monogenean parasites to evolve transmission strategies more targeted towards host interactions than towards species specific traits.

Monogenean parasites identify suitable hosts on the basis of various cues related to host physiology and behavior, such as shadows, chemicals, mechanical disturbance, and osmotic changes. Usually, these cues are generated by the activity of single species, but could also result from species interactions. For example, a predator hunting a school of fish may produce peculiar water turbulence, shadows, and specific chemicals, which are stimuli that have already been demonstrated capable of inducing mass hatching in monogeneans. Some monogenean parasites could have developed the ability to identify these cues, and to infect with similar probability a predator and its prey/s. If this hypothesis was true, it would have strong implications on evolutionary ecology, suggesting the existence of a peculiar situation, where some parasites have evolved high specialized host finding behaviors to become more generalist. Morevover, it would indicate that some monogenean parasites could be more vulnerable to coextinctions than suggested by the size of their host range, as their survival would depend on that of both the prey and the predator species.

Posted by: oikosasa | November 18, 2014

Sexual size dimorphism in island plants

Variation in size between sexes is something that we associate mainly with animals. But what about plants? Do female plants have larger leaves than males? Find out in the Early View paper in Oikos “Sexual size dimorphism in island plants: the niche variation hypothesis and insular size changes” by Patrick H. Kavanagh and Kevin C. Burns. below is their summary of the study:

Sexual size dimorphism (SSD) is common throughout the animal kingdom. Size differences between the sexes are often extreme and in many cases one sex may be twice the size of the other. While most plants are hermaphroditic, approximately 7% of flowering plants are dioecious (separate male and female individuals). SSD is also common in dioecious plants, yet has received far less attention than SSD in animals. The niche variation hypothesis predicts the degree of SSD to increase for insular populations as a response to increased intraspecific competition.   Many animal taxa conform to this prediction, however SSD of island plant populations had not been investigated.


We investigated differences in SSD between related island and mainland plants by using herbarium material. Specifically, we quantified the sizes of leaves and stems for plants from the New Zealand mainland and surrounding offshore islands. Our results suggest that the degree of SSD is not predictable for island plants, contrary to predictions of the niche variation hypothesis. Furthermore, SSD was consistently female biased on the mainland, however the direction of SSD was not predictable on islands. Our results suggest that both sexes are under selection for increased size on islands. This may contribute to SSD being unpredictable due to the sexes responding to selection at different rates. However, further work is needed to gain a better understanding of SSD in island plant populations.

In our new paper “Marine biodiversity and ecosystem functioning: what’s known and what’s next?” just published online early in Oikos, we synthesise our current understanding of the functional consequences of changes in species richness in the marine realm. For those familiar with the field of biodiversity and ecosystem functioning, the first question might well be: do we really need yet another meta-analysis on this topic? I mean, really. There have been several meta-analyses published in recent years. Do we really need this work?

Well, our answer to the question is yes. Here’s why.

This paper started while we were synthesising data for general biodiversity-ecosystem functioning relationships at NCEAS  in Santa Barbara, USA. We realised that much data from the marine side was missing, as many of those studies did not fit the inclusion criteria set up for our original database. Previous meta-analyses1, 2 focused solely on how richness influences resource capture and/or the production of biomass. Marine studies, however, all over the map in terms of what functions they measured: resource use, biomass production, nutrient fluxes, trophic cascades, and so on.

gamfeldt photo 4

Panel with a sessile invertebrate community. Photo credit: Jarrett Byrnes.


So what’s the full picture of how biodiversity-ecosystem influences functions in the ocean – from primary production to biogeochemical cycles?

We got our hands on 110 marine experiments that manipulated the number of species and analysed some ecosystem response. In general, our analyses generally confirm previous findings that the average mix of species uses resources more efficiently and produces more biomass than the average monoculture. We honestly weren’t sure how this was going to fall out, and find great comfort in the generality of the result.

gamfeldt photo 2

Soft sediment microcosms, Sweden. Photo credit: Karl Norling.



In contrast, we find a different shape to relationship between biodiversity and ecosystem functions than has been seen previously. The relationship between species richness and production is best described as linear. The relationship between species richness and consumption appear to follow a power function. We find this by using new and more powerful techniques to describing the shape of relationships across multiple studies that we hope future researchers will use as well. (And, yes, we give you all of our code so that you can follow along at home!)


A seagrass field experiment in Finland. Photo shows a polyculture with three species. Photo credit: Camilla Gustafsson.


We also identify several gaps in our understanding of marine biodiversity and ecosystem functioning that are ripe for future investigation. First, the number of studies focusing on biogeochemical fluxes is still tiny. We need more. Second, we need more studies in pelagic and salt-marsh environments. Third, we still have only a handful of studies focused on predators. Fourth, the effects of increases in species richness (e.g. due to invasives or range shifts) are poorly understood. And last, we really only looked at relatively simple experiments, using on average only 3 species! We sorely need experiments targeting how spatial scale and heterogeneity, realistic local extinction scenarios from natural (read: large!) species pools, and functional and phylogenetic composition alter the relationship between biodiversity and ecosystem function.

To sum: there’s much work to be done, and we look forward with high hopes to the next generation of experiments exploring the consequences of changes in marine biodiversity.

gamfeldt photo 1

Three species of crab, used in the experiment in Griffin et al. 20083. Photo credit: Pippa Moore.


Now, if you had to explain this study to your mom or dad: the world has an incredible number and variety of different species, but we are losing them due to things like fishing, habitat destruction, and other threats from humanity. We need to understand what the consequences of these extinctions are for healthy and productive ecosystems, which is why researchers conduct experiments where they remove species and see what happens. We summarized data from 110 such experiments and found that losing species, on average, decreases productivity and growth, as well as a myriad of other processes related to how marine organisms capture and utilize resources, like nutrients. These processes ultimately put food on the dinner table and give us clean water. What is most interesting is we expected these declines to be non-linear based on previous studies: you can lose some species up to a point, then it starts to go downhill. The results from our analysis suggest that, for some processes, every species matters! Thus it is imperative that we protect and conserve biodiversity in our world’s oceans.

Lars Gamfeldt and co-workers


  1. Cardinale, B. J. et al. 2006. Effects of biodiversity on the functioning of trophic groups and ecosystems. – Nature 443: 989-992.
  2. Cardinale, B. J. et al. 2011. The functional role of producer diversity in ecosystems. – American Journal of Botany 98: 572-592.
  3. Griffin, J., de la Haye, K., Hawkins, S., Thompson, R. and Jenkins, S. 2008. Predator diversity effects and ecosystem functioning: density modifies the effect of resource partitioning. – Ecology 89: 298-305.


Posted by: oikosasa | November 12, 2014

Are mismatches the norm?

Conservation biologists and climate change researchers are worried by the observed phenological changes, that is, timing of biological events. These concerns are partly motivated by the expected species-specific and thus potentially non-parallell phenological shifts among interacting species, leading to what is often-named ’mismatches’ for ’plants (that) are finely tuned to the seasonality of their environment’.
These concerns are rarely accompanied by empirical data showing that phenological change leads to changes in fitness or population dynamics, and most often they focus on a single phase in the plant’s annual cycle. In our study, we observed the timing of flowering, fruiting, dispersal and germination and their effects on fitness components in the insect-pollinated, and bird-dispersed shrub Frangula alnus (Rhamnaceae).

Frangula alnus bicolored fruit display

Frangula alnus bicolored fruit display

In our study “Are mismatches the norm? Timing of flowering, fruiting, dispersal and germination and their fitness effects in Frangula alnus (Rhamnaceae)” we found that the effects of earliness (in phenological terms) varies between different phases and between different fitness components. Thus, we argue that the timing and temporal distribution of every single phase, e.g., flowering, fruiting or germination, is not at all finely tuned, but a robust compromise to selection pressures varying between phases and years.

Kjell Bolmgren and Ove Eriksson

Posted by: oikosasa | November 11, 2014

Elevation effects on body size

The higher up, the smaller the insects…or? Dispersing insects might be different. Read more in the Early View paper “Dispersal potential impacts size clines of grasshoppers across an elevation gradient” by Richard Levy and colleagues. Below is the author’s own summary of the study:

Insects found across elevation gradients that experience seasonality are commonly observed to become smaller with increased elevation. This results primarily from a reduction in season length at higher elevations, which selects for individuals that mature as early as possible, despite losing the benefits of a larger body. However, our study finds that this pattern can be completely negated in species of grasshoppers that exhibit morphologies and behaviors that increase their dispersal. To see if the nullification of this evolved pattern influenced the reproductive fitness of large bodied, high elevation grasshopper populations, we brought females back from the field and allowed them to lay clutches of eggs in the laboratory. The grasshoppers were then dissected and the functionality of their ovarioles (female insect reproductive organs) was analyzed. While we did find that ovariole functionality decreased due to higher dispersal, we were unable to measure any effect on the size and number of eggs laid. Overall, our study provides evidence that dispersal among populations can reduce or counter traits evolved to best suit local conditions.

Ovarioles from Melanoplus pellucida

Ovarioles from Melanoplus pellucida



Melanoplus dodgei from alpine site

Melanoplus dodgei from alpine site

Posted by: oikosasa | November 7, 2014

Multifractals in intertidal biofilms

Ecologists strive to understand the causes of the observed variability in population abundance and distribution. 1/f noise models and multifractals provide complementary conceptual and analytical frameworks to characterise variability in temporal and spatial series of environmental and ecological data. In our paper, “Multifractal spatial distribution of epilithic microphytobenthos on a Mediterranean rocky shore”, we combined these techniques to investigate the spatial distribution of epilithic microphytobenthos (EMPB) forming biofilms on rocky intertidal surfaces.


Marine biofilms, which mainly consist of photosynthetic organisms (diatoms, cyanobacteria and spores of macroalgae) embedded in a polysaccharide matrix, are important but almost neglected components of rocky intertidal habitats. Indeed, they substantially contribute to coastal primary productivity, provide food for grazing gastropods and facilitate the settlement of algal propagules and larvae of sessile invertebrates. Previous studies investigating the spatial distribution of soft bottom biofilms and periphyton communities highlighted that these microscopic organisms form complex multifractal spatial structures. On the bases of these results we hypothesized that power laws and multifractals could best describe the spatial distribution of rocky intertidal biofilms. We tested this hypothesis applying spectral analysis and multifractal geometry to nearly-continuous EMPB biomass data, obtained from calibrated colour-infrared images.


Our results support the hypothesis that 1/f noise spatial patterns are also multifractal. We interpreted these findings from two different but not mutually exclusive perspectives: either as the result of the superimposition of several biotic and abiotic processes acting at multiple spatial scales or as the hallmark of self-organization. Both interpretations stress the importance of local biotic interactions, either positive or negative, in shaping spatial pattern of distribution of EMPB biomass, while differing in the way environmental processes are supposed to affect microalgal abundance. The first interpretation is that environmental processes associated with temperature, insolation and moisture exert a direct effect on EMPB. Conversely, under self-organization, the influence of these abiotic variables is indirect, being mediated by the presence of the polysaccharide matrix in which microalgal cells are embedded.

Martina Del Bello

Posted by: oikosasa | November 4, 2014

Editor’s choice November

DriesAs announced in the August issue, Oikos is publishing meta-analyses at an increasing rate, and similar to the transformative capacity of the Forum section, a dedicated section associated with formalized, replicable systematic reviews and meta-analyses will also advance discovery and integration via effective curation. Chris Lortie will act as EiC for this section, and we strive to make all decisions and reviews with one month (provided referees respond in a timely manner), and referees will be selected to review not only the topic explored but also the elements of synthesis included. These efforts will be open-access published as editor’s choice to stimulate positive practices in our field more broadly and to facilitate longitudinal cross-study contrasts of ecological syntheses. Ecology is a very diverse discipline, and big-science ecology needs big bridges between our synthetic discoveries. Granda and colleagues’ meta-analysis on the physiological responses of woody plants to extreme climatic conditions was therefore selected as EiC for November. Understanding responses of species to winter cold and summer drought extremes is especially relevant in face of ongoing climate change. The authors compiled the existing literature on these responses of woody plants from temperate zone and show that that deciduous angiosperms were most sensitive to climatic stress and that evergreen species show less pronounced seasonal responses in both leaves and stems than deciduous species.

The October issue has been dedicated to a set of integrative research papers that bring synthesis on the functioning of soil food webs brought together by one of our editors Ulrich Brose. You can read more on this here. We jointly published with this special issue the forum paper of Fabrizio and colleagues as editor’s choice. They provide a concise synthesis on the role of top predators in food webs. Ecological research on the role of top predators in food webs is becoming increasingly important and popular in terrestrial (see for instance Boersma et al) and marine systems (e.g. Goyert et al; Rizzari et al. ) but also from a more theoretical point of view (e.g. Berg et al.). This exponentially expanding literature is, however, strongly associated with a rapid disintegration into specialized, disconnected subfields for study (e.g. vertebrate predators versus invertebrate predators, community ecology versus biological control etc.). The authors argue that this results in a loss of coherent, integrated understating of the role and importance of these species in ecosystems.

Posted by: oikosasa | October 24, 2014

Everything is connected – in nature too

You might, sometimes, have heard the phrase ‘everything is connected’. Maybe you are thinking about computers and mobile phones, but in fact this statement is particularly true in nature. For instance, we know that species are not isolated entities, instead they are part of communities in which multiple different species are interacting with each other. Some of these interspecific interactions are cooperative and positive for all interacting partners, and are called mutualistic interactions. Virtually all species on Earth are involved in one or more mutualistic interactions. Specifically, the interactions between plants and their pollinators may be some of the most studied ones, as nearly 85% of plants rely on animals for pollination service. In the last 20 years the study of pollination interactions using network analysis has been a hot topic in ecology. Networks have proven to be a useful tool to unravel patterns in plant-pollinator interactions at the whole community level. Usually, almost all plant-pollinator networks are constructed at the species-level (species-based networks), i.e. nodes in the network are plant and animal species and links represent the interactions occurring between them (e.g. flower visits). However, species are composed of populations of individuals and those individuals are the true actors establishing interactions in nature. Even more interesting is the fact that conspecific individuals are phenotypically and behaviourally diverse with respect to, e.g. size, sex, age, and social status, which also might imply that their foraging decisions become different. Most ecological networks studied to date have not considered this intraspecific variation in interactions, despite the importance of individual variation within natural populations addressed in the theory of evolution by natural selection. For that reason, moving from species-based networks to individual-based networks, to disentangle a process, which can be defined as network downscaling, is probably one of the major challenges right now in ecological network research.


Network downscaling. In traditional species-based networks each node represents a species (red nodes are pollinators and green ones are plants), but if we decompose a species into its constituting individuals we can obtain an individual-based network. In the figure, downscaling is only represented for the pollinator subset.

Network downscaling. In traditional species-based networks each node represents a species (red nodes are pollinators and green ones are plants), but if we decompose a species into its constituting individuals we can obtain an individual-based network. In the figure, downscaling is only represented for the pollinator subset.


In an attempt to fill this gap of knowledge, we got the idea of downscaling an entire pollination network to the individual level for the pollinator subset and explore network patterns at both interacting scales: species and individuals. This was possible with the study of pollen loads of insect individuals. Insect flower visitors in two mountain shrub communities from Mallorca (Balearic Islands) were captured, and later in the laboratory, pollen carried by each one was identified and quantified under the microscope. It was a highly time consuming and difficult task, but it paid well off as it provided a record of the flowering species visited by each individual pollinator over time. Data revealed that generalized species in the plant-pollinator network are composed of specialized and idiosyncratic individuals. The high heterogeneity in individual foraging behaviour and the high individual specialization of pollinators are obviously hidden in traditional species-based networks, and thus determine differences in several topological properties between species-based and individual-based networks. Particularly, the modular structure – a broadly described pattern in pollination networks which consists of densely connected groups or cliques of nodes with sparse connections to other groups– is not consistent across networks at the two scales. We found that modularity increases when downscaling networks to the individual level, and we confirmed this result using different modularity detection algorithms. In contrast to the view of modules as a set of taxonomically related species or species with convergent morphological traits in species-based networks, modules in individual-based networks are groups of functionally different pollinators distantly related but with overlapping pollen niches. Thus, interestingly, conspecific individuals are distributed in different modules. Modules showed to have a strong phenological component, and attributes related to the phenophase of plants and individuals even determined the topological roles of nodes in the network. Only when downscaling to the individual level it was possible to detect a dynamical interaction switching within-species and a module turnover throughout the flowering season, thus modules of individuals assembled and disassembled over time.

Study site. The study was conducted on two locations in Puig Major (1445 m), the highest mountain in Mallorca (Balearic Islands).

Study site. The study was conducted on two locations in Puig Major (1445 m), the highest mountain in Mallorca (Balearic Islands).

Methods. Pollinator observations were conducted in the field. Insects visiting flowers were captured and, later, their pollen loads were analyzed in the lab.

Methods. Pollinator observations were conducted in the field. Insects visiting flowers were captured and, later, their pollen loads were analyzed in the lab.


In conclusion, findings reported in our study, “Increasing modularity when downscaling networks from species to individuals”  (Tur et al.) highlight that network patterns differed across the individuals and the species scales, because much within-species variation exists. This implies that it is not always possible to deduce structure at one hierarchical level from information about structure at an adjacent level. Combining the study of networks at both scales offers the possibility of uncovering important properties and processes, which might influence network stability, dynamics and the outcomes of interactions.

Distribution of conspecifics into modules. One of the objectives in our study was to investigate whether individual-based networks were modular and if this was true, to analize how conspecific individuals were distributed among modules. There are two possibilities: (a) all conspecific individuals belong to the same module, or alternatively, (b) conspecific individuals belong to different modules. In most species we found ‘b’.

Distribution of conspecifics into modules. One of the objectives in our study was to investigate whether individual-based networks were modular and if this was true, to analize how conspecific individuals were distributed among modules. There are two possibilities: (a) all conspecific individuals belong to the same module, or alternatively, (b) conspecific individuals belong to different modules. In most species we found ‘b’.


Module turnover. When downscaling from species to individuals, a module turnover associated to seasonality was identified, so that at a given moment of the season there is predominance of a particular module of individuals. The complete individual-species network and the different slices of each month are shown in the figure.

Module turnover. When downscaling from species to individuals, a module turnover associated to seasonality was identified, so that at a given moment of the season there is predominance of a particular module of individuals. The complete individual-species network and the different slices of each month are shown in the figure.

By Christina Tur



Posted by: oikosasa | October 21, 2014

Marsupial browsing effects insect damages

Yes, made it through the wallaby attack!, No, no, no- no reason to celebrate Eucalyptus trees. Less marsupial browsing – opens up for more insects. Life is just not easy. Read more in the Early View paper “Direct and indirect effects of marsupial browsing on a foundation tree species” by Christina L. Borzak, Julianne M. O’Reilly-Wapstra and Brad M. Potts. Below is their summary of the study: Herbivores have impacts on plant survival, growth and form and these induced changes can have important flow-on consequences to subsequent organisms. Although a large number of studies in eucalypt systems have previously investigated vertebrate feeding preferences and the direct impacts of herbivory, few studies have focused on how herbivores interact to directly affect each other’s feeding preferences, and even less have addressed the indirect plant-mediated effects of herbivores. We investigated the direct and indirect effects of uncontrolled browsing by marsupial herbivores including the common brushtail possum (Trichosurus vulpecula), Bennetts wallaby (Macropus rufogriseus) and the red-bellied pademelon (Thylogale billardierii), in a Eucalyptus system known to have extended community and ecosystem genetic effects. In a common garden trial containing 525 full-sib Eucalyptus globulus families from an incomplete diallel crossing program located in north-eastern Tasmania, Australia, we assessed the genetic basis to herbivore preferences, the impact of a single and repeated marsupial browsing event on tree fitness and morphological traits and the associated indirect plant-mediated effects on a subsequent herbivore, autumn gum moth (Mnesampela privata).

We found that marsupial browsing was not influenced by plant genetics, but spatial components instead affected the pattern of damage across the trial. Marsupial browsing had significant impacts on tree development, morphology and survival, resulting in reductions in survival, height and basal area, an increase proportion in multiple stems, delays in flowering as well as delays in phase change from juvenile to adult foliage. Fitness impacts were minimal in response to a once-off browsing event, but effects were exacerbated when trees suffered repeated browsing.

Trait assessments under way at the Eucalyptus globulus trial site by authors Christina Borzak (left) and Julianne O’Reilly-Wapstra (right).

Trait assessments under way at the Eucalyptus globulus trial site by authors Christina Borzak (left) and Julianne O’Reilly-Wapstra (right).

Assessments of autumn gum moth damage showed that their presence was linked to marsupial browsing, with browsed plants being less susceptible to the insect herbivore. The majority of the effect was attributed to the indirect effects of browsing on tree height, where AGM were attracted to taller trees that were not browsed. Such indirect effects have the potential to influence biotic community structure on a foundation species host-plant, and the evolutionary interactions that occur between organisms and the host-plant themselves.

Predicting herbivore intensity in disturbed habitats is not as easy as it might seem… Results were a bit surprising in “Land-use legacies and present fire regimes interact to mediate herbivory by altering the neighboring plant community” by Philip G. Hahn and John L. Orrock. Below is the author’s summary of the study:

The southeastern United States was once teaming with biodiversity in the sprawling, open pine savannas that stretched from Virginia to Texas. Post-settlement, these biodiversity hotspots were quickly reduced to less than 3% of their original extent, largely through conversion to agriculture and fire suppression. More recently, many agricultural fields have been abandoned and replanted with pine trees. Although these degraded woodlands harbor low levels of biodiversity, they offer tremendous potential to restore lost species. Particularly, ecologists know very little about interactions among plants and insects in these degraded ecosystems. Hypothetically, insect herbivores, such as grasshoppers, could be suppressing plant diversity in these post-agricultural woodlands by preferentially consuming more palatable remnant wildflowers that attempt to reestablish.


The sun rises over a rare remnant longleaf pine savanna, fueling a motley array of biological interactions.

We tested this idea by transplanting native plants into herbivore exclosures within longleaf pine stands on historic agricultural sites. In order to compare disturbed and undisturbed longleaf pine savannas, we also located several stands of remnant longleaf pine savanna. Because some of these stands experienced woody encroachment due to fire suppression, we crossed fire frequency with historical land use as a component of our experimental design. This created a gradient of degradation with either low or high fire frequency stands within post-agricultural or remnant woodlands.

After measuring herbivore density and herbivory rates on our experimental plants for a field season, we found that sites with low levels of plant cover supported small populations of herbivorous grasshoppers, which resulted in low herbivory rates on our experimental plants. These sites were usually degraded by historic agriculture and were extremely fire suppressed.


Sites representing the range of neighboring plant cover at our experimental sites. Insect exclosures or control cages (with holes) were placed over a set of experimental plants.


There were more grasshoppers at sites with extremely high levels of plant cover. Herbivory rates were expected to be higher at these sites because there were so many grasshoppers. As it turns out, herbivory rates were actually low because there were many more neighboring plants for grasshoppers to consume. In other words, high abundance of neighboring plants swamped out the negative effect of herbivory on the focal plants. These sites with low herbivory rates tended to be frequently burned remnant sites, meaning that remnant sites can support high populations of both plants and grasshoppers, while minimizing the negative effects that herbivores have on plants.

We found the greatest herbivory rates at intermediate levels of plant cover, where grasshoppers were also in intermediate abundance. These sites tended to historically be used for agricultural or were fire suppressed remnants. In other words, moderately degraded sites had the highest rates of herbivory.


Sites representing the range of neighboring plant cover at our experimental sites. Insect exclosures or control cages (with holes) were placed over a set of experimental plants.

Data being generated

By demonstrating that past and present human activities play a key role in present-day plant-herbivore interactions, our work has several important implications for basic and applied ecology. The findings provide a starting point to predict when and where herbivore density or neighboring plants will be important drivers of herbivory. The results also have implications for the recovery of biodiversity in post-agricultural lands and other systems affected by human disturbance by generating predictions about which habitat types will be more susceptible to herbivores.

Posted by: oikosasa | October 14, 2014

The struggle for safety: caterpillar against birds

Aptly described by the naturalist Arthur-Miles Moss, the life of a caterpillar is a virtual struggle for safety from formidable predators, ruthless parasites, and fatal pathogens. To cope, caterpillars possess an array of anti-predator adaptations, or defenses, which aid them in the struggle. An individual caterpillar might employ camouflage, chemical defenses, hairs, spines, and aggressive behaviors to escape or repel its enemies. Despite the fact that these defenses constitute some of the classic examples of adaption, we still know very little about their effectiveness against predators in a natural community context.


The bold, gaudy colors of certain insects have arrested the eye of many a naturalist. Alfred Russel Wallace and Henry Bates, who collected butterflies in the Amazon, proposed an ingenious evolutionary explanation for the flashy appearances of many insects. They argued that conspicuous colors on insects are actually warning signals to would-be predators, such as birds, which advertise underlying chemical defenses. The stronger the signal, the clearer would be the message: “Don’t mess with me, I’m poisonous.” Thorough experimental confirmation of this idea, called aposematism, did not come until the second half of the 20th century.


Figure 1. Aposematic caterpillar of the monarch butterfly on its milkweed host plant. Photo by Michael S. Singer.

The great Victorian naturalists likewise surmised that camouflage was another important defense of insects against bird predation. Observations of caterpillars, katydids, and walking sticks in their natural environments revealed a wondrous precision in the match between the insect’s appearance and the vegetation upon which it lived. Darwin’s and Wallace’s theory of natural selection was the best scientific explanation: only the best camouflaged individuals of each species would escape detection by predators.


Figure 2. Camouflaged inchworm caterpillar on its host plant, manzanita, in southern Arizona. Photo by Michael S. Singer.

Figure 2. Camouflaged inchworm caterpillar on its host plant, manzanita, in southern Arizona. Photo by Michael S. Singer.


Over the last 50 years, many additional, important observations and experiments have reinforced these evolutionary theories of prey defense. But the vast majority of experimental studies used captive avian predators or artificial prey (such as dead or artificial insects) exposed to wild birds, leaving some question about the effectiveness of aposematism and camouflage in natural predator-prey interactions, which are notoriously difficult to observe directly in the wild. In the mean time, new techniques and technologies have emerged that allow researchers new modes of studying prey defenses in the wild.

Figure 3. Black-capped chickadee with a captured caterpillar in one of the forest sites used to study bird predation of caterpillars in Connecticut. Photo by Christian Skorik.

Figure 3. Black-capped chickadee with a captured caterpillar in one of the forest sites used to study bird predation of caterpillars in Connecticut. Photo by Christian Skorik.


Enter the new study by Lichter-Marck and colleagues, “The struggle for safety: effectiveness of caterpillar defenses against bird predation.” This study used a bird-exclusion field experiment set in the forests of Connecticut, USA to test evolutionary theories of prey defense in the context of a natural ecological community. Over four years, the research team surrounded hundreds of experimental tree branches with garden-variety bird-proof netting, matching each experimental branch with a control branch lacking netting. The netting was applied at the beginning of each growing season, and allowed caterpillars to come and go while preventing access to insectivorous birds. Three weeks later, the researchers returned to each branch and collected the caterpillars living on them. By experiment’s end, the caterpillar species with the largest proportional increase in numbers in experimental branches (protected from bird predation) relative to control branches (open to bird predation) were inferred to suffer the most bird predation. By measuring the defensive traits of each caterpillar species and correlating them with the inferred magnitude of bird predation, the researchers could determine which traits were most effective as defenses against bird predation.



Figure 4. Red maple branch covered with a bird-exclusion net, one of the experimental branches used in this study. Photo by Christian Skorik.

Figure 4. Red maple branch covered with a bird-exclusion net, one of the experimental branches used in this study. Photo by Christian Skorik.


This unique methodological approach supported the main prediction of aposematism theory: among the 38 species of caterpillars that were numerous enough to analyze, those that possessed warning signals, such as bright coloration, received the most protection from birds. But the study revealed another, critical aspect of the warning strategy of defense: stereotypical resting location. The caterpillar species most protected from birds combined warning signals with highly stereotypical resting locations on the plant. That is, their appearance and their location together provided the warning signal to birds. This finding highlights the relatively neglected behavioral aspect of warning strategy of defense.


Figure 5. A warningly-signaled caterpillar species (Nola triquetrana) on its host plant, witch hazel, at one of the forest sites used in this study. Photo by Michael S. Singer.

Figure 5. A warningly-signaled caterpillar species (Nola triquetrana) on its host plant, witch hazel, at one of the forest sites used in this study. Photo by Michael S. Singer.


Yet the majority of the 38 caterpillar species did not possess warning signals, instead employing camouflage as their primary defensive strategy. Because visual camouflage can result from several different tricks in appearance (disruptive patterns, countershading, etc.), the magnitude of camouflage can be difficult to measure. The researchers turned to an increasingly used method, the human proxy predator assay. In this assay, human participants were shown digital images of the caterpillar species resting on their host plants, and a computer program was designed to record how quickly each participant located each caterpillar with an accurate click of the mouse. The longer it took, on average, to find a caterpillar species, the greater the magnitude of camouflage was inferred.


Figure 6. A camouflaged caterpillar species (Catocala ultronia) on its host plant, black cherry, at one of the forest sites used in this study. Photo by Michael S. Singer.

Figure 6. A camouflaged caterpillar species (Catocala ultronia) on its host plant, black cherry, at one of the forest sites used in this study. Photo by Michael S. Singer.


In support of evolutionary theory of prey defense, the study found that the caterpillar species with the greatest inferred magnitudes of camouflage received the most protection from birds. Once again, this part of the study revealed a behavioral twist. A caterpillar species’ frequency of behavioral responsiveness to attack, measured independently, worked against the effectiveness of camouflage. This finding suggests that effective camouflage requires not only an appearance that matches the prey’s background, but also behavioral maintenance of the cryptic posture in the face of physical disturbance.

The authors, through Michael S. Singer

Posted by: oikosasa | October 10, 2014

A voyage into soil darkness

While most people know the aboveground part of forest ecosystems, very few have caught a glimpse of the belowground environment that comprises a highly diverse fauna. The number of species co-occurring on less than a square meter habitat ground (or a cubic meter of habitat volume) exceeds that of the aboveground compartment by far. In consequence, forest soil communities have been called “poor man’s rainforest”. Nevertheless, we still do not know much about the animals living in these “next-door” habitats and the structure of their communities.


Impression of a central European beech forest. Much more is known about the aboveground animals and their interactions than about the belowground communities that carry out the critically important ecosystem functions of litter decomposition and nutrient recycling.


Why is our knowledge about forest soil communities so limited? Progress in our understanding of soil communities and processes has been hampered by the chronic lack of data for complex soil food webs of high resolution. This is caused by aggregation of populations in coarse functional groups, whose species often span multiple trophic levels from primary to secondary or tertiary predators. In addition, soil is an opaque medium leading to a limited visibility of interactions. Further, detritivores typically ingest a multitude of intermingled resources hampering identification of what the animals actually digest and live on. In the recent years, new molecular methods have emerged providing the possibility to unravel belowground interactions and the complex structure of forest soil food webs.


A soil core provides an impression of the complex structure of the belowground habitat. This environment comprises a highly diverse and complex animal community spanning several trophic levels.

A soil core provides an impression of the complex structure of the belowground habitat. This environment comprises a highly diverse and complex animal community spanning several trophic levels.

The special issue “Into darkness” comprises several studies of central European beech forest soil communities. The studies included in this special feature fill employ state-of-the-art methods to unravel general feeding guilds by stable isotopes (Klarner et al.) as well as specific directed feeding interactions by molecular gut content and fatty-acid analyses (Ferlian and Scheu, Günther et al., Heidemann et al.). This allowed the construction of the first highly-resolved complex soil food webs (Digel et al.) and analyses how they respond to external drivers such as the nutrient stoichiometry of the basal litter (Ott et al.) and climate change (Lang et al.). Together, they provide a unique impression of a voyage into darkness.

Ulrich Brose, Editor of the Oikos Issue “Into Darkness”



Posted by: oikosasa | October 7, 2014

Drought – increase or decrease herbivore abundance?

Grasshoppers tend to increase in abundance during drought, no decree, or increase…Find out which and when in the Earl View Oikos paper “Water stress in grasslands: dynamic responses of plants and insect herbivores” by Paul A. Lenhart and co-workers. Below is their summary of the study:

When I first saw the climate projections from NOAA in 2011 that there would be a severe La Niña-fueled drought I was worried that my fieldwork season would be a bust. In 2011, Texas, as well as much of the south central United States of America, suffered through the worst seasonal drought since modern record keeping began in 1895. The drought had severe economic and ecological impacts across the region, but I was focused on my main study organism: grasshoppers. These insects are a very important component of grassland ecosystems, and for the past two years I, together with my co-supervisors (Micky Eubanks and Spencer Behmer), had worked in the grasslands and savannah of central Texas studying a vibrant grasshopper community, consisting of over 56 species. I was working to understand the diet breadth of some of the key species, including their macronutrient regulation behavior, while also quantifying competitive dynamics between these species.


Examples of grasshopper diversity. Clockwise from the top left: Melanoplus packardii, Hadrotettix trifasciatus, Acrolophitus hirtipes, Phaulotettix eurycercus.


Prior to 2011, one of our sampling seasons (2009) was slightly drier and we found a decrease in grasshopper density and abundance. This went against many previously published observations of grasshoppers and other insect herbivores having larger populations in drier years. The proposed mechanism in the literature is that plant nutrient content actually increases with water stress. However, studies that measure the effect of water stress on plant nutrients typically use greenhouse-reared plants or crop species, and generally measure plant quality as simply a function of nitrogen content. We now know that plant dietary quality is much more complex, and in particular that herbivores actively regulate their protein and carbohydrate intake. Therefore, we decided to change course from our originally planed competition experiment. Instead, we took advantage of the coming drought to conduct a manipulative study in order to gain insights into what happens to native plants, and herbivore behavior, when the rains do not come

            We started our experiment early enough in the season to quantify, over time, the effect of water stress on the native grassland plant’s quality, quantity, and diversity. We marked off small open plots distributed across the grasslands of the Balcones Canyonlands National Wildlife Refuge. Half of these plants were left alone to suffer through the drought and we watered the other half [laboriously] by hand to mimic average summer rainfall. We did this through the growing season and took plant samples and visual grasshopper surveys monthly; in each plot individual grasshoppers were identified to species by sight. After completing each grasshopper survey, we measured grass and forb species richness, and took samples back to the lab to assess biomass and macronutrient content. Specifically, we quantified both digestible protein and nonstructural carbohydrate content in bulk samples of grass and forb tissues using biochemical assays.


Behind the scenes of watering plots by hand in the field.

Behind the scenes of watering plots by hand in the field.


            At the end of the summer we found that drought reduced grasshopper abundance and diversity, relative to our water supplemented plots. Using our knowledge of different grasshopper species diets, we grouped species into functional feeding groups and found that functional groups responded differently to our watering treatments. Forb specialists seemed unaffected by the drought while grass-feeders and mixed-feeders (grass+forbs) were significantly less numerous in the unwatered plots. These different grasshopper responses were due to their particular feeding biology and the fact that grass and forbs responded differently to water stress. We go into more detail in the manuscript, but in short, forbs decreased in diversity and experienced a significant shift in their macronutrient profile over time, becoming less protein biased. In contrast, grass biomass was reduced by water stress, but grass protein-carbohydrate content was similar between our two water treatments.


A freshly watered plot in a parched grassland.

A freshly watered plot in a parched grassland.


Our results are significant because we used naturally-growing, drought acclimated plants, and quantified protein-carbohydrate content – which are the two most important nutrients that affect insect herbivore feeding behavior and performance. Our research provides valuable data on how plant macronutrient content, biomass, and diversity co-vary in the field, and such data can be used to parameterize models that can help us better understand how generalist herbivores forage and perform under drought conditions, which are predicted to become more common as climate change intensifies. Although more work is required, we envision the use of remote sensing technology, measuring plant quality, biomass, and diversity, to better manage insect pests in rangeland ecosystems.


Mixed-grass oak savannah on the Balcones Canyonlands National Wildlife Refuge during a wet summer.

Mixed-grass oak savannah on the Balcones Canyonlands National Wildlife Refuge during a wet summer.

Posted by: oikosasa | October 3, 2014

What exactly can network models predict?

Ecological networks quantify the diversity of direct and indirect interactions taking place in nature. However, due to their complexity, ecologists rely heavily on the use of metrics to summarize aspects of network structure thought to be of biological importance. Many of these structural features are non-random and strongly conserved across diverse habitats and species assemblages, begging the question: what factors determine network structure? The most successful hypotheses to explain these patterns are the neutrality and biological constraints hypotheses, which posit that species interactions can be explained by trait mismatches, and relative abundances respectively. In the Early View paper “Species traits and relative abundances predict metrics of plant-pollinator network structure, but not pairwise interactions” in Oikos, we Colin Olito and Jeremy W. Fox, evaluate the relative ability of trait-based and neutral models of species interactions to explain the structure of a temporally resolved alpine plant-pollinator visitation network.


An unidentified muscid visiting Erigeron peregrinus. Although their charm often goes unappreciated, flies are by far the most diverse and abundant pollinators in the alpine. Interestingly, many of their behaviours that facilitate pollination differ markedly from more intensively studied foraging pollinators, such as bumblebees. Understanding their crucial role in alpine and high-latitude plant-pollinator communities will require a greater understanding of both their reproductive and foraging biology. Photo credit: Martin Fees.

An unidentified muscid visiting Erigeron peregrinus. Although their charm often goes unappreciated, flies are by far the most diverse and abundant pollinators in the alpine. Interestingly, many of their behaviours that facilitate pollination differ markedly from more intensively studied foraging pollinators, such as bumblebees. Understanding their crucial role in alpine and high-latitude plant-pollinator communities will require a greater understanding of both their reproductive and foraging biology. Photo credit: Martin Fees.

As our title suggests, species traits and relative abundances successfully predicted every metric of network structure tested, but failed to predict observed interactions. That is, a variety of models can predict network metrics well, but for the wrong reasons. We explore the implications of this contrast, and highlight potential problems with the use and interpretation of network metrics. We also found that species phenologies (the timing of flowering or pollinator activity) always out-performed neutral models at predicting pairwise interactions, and discuss limitations of neutral models of network structure, particularly when species interactions are under-sampled. We suggest that future progress in explaining the structure and dynamics of ecological networks will require new approaches that emphasize accurate prediction of species interactions rather than network metrics, and better reflect the biology underlying species interactions.

Sampling plant-pollinator interactions in a low-alpine meadow in Kananaskis Country, Alberta, Canada. Photo credit: Martin Fees.

Sampling plant-pollinator interactions in a low-alpine meadow in Kananaskis Country, Alberta, Canada. Photo credit: Martin Fees.

Posted by: oikosasa | September 30, 2014

What causes dialects in bat?

Bat acoustic signals might seem rather simple. yet, there are individual differences. The background to these variations are studied in the Early View paper “Geographical variation in echolocation vocalizations of the Himalayan leaf-nosed bat: contribution of morphological variation and cultural drift” by Aiquing Lin and co-workers. below is their summary of the study:

Animals’ acoustic signals often vary geographically—but how and why? We studied the geographical variation in echolocation vocalizations of a widespread bat species Hipposideros armiger sampled from 17 localities in South China. We asked whether there was detectable population divergence in the vocalizations and whether the acoustic divergence was related to the variation in morphological (forearm length), climatic (mean annual temperature, mean annual relative humidity, and mean annual precipitable water), geographical (latitude, longitude, elevation, and geographical distance), or genetic (genetic distance and population genetic structure) factors. We found remarkable geographical variation in the peak frequency of echolocation pulses of H. armiger, which clustered into three groups: Eastern and Western China, Hainan, and Southern Yunnan. The acoustic divergence was significantly related to morphological differences and geographical distance, but not significantly related to climatic (after controlling for morphological distance) or genetic variation. We also found a correlation between population differences in morphology and climatic variation (mean annual temperature). Our results suggest the action of both indirect ecological selection and cultural drift promote divergence in echolocation vocalizations of individuals within geographically distributed populations.


Figure 1 Himalayan leaf-nosed bat (Hipposideros armiger)



Figure 2 Oscillogram (above) and Spectrogram (below) of an echolocation pulse of Hipposideros armiger. fpeak = peak frequency.

Figure 2 Oscillogram (above) and Spectrogram (below) of an echolocation pulse of Hipposideros armiger. fpeak = peak frequency.

Posted by: oikosasa | September 26, 2014

Costs of living in a nest

Male of the harvestmen Zygopachylus albomarginis (with yellow ink marks) inside his mud nest, while a female approaches from the outside [Credit: Gustavo S. Requena]

Male of the harvestmen Zygopachylus albomarginis (with yellow ink marks) inside his mud nest, while a female approaches from the outside [Credit: Gustavo S. Requena]

Nests are extremely important for males’ fitness when reproduction and parental care are associated with these structures. The possession of a nest and its conditions may determine male attractiveness (due to female reproductive decisions) and offspring survival (due to protection against adverse biotic and abiotic conditions). Nest construction and maintenance, however, may also impose costs to males: nest-related behaviors may demand time and energy or may increase mortality risks. The costs and benefits approach is usually used to understand the evolution and maintenance of behavioral traits, and we explored this framework in a study with the Neotropical harvestmen Zygopachylus albomarginis.


During the breeding season, nesting males of Z. albomarginis spend several months building, repairing, cleaning and defending their mud nests. After mating, females abandon the eggs under the protection of males, who actively defend them against predators and fungal infection. Although nest defense, nest maintenance, and offspring protection contribute to different components of males’ fitness, they are performed concomitantly and entail similar behaviors. For instance, when a nesting male chases away a conspecific individual, he defends the possession of his nest at the same time he protects the offspring against a potential egg predator (see video below). Moreover, nest maintenance requires males to remove debris and prevent fungal growth inside the nest, actions that also contribute to protect eggs against infection.


VIDEO: [Credit: Gustavo S. Requena]


In our Early View Paper “Lack of costs associated with nest-related behaviors in an arachnid with exclusive paternal care”, we quantified the costs of nest-related behaviors in Z. albomarginis under natural conditions. Because males are mainly constrained to forage in a small area close to the nest for up to five months, we expected high energetic costs of being associated with a nest. However, we did not find any evidence of decline in the physical conditions of nesting males over time. Interestingly, males may spend several days eating fungal hyphae growing inside their nests, which we suggest constitutes an important food resource to stationary individuals and compensates for energetically costly activities performed for so long periods.



 At the left, we can see a male inside his nest on a fallen trunk without fungus infestation. At the right, the trunk is covered by fungus fruiting bodies, except inside the nest. Nest-cleaning behavior maintains hygienic conditions inside the nest at the same time it provides food to the male, which feed upon the fungus hyphae. [Credit: Gustavo S. Requena]

At the left, we can see a male inside his nest on a fallen trunk without fungus infestation. At the right, the trunk is covered by fungus fruiting bodies, except inside the nest. Nest-cleaning behavior maintains hygienic conditions inside the nest at the same time it provides food to the male, which feed upon the fungus hyphae. [Credit: Gustavo S. Requena]


Due to contest injuries over the possession of a nest or its conspicuousness, we also expected high mortality risks associated with nest-related behaviors. The survival probabilities of stationary nesting males, however, were higher than the probabilities of vagrant individuals not associated with nests surviving. This pattern of differential mortality dependent on Z. albomarginis movement activity may be explained by the potential higher chances of encountering predators while moving, particularly walking among trees and crossing the leaf litter.


Given that females lay eggs exclusively inside nests and the costs of nest maintenance and defense are extremely low (if not absent), the million dollars question is “why do not all males have a nest?” Males add salivary secretions to the mud at the moment they build the nests. One possibility, therefore, is that the production of such secretion is costly and only males in good body condition would be able to invest in nest construction. Although the costs of performing this activity was not evaluated in our study, the fact that vagrant males may occupy an empty nest or even aggressively attack a nesting male and take over his nest suggests that some individuals rely on usurpation as an alternative reproductive tactic to acquire nests.



Male resting inside his nest, which contains several black eggs (indicating advanced embryonic development) [Credit: Gustavo S. Requena]

Male resting inside his nest, which contains several black eggs (indicating advanced embryonic development) [Credit: Gustavo S. Requena]

The authors through Gustavo S Requena

Posted by: oikosasa | September 23, 2014

Towards the catastrophic collapse?

What happens when climate change destroys too many habitats? In the Early View paper “Robustness of mutualistic networks under phenological change and habitat destruction” Tomás A Revilla and co-workers present a model predicting potential outcomes.
Below is a short summary of their model and paper:
There is concern that climate change will disrupt the temporal schedules of interactions between plants and their pollinators or seed dispersers. This can make communities vulnerable to other ecological threats, for example habitat destruction. In our paper “Robustness of mutualistic networks under phenological change and habitat destruction”, we studied the simultaneous effects of phenological shifts and habitat destruction on the diversity and structure of mutualistic metacommunities.
We created a spatially-explicit model, in which the network of mutualistic interactions is locally determined by species occupancies, over a finite number of randomly distributed sites. The strengths of the interactions depend on the amount of phenological overlap between the species, affecting local survival. Our model uses empirical data on plant and pollinator phenologies recorded a century ago by Charles Robertson, and in present times and in the same area by Burkle et al (DOI: 10.1126/science.1232728), giving us the opportunity to simulate projected as well as historical changes in phenology. Habitat destruction was simulated by removing sites from the model.

Interactions depend on local species presences and phenological overlaps (in days). Extinctions are caused by site destruction (X) and interaction weakening, e.g. pollinator 1 becomes disconnected and plant 2 losses many interaction-days.

Our results show that habitat destruction causes the gradual erosion of local diversity. A catastrophic collapse in global diversity finally happens when the number of non-destroyed sites becomes too low, and the distances between them too large for recolonization. Recovering from such collapses could be difficult due to the positive feedbacks characterizing mutualisms, which promote alternative stable states.
Under phenological shifts interactions become weaker on average, increasing local extinction rates. When habitat destruction and phenological shifts occur together, they act synergistically: many sites become barren even though they are not destroyed, but for practical purposes these sites behave as if they were destroyed, making metacommunities even more vulnerable to habitat destruction.
Decrease in local and global diversity in response to habitat destruction, for 0, 10, 20 and 30 days of average advance in species phenologies.

Decrease in local and global diversity in response to habitat destruction, for 0, 10, 20 and 30 days of average advance in species phenologies.

Previous research has shown that connectance and nestedness can make mutualistic communities robust against cascading extinctions. We discovered that in effect, these network properties gradually decline with habitat destruction, leaving a very small core of highly connected sites holding the metacommunity before the final collapse. Small alterations in phenology can raise connectance a little bit, due to a few generalist species being able to make new interactions, but overall, large alterations tend to reduce connectance.
We can conclude that the robustness of mutualistic metacommunities against habitat destruction can be greatly impaired by the weakening of mutualistic benefits caused by the loss of phenological overlap.
Image credits: Michel Loreau

Image credits: Michel Loreau

We wish to thank Jacob Johansson, Niclas Jonzén and Jan-Åke Nilsson for inviting us to contribute with our paper to the special issue about “Phenological change and ecological interactions”.
Posted by: oikosasa | September 19, 2014

Finding the food in complex environments

Many animals locate resources and orient in rather complex environments like vegetation, coral reefs or leaf litter. How does the presence of a stimulus affect animal movement in such complex environments? And what is the relative contribution of a stimulus vs. the complexity of the environment on animal movements? Find out in the Early View paper “Relative roles of resource stimulus and vegetation architecture on the paths of flies foraging for fruit” by Oriol Verdeny-Vilalta and co-workers.

Below is the author’s summary of the study:

To answer the questions above, we developed a novel method using random walks on graphs to accurately estimate both the perceptual range and the attraction strength from 3D movement trajectories of individuals. The perceptual range gives us an idea of the maximal distance at which a stimulus source biases the movement. The attraction strength measures how the attraction of the stimulus varies at different distances within the perceptual range. Additionally, the methodology enabled us to calculate the relative roles of the architecture of vegetation and of the strength of attraction of a stimulus on the movement of individuals. We applied the methodology to estimate perceptual range and strength of apple maggot flies (Rhagoletis pomonella) foraging for artificial fruit in apple trees of varying complexity.



Figure: Main steps (a-d) followed to study animal orientation in complex vegetation structures.


In our study we have shown that, conditional on visiting the stimulus location, the presence of the fruit affects animal movement much more than the plant architecture. Moreover, we found that plant complexity makes a minor contribution to defining the perceptual range, but a large contribution to the attraction strength. Thus, we highlight the importance of estimating not only the perceptual range but also the attraction strength of animals, which has been traditionally neglected. Our findings have implications for studying foraging ecology and landscape connectivity. For example, several dispersal models developed to study landscape connectivity incorporate the perceptual range of individuals, but the distinction between perceptual range and attraction strength is still lacking. We expect that landscape connectivity will be higher in animals showing higher attraction strengths for equal perceptual ranges. Given that animals use their sensory systems to make informed decisions and that they move and interact in heterogeneous environments, our approach might be of relevance to the myriad of animals walking and searching in complex environmental structures.

When habitat is lost so are species. One way of investigating the processes underlying this pattern is to pay attention to the identity (not only the number) of species. What happens to between-site differences in species composition when habitat loss transforms formerly continuous habitat into habitat fragments?

Who consults widely applied theoretical frameworks (e.g. theory of island biogeography) to answer this question will come to the conclusion that between-site differences in species composition – i.e. beta-diversity – should increase following habitat loss due to a strong influence of chance on the extinction process. Species are assumed to be ecologically equivalent (all have the same chance of getting extinct) and ecological drift (stochastic changes is species abundance) to increase in importance when populations are small. Further, chance makes it unlikely that populations surviving in different habitat remnants belong to the same species, and homogenization is hindered by isolation.


Who, on the other hand, consults empirical work will find that for various groups of plants and animals it is common to observe that, of the diverse set of species in continuous habitats, it is frequently the same small set of species that persists after habitat loss. Apparently, only certain resistant species are able to survive in fragments, thereby making the species composition in fragments deterministically more (and not less) similar, indicating – in contrast to theoretical models – low influence of chance on species extinction.

In our study “Ecological filtering or random extinction? Beta-diversity patterns and the importance of niche-based and neutral processes following habitat loss we investigated how the importance of different processes changes with habitat loss relying on a large database of small mammals in the Brazilian Atlantic Forest. We used a null model approach to quantify beta-diversity and make inferences about the relative importance of niche-based (deterministic) and neutral (stochastic) processes on community assembly at landscapes with varying degree of habitat loss.


Our results did not support a positive relationship between beta-diversity and habitat loss, as predicted by commonly-used theoretical frameworks. Rather, when considering exclusively species composition (disregarding their abundance), beta-diversity was independent from habitat loss, with small mammal communities being more similar than expected by chance in deforested as well as continuously-forested landscapes. However, when species abundance was taken into consideration, we observed a drastic decrease in beta-diversity with habitat loss (i.e. biotic homogenization), thereby indicating an increase (rather than a decrease) in the importance of deterministic processes at landscapes with high degrees of habitat loss. Finally, we observed a drastic change in species composition in a highly deforested landscape, with communities being not just a rarefied sample but rather disproportionately dissimilar to the communities in continuously-forested landscapes.


These results indicate that habitat loss can be seen as a strong ecological filter and species extinction is clearly more influenced by deterministic than by stochastic processes. Against this background, the incorporation of relevant species traits into theoretical models seems to be a useful step forward for the practical relevance of these models. Moreover, pro-active measures seem to be essential to prevent tropical landscapes to go beyond critical levels of habitat loss.

The authors through Thomas Püttker

Posted by: oikosasa | September 12, 2014

White stork modelling

Understanding lifetime tracks and fitness of long distance avian migrants. This is the title of our DFG-funded German-Israeli Project Cooperation and it is also our quest for several years. Within this project, we aim to explore how movement, survival and reproduction reflect an optimal response to the environment. Evidence is drawn from both theoretical and empirical analyses. Migrants like white storks are particularly interesting for studying these questions as they move large distances and may experience different environmental conditions in different parts of the world with more or less strong impacts on their fitness (carry-over effects). Small-scale movement and behavior and their impact on local population dynamics are equally interesting. Latest technologies allow us unprecedented insights into the life of animals. For example, ultra-light GPS tags allow tracking individuals with very high temporal resolution and over several years, and acceleration measurements allow classifying behavior from distinct acceleration signals. These data together with careful monitoring provide the means for better understanding movement phenomena and their consequences for population dynamics and fitness. Juni11 103 Mit_Sender

My main focus within the project are developing behavior-based models for different life-cycle stages (e.g. breeding, migrating, wintering) as well as annual-cycle models that allow studying carry-over effects on individual fitness and population dynamics. Thereby, optimality is an important topic. From evolutionary perspective, fitness-maximizing, optimal behavioral strategies should evolve, determining for example when an individual should start reproducing or start migrating within the annual cycle. On finer temporal and spatial resolution, optimal foraging strategies should evolve which are the focus of our study ‚Individual-based modeling of resource competition to predict density-dependent population dynamics: a case study with white storks‘ (Zurell et al.). Here, we aimed to better understand how density-dependent demographic rates may evolve from home range behavior. To this end, we built an individual-based model for foraging white storks that incorporates both physiology and behavior. We expected that the form of density dependence may differ between different home range behaviors. To our surprise, we also found that it may differ strongly between landscapes with the same degree of fragmentation and the same overall resource availability. This phenomenon is strongly affected by the behavioral trade-offs and by imperfect detection of resources. Thereby, simulated patterns corresponded surprisingly well to empirical patterns although the model was not calibrated. For predicting population or even community dynamics under changing environmental conditions, it seems crucial to better understand these interactive effects of behavior and local environment.

We heartily invite you to play around with the model code (available at and adapt it to your needs. As you will see, the model also allows exploring additional aspects of movement ecology, for example studying movement paths or density-dependent home range structures in more detail.

Posted by: oikosasa | September 11, 2014

Can viruses alter host behavior?

How can viruses alter the behavior of the hosts? And to what effect? Find out in the Oikos Early View paper “Virus infection alters the predatory behavior of an omnivorous vector” by Candice A Stafford-Banks and colleagues. If you click on the link below, they will tell you all about it in a short presentation.



Oikos presentation Stafford-Banks

Posted by: oikosasa | September 9, 2014

Better being early?

If invaders do better by early arrival and growing, will native species also benefit from being early? Not necessarily, as found in the Early View paper “Priority effects vary with species identity and origin in an experiment varying the timing of seed arrival” by Elsa E. Cleland and co-workers. Below is their summary of the study and a photo of the students helping out with field work.

Studies show that exotic species differ in phenology (i.e. are active at different times in the season) from the native species in the communities they invade. In Southern California many of our common invaders are exotic annual grasses and forbs that germinate earlier with the onset of winter rains than native herbaceous species. Hence, exotic species might benefit from emerging earlier in the season, allowing them to pre-empt space and other resources to suppress later emerging species, a kind of seasonal priority effect. We tested this hypothesis in an experiment varying the “arrival” time of pairs of species, by placing seeds of focal species into pots of field-collected soil either simultaneously or one week apart. In contrast to our expectations, native species benefited from earlier arrival more often than exotic species. An important implication of this finding is that giving native species a long “head start” likely aids in ecological restoration efforts.

Then, if being active early is so beneficial, why don’t native species have earlier phenology? Isn’t there sufficient selective pressure to favor earlier phenology in native species? Two additional aspects of our experiment support this idea. First, our results show that different species have various strength and even direction of priority effects. In diverse communities where the identity of neighbors will differ among individuals in the population, this could dampen directional selection on phenology. Second, we found that no significant disadvantage to arriving later when compared to being planted at the same time as a competitor. Thus, for native species that tend to have later emergence time than exotic competitors, there seem not to earlier emergence, as this still exposes them to similar levels of competition.

A final aspect of our experiment that is noteworthy; it was planted and harvested by 36 students enrolled in an undergraduate Ecology Lab course at the University of California, San Diego taught by the lead author (the co-authors on this manuscript were the Teaching Assistants for the course). Teaching evaluations and surveys showed that the students enjoyed contributing to original research, and the amount of preparation and oversight necessary to ensure data quality was not much greater than for any of the other lab activities where data were not destined for publication; a clear “win-win” for both the faculty and the students. Hence, our results demonstrate the synergies can arise by merging undergraduate teaching with faculty research programs.

Undergraduate students contributed to this study by aiding in both planting and harvesting. Here they are shown planting seeds for species pairs at the start of the experiment.

Undergraduate students contributed to this study by aiding in both planting and harvesting. Here they are shown planting seeds for species pairs at the start of the experiment.


Posted by: oikosasa | September 5, 2014

Pollinator decline effects on plants

How pollinator decline affect plant-plant interactions for pollinator is studied in the Early View article ‘Experimental reduction of pollinator visitation modifies plant-plant interactions for pollination’ by Amparo Lázaro and co-workers.

Several studies have indicated a widespread pollinator decline, caused mainly by land-use changes, degradation of natural habitats, fragmentation and habitat loss. Since the majority of plant species are dependent on animal pollination for reproduction, pollinator decline may influence plant reproduction and the persistence of plant populations. However, a pollinator decline may also affect the way plants interact for pollination because these interactions depend on the abundance of plants and pollinators in the community.

To simulate a pollinator decline we set up a novel experiment to reduce pollinator visitation in two communities (one lowland and one alpine) in Southern Norway (see also Lundgren et al. 2013). In the experiment we compared control plots with plots where pollinator visitation had been reduced by means of dome-shaped cages constructed by bending two PVC-tubes diagonally and covering them with fishnet. The fishnet was totally transparent, so flowers were fully visible from outside the net. In order to allow flower visitors inside cages to exit easily, we left an opening between the mesh and the ground, and another opening in the top of the dome. This experiment effectively reduced pollinator visitation without modifying the composition or behaviour of pollinators, or other important biotic and abiotic variables.





Lázaro et al. (2014) shows that the reduction in pollinators modified plant-plant interactions for pollination in all the six species studied; although for two of them these interactions did not affect seed set. Pollen limitation and seed set data showed that the reduction of pollinator visits most frequently resulted in novel and/or stronger interactions between plants in the experimental plots that did not occur in the controls. Although the responses were species-specific, there was a tendency for increasing facilitative interactions with conspecific neighbours in experimental plots where pollinator availability was reduced. Heterospecifics only influenced pollination and fecundity in species in the alpine community and in the experimental plots, where they competed with the focal species for pollination. The patterns observed for visitation rates differed from those for fecundity, with more significant interactions between plants in the controls in both communities. This study warns against the exclusive use of visitation data to interpret plant-plant interactions for pollination, and helps to understand how plant aggregations may buffer or intensify the effects of a pollinator loss on plant fitness.

Posted by: oikosasa | September 2, 2014

From rich to poor – what happens in the soil?

What happens with plants, microbes and animals during soli transition from mull to mor? Find out in the Early View paper “Coordination of aboveground and belowground responses to local-scale soil fertility differences between two contrasting Jamaican rain forest types” by David Wardle and colleagues. below is their summary of the study:

There is much interest in understanding how long term decline in soil fertility, in the absence of major disturbance, drives ecological processes, or ‘ecosystem retrogression’. However, there are few well–characterized systems for exploring this phenomenon in the tropics. We studied two types of montane rain forest in the Blue Mountains of Jamaica that occur in patches adjacent to each other and represent distinct stages in ecosystem development, i.e., an early stage with shallow organic matter (‘mull’ stage) and a late stage with deep organic matter (‘mor’ stage). We measured responses of soil fertility and plant, soil microbial and nematode communities to the transition from mull to mor, and assessed whether these responses were coupled. For soil abiotic properties, we found this transition led to declining soil nitrogen and phosphorus, and reduced availability of phosphorus relative to nitrogen; this led to a shorter and less diverse forest. The resulting litter from the plant community entering the soil subsystem contained less nitrogen and phosphorus, resulting in poorer quality litter entering the soil. We also found impairment of soil microbes (but not nematodes) and an increasing role of fungi relative to bacteria during the transition. These results show that retrogression phenomena involving increasing nutrient (notably phosphorus) limitation can be important drivers in tropical systems, and are likely to involve aboveground–belowground feedbacks whereby plants produce litter that is less nutritious, impairing soil microbial processes and thus reducing the release of nutrients from the soil needed for plant growth. This type of feedback between plants and the soil may serve as major though often overlooked drivers of long term environmental change.

Pictures: Characteristic ‘mull’ forest (top left) and uppermost soil layer with significant mixing of organic material and mineral soil (bottom left); and characteristic ‘mor’ forest (top right) with uppermost soil layer consisting of a thick layer of organic matter (bottom right). Over time the ‘mull’ soil transitions to ‘mor soil’, characterized by less available nutrients and reduced availability of nitrogen relative to phosphorus; this in turn has important consequences for the vegetation and quality of litter that is returned to the soil.




A slug feeding on capsules of the Rough-stalked Feather-moss (Brachythecium rutabulum).



Herbivores can increase diversity in plant communities by consuming biomass and reducing light competition, thereby benefitting low growing species such as mosses and liverworts (bryophytes). Slugs and snails are important herbivores of forb species and might promote bryophyte diversity if they reduce forb abundance. They also feed on bryophyte capsules, which contain the spores, and it has recently been shown that these spores, can survive the digestive tracts of slugs and snails (endozoochory: internal transport of propagules). Slugs might therefore benefit bryophytes by dispersing their spores.


Moss protonema germinated from slug feces in a previous lab experiment (for details see Boch et al. 2013. Fern and bryophyte endozoochory by slugs. Oecologia 172: 817–822).

Moss protonema germinated from slug feces in a previous lab experiment (for details see Boch et al. 2013. Fern and bryophyte endozoochory by slugs. Oecologia 172: 817–822).


However, whether gastropod herbivory can reduce the dominance of vascular plants and thereby promote the germination and establishment of endozoochorously dispersed bryophyte spores has never been tested experimentally. Moreover, it is unclear whether these possible interacting effects can influence bryophyte species richness. In our study, “Endozoochory by slugs can increase bryophyte establishment and species richness” (Boch et al.) we tested for endozoochorous spore dispersal by slugs (Spanish slug; Arion vulgaris Moquin-Tandon; Arionidae), in combination with sowing of vascular plants, in a fully factorial common garden experiment. We built 30 slug enclosures of 100 cm × 20 cm and introduced either slugs previously fed with the sporophytes of 12 bryophyte taxa, control slugs previously fed with lettuce, or no slugs. We also sowed seeds of vascular plants into half of the enclosures.


Experimental setup in the Botanical Garden of Bern with helpers estimating cover values of bryophytes, herbs, and grasses, which then have been averaged for analysis.

Experimental setup in the Botanical Garden of Bern with helpers estimating cover values of bryophytes, herbs, and grasses, which then have been averaged for analysis.


Twenty-one days later bryophyte cover was on average 2.8 times higher (3.9% versus 1.4%) in the enclosures containing slugs previously fed with bryophytes than in the other treatments. After eight months slugs had substantially increased bryophyte species richness: there were 2.6 times more bryophyte species in the enclosures which had contained the slugs fed with bryophytes than in the other treatments. Sowing vascular plants into the cages did not affect the initial recruitment of bryophytes but after eight months high vascular plant cover did reduce bryophyte diversity. Our findings suggest that slugs are important dispersal vectors for bryophytes and that they can locally increase bryophyte populations and diversity through dispersing spores. They may also act to maintain bryophyte diversity by reducing the dominance of vascular plants.

Posted by: oikosasa | August 22, 2014

Frugivory and seed dispersal

What is it that determines if a bird should deposit a seed from a fruit in a specific place or not? I the Early View paper “Seed dispersal in heterogeneous landscapes: linking field observations with spatially explicit models”, Jessica E Lavabre ad colleagues combines modelling with empirical studies to find out! Below is the author’s summary of the study.

Frugivorous birds play a critical role in the population dynamics of many fleshy-fruited plants by defining the template for the establishment of new individuals. Because successful germination and subsequent seedling survival is highly dependent upon the micro-habitat where a seed arrives, it is crucial to understand which factors drive seed deposition. In our study, we aimed to take an important step forward in understanding the complex mechanisms that generate the spatial patterns of seed dispersal. Few studies have previously modelled seed dispersal in a real landscape, mostly because real vegetation structure is often highly heterogeneous. Here, we have taken advantage of a simple study system to parametrize mechanistic seed-dispersal models with empirical field data, and we built three models that test three seed-dispersal predictors: distance from the source tree, microhabitat type, and a combination of both distance from the source and microhabitat type.

To our greatest surprise, the third model, combining distance and microhabitat type, did not perform significantly better than the other two, simpler models. Additionally, our results suggested that what we had initially considered as one population could instead be two functionally distinct patches, with distinct seed dispersal dynamics. Altogether, these results reinforce the hypothesis that functionally distinct groups of frugivore species generate scale specific seed rain patterns.

LaVera LaVera2

Posted by: oikosasa | August 19, 2014

How much do asexual plants actually change?

A sexual reproduction system should confer higher mutation rates and hence evolutionary rate than asexual ones. Is it really so? Find out in the Early View paper “Asexual plants change just as often and just as fast as do sexual plants when introduced to a new range” by Rhiannon L. Dalrymple and colleagues. Below is their summary of the study:

Many of the world’s most invasive plant species can reproduce asexually. However, asexuality might be a double edged sword for introduced species. Shortly after introduction, asexual species might have the upper hand because they do not need a partner for promptly increasing in numbers and establishing populations in the new range. Classic theory tells us that sexual reproduction should fuel the processes of adaption through the creation of variation on which natural selection can act. While asexuality may be of advantage in the early phases of introduction, it may lead to an evolutionary dead end.

We measured the rate of changes in multiple asexual species distributed through Australia’s east coast and New Zealand. We have provided evidence that multiple asexual species have undergone rapid morphological changes in response to the novel environments in their introduced range. We then compared the proportion of asexual species that demonstrated a significant change in at least one trait, and the rates at which these changes progressed, to comparable data on sexual species. This was the first test of the difference in potential for rapid change afforded by sexuality, cross species and in the natural world. Our results were astounding: we found no significant difference in the rate or frequency of rapid changes between asexual and sexual species. That is, sex and genetic recombination do not increase the rate or potential for change in this context. Introduction to a novel environment, a population may experience strong selective forces. The new environmental conditions force rapid and significant changes in the phenotype of both asexual and sexual species. It appears that in the process of introduction – it may be adapt or fail, regardless of breeding system.

Asexual1 Asexual2

Posted by: oikosasa | August 15, 2014

Changing perspectives

The most exciting aspect of this study “Increase of fast nutrient cycling in grassland microcosms through insect herbivory depends on plant functional composition and species diversity” (Nietschke et al)- for me – was to take our experiences and results from the field site – the Jena Experiment that was designed for elucidating mechanisms of diversity effects – and to incorporate them into a microcosm experiment under well controlled conditions.

Here, we aimed at tracking the way of nutrients from the intact plant, over an insect herbivore and its feeding characteristics, into the soil, and over to another trophic level – And to judge the role of plant diversity and functional composition along that way.

  • Some aspects of the course showed very clearly (e.g. the release of nutrients with feeding and the relevance of the plant functional groups),
  • some were surprising (e.g. both throughfall pH and P increased with herbivory intensity and faeces accumulation – diversity having a similar effects, although independently of herbivore intensity),
  • and yet others were challenging (e.g. clear soil microbial responses only occurred at high levels of herbivory).

Finally, stepping back a little and taking our field site results into account, formed a broader picture and gave some new perspectives.

Besides the change of perspective the study brought about and the various methods we applied, it was very inspiring and rewarding to work together in a team of people that have realized quite different niches within Biodiversity Ecosystem Functioning-space.

Norma1 Norma2 Norma3 Norma4 Norma5

Posted by: cjlortie | August 14, 2014

New formal synthesis section for Oikos papers

Synthesis and integration are critical elements of knowledge synthesis. Using/reusing the work we have already done is a sign of maturation as a discipline, and a very positive step forward to accelerate inquiry by identifying research gaps, opportunities, and effectively summarizing the strength of evidence to date. We are not only poised for potentially profound novel tests of ideas in ecology, evolution, and environmental science, but we are collaborating in news ways, sharing datasets more freely, and more transparently sharing workflows and insights. Oikos supports this movement in all the ways that we can for now and hopefully even more dramatically as we evolve too.  Hence, we are launching a new formal synthesis section for publications.

Meta-analyses and systematic reviews are but two forms of synthesis. Nonetheless, these reviews are currently the most easily aligned with the traditional peer-reviewed ‘publication’ as paper model. This is admittedly a small step, but we need these contributions to inform evidence-based decisions not just for additional research but for management and application. We now have a section devoted to reviews that include quantitative summaries of evidence from within studies or aggregated datasets, i.e. include effect size estimates and appropriate statistics, and also includes systematic reviews that summarize the state of the art of research for a sub-discipline or topic at the scale of studies (i.e. attributes associated with the research, similar to the meta-data of the datasets but without the data from each study listed). We recognize there are many other forms of synthesis that we need to share, and consequently, we will work hard to ensure that we consider these contributions as well (i.e. how to effectively synthesize evidence in all forms, aggregate, and use datasets for novel synthesis).  In handling these papers, similar to all reviews really, we will strive for rapid turnaround, and if sent out for review, we will also work hard to ensure that referees expert in synthesis can provide you with input.

The editorial associated with this section is now OA and online.
Let’s work together to find that big picture.







Posted by: oikosasa | August 11, 2014

Do invasive species alter litter nitrogen release?

Can invasive species actually alter their environment so that more nutrients are available for them? Find out in the Early View paper “Non-additive effects of invasive tree litter shift seasonal N release: a potential invasion feedback” by Michael J. Schuster and Jeffrey S. Dukes. Below is their summary of the study:

Many woody invasive species change their environment to better fit their needs for resources, particularly soil nutrients like nitrogen. One way that they can do this is by accelerating the decomposition of leaf litter—an important step in recycling leaf nitrogen into a form that can be used by plants. However, much of what we know about the decomposition of invasive species’ litters, and their impacts on soil fertility, is based on observations of litter from an individual species decomposing by itself. This is problematic because litters rarely decompose by themselves in nature. More commonly, litters of multiple species are mixed together and decompose more quickly or more slowly than we would expect based on the decomposition rates of each species separately. Thus, we designed a litter bag experiment to examine how the litter of four invasive tree species decomposed differently when mixed with that of four native species, and how this difference might change as the invader became more dominant in the litter layer.

mixed litter (1) mixed litter2

One year and 448 litter bags later, we found some surprising results. Indeed, native-invasive litter mixtures commonly decomposed at different rates than would have been predicted, but whether mixtures lost mass faster or slower than the predicted rate did not follow a strong, consistent pattern. In contrast, the release of nitrogen from these mixtures followed a very clear pattern of being slowed early on, but catching up to or exceeding the amount of nitrogen that would have been predicted at the end of the experiment. Implicitly, native-invasive mixtures were consolidating the release of their nitrogen until later on in the decomposition process, a time that corresponded to the period during which plants, especially the fast-growing invasive species, require the most nitrogen. This pattern was stronger in mixtures comprised mostly of the invasive species and for invaders that produced more nitrogen-rich litter. These findings, in concert with others’ on invasive species and nutrient cycling, led us to suggest that these invasive species might be shifting the release of nitrogen from the litter layer to a time when they are better able to use that nitrogen, and that this might be an important contributing factor to the success of some invasive species.

Mixed litter3

Posted by: oikosasa | August 6, 2014

Non-native plant species benefit from disturbance

How are native and non-native-plants affected by various disturbances? Find out in the Oikos Early View paper “Non-native plant species benefit from disturbance: a meta-analysis” by Miia Jauni and colleagues. Below is the author’s summary of the study:

Disturbances, such as fire and grazing, are often claimed to facilitate plant species richness and plant invasions in particular, although empirical evidence is contradictory. Mixed results on the link between disturbance and plant invasions may be partly explained by differences in environmental and methodological factors among studies. To synthesize the literature on how plant species, both natives and non-natives, are affected by disturbances, we conducted a meta-analysis. More specifically, we examined how habitat and disturbance types, and methodological factors (study approach, the spatial and temporal scale of the study) modify the disturbance-diversity and disturbance-abundance relationships. We show that disturbance indeed facilitates the diversity and abundance of non-native plant species in communities where they are already present, while native plant species are less affected. However, the strength of the facilitative impact on non-natives depends primarily on disturbance type and on the measure used (species diversity or abundance), with grazing and anthropogenic disturbances leading to higher diversity and abundance of non-native plant species than other disturbance types examined.



Non-native plant species may be able to colonise disturbed patches more efficiently than native species.

Posted by: oikosasa | July 18, 2014

Food flow across ecosystems

How much of nutrients found in a lake actually originate from that lake? And from the surrounding grounds? From the ocean? In the new Oikos paper “Broad sampling and diverse biomarkers allow characterization of nearshore particulate organic matter” Alexander T. Lowe and colleagues study the flux of food across ecosystems. Below is their own summary of the paper:

The flux of food across ecosystem boundaries has important consequences for biological community and ecosystem dynamics. Nutrient poor environments are often subsidized by more productive adjacent habitats. For example, a kelp forest can support animals living in a deep submarine canyon through the transport of dead kelp. In marine and aquatic ecosystems, detritus, or decaying organic matter, produced through photosynthesis is thought to break down and mix into the water. This particulate organic matter (POM) can then be transported long distances by water motion, potentially feeding organisms living far away from the original location of photosynthesis.


Researchers from the Friday Harbor Laboratories collect ‘raw’ POM samples. The complex mixture in these jars was dissected using counts and diverse biomarkers. Photo: A. Lowe.

Researchers from the Friday Harbor Laboratories collect ‘raw’ POM samples. The complex mixture in these jars was dissected using counts and diverse biomarkers. Photo: A. Lowe.


The origin of this organic matter is impossible to visually identify once the source has broken down into microscopic detritus. In coastal oceans this food source could come from land or sea. It is therefore common to use biomarkers like stable isotopes or fatty acids to track organic matter through food webs. This type of food source tracking depends on the assumption that each source has a unique biomarker signature that does not overlap the signatures of other potential sources. Using this ‘unique signature’ approach, studies have found high utilization of terrestrial plant detritus in freshwater lakes and coastal marine ecosystems. Similarly, kelp particulate organic matter has been traced into suspension feeders in an array of marine ecosystems. We were interested in the availability of different food sources to organisms feeding on POM. Most studies focus on the consumers, reconstructing the assimilated food sources using mixing models and the unique signatures. This method is often criticized because it does not account for natural variability in the source signatures, which is rarely measured directly. So we took a different approach. We looked directly at the POM to address this question; being a group of ecologists faced with a problem, we tried to count our way out.


An example of the complex composition of suspended particulate organic matter. Photo: A. Lowe.

An example of the complex composition of suspended particulate organic matter. Photo: A. Lowe.


Instead of assuming fixed signatures for each source, we made detailed observations of the living and detrital components of the POM and compared them to the stable isotope and fatty acid signatures of same sample. We took advantage of the natural variability of the coastal, temperate environment of the San Juan Islands, Washington, USA to look at food sources available. We expected the distinct seasons to be characterized by variable mixes of riverine, marine benthic and pelagic, and terrestrial sources of productivity. This broad sampling design allowed us to look at the relationships among the abundance of each POM component and the stable isotopes and fatty acid signatures under a range of natural conditions.


Nearshore suspended detritus in various stages of decay at Eagle Cove, San Juan Island. Photo: A. Galloway 2012.

Nearshore suspended detritus in various stages of decay at Eagle Cove, San Juan Island. Photo: A. Galloway 2012.


What we found was 1) a lot of unidentifiable detritus and 2) an incredibly strong correlation among both biomarker methods and the phytoplankton component of the POM. Detrital particles numerically dominated every sample, which meant standard cell counts (especially those that ignore detritus) were potentially missing a big part of the story. So we adapted a point count method to estimate proportions of each category of POM (but did cell counts anyway). Phytoplankton abundance alone explained a lot of the variation in stable isotopes and fatty acids. Incorporating changes in the phytoplankton community explained even more of the variation in biomarker signatures. This signature overwhelmed the non-phytoplankton detritus, implicating phytoplankton as not only a major source of ecologically important fatty acids, but also as a major driver of the variation we see in biomarker signatures. Taking this result a step further we were able to establish the relationship between the proportion of each POM component and the biomarkers often reported to ‘identify’ them. Information that is critical for selecting and interpreting biomarkers. This simple method provided more information about source contribution than the ‘unique signature’ method and allowed us to pick apart the ever-present detritus. We advocate this method as a way to improve the use of biomarkers in complex ecological studies and to start making cross-system comparisons in order to better understand food sources available to POM consumers.


Suspension-feeding POM consumers (dominated by Balanus here) in shallow subtidal habitats of western San Juan Island. Photo: A. Galloway 2012.

Suspension-feeding POM consumers (dominated by Balanus here) in shallow subtidal habitats of western San Juan Island. Photo: A. Galloway 2012.

If you haven’t yet heard the story of the body-snatching parasite lurking among wildflower populations across the globe, you certainly would not be alone. There is no need for alarm – it is a natural part of its ecosystem, and has likely followed its current hosts’ evolutionary paths for millennia. As such, it offers the opportunity to understand ecological, evolutionary, and environmental effects on infectious disease in wild populations, such as those responsible for many emergent infections threatening agriculture, wildlife, and human health. This is what the Early View Oikos paper “Elevational disease distribution in a natural plant-pathogen system: Insights from changes across host populations and climate”  by Abbate & Antonovics is about. Below, is the rest of the summary of the study:


Silene vulgaris plants infected with a pathogen that replaces pollen with dark fungal spores.  The flowers’ dirty appearance earned the disease its name, “anther smut”.

Silene vulgaris plants infected with a pathogen that replaces pollen with dark fungal spores. The flowers’ dirty appearance earned the disease its name, “anther smut”.


Darwin and Linnaeus were among the first to notice the affected plants with their dirty appearance and altered genders. The tiny culprit, a complex of species-specific fungi in the genus Microbotryum, and its lovely array of flowering Pink Family hosts, has since risen to prominence as a model system for studying everything from genome evolution to how parasites compete for hosts. The fungus works its way through the whole plant and into the flowers, where it takes over the structures that would normally produce pollen (or induces their formation in plants that were otherwise female!) – forcing the plant to produce fungal spores instead. Insect pollinators visit these flowers, whose attractive petals and sugary rewards often appear completely normal, and are tricked into carrying those spores to the next host. As this pollination process is how plants mate, the fungus is essentially a sexually-transmitted infection, behaving epidemiologically similar to diseases of humans or animals driven by either sexual or vector-mediated contact. An infected plant is not killed but sterilized, and has little choice but to keep flowering, year after year, propagating the insidious disease.


A bee visits a diseased flower, which is likely still producing nectar. Fungal spores will travel on the bee to the next flower, hopefully (for the parasite) a new host to colonize.

A bee visits a diseased flower, which is likely still producing nectar. Fungal spores will travel on the bee to the next flower, hopefully (for the parasite) a new host to colonize.

For one particularly widespread host, the bladder campion Silene vulgaris, endemic disease had only rarely been found outside of high-elevation European alpine habitats, despite its weedy presence across the continent. Many diseases are limited to particular habitats within the larger range of their hosts. The most obvious and arguably important example is malaria, which is devastating in the tropics but largely absent from latitudes closer to the poles. Many studies have predicted that as the global climate warms, malaria risk will increase in more densely populated temperate zones, largely in response to shifts in vector distribution. However, others have questioned whether rapid aridification may also reduce risk in currently affected areas with less public health infrastructure. As understanding disease emergence hinges on un-answered academic questions about what factors drive the distribution of disease, we set out to test whether the presence of our little anther-snatcher in Silene vulgaris was similarly limited by environmental factors. It was equally possible that the host populations were simply not as abundant or connected at lower elevations, or that not enough botanists noticed or reported the disease while cataloging plant occurrence.


A bee visits a diseased flower, which is likely still producing nectar. Fungal spores will travel on the bee to the next flower, hopefully (for the parasite) a new host to colonize.

A bee visits a diseased flower, which is likely still producing nectar. Fungal spores will travel on the bee to the next flower, hopefully (for the parasite) a new host to colonize.

To do this, we went to the eastern French Alps, recording host population locations, size, density, and of course, disease. Back in the lab, we were able to use the GPS point of each population to get their proximity to one-another, as well as summaries of climatic conditions. What we found was that indeed, despite being common at high elevations, the disease was exceptionally rare in populations below 1300 meters in elevation. Furthermore, the cool temperatures, high precipitation, and more stable climatic conditions of diseased locations explained this distribution even after correcting for the fact that disease was most common in larger populations, which were relatively more frequent at higher elevations. This study sets up the opportunity to investigate environmental, evolutionary potential, vector distributions, and host resistance effects on the distribution of infectious disease in a natural model species that poses little risk to human health, wildlife, or agriculture. Such studies will be crucial to understanding, and ultimately anticipating, how climatic perturbations may impact disease dynamics and emergence.


Janis Antonovics (study author), enjoying an afternoon refreshment along the sampling route in the small alpine village of Les Terrasses, France.  Citizens, farmers, and even sheep herders in the local community were always interested in why we were there, and often offered more than just water and lettuce from their gardens – they are a wealth of knowledge on the history, land use, and even biology of their local flora and fauna.

Janis Antonovics (study author), enjoying an afternoon refreshment along the sampling route in the small alpine village of Les Terrasses, France. Citizens, farmers, and even sheep herders in the local community were always interested in why we were there, and often offered more than just water and lettuce from their gardens – they are a wealth of knowledge on the history, land use, and even biology of their local flora and fauna.


A bee visiting diseased Silene vulgaris, complete with visible dark fungal spores at the tips of protruding anthers.

A bee visiting diseased Silene vulgaris, complete with visible dark fungal spores at the tips of protruding anthers.

Infected Silene vulgaris at the Col du Galibier, France.

Infected Silene vulgaris at the Col du Galibier, France.

The view of our study area from La Meije glacier near La Grave, France.  Pictured: Jessie Abbate (L), lead author; Kerri Coon (R), field assistant and undergraduate researcher.

The view of our study area from La Meije glacier near La Grave, France. Pictured: Jessie Abbate (L), lead author; Kerri Coon (R), field assistant and undergraduate researcher.

Shorter (ie, Twitter) version:

Fungal anther-smut disease in Silene vulgaris is restricted to host populations in high-elevation alpine climates.


Related website: Field assistant and undergraduate researcher Kerri Coon’s tumbler blog, documenting the 56 days she spent with me in the field tracking some of these populations. 56 reasons to be a biologist:









































Posted by: oikosasa | July 10, 2014

Tuna-tern facilitation

How do seabirds use tunas to find more fish? Find out in the Early View paper “Facilitative interactions among the pelagic community of temperate migratory terns, tunas and dolphins” by Holly F Goyert and co-workers. Below is their short summary of the study:

In the Northwest Atlantic Ocean, researchers and fishers have been known to follow flocks of seabirds, particularly terns, in search of Atlantic bluefin tuna, Thunnus thynnus. We wanted to understand whether such “local knowledge” of tern-tuna associations, which has been described but not tested in the literature, is based on quantifiable community interactions. Marine biologists have speculated that these top predators have a commensal (i.e. mutualistic) relationship, such that terns benefit from feeding tunas, which draw attention to “bait balls”, then drive fish to the surface. We found positive, fine-scale, spatial and foraging (e.g. feeding) associations among tunas and terns (common, Sterna hirundo and roseate, S. dougallii), which supports our hypothesis that facilitation drives their ecological and behavioral interactions at sea, where tunas increase prey detectability and accessibility to terns.


common terns (Sterna hirundo) observed during a pelagic survey, 27 Sep 2006 (Photo:Marie-Caroline Martin).

common terns (Sterna hirundo) observed during a pelagic survey, 27 Sep 2006 (Photo:Marie-Caroline Martin).

The first author, Holly Goyert, observing for seabirds, marine mammals, and tunas during a pelagic survey

The first author, Holly Goyert, observing for seabirds, marine mammals, and tunas during a pelagic survey

Posted by: oikosasa | June 24, 2014

How do behavioral changes affect ecosystems?

That predators affect prey populations and vice versa is well known. But how does the prey’s behavioral responses to predators affect populations of the prey’s prey? This was studied by Bradley Carlson and Tracy Langkilde in the Early View paper “Predation risk in tadpole populations shapes behavioural responses of prey but not strength of trait-mediated indirect interactions”. Below is Bradley’s summary of the study and some photos from the experiment:

It is old news that different populations of the same organism often differ from one another in a number of characteristics. Populations may vary in color, size, morphology, behavior – just about anything. A common cause of such diversity is that populations face different environments that favor different traits. In particular, local predator communities (that is, how many predators there are and what species of predators) can be highly variable and can affect prey behavior. Where predators are abundant and dangerous, prey ought to behave cautiously by hiding, fleeing readily, or being inactive and secretive. When the risk of predation is low, prey ought to pursue opportunities to eat and reproduce.

My doctoral advisor, Tracy Langkilde, and I tested whether populations of wood frog tadpoles (Lithobates sylvaticus) from ponds with high predation pressure showed stronger behavioral responses to predators than tadpoles from ponds with low risk of predation. When they smell a predator in the water, wood frog tadpoles (like many tadpoles) typically become inactive, swimming less and hiding more. We think they’d rather be active so they can consume lots of food, grow large and fast, and turn into frogs before their pond dries. But, if there’s a predator around, it’s more important to not get eaten.


Wood frog tadpoles swim in a pond mesocosm.

Wood frog tadpoles swim in a pond mesocosm.


                We selected 18 ponds with wood frogs across Pennsylvania, representing a range of predator communities. Early in the year, I collected freshly-laid wood frog eggs to bring back to the laboratory so I could measure the behavior of the tadpoles. Later in the year (when tadpoles were swimming in the ponds), I used a net to collect random samples of animals from each pond. I’d then sort through the contents of the net to count the number of each kind of predator I found. These included dragonfly nymphs, newts, and other salamanders and insects. These data were used to assign each pond a value for how high the predation risk was; the more predators, and the more ‘dangerous’ these predators, the higher this number would be.

Brad Carlson uses a dipnet to sample a pond community.

Brad Carlson uses a dipnet to sample a pond community.

                But what about those eggs I brought back to the laboratory? We hatched them into tadpoles at a research farm at Penn State University and, when the tadpoles were old enough, introduced them to pond mesocosms. Pond mesocosms are artificial ponds, smaller than most natural ponds but big enough to be reasonably realistic environments. Our mesocosms were created by filling large cattle-watering tanks with water and adding standard amounts of dead leaf litter. We also added a small amount of pond water, introducing bacteria, algae, and other microorganisms which flourish and create functioning ecosystems. Each pond that we collected eggs from was represented by two of the mesocosms. One mesocosm had a cage floating in it, which contained a dragonfly nymph (Anax junius), voracious and hardy tadpole predators. Regularly feeding the dragonflies extra tadpoles in these cages ensured that the mesocosm water smelled of danger but that the experimental tadpoles wouldn’t actually be eaten. The other mesocosm served as a control (with an empty cage), allowing us to measure the normal, ‘baseline’ activity levels of these tadpoles. After a few weeks, I measured how active the tadpoles were by quietly walking around the mesocosms and counting how many tadpoles I could see (their ‘visibility’) and how many of those visible tadpoles were moving rather than still (their ‘movement rate’).

Brad Carlson observes tadpole behavior in mesocosms.

Brad Carlson observes tadpole behavior in mesocosms.

Our analysis revealed that tadpoles from ponds with higher predation risk responded more strongly to predators than tadpoles from other ponds, spending less time visible, and slightly tending to move less. A good explanation for this pattern is that tadpole behavior is adapted to local environments, with generations of selection in high predation environments ensuring strong responses to predators. This is interesting in its own right though not too surprising. What we really wanted to know was whether this kind of variation in tadpole behavior affects the pond ecosystem. In particular, antipredator behavior often leads to trait-mediated indirect interactions (TMII). In this case, the TMII is an effect of the presence of a predator (dragonfly) on the tadpole’s own food (periphyton, a film of algae and other microbes and detritus growing on submerged surfaces). Dragonflies shouldn’t directly interact much with periphyton, but they should indirectly via their effect on a trait of the tadpoles – foraging activity. If dragonflies cause tadpoles to be less active and thus eat less food, then their food (the periphyton) should increase in abundance. We expected, therefore, that populations of tadpoles that respond strongly to predators should produce larger increases in periphyton than less responsive tadpoles.

Pieces of filter paper with collected samples of dried periphyton (green and brown matter) from tadpole mesocosms.

Pieces of filter paper with collected samples of dried periphyton (green and brown matter) from tadpole mesocosms.


To test this we simply measured the mass of periphyton in the mesocosms by removing tiles we placed in their, scraping off the periphyton, and weighing it after drying. We found that, as expected, mesocosms with caged predators had much higher biomass of periphyton. However, the amount that periphyton increased when adding predators didn’t depend on how strongly the tadpoles responded to predators, nor did it depend on the predation risk in the pond from which the tadpoles came. In fact, tadpole behavior overall was a generally poor predictor of the amount of periphyton in the mesocosms.

This was unexpected, as we established a clear mechanism by which periphyton increases with predators (dragonflies decrease tadpole activity, decreased tadpole activity increases periphyton). Apparently, the amount the tadpoles actually eat is not perfectly linked to their activity: some tadpoles may become very inactive when predators are introduced, but still consume as much food as tadpoles that barely respond to predators. Do they switch their diets, or eat while hiding in refuges, or do most of their foraging at night? And, are tadpoles from ponds with more predators better at compensating for lower activity levels? These questions and more still remain, and will help us as we continue to try to understand how variation in behavior can impact ecosystems.

Posted by: oikosasa | June 17, 2014

Carbon flow between lakes and ground

How carbon moves from terrestrial food-webs to aquatic ones are studied in the new Early View paper “Boomerang ecosystem fluxes: organic carbon inputs from land to lakes are returned to terrestrial food webs via aquatic insects” by K. Scharnweber and co-workers. Below is their summary of the study:

The TERRALAC-project ( ran from 2010 to 2013 and was an interdisciplinary project, based at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany in collaboration with University of Potsdam and Technical University Berlin. Five PhD students worked in five subprojects. The overall aim of TERRALAC was to study the effects of terrestrial particulate organic carbon (tPOC) on shallow lake ecosystems. We wanted to find out, how terrestrial leaves that enter lakes in autumn are actually processed with the lake food webs and we wanted to test this on the natural spatial scale.


One prominent obstacle in studies on the effects and contribution of this allochthonous carbon on lake food webs is the problem of overlapping isotope values of the potential resources. Isotope signatures of the terrestrial carbon are often very similar to those of aquatic primary producers. TERRALAC chose a novel approach by using a tPOC tracer that is isotopically distinct, but also similar to the size and structure of natural leaves: maize (Zea mays) leaves.

Photo 1: Maize added into the littoral zone. The rope prevents it from floating into the open water.

Photo 1: Maize added into the littoral zone. The rope prevents it from floating into the open water.

In October 2010, we divided two small and shallow lakes, located in the rural area of Northern Germany approximately 100 km north of Berlin, in two equal halves. Both lakes are eutrophic and of similar size and depth. However, they have different alternative stable states: Gollinsee has turbid water and is dominated by phytoplankton, whereas Schulzensee has clear water and is dominated by macrophytes. We used plastic curtains to divide the lakes (Photo 1) from surface to bottom. With the help of many hands, we added the maize leaves into the littoral zone of the treatment sides of the lakes (Photo 2,3). By this approach we tried to mimic the natural input of tPOC by leaves in autumn.

Photo 2: Maize addition by hand in Schulzensee (October 2010).

Photo 2: Maize addition by hand in Schulzensee (October 2010).

Photo 3: Installation of the plastic curtain in Gollinsee.

Photo 3: Installation of the plastic curtain in Gollinsee.

Photo 4: Emergence trap used in our study.

Photo 4: Emergence trap used in our study.

In our article we present the flow of terrestrial carbon to lakes and back to its terrestrial surroundings via emerging insects (Figure 1). We focused on Chironomidae that have an aquatic-terrestrial life cycle and collected them as larvae, but also as adults using emergence traps (Photo 4). After emergence, they are known to become prey for terrestrial predators, for example for spiders that live in the riparian reed belts. Carbon isotope values of Chironomidae and spiders were significantly elevated in the lake treatment sides as compared to reference sides. As further demonstrated by isotope mixing models, contribution of maize was higher in Schulzensee, the lake where macrophytes are present. We conclude that structural complexity provided by the macrophytes may trap the leaves and by that enhance the food availability for the larval Chironomidae. In summary, we present the tight linkage between aquatic and terrestrial habitats and the cycling of organic matter across boundaries and borders.





Posted by: oikosasa | June 13, 2014

How does climate change affect pollination phenology?

What is climate change doing to plant–pollinator interactions? In the last decade, ecologists have focused on the possibility that climate change will shift the seasonal timing (phenology) of plants relative to their pollinators, reducing temporal overlap between interdependent species. In the Early View paper “Plant–pollinator interactions and phenological change: what can we learn about climate impacts from experiments and observations?” in Oikos , I evaluate the evidence that plant–pollinator overlap is changing as a result of climate change, and that such changes affect population persistence. I also discuss the strengths and limitations of different types of evidence for climate-change impacts. In particular, I explore the challenge of interpreting “temporal transplant” experiments, which manipulate the phenology of a subset of plants or pollinators in isolation, creating subpopulations of mistimed individuals in a matrix of unaltered phenology.

Observational data have shown us that, for the most part, plant and pollinator phenologies are advancing in parallel in response to warmer temperatures. While there are cases of plants blooming before their pollinators are active and consequently setting few seeds, there is no conclusive evidence yet that climate change is causing this “mismatch” to happen more often. There’s even less indication that pollinators are suffering from mismatch with plants—but pollinator fitness has received much less study. I suggest that there is much still to be learned about the direct effects of climate change (e.g., snowpack reductions, temperature extremes and fluctuations) on populations of pollinators and pollinator-dependent plants—effects that might be more demographically consequential than non-parallel shifts in phenology.

By Jessica K.R. Forrest


Photo of male Megachile on unopened flower head of Erigeron speciosus. © J. Forrest

Many animals are breeding earlier and earlier in response to a gradually changing climate – but what happens when a species encounters a dramatically different climatic regime such as a complete reversal of the summer-winter rainfall pattern? In our article,  Phenological shifts assist colonisation of a novel environment in a range-expanding raptor, we explore this question by investigating how variation in the timing of breeding and breeding success of black sparrowhawks Accipiter melanoleucus relates to weather patterns during their colonisation of the Cape Peninsula of South Africa.


Heavy rain can have pronounced effects on breeding success in raptors, flooding exposed nests and potentially impairing the ability of parents to hunt. In the eastern and north-eastern areas of South Africa the majority of rain falls in the summer months and black sparrowhawks breed during the dry and relatively cool winter. During the last half a century a number of bird species have gradually expanded their range south-westwards within South Africa bringing them into contact with a dramatically different weather pattern: in the south-western regions the rain falls mainly during winter, a complete reversal of what occurs elsewhere in their range.


In the 1990s, the first black sparrowhawks were recorded breeding on the Cape Peninsula, in the shadow of the iconic Table Mountain. Rising to over a 1000m, this huge lump of sandstone generates exceptionally high levels of rainfall in the immediate lee of the prevailing winds, during the winter months as deep depressions roll in off the Southern Atlantic Ocean.

In 2001 a long-term study of the black sparrowhawk population[] on the Cape Peninsula was initiated. In the first year fewer than a dozen nests were monitored but as the population expanded so did the project. A team of dedicated volunteers now follows the breeding success of over 50 nests each year. This hard work has generated a fantastic data set with which to explore how the unusual weather of the Cape Peninsula affects breeding phenology and the role that shifts in breeding phenology has played in the growth of this population.

Raptors3 raptors1

We found that black sparrowhawks on the Cape Peninsula commence breeding up to three months earlier than eastern and north-eastern populations, and that breeding was suppressed during the months of heaviest rainfall. Earlier breeding attempts also produced more chicks. As a result of the shift in timing of breeding, the probability of population extinction was reduced by 23%, suggesting that this phenological shift could have assisted the colonisation of the Cape Peninsula. However contrary to expectations we found no strong evidence that black sparrowhawks were responding to local variation in rainfall within the Peninsula study area. We suggest that shifts in breeding phenology may be driven in part by other novel processes encountered during colonisation, such as interspecific competition for nest sites and lower temperatures during late summer than is the case in the rest of their range.


Clearly there is more to learn about how black sparrowhawks cope with the differing environments they encounter as they have spread westwards. Looking forward, PhD student Gareth Tate is using nest cameras and satellite tracking technology to investigate in further detail how weather influences hunting behavior.

The Authors through Arjun Amar


Posted by: oikosasa | June 4, 2014

Editor’s Choice June

DriesPapers published in Oikos should meet the principal criteria to generate synthesis in ecology. Synthesis can be created in different ways and definitively obtained when long-term data sets, novel analytical tools and good hypotheses are merged. The first editor’s choice for the June issue is the paper by Karen Lone and colleagues on multi-predator landscapes of fear. Motivated by challenges to manage large carnivores in Scandinavia in relation to human conflict, the authors used an extensive dataset containing Lidar data, behavioural data and hunting information to demonstrate the interactive impact of multiple predators on predation risk of a single prey species. By means of this integrative approach, the authors demonstrate a predation risk from humans and lynx on roe deer in areas with a high vegetation cover, but an additive impact in more rocky locations. As such, the study demonstrates the complexity of predator-prey interactions in real landscapes, and clearly emphasises the need and value of individual-based ecology to understand interactions in (simplified) foodwebs.

Oikos has decided to highlight meta-analysis papers because of their principal role to create synthesis in ecology.  Chris Lortie will be Editor-in-Chief for this category of papers (look out for his editorial in the August issue). Meta-analysis papers will be published OA for three months after publication. Ward and colleagues tested the performance of time-series forecasting models for natural animal populations based on more than 200 datasets of vertebrate surveys. Such a meta-analysis is considered essential because of the increasing demand to forecast population dynamics under different global change scenarios. While forecasting approaches using non-mechanistic statistical models have greatly evolved the last decades in population biology, still a limited amount of such models are commonly used. By performing a statistical competition experiment, the authors tested the predictive performance of 49 different forecasting models and found simple models to behave well after all, although increasing model complexity fitted time series better in case of species with cyclic population dynamics.

Editor’s choice papers are free online for three months!

The ants underfoot forage faster as the ground warms each day. Likewise, the fish in nearby streams and the worms and other organisms that parasitize these fish speed their activities as the water warms. Above the stream, dragonflies engage in more aerial battles each hour for prime perches as the air warms. We have all witnessed these quickenings, as insects and other ectotherms, organisms whose body temperature are primarily environmentally determined, become more active and interact more quickly in hotter environments. How much faster do such biotic interactions increase with temperature, and why?  This is studied in the Early View paper “Rates of biotic interactions scale predictably with temperature despite variation” by Bill Burnside and co-workers. Their sumary of the study continues here:


Anderson Mancini, Creative Commons – Flickr


In this meta-analysis, we look across taxa and habitats to assess the temperature dependence of biotic interaction rates, such as herbivory and competition, between two species. We hypothesize that these rates will increase approximately exponentially with temperature, mirroring the temperature dependence of respiratory metabolism generally. This hypothesis is inspired by the metabolic theory of ecology, which suggests that many ecological patterns and processes are functions of individual metabolic rates of the organisms involved. These rates vary characteristically with body temperature, which affects the rates of cellular chemical reactions. Biotic interactions are metabolic because they involve exchanges of energy and materials between organisms and their environment and because they are inspired by basic metabolic demands, like the need to eat.

Matthew Britton, Creative Commons - Flickr

Matthew Britton, Creative Commons – Flickr


This work was intriguing because even though we were not interacting ourselves with all the amazing organisms in our analysis, like those pictured here, we did not know what we would find. The studies were often focused on a related question and just happened to include temperature as a variable or did not include graphs, so it was tough to visualize how some rates varied with temperature. And the rate terms varied by their nature, from the rate ground beetles catch and eat fruit flies to the rate sea lice parasitize fish.

Watershed_Watch, Creative Commons - Flickr

Watershed_Watch, Creative Commons – Flickr


Seeing the results for the first time—the generally parallel lines, each with a slope indicating how interaction rate scales with temperature—was amazing. Our results generally supported our hypothesis, but there was also a great deal of variation. We could only find a fairly small sample of studies on most interaction types, which probably accounts for some of this variation, but organisms vary in their level of thermal performance and peak response, among other traits, which surely accounts for variation when different species interact.

Understanding how temperature affects biotic interaction rates is more important than ever in our warming world. The answers won’t be entirely straightforward and may vary among places, species, and communities, but this study offers basic insight to inform our search. 

Vernal pool ecosystems emerge from winter and spring rains that fill shallow depressions in the earth, resulting in small patches of wetlands spread throughout the Central Valley of California, USA. These pools act as a crucial habitat for a diverse community of annual plants, many of which are endemic to the region. Many of these species complete the majority of their entire lifecycle within the short duration that the pool exists. As the standing water evaporates, a stunning array of wildflowers is produced from the plant species that co-occur in these temporary ecosystems. The endemic diversity of these pools, coupled with the short lifespan and small size of the species, makes them an ideal model system for testing questions of community assembly. In the study Functional trait differences and the outcome of community assembly: an experimental test with vernal pool annual plants” in OikosNathan Kraft, Greg Crutsinger, Elisabeth Forrestel and Nancy Emery measured functional trait differences between species in the pools and tested whether these characteristics could be used to predict the outcome of interactions among plant species and, ultimately, the processes structuring the vernal pool communities.

photo 1_smaller

Kraft and colleagues used a greenhouse experiment with eight different annual vernal plant species and grew them together in all pairwise combinations, so that every species had a chance to interact with every other species. They also submerged these combinations in tubs of water for different amounts of time to mimic growing at different depths in a vernal pool. Prior work in this system has found that the recession of water in the spring generates a gradient of species composition along the sides of vernal pools. The authors observed that plant species tended to do better when they had larger leave size, lower specific leaf area (fresh leaf area divided by dry leaf mass), and greater investment in lateral canopy spread than their neighbors. It also turns out that not all individuals within species are equal in these interactions. The authors took an additional step relative to many trait-based studies and measured functional traits for all individuals in the experiment. Models that incorporated individual trait differences did a better job of predicting the outcome of the interactions than models using only species average trait values.

The results of this study suggest that plant traits can be used to help understand the outcome of interspecific interactions in vernal plant communities. It’s also clear that individual trait differences matter. If resources allow, researchers can boost their predictive power by considering trait differences among individuals, instead of focusing on the average traits for different species, which has been the standard practice. There is still more work to be done to understand how vernal pool communities are assembled, as the patterns Kraft and colleagues observed in greenhouse did not strongly match patterns seen in natural pools, suggesting other undiscovered factors are also contributing to species distributions. Ongoing work from Nancy Emery and others will continue to shed light on the processes structuring these fantastic communities in the near future!

Posted by: oikosasa | May 23, 2014

Winter is coming! How do plants react?

How do plants react to seasonal extremes? Find out more in the new Early View paper “Leaf and stem physiological responses to summer and winter extremes of woody species across temperate ecosystems” by Elena Granda and co-workers. Read Elena’s summary of the study here:

Our paper presents evidence that winter stress in the temperate region is more extreme than summer for forests that do not experience summer droughts, but also for those where summer drought combines with winter freezing. In this study we compiled existing literature to identify overall trends of the impact of seasonal extremes on plant performance (leaf and stem physiological responses). We further compared the general patterns over the temperate region with a continental Mediterranean case study subject to intense summer droughts and winter freezing.


Continental Mediterranean and riparian forests at Alto Tajo Natural Park (Spain) during a) summer drought and b) winter freezing

 Although it is known that winter cold limits plant performance, as is also the case for summer drought in dryland ecosystems, our study revealed that across temperate forests: (i) winter is commonly an equal or even stronger stress than summer, including particular cases of Mediterranean vegetation; (ii) many species are able to maintain stomata open during winter, favoring carbon gain over most of the year; (iii) stomatal conductance and xylem hydraulics show a coordinated seasonal response at sites without summer droughts, and (iv) deciduous angiosperms are the most sensitive to climatic stress.

These results suggest that the differences among functional types in seasonal dynamics of physiological performance are strong enough to advocate their importance in determining ecosystem productivity throughout the year, especially in ecosystems where carbon gain is limited to a few months. These patterns present a baseline against which to compare shifts for key plant species and communities with ongoing climate change.

Posted by: oikosasa | May 20, 2014

How common is bird-pollination in Europe?

Bird pollination in Europe? Really? Well, find out in the Early View paper Flower visitation by birds in Europe” by Luis P da Silva and co-workers. Below is Luis summary of the paper:

Birds are among the most studied animal groups and are most likely the one that attracts more general public attention. These winged animals, are known throughout the world for their interactions and coevolution with plants. They are known to be very important for several plant groups, providing seed dispersal and promoting sexual reproduction through pollination.

When anyone hears about birds pollinating plants, their first thoughts go to hummingbirds, which are very specialized bird pollinators that are able to hover. However, there are other bird groups that are not considered so specialized in pollination (and unable to hover), but well known to pollinate plants, as the honeyeaters. This bird group, somewhat specialized in taking nectar and consequentially pollinate flowers, are present in almost all over the world, except in Antarctica and Europe. If in Antarctica that is not unexpected, in Europe (and actually in almost all the Western Palearctic) that seems a little odd, that how such ubiquitous and abundant food source is not recognized to be regularly exploited by any bird species. With this, insects are often considered the only ecologically relevant pollinators in Europe. Nevertheless, generalist birds are also known to visit flowers and in some cases to successfully pollinate plant species around the world.


In Europe there are several scattered publish records of flower visitation by birds. We carried out a fine literature search and compiled our own observations to estimate the extent, richness and ecological relevance of this mutualistic interaction. These interactions were not only of direct feeding observation, but also from pollen found attached to feathers, in faeces and stomach contents. We found that 46 bird species visited flowers of at least 95 plant species, 26 of these being exotic to Europe, yielding almost 250 specific interactions inside Europe. Additionally, we registered four more European bird species interacting with 12 different plant species outside Europe. Despite these numbers, only six plants species, both native and exotic, were confirmed to be efficiently pollinated by birds in Europe. We argue that the ecological importance of bird-flower visitation in Europe is still largely unknown, particularly in terms of plant reproductive output. We suggest that nectar and likely pollen are important food resources for several bird species, mainly during winter and spring, especially for tits (mainly Cyanistes), Sylvia and Phylloscopus warblers. The prevalence of bird flower-visitation, and thus potential bird pollination, is slightly more common in the Mediterranean basin, which is a stopover for many migrant bird species, which might actually increase their rule as potential pollinators by promoting long-distance pollen flow. We argue that research on bird pollination in Europe deserves further attention to explore its ecological and evolutionary relevance.

Posted by: oikosasa | May 16, 2014

Who eats the sea urchin?

Different factors that effect predation in marine habitats are explored in the Early View paper Differences in predator composition alter the direction of structure-mediated predation risk in macrophyte communities by Simone Farina and co-worker’s. Below is Simone’s short summary of the study as well as a reflection on the history that lead to the study.


Using sea urchins as model prey we examined the role of structural complexity in mediating predator-prey interactions across three bioregions: Western Mediterranean Sea, Eastern Indian Oceanand Northern Gulf of Mexico. As expected biomass of the habitat structure and fish predator abundance were the main determinants of predation intensity. Interestingly though, the direction of structure-mediated effects on predation risk was markedly different between habitats and bioregions. In Spain and Florida, where predation by fish was high, structure served as critical prey refuge, particularly for juvenile sea urchins. In contrast in Western Australia predation was generally higher inside the structure where bottom predators were more abundant.

Gallery 1: Meditarrenean Sea

In this sense, as a Mediterranean student I will never forget the days of field work in WA. We had to look for Heliocidaris erythrogramma, the Australian model prey, the equivalent of Paracentrotus lividus in the Mediterranean Sea, and it was like looking for sparrows in the Amazon rainforest. Kelp was full of huge gastropods. Every time I moved a few shoots I had the impression of seeing any kind of tail run away quickly. It was very easy to get distracted from my search. One time we found a huge Coscinasterias calamaria (sea star) remained attached to a fishing line used to mark one of the sea urchins after having swallowed it entire. In seagrass ecosystems (Amphibolis griffithii and Posidonia sinuosa) we observed many little and coloured sea stars (Patiriella brevispina) approaching day by day to our sea urchins and finish them one after the other. In Australian seagrass ecosystems, working to head down supposed some time to have a look around to check for unexpected arrivals of huge sting ray, as ufos flying over a landscape. Macrophyte communities that we explored in Australia are totally different from the Mediterranean ones, so it does not surprise me that structures works in different ways. It was a really pleasure to carry out this experience and I think I’ll have a good story to tell my grandchildren.

Gallery 2: Australia

Posted by: oikosasa | May 13, 2014

How does the habitat effect body size?

By comparing fish communities with bird communities Kirsten Nash an co-workers assess the appropriateness of different size distribution indices used in a variety of studies. The analyses resulted in the Oikos Early View paper Habitat structure and body size distributions: cross-ecosystem comparison for taxa with determinate and indeterminate growth”. Below is the author’s own summary of the paper:

How the study arose:

The study resulted from conversations at the working group ‘Understanding and managing for resilience in the face of global change’, hosted and funded by the USGS Powell Centre for Analysis and Synthesis in Fort Collins, Colorado ( The working group came about through a chance meeting between myself and one of the other authors of our study (Shana Sundstrom) at the 2011 Resilience Conference in Arizona. We were both starting our PhDs addressing similar questions but in different ecosystems: Shana primarily focuses on birds, whereas my research looks at coral reef fish communities. Participants in the working group come from Canada, USA, Sweden, South Africa and Australia.


Photo of the ‘Managing for resilience’ working group at the USGS Powell Centre in Fort Collins, Colorado:

Description of our group’s focus and short summary of the paper:

An ecosystem can be in a number of different forms or states, such as a reef covered in many corals vs. a reef with large areas of thick algae and little coral. The ability of an ecosystem to remain within its initial form (e.g. coral reef) when impacted by disturbances resulting from rapid global change, rather than shift to another state (e.g. algae reef), is called the resilience of the ecosystem. Until recently, a lot of the work that has looked at the resilience of ecosystems has been theoretical. Our working group focuses on putting this theory into more practical terms, i.e. understanding how likely is it that the ecosystem will remain in its current form over time, as this directly affects natural resources on which we rely for food, water, etc.


An important step in this process was to take methods that had initially been developed to look at terrestrial taxa, and explore their appropriateness for marine systems, to ensure that our work was relevant across a range of ecosystems and wasn’t limited to terrestrial studies. The result was our study recently published in Oikos, where we compare the ability of different animal size measurements to highlight the effect of a variety of habitats on the bird and fish communities that live within them. We completed fieldwork on fish communities inhabiting reefs of the Great Barrier Reef and the Seychelles and used published data for bird communities in Borneo (Cleary et al., 2007, Ecological Applications) and the Lofty Ranges, Australia (courtesy of Hugh Possingham and the Nature Conservation Society of South Australia). The study shows that both average and individual size measurements are useful for showing the effect of habitat on bird communities. In contrast, for fishes, we need to incorporate the size measurements of all individuals within a community to show the effects of habitat on fishes.



Posted by: chrislortie | May 12, 2014

Sharing data issues for ecology and evolution

An open future for ecological and evolutionary data?
Amye Kenall*, Simon Harold and Christopher Foote

A very succinct and accurate editorial on the future directions needed for data sharing in ecology & evolution was recently published at BMC Ecology. The key issues for ecology & evolution were identified and included the following:

Opportunity cost: available data reduces costs, provides opportunities, and serves local stakeholders

Shared benefits: new forms of collaboration, discovery, and accelerated synthesis will emerge

Blood, sweat and tears: long-term datasets are hard won and it is difficult to let them go (public)

A bigger picture: macroecology and other relatively new ways of doing ecology need open data

Data management: metadata is critical, always, and for ecology/evolution in particular because the ‘lab’ is often outdoors

Credit where credit is due: many new reputation economy tools and reward systems are in place to serve as incentives for all scholars not just for ecology and evolution

Transparency and trust: we review the manuscripts of others, why should datasets be free from scrutiny?

Each issue is well described. The one that clicked the most for me was the ‘blood, sweat, and tears’ argument as I have heard it made by many ecologists. Fieldwork can be grueling.  We also hope that we will reuse our own datasets many times. However, I suspect that we do not. Let them go free. As a personal goal, I have mentioned the idea of #ecodataweek on twitter as a good push to myself via a bit of friendly competition to get stuff out there more too.

Oikos also partners with Dryad, and I hope that this editorial likewise inspires you to publish your data.  I would also love to see a more in-depth set of analyses for Oikos readers on these topics and how they relate to the journal mission of ‘novel synthesis’ and synthesis science in general.

Additional resources to consider:
1. DataONE for data management planning tools and a list of best management practices for data documentation (pdf link is best).
2. A good read on how our discipline has likely crossed into the realm of big data.
3. The pivotal nature of data sharing to the future of publishing in ecology.
4. Re-read the file-drawer problem papers such as the Csada et al. 1996 Oikos paper on the topic (classic for ecology but still feels modern with issues of limited OA and accessible data).





Posted by: oikosasa | May 9, 2014

Linking theory with empirical studies of competition

“Nature is much more complicated” than predicted in theoretical  models. In the Early View Paper “Matrix models for quantifying competitive intransitivity from species abundance data”, Werner Ulrich and colleagues try to fill a gap between models and mother nature when it comes to competition interactions.

Below is their summary of the study:

Ecologists have devoted much effort to inferring competitive processes from observed patterns of species abundances, morphology, and particularly from changes in the spatio-temporal distribution, (i.e. species co-occurrences). Classic assembly rules models, derived from the principle of competitive exclusion, predict that differences in competitive abilities should cause non-random patterns of species occurrences among sites and generate inequalities in species abundances within sites. Competitively inferior species are predicted to occur less frequently and at lower abundance, and an important and largely unresolved question is how such species can persist in a community over long time periods (also known as Darwin’s paradox and nicely explained in .

Simple competition models assume that species can be ranked unequivocally (A>B>C…>Z) according to their competitive strength. However, nature is much more complicated and intransitive competitive networks can generate loops in the competitive hierarchy (e.g. the rock-scissors-paper game, in which A>B>C>A). Importantly, such loops allow weak competitors to coexist with strong ones. Additionally, the structure of such loops might be modulated by environmental factors. Experimentally competitive strength can be tested with simple two-species systems. Testing competitive interactions in many-species systems requires an increasing number of species exclusion experiments.


Up to now no comprehensive theoretical framework existed to infer competitive loops from observed patterns of species abundances. Existing mathematical models based on presences and absences of species (on perfect competitive exclusion) have rarely been applied to empirical data. Our new paper in Oikos seeks to fill this gap in our knowledge. We introduce a statistical framework for evaluating the contribution of intransitivity to community structure using species abundance matrices that are commonly generated from replicated sampling of species assemblages. We use a stochastic back-engineering procedure to find a transition probability matrix that predicts best observed distributions of abundances in an ordinary Markov chain approach. We then use a probabilistic argument to convert this transition matrix into a pairwise competition matrix that contains the information of competitive strength between all species in the community. Our approach can be used for abundance data, time series, and abundances in combination with environmental data. Our case study on necrophagous flies and their hymenopteran parasitoids revealed strong hints towards instable competitive hierarchies. In other words the competitive outcome in these communities (having at least five species) strongly depended on environmental conditions but also on the spatial structure of fly and parasitoid occurrence.


We hope that our new approach sparks a fresh new look at competitive interactions in ecological communities and helps to assess and appreciate the importance of intransitivity for the coexistence of species in natural communities. We fell the need for joint approaches that link existing methods using the temporal and spatial co-variation in species abundances and occurrences with methods (like our) that reconstruct competitive hierarchies. 

Posted by: oikosasa | May 7, 2014

Mixture models instead of bimodality?

Bimodality – the characteristic of a continuous variable having two distinct modes – is of widespread interest in data analysis. This is because, in some cases, we can use the presence or absence of bimodality to infer something about the underlying processes generating the distribution of a variable that we are interested in studying. In ecology, tests of bimodality have been used in many different contexts, such as to understand body size distributions, functional traits, and transitions among different ecosystem states. But a lack of evidence for bimodality has been reported in many studies. Our paper “Masting, mixtures and modes: are two models better than one?”, now shows that a widely-used statistical test of bimodality can fail to reject the null hypothesis that focal probability distributions are unimodal. We instead promote the use of mixture models as a theory oriented framework for testing hypotheses of bimodality.

Our interest in this problem arose with the publication of Allen et al. 2012 Oikos 121: 367–376. The paper impressive synthesised 43 years of seeding patterns in a New Zealand mountain beech Nothofagus forest. Seed production in these trees is interesting because a population can go several years without reproducing and then all the individuals in a population will do so. Such intermittent and synchronous reproduction is also known as mast seeding.


Dense Nothofagus forests (dark green) dominate mountain-sides in Fiordland, New Zealand

Dense Nothofagus forests (dark green) dominate mountain-sides in Fiordland, New Zealand


In their paper, Allen et al. tried to infer the importance of resource limitation in driving mast seeding patterns by describing various characteristics of seed production. A key finding was that they could not reject the null hypothesis that the distribution of annual seed production was unimodal using an empirical calculation known as Hartigan’s dip test. Allen et al. 2012 concluded that few studies could ‘robustly test for bimodality’ because they lacked ‘long time-series’ and used questionable methods.



Nothofagus solandri var. cliffortioides trees in Fiordland, New Zealand

The bimodality result inspired a lot of reflection. We were interested in why we might consider an annual count that takes values larger than zero at more than 2 year intervals to ever even present two modes. Populations should only have one mode because the single most frequently observed seed count will be relatively low, on average, in most years. Thus, plants can never be bimodal when a single distribution is fit to seed counts – even if they alternate annually, on average, between the absence and presence of seed production. This is because of the underlying statistical nature of seed counts, which we describe in further detail in our paper.

Stepping back further from the problem, we began to realize that there is a need to consider multiple probability models, each with at least one unique mode, where plants flower at >2 year intervals.We illustrate these ideas by analysing 37 years of data from five grass species in New Zealand. Critically, we found clear evidence for bimodality using mixture models that associate distinct probability distributions with medium- and high- versus non- and low-flowering years. We expect these patterns to be driven by different processes and hence, modelled by different probability distributions. We found no evidence for bimodality with Hartigan’s dip test that assumes a single probability distribution can be fitted to all the data. Our findings show the importance of coupling theoretical expectations with the appropriate statistical tools when predicting the responses of ecological processes.



Snow tussock Chionochloa rigida flowering in Fiordland, New Zealand

The authors through Andrew J. Tanentzap

Posted by: oikosasa | May 3, 2014

Fever, starvation or being eaten?

Talk about choosing between pest and cholera! A bird on migration, has a tough job, and with a virus infection in the body, the job is even worse! In the early View paper “A tradeoff between perceived predation risk and energy conservation revealed by an immune challenge experiment”, by Andreas Nord and co-workers.

Below is Andreas summary of the study:

Birds, like man, must maintain a high and even body temperature to function properly. This is a challenging task, not least in winter when the internal temperature may be some 50-60 °C above that of the surroundings. It is not surprising that this challenge requires high food intake. In fact, a blue tit, which is a common garden and forest bird across Europe, must sometimes put on 10 % of their body weight as fat on a daily basis during cold winter days. A human of average weight would have to eat some 200 hamburgers to ingest the same amount of fat.
As if this was not enough, the time of peak food demand often coincides with the time when food resources are the most difficult to obtain. To overcome such hardships, many animals actively reduce their body temperature at night (nocturnal hypothermia), a process that substantially lowers their energy demands. Yet the use of nocturnal hypothermia is often not enough to avoid the risk of starvation, because the demands from other body functions compete for the same fat reserves. In these situations, birds may have to prioritize surviving the night by reducing the use of other costly functions, such as the immune defense system. In other words, because food availability is limited in winter, it may not be possible to maintain sufficient amounts of body fat at the same time as an adequate defense against invading pathogens.
This was the subject for our study, in which we investigated how an activated immune defense system impacted on the use of nocturnal hypothermia and behavioral strategies for minimizing energy expenditure in wild blue tits in southern Sweden. Contrary to our expectations, an immune response did not cause birds to change their use of nocturnal hypothermia, which could indicate that any energy costs of the immune defense system are not large enough to interfere with energy conservation processes. However, birds with an ongoing immune response showed a different behavior compared to healthy birds, which was manifested as an increased use of sheltered roosting sites when the immune response was at its peak. Using such roosting sites often confers energy savings, because birds are less exposed to wind and temperatures are higher than those outside. However, sheltered roosts often come at the expense of increased predation risk, because these roosts may be easier to locate and escape prospects are typically relatively low upon detection.
We interpret this increased risk taking behavior in sick birds as consequences of a higher need to exploit the energetic benefits of sheltered roosts. Because this required birds to accept a higher predation risk at night, our results may indicate that energy stress from less efficient thermo-regulation poses a higher mortality risk for sick birds than does any predation risks pertaining to sheltered roosting sites. This was not the case for healthy birds, whose thermo-regulatory capacity was not impaired by an ongoing immune response. These birds instead actively avoided sheltered roosts, because their main source of overnight mortality might indeed have been the risk of predation.

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