Posted by: oikosasa | November 23, 2012

Heavy work in community ecology

When I did an undergraduate project at Silwood Park, my supervisor, Hefin Jones used to say that ecological research is about 10 % inspiration and 90% transpiration. And this is exactly what Winfried Voigt report about in his story about his and his colleagues Early View paper “Bottom–up and top–down forces structuring consumer communities in an experimental grassland” (Rzanny et al.). He story also shows what might be the outcome of large collaborative projects.

Gathering up sufficient and useful data for answering questions concerning entire communities is always a strenuous and costly job. It is actually only feasible when working in cooperation with a large team. We were lucky to be involved in an appropriately large team, the Jena Experiment, a controlled biodiversity experiment []. The dimensions (10 ha, 80 plots each 20×20 m with a set plant species diversity of 1, 2, 4, 8, 16 and 60 as well as 1,2,3 or 4 functional plant groups, and numerous smaller plots) are unique and  turned out to be the right platform for asking “bigger” questions about structure and function in grassland communities. A lot of periodic work, in particular weeding, was done by a capable maintenance staff (5 professional gardeners and numerous student assistants) so that the only thing left to do was to collect our own data. Nevertheless, for our closer small team (with some support  of a few student assistants) even that was quite a strenuous job but all the effort and suffering (not to say blood, sweat and tears) are not  always obvious if one looks at the end result on just a few hundred Kbyte of data we hold in an excel file. We collected arthropods 5 times, from May to October in 2005, on 5 quadrats randomly placed within the core area of 50 big plots (16 monocultures, 16 four-species, 14 sixteen-species and four 60-species mixtures) using suction samplers combined with biocenometers (see photograph) as well as using pitfall traps. All in all, we successfuly accrued 322 arthropod species/taxa with 81658 individuals that we could exploit in the end.

Identifying the key factors for structuring ecological communities is at the heart of ecological research. Most studies dealing with this question on community level rely on lumped, aggregated variables such as summed species abundance, biomass or diversity measures or they confine to a small part of species usually one or a few taxa.

We already developed a different approach 10 years ago by assigning all species to functional groups representing approximately the entire community. Because we hold all functional groups as matrices containing abundances of their members (species), we acquire a sufficient simplification, while retaining full species information. As we see ecological communities as a system of interdependent functional groups, we performed an exploratory multivariate analysis, explicitly addressing species composition of functional groups. We used species resolved plant biomass and arthropod abundance data from the Jena Experiment to estimate the dependencies among plant – and consumer functional groups, thereby accounting for spatial effects and differences in soil conditions.

Using a set of five groups of biotic and abiotic predictor variables (Plants, Herbivores/Detritivores, Carnivores, Soil, and Spatial patterns), we aimed to determine the independent and shared fractions of variation explained by these variables in the composition of all consumer functional groups. Depending on the trophic level of the predictor variables, we quantified the relative roles of top-down and bottom-up effects.

It turned out that legume composition explaines the highest fraction of variation in virtually all consumer functional groups, indicating that legumes play a key role in controlling multiple ecosystem processes. Both plant species richness and plant functional richness show significant effects on (nearly) all functional groups, however, the fraction of variation explained is always exceeded by the fraction explained by plant community biomass. Carnivore composition explain significant fractions of variation in many functional groups, the same applies for the soil and space variables. Consequently, we conclude that bottom-up effects seem to play the most important role in structuring the consumer communities in our experimental system, but at the same time top-down effects are still important for the majority of arthropod functional groups.

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