Posted by: oikosasa | February 3, 2015

Invaders in plant-pollinator communities

The introduction of a new species to an ecological community can initiate a chain of events that results in a significant change to the community’s composition. For instance, the introduction of a pollinator species can facilitate the colonization of new plants that rely on the new pollinator for reproduction. Conversely, a pollinator species may drive down the population levels of certain species—e.g., if it aggressively robs a plant of its nectar without pollinating it.

How do communities respond to these invasions, and what lessons can be learned about the underlying properties of ecological communities in response to such invasions? In “Plant-pollinator community network response to species invasions depends on both invader and community characteristics,” the authors investigate the relationships between invasive species and community characteristics in shaping a plant-pollinator community’s response to an invasion.

Monarch butterfly (Danaus plexippus) on invasive plumeless thistle (Carduus acanthoides). Photo credit: Laura Russo

Monarch butterfly (Danaus plexippus) on invasive plumeless thistle (Carduus acanthoides). Photo credit: Laura Russo

The study makes use of a computational model that was originally used to investigate the process by which stable plant-pollinator communities form. The use of such models is attractive for two main reasons. First, a model that recapitulates real-world behavior offers insight into the mechanisms that operate in nature; second, computational models allow rapid and widespread exploration that would be time-consuming, costly, and in some cases impractical to perform in nature. As such, computational models are well-positioned to speed up the process of scientific discovery by providing novel and informative predictions and insights into the properties of the systems being modeled.

The model itself is used to generate simulated plant-pollinator communities with properties drawn from the empirical literature. Interactions may be true mutualisms (beneficial to both species) or detrimental to one species and beneficial to another (e.g., insects that visit flowers for nectar without pollinating the plant and plants that trick pollinators without providing them with nectar rewards). Colonization or maintenance of a species in the community is possible if its beneficial interactions outweigh its detrimental interactions; otherwise, the species goes extinct.

The model predicts that invasive species with properties that are very different from the native species in the region (e.g., supergeneralists that benefit the species with which they interact) are more likely to drive significant changes in the number of species colonizing the community. When an invasive species increases the species richness of the invaded community, there is a corresponding increase in the community’s nestedness and a decrease in the community’s connectance. Nestedness is a measure that accounts for the tendency of the community to be composed of (1) generalist species that interact with many species and (2) specialist species that interact with a subset of generalists. Connectance is the number of observed interactions relative to the number of possible interactions. This predicted divergence in nestedness and connectance is in agreement

with recent empirical work, and stands in contrast to the correlation of these two measures when considering the process by which communities stabilize.

This finding is relevant to the active discussion among researchers concerning the relationship between nestedness and connectance. By investigating the differing behavior of these properties in the context of species invasion, this paper supports the argument that nestedness and connectance are complementary properties that provide a more accurate picture of a community together than either measure provides alone. These findings are most strongly supported in the context of invaders that increase the number of species colonizing the community. As these invaders tend to participate in many species-species interactions, this paper also highlights the important role of generalist species in shaping the structure and dynamics of ecological communities.

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