It's complicated: Exploring top-down and bottom-up controls in a multi-trophic level system

Background/Question/Methods

What controls food web dynamics and ecosystem function is an area of continued research and debate. Due to the complexity of ecosystems and communities, we continue to face challenges in untangling the relative importance of different species interactions on the rest of the community and that community’s function. One such example is top-down and bottom-up controls on food web dynamics, where there is conflicting evidence of how important either of these controls are on the function of an ecosystem. Increasing our understanding of what controls food web dynamics is important specifically in this time of unprecedented global change. As many ecosystems face pressure from varying anthropogenic stressors, such as habitat loss and forest fragmentation, there is added urgency to understand the mechanisms that control food webs in order to predict the consequences of these changes. Using a small tropical, aquatic decomposer ecosystem, we conducted multiple experiments observing the impact of forest fragmentation on these systems, both in terms of the top predator and the leaf litter input that provides nutrients to the community. We were able to characterize the entire invertebrate community at different trophic levels as well as measure leaf litter loss, a proxy for decomposition rate.

Results/Conclusions

We found conflicting impacts of forest fragmentation; fragmentation had observable effects at both the top and bottom of the food web. We were more likely to find our top predator in ecosystems in intact forest areas compared to small forest fragment areas. Leaf litter found in either large fragmented forests or intact forests experienced less mass loss than leaf litter from small fragmented forest. Our preliminary analysis of the intermediary trophic levels shows that these changes to either end of the food web does not necessarily lead to chain reactions that are consistent across different species. These results highlight the continued challenge to predicting the impact of habitat change on ecosystems. Further work in incorporating the varying scales that species experience at different trophic levels may help in solidifying predictions in the future.