Non-trophic interactions reduce modularity in a biome-wide food web of the North American boreal forest tetrapods

Background/Question/Methods

The prevalence and organization of non-trophic interactions in real ecosystems of any geographic extent is a current knowledge gap that inhibits our understanding of ecology, especially at the scale of an entire biome. Ecological networks allow us to examine patterns of interactions that give insight into how the entire system functions, and can be used to consider trophic and non-trophic interactions concurrently. To make the most of new network methods and existing ecological knowledge, we searched natural history and ecology literature and classified (by type and season) all known trophic and non-trophic interactions between species in the 600-million-hectare boreal region of North America. We assembled these interactions into the largest geographic scale trophic and non-trophic multiplex ecological network to date to gain a better understanding of the holistic structure of this ecosystem. Our analysis focused on the effects of non-trophic interactions for network modularity, which is well-known to relate to how perturbations flow through networks and can have ‘real world’ implications in addition to being of theoretical interest. Specifically, we asked whether non-trophic interactions “reinforce” modularity that exists in the trophic interaction network, or if they reduce modularity by creating non-trophic links between trophic modules.

Results/Conclusions

Analysis of our dataset, which contains 423 species connected by 5037 interactions spanning the vast boreal region of North America, revealed remarkable differences in network topology when non-trophic interactions were considered in addition to the trophic interactions. In particular, we found that inclusion of non-trophic interactions decreases network modularity in both summer and winter seasons. Failure to understand these non-trophic interactions has inhibited our ability to study and manage the effects of disturbance in North America’s boreal forest, a complex ecosystem where many different interactions and interaction types are relevant at a given time. Stark seasonal differences in network structure also suggest that as species ranges and interactions shift under climate change, boreal conservation can only be enhanced by knowledge of seasonal species interaction structure, especially overlooked winter interactions. Our work also highlights the importance of natural history for informing ecological research as well as the importance of seasonal, trophic, and non-trophic contexts that conclusions about real ecosystems must consider.