Untangling the complex food webs of tropical rainforest streams

Abstract

1- Describing food webs with precision and detail is critical for understanding the functioning of ecosystems. Streams in the Brazilian Atlantic Rainforest stand out as one of the most threatened ecosystems worldwide and yet, little is known about whether these systems house stable food webs and what are the trophic properties associated with that. 2 - In this study, we applied a Bayesian Hierarchical approach unifying three sources of information (gut content analysis, literature compilation, stable isotope data) to derive a well-resolved meta food web and five individual food webs from an Atlantic Forest catchment in Brazil. Together with data on population-level biomass and individually measured body mass, we applied a bioenergetic model and assessed their dynamical stability using a Lotka-Volterra approach. From the meta food web, we simulated individual population extinctions and their impacts on the food web stability. Finally, we calculated network dissimilarity indices in order to estimate the relative importance of regional processes for the maintenance of the meta food web.3 - The hierarchical model resulted in a meta food web with 307 probable links among the 65 components, from basal resources to consumers (aquatic insects, shrimps, and fish). At the regional scale, a surprisingly large proportion of total energy flow (70%) occurred through a set of ten invertebrates with large variation in body masses, from chironomids to shrimps. The remaining 30% of total energy flow relied on 51 different taxa supporting a significant dependency on a diverse community.4 - We found that these food webs were robust against disturbances, with dynamic resilience of the bioenergetic models. The simulations of population extinctions resulted in scenarios with stable food webs indicating that these systems do not rely strongly on keystone species for their resilience. Finally, approximately 40% of the interactions changed among local food webs mostly due the absence of predators in individual streams.5 - At the local scale, stability was likely maintained due to the high diversity of organisms with different body-sizes buffering against size specific disturbances. At the regional scale, high total diversity should entail the colonization of redundant taxa at a high rate, while high beta diversity of predators should enhance spatial asynchrony and regional stability. Here we integrated for the first time a hierarchical model to rebuild a detailed food web with methods for estimating energy fluxes within it. This innovative approach provides a baseline for the structure and functioning of tropical rainforest streams, setting the ground for future studies on how these ecosystems may respond to environmental change.