The commonality of core biological groups across freshwater food webs

Assessing how organisms interact is fundamental to understand biodiversity patterns. Like many ecosystems, most freshwater environments are susceptible to spatial and temporal variations, especially floodplains. In spite of the expected variability, the structure of interactions in food webs is a paramount aspect to uncover processes related to ecosystem functioning. Here, we surveyed four large floodplains to identify the biological groups that are protagonists across different freshwater food webs. We used limnological data on physical and biochemical variables, combined with biological information on 10 communities grouped into 41 trait-based categories. Despite the expected differences among floodplains and between hydrologic periods (dry and flood), we identified that complex freshwater food webs shared core network compartments. Using the eigenvector centrality as a measure of importance to the whole food web, we found that both primary producers (i.e., phytoplankton) and potential top predators (i.e., fish) were the core groups in freshwater trophic networks. These biological groups were consistently among the most linked compartments in all reconstructed food webs. Considering variations among food webs, we found that most differences were dictated by how compartments interact, rather than by their co-occurrences. Perhaps equally important is that omnivory was important for the reconstructed food webs, drawing attention to this interaction type as a fundamental aspect to unveil ecosystem processes. Although more empirical research is required to fully understand the importance of biotic interactions in freshwater food webs, our findings underline that ecosystems are more likely to collapse when disturbances affect the boundaries of the energy paths.