Many frugivorous animals act simultaneously as seed predators and dispersers, and shifts in the abundance and mean phenotype of the frugivore population can alter the prevalence of antagonistic vs mutualistic interactions with seeds.

Here, we evaluated how a reduction in the abundance and species richness of large-bodied frugivorous fishes affects their interactions with plants in a Neotropical floodplain. Fisheries selectively harvest large frugivorous fishes, which are the most effective seed dispersers. Thus, functional extinctions of fish mutualists can cascade to secondary extinctions of plants and their biotic associates, and ecosystem-level change.

We evaluated functional responses of wetland communities to changes in fish body size by modeling the robustness of plant-frugivore networks to different extinction scenarios, using fish body size as a functional trait influencing seed dispersal and seed predation. We built ecological networks for frugivory, seed dispersal and seed predation.

All networks lacked modularity and showed reduced nestedness compared with randomized networks. The three networks indicated that frugivorous fish have wide niches and low to intermediate degree of specialization. Network robustness to extinction of frugivorous fish decreased with selective loss of large individuals.

Synthesis and applications. We predict that reductions in the abundance of large-bodied fish species will enhance the dominance of plant species with small seeds and decrease plant diversity. The functionally most important species in our networks are the most susceptible to overfishing. Therefore, sustainable management in these ecosystems needs to contribute to protecting interactions and, consequently, ecological, and evolutionary processes related to maintaining biodiversity in vast wetland forests.