Structural but not functional resistance of frugivore-plant interaction networks to the defaunation process

Abstract

Defaunation is the process of sequential loss of larger animal species caused by anthropogenic activities such as deforestation and hunting. This phenomenon has potential to disrupt the ecosystem stability, as well as their functional performance through the disruption of trophic interactions. We evaluated the effect of defaunation on structure and function in frugivore-plant interaction networks. We characterized interaction networks in two areas in northern Colombia and used a simulation model of species extinction in three different scenarios. The first scenario eliminated species based on body size, measured by body mass, according to the progressive effect of non-random species loss (defaunation); the second scenario eliminated species according to their contribution to network structure (CNS); and finally, the third scenario eliminated species according to their seed dispersal potential (SDP). Based on these simulations we evaluated the effect of species loss on the structural patterns (nestedness and modularity) and the functional diversity of the frugivore community, through the indexes of functional richness (FRic) and functional evenness (FEve). The loss of species with larger body sizes increased nestedness and did not affect modularity, whereas in the CNS scenario both patterns were affected. The FRic index decreased in the first stages of extinction by defaunation and by SDP, while the FEve index did not suffer significant variations in any scenario. The combination of interaction network analysis with functional diversity indices allows direct quantification of the robustness of network structural patterns and the vulnerability of the functional capacity of frugivore communities in the face of defaunation.