Mutualistic networks support many ecological functions, such as seed dispersal by tropical forest vertebrates. A central problem in ecology is to understand how land use change affects plant-animal interactions that lead to effective seed dispersal services. Recent work shows that seed dispersal networks are highly heterogeneous, whereby some frugivorous bird species are more critical to network organization seed dispersal services than others. Here, we explore how habitat loss and then the potential local extinctions of highly frugivorous birds would affect the organization of seed dispersal networks. For this aim, we examined the organization of 22 empirical seed dispersal networks documented in tropical communities varying widely in the total amount of forest habitat persisting across different landscapes. We performed pathway proliferation analysis and simulated extinctions to evaluate the role of highly frugivorous birds in network organization.
Results/Conclusions
All seed dispersal networks were very similar in their organization, but networks from small forest fragments consistently supported a lower species richness of seed dispersers. This pattern was particularly evident for highly frugivorous birds which were almost entirely absent from small forest fragments. Network analysis showed that highly frugivorous birds form the core of interacting species, supporting the pathways that directly or indirectly connect all species within a given seed dispersal network. Extinction simulations suggested that the differential losses of highly frugivorous birds would lead to more severe disruptions in these pathways than would random extinctions. Our combined results support the notion that habitat loss erodes the diversity of highly frugivorous birds, reducing the integrity of seed dispersal networks. Because trophic cascades are likely to propagate across network pathways, we hypothesize that structural changes promoted by the loss of highly frugivorous birds following habitat loss would have implications for both ecological and evolutionary cascades in mutualistic networks.