Waterbird seed‐dispersal networks are similarly nested but less modular than those of frugivorous birds, and not driven by functional traits

Abstract

Frugivory is widely recognized as vital for the dispersal of many plants. Moreover, plant species and their frugivorous dispersers form seed‐dispersal assemblages whose structure has important implications for the persistence and stability of the community. However, dispersal interactions between plants and non‐frugivorous animal groups such as waterbirds remain largely understudied. We aimed to characterize the structure of waterbird seed‐dispersal networks, assess if this structure is similar to that of networks formed between frugivorous birds and fleshy‐fruited plants, and identify bird or plant functional traits important for the maintenance of network structure. We used network analyses and data from four community‐level studies on waterbird gut contents, including 12 bird and 88 plant species. We compared the network structure of waterbirds to those from previous studies of frugivorous birds. We also related the contribution of each species to the network structure with functional traits (e.g. size, habitat requirements, diet). Waterbird seed‐dispersal networks are similarly nested (i.e. specialists interact with a subset of those species that interact with generalists) but less modular (i.e. fewer semi‐independent groups of highly interacting species) than those of frugivores. Dabbling ducks, diving ducks and rallids did not separate into modules. The contribution of bird or plant species to network structure was not related to any functional trait. Seed‐dispersal networks of waterbirds share some organizational patterns with those of frugivores, but the underlying processes are not related to functional traits. This is probably related to fundamental differences between waterbirds and frugivores in how seeds are ingested. Differences in the functional role of waterbirds for seed dispersal are likely driven by other processes such as differences in population size, movement, ecology or gut processing of seeds.