Macroecological trends in nestedness and modularity of seed-dispersal networks: human impact matters

Aim We aim to characterize the macroecological patterns in the structure of mutualistic seed-dispersal networks. Tropical areas hold more species than temperate ones. This difference in species number may favour ecological processes that minimize interspecific competition in species-rich areas. There is theoretical evidence that both modularity (i.e. the presence of semi-independent groups of highly interacting species) and nestedness (i.e. specialists interact with a subset of the species interacting with generalists) reduce the effects of competition. Thus, we expect high degrees of modularity or nestedness at low latitudes in seed-dispersal networks. Moreover, we test whether climate, topography and human impact influence network structure. LocationThirty-four qualitative and 21 weighted seed-dispersal interaction networks located world-wide. MethodsWe related the degree of modularity and nestedness of seed-dispersal interaction networks with latitude. To disentangle the macroecological drivers of network structure, we also associated modularity/nestedness with species richness, altitudinal range, human impact and an array of climate predictors: precipitation, temperature, precipitation/temperature seasonality and historical climate-change velocity and anomaly. ResultsBinary networks showed stronger macroecological patterns than weighted networks. Latitude was unrelated to the structure of seed-dispersal networks, but more nested assemblages were species rich and were located in areas with a high degree of human impact, high temperature seasonality, low precipitation, and, especially on the mainland, high stability in precipitation. Modular networks were species rich and found in areas with low human impact. For both nestedness and modularity, the effects of species richness and human impact were especially strong and consistent. Main conclusionsAs for previous macroecological studies of mutualistic networks, we found that the structure of seed-dispersal assemblages was related to current and historical climate. The largest influences on nestedness and modularity, however, were the number of competing species and the degree of human impact. This suggests that human disturbance, not just climate, is an important factor determining the structure of a seed-dispersal network.