The Structure of Plant-Animal Mutualistic Networks

This chapter discusses the structure of mutualistic networks. Despite their apparent complexity, mutualistic networks show repeated, universal structural patterns independent of species composition, size, and other ecological details. First, mutualistic networks are very heterogeneous: whereas the majority of species have only one or a few interactions, a few species are much more connected than expected by chance. Second, mutualistic networks are highly nested, that is, specialists interact with well-defined subsets of the species generalists interact with. Third, mutualistic networks are built on weak, asymmetric interactions among species. Fourth, mutualistic networks have a strong small-world property; that is, they simultaneously have a short path length among any pair of species and a high clustering coefficient. Finally, these networks are significantly modular—that is, there are small groups of species with morphological convergence of traits that interact strongly among themselves and more loosely with species from other modules. These modules can be regarded as the basic building blocks of mutualistic networks and their coevolutionary units.