The nested assembly of plant–animal mutualistic networks

TLDR

Plant–animal mutualism studies typically involve few species, and little is known about the structural organization of species‑rich networks, which is important for biodiversity maintenance. The authors analyze 52 mutualistic networks to demonstrate that they are highly nested, with specialists interacting only with subsets of generalists. The nested assembly generates highly asymmetrical interactions, organizes the community around a central core, is not random or compartmentalized, and increases with network complexity, indicating a nonrandom pattern relevant to biodiversity organization and persistence.

Abstract

Most studies of plant-animal mutualisms involve a small number of species. There is almost no information on the structural organization of species-rich mutualistic networks despite its potential importance for the maintenance of diversity. Here we analyze 52 mutualistic networks and show that they are highly nested; that is, the more specialist species interact only with proper subsets of those species interacting with the more generalists. This assembly pattern generates highly asymmetrical interactions and organizes the community cohesively around a central core of interactions. Thus, mutualistic networks are neither randomly assembled nor organized in compartments arising from tight, parallel specialization. Furthermore, nestedness increases with the complexity (number of interactions) of the network: for a given number of species, communities with more interactions are significantly more nested. Our results indicate a nonrandom pattern of community organization that may be relevant for our understanding of the organization and persistence of biodiversity.