The nested assembly of plant–animal mutualistic networks

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.

High richness of ectomycorrhizal fungi and low host specificity in a coastal sand dune ecosystem revealed by network analysis

“…2003; Bascompte and Jordano 2007; Bastolla et al. 2009), including mycorrhizal interactions (Bahram et al.…”

“…The absence of a positive nested pattern could be attributed to the fact that we sampled only four host species, which could be insufficient to statistically detect nestedness (Bascompte et al. 2003). Larger networks were generally found to be more nested than smaller networks (Bascompte et al.…”

“…In nested networks, specialized species are mainly associated with the generalist species of the other class but not with other specialists (Bascompte et al. 2003). High nestedness is thought to enhance the diversity and resilience of ecological communities (Burgos et al.…”

“…2012), particularly by reducing interspecific competition and facilitating species coexistence (Bascompte et al. 2003; Bastolla et al. 2009).…”

“…The structure of mutualistic networks (e.g., plant–pollinator networks) has been commonly described as highly nested (e.g., Bascompte et al. 2003; Burgos et al. 2007; Thebault and Fontaine 2010).…”