A neutral‐niche theory of nestedness in mutualistic networks

Abstract

Recently, there has been a vigorous interest in community ecology about the structure of mutualistic networks and its importance for species persistence and coevolution. However, the mechanisms shaping mutualistic networks have been rarely explored. Here we extend for the first time the neutral theory of biodiversity to a multi trophic system. We focus on nestedness, a distinctive pattern of mutualistic community assembly showing two characteristics, namely, asymmetrical specialization (specialists interacting with generalists) and a generalist core (generalists interacting with generalists). We investigate the importance of relative species abundance (RSA) for the nested assembly of plantÁanimal mutualistic networks. Our results show that neutral mutualistic communities give rise to networks considerably more nested than real communities. RSA explains 60Á70% of nested patterns in two real communities studied here, while 30Á40% of nestedness is still unexplained. The nested pattern in real communities is better explained when we introduce interactionspecific species traits such as forbidden links and intensity of dependence (relative importance of fruits for the diet of a frugivore) in our analysis. The fact that neutral mutualistic communities exhibit a perfectly nested structure and do not show a random or compartmentalized structure, underlines the importance of RSA in the assembly of mutualistic networks.Recent studies demonstrate that mutualisms among free living species often form nested networks, i.e. those species with fewer interactions are preferentially associated with a subset of species that interact with the most connected ones (Bascompte et al.

Processes entangling interactions in communities: forbidden links are more important than abundance in a hummingbird–plant network

“…These studies also showed that the observed interactions are better predicted by simulations when other factors are included together with the abundance, such as phenological overlap [9,11,12], morphological matches among species [14] or the phylogenetic relationship, which ultimately influences the traits of the species [15]. In this sense, species traits affect the possibility of interactions by creating forbidden links in the community, although their importance seems to be secondary in comparison with species abundance.…”

“…In the context of ecological networks, neutral-based processes (often treated as neutrality, [9]) presume that species are ecologically equivalent. This means that individuals interact randomly in the community independently of their traits (specializations), thus more abundant species interact with more partners and with higher frequency than rarer species [8][9][10][11][12]. On the other hand, the concept of forbidden links assumes that niche-based processes constrain the interactions by means of matches and mismatches on the ecological traits of species [1,[3][4][5][6]9,10,13].…”

“…This procedure ignores the intrinsic dependence between the frequency of interactions and the 'abundance' generated. Ideally, independent measures of abundance should be collected directly in the field while the network data are collected [11]. Overcoming and investigating the effects of these limitations are then necessary steps towards broader generalizations.…”

Connecting Resource Tracking by Frugivores to Temporal Variation in Seed Dispersal Networks

“…However, this hypothesis does not fully account for some properties observed in networks, such as, the extent that species with a lower degree have interactions with a subset of the partners of species with a high degree, referred to as nestedness. For example, relative species abundance explains only 60-70% of the nestedness of two networks for plant-frugivore communities in southern Spain, with the 30-40% unexplained nestedness suggesting that the abundance and forbidden links hypotheses (and likely more) are not mutually exclusive and may work in concert to determine network structure and variability (Krishna et al, 2008).…”

“…Resource tracking can create scenarios where local abundances of frugivores increase as certain resources increase, which may, in turn, increase the likelihood of interactions with multiple plant species in the area that are fruiting at the same time; a scenario that is predicted by the abundance hypothesis. Multiple studies have suggested that resource tracking may play a role in the temporal dynamics of SD networks (Krishna et al, 2008;Carnicer et al, 2009;Yang et al, 2013). However, the influence resource tracking has on SD networks has not been explicitly explored, and, therefore, theory of how resource tracking may influence networks is still being developed (Figure 1).…”