Asymmetries in specialization in ant-plant mutualistic networks

Abstract

Mutualistic networks involving plants and their pollinators or frugivores have been shown recently to exhibit a particular asymmetrical organization of interactions among species called nestedness: a core of reciprocal generalists accompanied by specialist species that interact almost exclusively with generalists. This structure contrasts with compartmentalized assemblage structures that have been verified in antagonistic food webs. Here we evaluated whether nestedness is a property of another type of mutualism-the interactions between ants and extrafloral nectary-bearing plants--and whether species richness may lead to differences in degree of nestedness among biological communities. We investigated network structure in four communities in Mexico. Nested patterns in ant-plant networks were very similar to those previously reported for pollination and frugivore systems, indicating that this form of asymmetry in specialization is a common feature of mutualisms between free-living species, but not always present in species-poor systems. Other ecological factors also appeared to contribute to the nested asymmetry in specialization, because some assemblages showed more extreme asymmetry than others even when species richness was held constant. Our results support a promising approach for the development of multispecies coevolutionary theory, leading to the idea that specialization may coevolve in different but simple ways in antagonistic and mutualistic assemblages.

Figure 1

(a) Different types of hypothetical plant–animal networks: random networks, in which there is no community-level patterns of specialization; compartmentalized networks, in which there is symmetrical specialization; and nested networks in which there is asymmetrical specialization, i.e. specialists interact with the core of generalist species. (b) EFN networks: LM, La Mancha; SB, San Benito; ZA, Zapotitlán; XL, Xalapa. Open nodes are ant species and closed nodes are extrafloral nectary-bearing plant species. Only the first three networks (LM, SB and ZA) show high degree of nestedness. Networks were drawn in Pajek (http://vlado.fmf.uni-lj.si/pub/networks/pajek/).

Figure 2

(a) Frequency of different degrees of nestedness observed in published studies on pollination, seed dispersal, and analysed EFN networks. Black columns indicate pollination networks, white columns indicate seed dispersal networks, and grey arrows indicate the values for analysed EFN networks (networks labelled as in figure 1). (b) The relationship between degree of nestedness and network size (i.e. number of interacting species). Closed circles indicate observed values of nestedness for each EFN network. Open squares indicate the average nested value predicted by null model I and open circles indicate the average value predicted by null model II (see text for further details). Error bars indicate 95% confidence interval (10 000 replicates for each null model).

Figure 3

The proportion of significant nestedness found in different types of interaction: EFN, extrafloral nectaries; SD, seed dispersal; PL, pollination; FW, food webs (antagonistic networks). NS, non-significant nestedness; p<0.05=significant nestedness.

Figure 4

Nestedness for real EFN and rarefied networks. Ensembles of rarefied networks were generated by randomly removing ant and plant species using two different procedures: assuming that each plant or ant species has the same probability of removal (white columns) and assuming that the probability of a species being removed is negatively correlated with the degree of generalism (grey columns). Arrows indicate the actual degree of nestedness of the smaller network. The p indicates the probability of a rarefied network showing nestedness equal to or more extreme than the real, smaller network: Xalapa and (a) rarefied La Mancha networks, (b) rarefied Zapotitlán and (c) rarefied San Benito; San Benito and (d) rarefied Zapotitlán, (e) rarefied La Mancha and (f) Zapotitlán and rarefied La Mancha.