Interspecific interactions evolve as a geographic mosaic, generating not only differences across space and time in traits and ecological outcomes but also in the number of species involved. The network structure of these interactions influences the nature of these ecological and evolutionary outcomes. Mutualistic networks exhibit complex but predictable patterns of interaction, frequently involving many species that form complex networks of interdependence. Recent analyses have shown that mutualistic networks among free-living species are nested. The overall pattern is one of weak and asymmetric links among species. These network patterns may be explained by ecological variables and evolutionary history but few data exist on their spatial and temporal stability. Several questions have been addressed using ant-plant interactions: Are ant-plant mutualistic interactions nested? Which ecological conditions may result in differences in nestedness among communities? Does interaction intimacy affect network structure and species richness? Do networks change in time? Most of the results are based on the analysis of the structure of a multispecific network of interacting ants and extrafloral nectary-bearing plants sampled between May 1989 and April 1991, and again between October 1998 and September 2000 in La Mancha, Veracruz, Mexico. Data analysis and statistics were performed using: Microsoft Excel, Pajek, Aninhado, UCINET, and XL-Stat 2008.
The structure of ant–plant networks exhibits nested patterns. Interaction intimacy markedly affects the structure of these networks. When interaction intimacy is high, the interactions are compartmentalized, when interaction intimacy is low, they exhibit nested patterns. These 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, and that simple features of mutualistic interactions are likely to have important consequences for the coevolutionary process and the patterns it generates in the organization of biodiversity. When compared in time, the nested topology of the network remains similar, group dissimilarity increases, standardized number of interactions for ant species increases and shifts towards higher values for plant species, more ant species and less plant species constituted the core of the more recent network, and presence of invasive/ruderal species increased while their contribution to nestedness remained the same. Generalist species characterized these patterns and appeared to maintain the stability of the network, since the new invasive/ruderal species incorporated in the communities were linked to this core of generalists: the overall network structure remains unmodified.