PMID: 14583855;Abstract:
One approach to understanding how mutualisms function in community settings is to model well-studied pairwise interactions in the presence of the few species with which they interact most strongly. In nature, such species are often specialized antagonists of one or both mutualists. Hence, these models can also shed light on the problem of when and how mutualisms are able to persist in the face of exploitation. We used spatial stochastic simulations to model the ecological dynamics of obligate, species-specific mutualisms between plants and pollinating seed parasite insects (e.g., yuccas and yucca moths) in the presence of one of two obligate antagonist species: flower-feeding insects (florivores) or insects that parasitize seeds but fail to pollinate (exploiters). Our results suggest that mutualisms can persist surprisingly well in the presence of highly specialized antagonists but that they exhibit distinctly different temporal and spatial dynamics when antagonists are present. In our models, antagonists tend to induce oscillations in the mutualist populations. As the number of per capita visits by antagonists increase, the system's oscillatory dynamics become more extreme, finally leading to the extinction of one or more of the three species. When the antagonists exhibit high per capita visitation frequencies and long dispersal distances, significant spatial patchiness emerges within these tripartite interactions. We found surprisingly little difference between the ecological effects of florivores and exploiters, although in general florivores tended to drive themselves (and sometimes the mutualists) to extinction at parameter values at which the exploiters were able to persist. These theoretical results suggest several testable hypotheses regarding the ecological and evolutionary persistence of mutualisms. More broadly, they point to the critical importance of studying the dynamics of pairwise interactions in community contexts.
Abstract:
Currently, there is little information transfer between empiricists working on cooperative interactions between species (mutualism) and theoreticians who model possible scenarios for the evolution and maintenance of cooperation between unrelated individuals. Furthermore, both theoretical and behavioral approaches often fail to consider ecological parameters that influence behavior. Our goal is to present the wealth of empirical knowledge (both behavioral and ecological) on mutualistic systems in a structure that may facilitate communication between empiricists and theoreticians. We have chosen eight broad categories of mutualisms that have been intensely studied and that are relatively well understood. For each system, we assess possible states of 12 parameters that can help theoreticians to construct game structures of mutualisms that are built on current empirical knowledge. We point out how ecological variables may influence behavioral decisions in ways not identified by our parameters. Finally, we elucidate similarities between mutualistic systems with respect to game structures that may not be expected given the diversity of mutualisms with respect to ecological and evolutionary background. On the basis of these results, we promote an interactive approach with models based on empirical knowledge, amenable to further testing. © 2004 Elsevier Inc. All rights reserved.
Abstract:
The services provided within a community can change as the species composition of that community changes. For example, ant-seed dispersal mutualisms can be disrupted in habitats dominated by invasive ants. We propose that this disruption is related to changes in mean ant body size, given that invasive ants are smaller than most native seed-dispersing ants. We demonstrate that the mean and maximum distances that ants transport seeds adapted for ant dispersal increase with worker body size, and that this relationship is an accelerating power function. This pattern is consistent among three ant subfamilies that include most seed-dispersing ants as well as most invasive ant species, is generalizable across ant species and communities, and is independent of diaspore mass. Using a case study, we demonstrate that both the mean body size of seed-collecting ants and seed dispersal distances are decreased in sites invaded by Solenopsis invicta, the imported red fire ant. Furthermore, we demonstrate that the mean size of seed-collecting ants at a seed depot or within a community is a useful predictor of mean seed dispersal distances at those sites. Last, we show that small seed-collecting ants and decreased seed dispersal distances are common features of sites occupied by invasive ants. The link between ant body size and seed dispersal distance, combined with the dominance of invaded communities by typically small ants, predicts the disruption of native ant-seed dispersal mutualisms in invaded habitats.
