Document: NAT-3-52-19

Contingent community structure: How resource-dependent interactions influence desert ant communities.

SANDERS, N.J.* and D.M.GORDON

Stanford University, Stanford, CA 94305-5020 USA ¹

Abstract:
Interspecific competition is an important influence on the structure of communities. But the mechanisms of competition are not well understood, and the importance of competition can vary both spatially and temporally. In desert ants, competitive interactions with Myrmecocystus mimicus and M. depilis lead to significant changes in the resource use of Aphaenogaster cockerelli. Our experiments showed that the influence of Myrmecocystus neighbors on A. cockerelli was dependent on resource type. When colonies competed at patches stocked with protein resources, Myrmecocystus foragers tended to displace A. cockerelli foragers. But, in patches stocked with seed resources, A. cockerelli displaced Myrmecocystus foragers. The mechanism for this seems to be that Myrmecocystus foragers are more aggressive and actively displace A. cockerelli foragers from protein resource patches. We then examined how such resource-dependent species interactions influence the structure of communities. It is generally accepted that ant communities are structured in transitive dominance hierarchies as a result of competitive interactions, where ecologically dominant species are those species that are more abundant at resource patches than would be expected given their actual abundance in the community. But if the outcome of competitive interactions is resource-dependent, then ecological dominance, and therefore community structure, is also likely to be resource-dependent. We tested this hypothesis by offering protein and seed resources in eight replicate desert ant communities of similar diversity and composition. Though the most abundant species tended always to be ecologically dominant, there was considerable variation in dominance hierarchies among communities. Interestingly, there was no overall effect of resource type on ecological dominance. Rather, species responded to resources differently, and the influence of resource type on ecological dominance varied among communities. Since ecological dominance is determined by the outcome of interactions at resources, and interactions vary with resources, we suggest that the organization of desert ant communities is contingent upon resource-dependent interactions.

Keywords: competition, foraging behavior, patch use, niche shift

This abstract is being presented at: 4:15 PM in session:
Oral Session #53: Terrestrial Invertebrate Ecology.