Competitive effects of a dominant arboreal ant (Azteca instabilis) on ground foraging ant diversity and community structure in a coffee agroecosystem

Background/Question/Methods   Interspecific competition can limit diversity and dictates coexistence at local scales. Generally, only one or two strong competitors can dominant a local environment. However, when the competitive effect of a dominant species is reduced, species of lesser competitive ability are released and may occupy territory otherwise uninhabitable. In neotropical ground-foraging ant communities, local diversity is high, but is also often dominated by two or three species. In the Soconusco region of Chiapas, Mexico, we investigated the effects of Azteca instabilis, an aggressive arboreal-nesting ant, on the dominant competitors of the ground-foraging ant community in a coffee agroecosystem. In a total of 8 field sites (ranging in size from 20x20m to 48x48m) across 3 different management intensities (high shade, moderate shade, low shade) we used tuna baits at every 2 or 4 meters to determine the spatial distribution of ground-foraging ant species around clusters of A. instabilis nests.

Results/Conclusions   Results indicate that there is a significant negative relationship between species richness of each site and the increasing distance from the nearest A. instabilis nest in both the high shade and moderate shade management intensities. At the same time there is a positive relationship between the presence of the dominant ground-foraging competitors and increasing distance from an A. instabilis nest. In low shade environments, however, the relationship is reversed and there is a positive relationship between distance from A. instabilis and the local species richness. This suggests that the influence of A. instabilis on the diversity of the ground-foraging ant community is dependent on the foraging distance or nest size of A. instabilis and/or the species composition in an agroecosystem, and further, that both of these factors vary with management intensity.