Ground-foraging Ant Diversity and the Role of an Aggressive Ant (Azteca instabilis) in Coffee Agroecosystems

Abstract

Conventional ecological theory on competition and competitive exclusion states that competition should limit diversity. However, diversity of all species is more common than competitive exclusion would suggest, especially in the tropics. Ants are a great study organism to ask questions of diversity and competition due to their high diversity and their relatively sessile nature of their nest. Similarly, agroecosystems offer a good habitat to study patterns of diversity due to the relative homogeneity of the habitat within each management regime. We studied the patterns of alpha diversity (local species richness) of ants from two different assemblages (coffee-foraging and ground-foraging) in the presence of a dominant and territorial ant species (Azteca instabilis) in a coffee agroecosystem. We hypothesized the that alpha diversity will be the highest at intermediate distance from the A. instabilis nests because close to the nests, the aggressive Azteca ants are successful at excluding other species, but far away form A. instabilis the normal competitive exclusion operates and reduces diversity to one or just a few species. We surveyed eight sites across three management intensities (high, moderate and low shade) for coffee-foraging ants (2 of 8 sites) and ground-foraging ants (all sites) and examined the species richness of each assemblage against the distance from the A. instabilis nest. We found no significant relationship between species richness and distance from A. instabilis for the coffee-foraging ant species. However, there was a consistent negative trend across high and moderate shade. This relationship was significant in five of six sites in the high and moderate shade plots. Species richness declines with increasing distance from the dominant ant (A. instabilis). In the low shade, the relationship was reversed in one site (positive trend) and nonexistent in the other. While correlative, this data suggests that competition may be shaping the assemblage patterns found. Furthermore the interactions between A. instabilis and other ground-foraging ants may actually increase ant species richness.