Genetic Variation and Cluster Formation of the Ant Azteca in Coffee Agroecosystems

Abstract

The spatial distribution of organisms is an important concept in ecology and it has been recently recognized that large-scale spatial patterns may depend only on local-level interactions. The ant Azteca instabilis has been shown to have a spatially aggregated distribution despite a homogeneous environment in a coffee agroecosystems with evenly planted shade trees,. It has been argued that this self-organized pattern is the result of endogenous local forces that include a process of local expansion and density dependent control. In this study we sample ants from a 45-hectar plot within a coffee plantation and used behavioral assays of aggression, mitochondrial sequence data, and five microsatellites to elucidate processes contributing to this self-organized pattern. High levels of aggression between groups and strong genetic divergence provide strong evidence for the presence of two evolutionary distinct units in this system. Low levels of aggression among nests within a cluster, lack of genetic differentiation, and significant isolation by distance all support a local nest expansion process through budding. Significant genetic differentiation between most clusters and lack of isolation by distance at the cluster level indicates that new cluster establishment could be happening via flying queens from other areas. Lack of differentiation between some clusters, however, suggest that large scale budding or short distance dispersal could also be responsible for the establishment of new clusters. This study confirms the existence of two evolutionary units co-inhabiting the 45-hectare plot. Furthermore, it confirms the formation of clusters through a colony budding process. However, the establishment of new colonies from which the cluster eventually emerge could be either through new flying queens from a regional pool, queens flying short distances or from long distance bubbing.