Persisting Hyper-abundance of Leaf-cutting Ants (Atta spp.) at the Edge of an Old Atlantic Forest Fragment

Leaf-cutting ants (LCAs) profoundly benefit from edge creation in Neotropical forests, where they act as a keystone species and disturbance agent. In view of their poorly explored population dynamics, the question arises whether high densities of LCAs are a transitional or a persisting phenomenon. We studied the temporal variation of LCA colony densities at the edge of the Brazilian Atlantic forest. At physically stable edges of an old forest fragment, densities of Atta cephalotes and Atta sexdens (11 and five times higher in a 50 m edge zone in comparison with the forest interior) persisted over a 4-yr interval (2001–2005) with no significant difference in densities between years. Species-specific per colony growth rates ranged from 12 to −5 percent/yr, suggesting that populations were approximately at equilibrium. High rates of colony turnover (little less than 50% in 4 yr) indicated an average colony life span of about 7 yr—a life expectancy considerably lower than previous estimates for Atta colonies. Stable, hyper-abundant populations of LCAs accord with the constantly high availability of palatable pioneer vegetation (the preferred food source of LCAs) at forest edges and are expected to persist in time as long as forests are characterized by high edge to interior ratios, with potentially long-lasting consequences for the ecosystem.