Colony Dynamics

Abstract

The density, distribution, and size of colonies of leaf-cutting ants (Atta spp. and Acromyrmex spp.) in a particular area is of great ecological importance for their potential impact on the ecosystem through trophic, as well as nontrophic processes. Their effects as herbivores range from selective and patchy damage of individual plants up to landscape scale influence on the plant community. Their influence on the environment includes soil turnover by soil deposition from nest-building activities, nest-clearing, i.e., removal of all foliar vegetation in the nearest vicinity on and above the nest, and the accumulation of biomass in the fungus garden and in refuse mounds. The spatial and temporal distribution of Atta colonies varies considerably between and within their habitats. In natural forests, nest densities range from only 0.05 colonies/ha in Amazon forests (Jaffe and Vilela 1989) up to 18 colonies/ha in a forest in Pará, Brazil (Ribeiro and Woessner 1979). In cultivated areas densities can be even higher. As many as 30 colonies/ha have been reported in plantations in Brazil (Ribeiro and Woessner 1979; Jaffe 1986). Unfortunately, most authors do not specify the size of the colonies, i.e., one cannot directly draw conclusions about the magnitude of their ecological impact. Thus, the extremely high densities reported for Acromyrmex (up to 200 colonies/ha in a Eucalyptus forest, Mendes Filho in Cherrett 1989) do not necessarily imply a similarly high effect on the forest, because this genus usually has much smaller colonies than Atta (Cherrett 1989). Within forest ecosystems, nest densities generally tend to be higher in rather open habitats, like gaps, forest edges and clearings. In closed forests the densities decline with the maturity of the stand. Therefore, it has been proposed that high densities are indicative of disturbance (Vasconcelos and Cherrett 1995).