The importance of treefall gaps as foraging sites for leaf-cutting ants depends on forest age

Abstract

Variation in local abundance of herbivores is influenced by several abiotic and biotic factors, but it may often be determined by the local availability of plant resources (Quinn et al. 1997). Leaf-cutting ants, common defoliators of plants in the Neotropics, are more abundant in early successional forests than in old-growth forest (Jaffe & Vilela 1989, Vasconcelos & Cherrett 1995). A recent study showed that the high density of mature leaf-cutting ant nests in early successional forests was mainly determined by the availability of pioneer plant species, largely preferred by the leafcutters (Farji-Brener 2001). The dominance of pioneer species in early successional forests decreases foraging costs for leaf-cutting ant colonies, and thus these habitats can support a high density of mature colonies. On the other hand, the effective defence mechanisms of shade-tolerant species, which dominate primary habitats, could explain the low density of mature leaf-cutting ant colonies in old-growth forests (palatable forage hypothesis, see Farji-Brener 2001). According to this hypothesis, it is proposed that leaf-cutting ant colonies in old-growth forests should concentrate their foraging effort in treefall gaps, in which sun and pioneer plant species (preferred by leafcutters), are readily available (Farji-Brener 2001, Nichols-Orians 1991a, b; Wetterer 1994). However, this prediction of the palatable forage hypothesis has not yet been explicitly documented.