Leaf-cutting ants as ecosystem engineers: topsoil andlitter perturbations around Atta cephalotes nests reduce nutrient availability

1. Despite considerable research into the effects of leaf-cutting ant nests, the potential occurrence of low-nutrient soils at nest sites has hitherto gone undetected. 2. Leaf litter cover and topsoil conditions (organic carbon, total nitrogen, soil acidity, cation exchange capacity) were assessed along transects running from nests of eight adult Atta cephalotes colonies into the understorey of Atlantic forest to examine the extent of nest effects. 3. Nests were virtually free of leaf litter (c. 150 gm(-2)) and litter cover increased along a saturating curve with nest distance, reaching 1300 gm(-2) in the undisturbed forest. Soil acidity and nutrient concentrations were strongly correlated with leaf litter cover (r = 0.66-0.72) for both soil types occurring in the study region. Total soil nitrogen concentration varied from 0.2 g kg(-1) in sandy nest soil and 0.5 g kg(-1) in clay-rich nest soil to 0.5 and 1.3 g kg(-1), respectively, at a distance of 24m from nests, while soil carbon concentration ranged from 2.1 to 6.1 g kg(-1) (sandy soil) and 4.5 to 15.7 g kg(-1) (clay-rich soil) over the same distance. Nest-associated variations in edaphic parameters suggest that each colony affected an area of up to 0.5 ha. 4. In contrast to the common perception that leaf-cutting ant activities increase nutrient availability, our results suggest that their territories are characterised by reduced levels of leaf litter and, consequently, soil nutrients. 5. The observed nutrient depletion must be taken into account when considering the potential impacts of ant nests for plant regeneration.