Are the impacts of an ecosystem engineer context-dependent? Tests with the leaf-cutter ant Atta laevigata in a Neotropical savanna

Species that transform habitats or create new ones are known as Ecosystem Engineers. It has been posited that ecosystem engineers benefit other species by ameliorating conditions at the stressful end of environmental gradients, but that this beneficial impact diminishes as conditions along the gradient become more benign. We tested this hypothesis along a canopy-cover gradient in Brazil’s Cerrado with a widespread engineer of Neotropical ecosystems – the leaf-cutter ant Atta laevigata. We tested for changes in environmental conditions influencing seedling establishment, seedling abundance, and seedling diversity at different distances from ant nests. Local environmental conditions are influenced by both canopy cover and proximity to nests. Furthermore, the negative effect of Atta on seedlings overwhelms that of the gradient, but these impacts restricted to the area covered by nest mounds. The results demonstrate that some engineers amplify rather than reduce stress along environmental gradients, but that these impacts can be context-dependent and spatially-complex.