Cutting more from cut forests:: Edge effects on foraging and herbivory of leaf-cutting ants in Brazil

Edge-mediated changes in species composition are known to result in modified species interactions. Because of the crucial trophic position of herbivores and their far-reaching impact on plant communities, it is important to understand how edge influences herbivory. In the present paper, we investigated whether and how leaf-cutting ant foraging is altered in the forest edge, as this habitat is characterized by an increased proportion of pioneer species. We assessed basic foraging data as well as the herbivory rate (i.e., the proportion of the leaf material harvested by a colony in relation to the available leaf area in the foraging area) of Atta cephalotes colonies at the edge versus interior sites of a large remnant of the Atlantic forest in Northeast Brazil. Our results indicated clear edge effects on leaf-cutting ants: equally sized A. cephalotes colonies located at the forest edge removed about twice as much leaf area from their foraging grounds than interior colonies (14.3 vs. 7.8%/col/yr). This greater colony-level impact within the forest edge zone was a consequence of markedly reduced foraging areas (0.9 vs. 2.3 ha/col/yr) and moderately lower leaf area index in this habitat, whereas harvest rates were the same. Our results suggest that forest edges induce increased leaf-cutting ant herbivory, probably via the release of resource limitation. Together with the increase of leaf-cutting ant populations along forest edges, this may amplify environmental changes induced by habitat fragmentation.