Nest density of Atta sexdens (Linnaeus, 1758) in Atlantic Forest restoration sites depends on the surrounding landscape

Abstract Herbivory is an important ecological filter, affecting plant establishment in restoration sites. One group of herbivores whose abundance has been increasing with environmental changes are the leaf-cutting ants (LCA). Here we evaluated the influence of the surrounding landscape on Atta sexdens nest density in restoration sites, by testing the hypothesis that sites farthest from forest fragments or with less surrounding habitat cover have higher nest density. The study was conducted in eleven reforestations with native species, amidst an agricultural matrix in southern Brazil. For each site, we estimated LCA nest density (active, inactive and total) and landscape metrics (distance to nearest forest fragment, surrounding habitat area and an index combining both distance and surrounding habitat area, the Proximity Index). There were negative relationships between active and total nest density and surrounding habitat area. These results suggest that increased isolation from forest fragments is a factor contributing to the relaxation of top-down control. Therefore, the increase in A. sexdens population density in restoration sites is a result, at least in part, of low pressure from natural enemies, since LCA are not limited by resource availability.