Forest edge orientation influences leaf-cutting ant abundance and plant drought stress in the Brazilian Atlantic forest

1 The release from bottom-up control is a key process promoting the proliferation of leaf-cutting ants (LCA) along neotropical rainforest edges. Considering the preference of LCA for drought-stressed plants, edge-induced drought-stress in plants could be one of the mechanisms behind this bottom-up response. We hypothesized that plants growing along the forest edge suffer higher levels of drought stress, which makes them a more suitable food resource for LCA and affects LCA colony density in a Brazilian Atlantic rainforest remnant. Additionally, we investigated whether this effect is stronger in north-facing edges because the radiation load is higher in this edge aspect in the southern hemisphere. 2 We examined LCA colony density and plant relative water content (RWC) by relating them to edge distance and edge orientation. 3 We recorded 57 LCA colonies in which the density showed an approximately six-fold increase within the first 50m of the forest compared with distances > 150 m. North-facing edges presented an approximately four-fold higher colony density compared with other edge aspects. Among the 296 plant individuals sampled, the average RWC was unrelated to edge distance, although plants growing near north-facing edges had approximately 10% lower RWC values compared with in south-facing edges. We also found that, considering edge orientation, the lower the average RWC, the higher the LCA colony density. 4 We conclude that drought stress-induced osmoregulation of plants varies with edge orientation and also that LCA may profit from changes in leaf quality in terms of an increased population density along edges, where plant individuals face higher drought stress levels.