How proximity to roads influence the local spatial dynamics and population growth rates of a Neotropical herbivore?

Background/Question/Methods Roads are widespread and increasingly important forms of alteration in most terrestrial landscapes. The majority of road studies suggest their effects are deleterious for most species. However, an overlooked and pervasive issue is the potential for some species to be attracted to and benefit from road creation. In this study we examined the effects of roads on a dominant Neotropical herbivore and crop pest, the leaf-cutter ant Atta laevigata, in the endangered Brazilian Cerrado. Evidence suggests that colonization is enhanced and recently established colonies have increased performance near roads, which have been noticeably more common due to the intensification of human activities in the Cerrado. However, it is unclear how the spatial distribution and population dynamics of these ants are affected by road proximity. We tested the hypothesis that roads attract leaf-cutter ant colonies and therefore enhance population growth rate at the local scale. During four consecutive years, we marked, mapped and monitored all mature A. laevigata colonies (nest mound size > 2 m²) in a 19-ha Cerrado site surrounded by dirt roads. In addition, we established demographic plots where colonies < 2 m² were monitored. We assessed the spatial distribution of mature colonies using Ripley’s K-function. We then compared the distance of colonies from roads with the average distance expected by a random distribution of points. Furthermore, we tested whether mature-colony survival and predation probabilities – potential underlying mechanisms – were influenced by proximity to roads. Finally, we constructed population models to determine and compare population growth rates near and far from roads.

Results/Conclusions We found that colonies were clustered in space at larger inter-point distances (120-150 m) and also significantly closer to roads than what would be expected by chance (P<0.0001). Overall, nearly 60% of the colonies were located within 30 m of a road. However, we found that survival and predation probabilities were not significantly related to road proximity. Furthermore, the population near roads was growing 65% faster than its counterpart far from roads. These results suggest that disturbances such as road construction have a profound positive influence on A. laevigata spatial distribution and population dynamics at the local scale. Other mechanisms such as vegetation composition and the performance of newer colonies may help explain this pattern. Nevertheless, with the expansion and improvement of road networks across the Cerrado, these ants are likely to expand in range and their many ecological and economic effects will potentially be exacerbated.