Roads increase population growth rates of a native leaf-cutter ant in Neotropical savannahs

Abstract

A challenge in applied ecology is understanding the responses of native species to roads, which are ubiquitous drivers of environmental heterogeneity. While most studies highlight their negative effects, the potential for native species to increase in abundance near roads has been underappreciated, and the demographic mechanisms underlying abundance changes remain unclear. Elucidating these mechanisms requires first determining how demographic vital rates and life-history stages interact to influence population growth. One can then assess how roads alter the relative importance of the alternative demographic pathways – increased performance of early vs. late life stages – suggested as the principal drivers of increased population growth. We parameterized demographic models with 3 years of census data and conducted perturbation analyses to evaluate how proximity to roads altered abundance and demography. To do so, we used the leaf-cutter ant Atta laevigata as our model organism. Leaf-cutters are native ecosystem engineers and dominant herbivores in Neotropical ecosystems and also amongst the longest lived insects in nature. Our study was conducted in Cerrado savannahs, where road networks are rapidly expanding and Atta species are major agricultural pests. We found that 35–45% of adult colonies were located within 15 m of roads, which is significantly closer than expected at random. Population growth rates (λ) were >1 both near and far from roads, but λ near roads were significantly (41%) higher. Elasticities of λ to early survival and growth rates were proportionally much higher than those of adult stages, irrespective of habitat. Life-table response experiment results indicate that the higher λ near roads was due primarily to the much higher colonization and early-life performance there. Synthesis and applications. Road networks are proliferating throughout biodiversity-rich regions. Developing conservation and management guidelines for species whose abundance is altered by roads requires understanding the demographic mechanisms that underpin these changes. We demonstrate that the higher growth rate of roadside Atta laevigata populations is due to increased early-life performance. Thus, the expansion of road networks could have major ecological and economic consequences by facilitating the increased abundance of ecosystem engineers and agricultural pests. Accounting for A. laevigata early life stages and careful planning of road placement should improve management strategies of protected areas and agricultural systems in Neotropical savannahs.