What is the relative importance of different life-history stages in intra- and inter-population responses to disturbance? Tests with a long-lived keystone herbivore in Neotropical savannas

Background/Question/Methods

Demographic studies of long-lived organisms generally suggest that adult survivorship and growth have the largest influence on population growth, but recent work indicates that vital-rates related to reproduction and recruitment can also have major impacts. However, the relative importance of these mechanisms for population dynamics and how general are these patterns across space remains poorly understood, especially in light of the myriad anthropogenic alterations worldwide. Roads are key forms of environmental disturbance and heterogeneity, and while studies suggest their effects are deleterious some species may be attracted to and thrive in habitats near roads. We reconcile population dynamics, demographic consistency and anthropogenic disturbances by evaluating how these alternative pathways alter the demography of long-lived organisms and how these patterns are influenced by proximity to roads. For this we use leaf-cutter ants (Atta spp.) in the Brazilian Cerrado as a model system. Leaf-cutters are dominant herbivores, crop pests and ecosystem engineers in this Neotropical savanna permeated by increasingly denser road networks. We hypothesize that road proximity increases population growth due to enhanced early-life performance in these habitats. First, we compared the distance from roads of adult colonies to that expected by random point distributions. We then constructed demographic models based on three years of censuses, sensitivities and life-table response experiments (LTREs) to compare population growth near and far from roads. Finally, we determine how demographic rates near roads vary across multiple populations.

Results/Conclusions

Overall, 35-45% of the adult colonies were located within 15 m of a road, which is significantly closer than expected by a random distribution. Growth rates (λ) were >1 both near and far from roads, but λ for the population near roads was ~80% higher. Elasticities of λ to survival and growth rates at early-stages were proportionally much higher than those of adult stages, irrespective of habitat and transition. LTRE results indicate that the higher λ near roads was due primarily to the increased establishment of incipient colonies. Furthermore, colony abundance in multiple populations was generally very high although vital rate variability across populations is still under analysis. Our results are in sharp contrast to the dominant paradigm describing the demography of long-lived species, also suggesting that disturbances such as roads have profound positive influence on Atta populations both locally and regionally thus potentially exacerbating their effects. We hypothesize these patterns could be characteristic not only of other social insects but also invasive species favored by anthropogenic disturbances.