Human disturbance and aridity influence biomass harvesting by leaf-cutting ants with impacts on nutrient dynamics in a Caatinga dry forest.

Abstract

Human activities have converted mature forests into mosaics of successional vegetation and chronically disturbed habitats, altering the patterns of population distribution, foraging ecology and thus, the flow of matter and nutrients through ecosystems. Although the effects of human disturbance are mostly harmful, hyperabundant native generalist species can emerge and increase their populations under disturbance, such as leaf-cutting ants (LCA), prominent herbivores that are considered ecosystem engineers. Here, we examined the population response of two LCA species of the Caatinga dry forest (Acromyrmex balzani and A. rugosus) to increasing levels of chronic anthropogenic disturbance and aridity, and assessed the foraging activity, biomass and nutrients harvested by their colonies. We found that colony densities increased at more disturbed habitats, varying considerably from 0 to 81 nests/ha, but aridity had no effect. The two species exhibited markedly different foraging activities (44.66 ± 28.76 and 294.6 ± 260.53 ants foraging daily), with the foraging rate increasing in more arid conditions for a species with smaller nests, but with no response to disturbance. Biomass consumption varied distinctly between species, ranging from 0 to 4.81 g (7.24 kg ha.year^-1, in A. balzani) and from 5.6 to 74 g (174.39 kg ha.year^-1, in A. rugosus). Furthermore, there was no effect of disturbance and aridity on the biomass harvesting of individual colonies. However, there was a considerable increase in the biomass harvested by the populations of colonies in the plots (i.e. accounting for colony densities). Moreover, the species A. balzani foraged upon more nutrient-rich material at more disturbed and arid habitats, with plant material containing higher concentrations of N, Ca, S, Sr, Fe and Mn, as well as a lower C:N ratio in these areas. Our results suggest that Acromyrmex species (1) can achieve larger populations in more disturbed habitats, though not directly associated with aridity, (2) operate as a key herbivore able to fit harvesting/diet through the entire environmental gradient and forage complementarily (monocot vs. dicot) and (3) reallocate expressive amount of forest biomass, resulting in temporary nutrient sinks with potential impacts on Caatinga resilience.