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

Human activities have converted mature forests into mosaics of
successional vegetation and chronically disturbed habitats, altering the
patterns of populations 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 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.yr-1, in A. balzani)
and from 5.6 to 74 g (174.39 kg ha.yr-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 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 complementary (monocot vs. dicot), (3) reallocate
expressive amount of forest biomass, resulting into temporary nutrient
sinks with potential impacts on Caatinga resilience.