Effect of intraspecific competition on the demography of leaf-cutting ants: a matrix model approach

Abstract

Intraspecific competition is a pervasive phenomenon with important ecological and evolutionary consequences in ants. However, its effect at population level remains less known. We investigated the effect of intraspecific competition on the demography of the leaf-cutting ant Acromyrmex lobicornis using a stochastic matrix demographic model parameterized with 3 years of census data. Given that competition is a negative interaction with potential consequences on fitness, we expected that nests that share their foraging area with conspecific nests would have a lower population growth rate than nests that did not. The stochastic growth rate of all sampled nests showed positive values, but with differences according to their competitive condition. Nests that did not share their foraging area showed a 34% annual growth, while nests that shared their foraging area with another conspecific nest showed only 13%. This difference appears to be related to a reduced probability that small nests grow to medium size in the competitive condition, this transition being the one that contributes the most to the population growth rate. These results suggest that competitive interactions often restrict the growth of small nest sizes, supporting previous evidence that proposed young ant colonies as the most vulnerable demographic stage. The known pattern of low overlap in ant foraging areas could be a consequence of a lower population growth rate of nests under competitive conditions. This illustrates how selective pressures on individuals (e.g., ant nests) can influence demography, emphasizing the role of intraspecific competition at population level and the potential consequences for species density and geographical ranges.