The effects of fire on ant trophic assemblage and sexallocation

International audienceFire plays a key role in ecosystem dynamics worldwide, altering energy flowsand species community structure and composition. However, the functionalmechanisms underlying these effects are not well understood. Many grounddwellinganimal species can shelter themselves from exposure to heat andtherefore rarely suffer direct mortality. However, fire-induced alterations to theenvironment may change a species’ relative trophic level within a food web andits mode of foraging. We assessed how fire could affect ant resource utilizationat different scales in a Mediterranean forest. First, we conducted isotopic analyseson entire ant species assemblages and their potential food resources, whichincluded plants and other arthropods, in burned and unburned plots 1 yearpostfire. Second, we measured the production of males and females by nests ofa fire-resilient species, Aphaenogaster gibbosa, and analyzed the differences inisotopic values among workers, males, and females to test whether fire constrainedresource allocation. We found that, in spite of major modifications inbiotic and abiotic conditions, fire had little impact on the relative trophic positionof ant species. The studied assemblage was composed of species with awide array of diets. They ranged from being mostly herbivorous to completelyomnivorous, and a given species’ trophic level was the same in burned andunburned plots. In A. gibbosa nests, sexuals had greater d15N values than workersin both burned and unburned plots, which suggests that the former had amore protein-rich diet than the latter. Fire also appeared to have a major effecton A. gibbosa sex allocation: The proportion of nests that produced male broodwas greater on burned zones, as was the mean number of males produced pernest with the same reproductive investment. Our results show that generalistants with relatively broad diets maintained a constant trophic position, evenfollowing a major disturbance like fire. However, the dramatically reduced productionof females on burned zones compared to unburned zones 1 year postfiremay result in considerably reduced recruitment of new colonies in the midto long term, which could yield genetic bottlenecks and founder effects. Ourstudy paves the way for future functional analyses of fire-induced modificationsin ant populations and communities