Influence of temporal fluctuations in seed abundance on the diet of harvester ants (Pogonomyrmexspp.) in the central Monte desert, Argentina

Abstract

AbstractHarvester ants usually go through temporal fluctuations in environmental seed abundance and composition which could influence their behaviour and ecology. The aim of this study was to evaluate how these fluctuations influence the diet ofPogonomyrmex rastratus,P. pronotalisandP. inermis(Hymenoptera, Formicidae) in the central Monte desert during three consecutive growing seasons. Although seeds were the main item in the diet, these ants turned more generalist when seed abundance of the most consumed species (grassesAristidaspp.,Trichloris crinita,Pappophorumspp.,Digitaria californicaandStipa ichu) was low. Accordingly, diversity of items in the diet decreased with seed abundance in a logarithmical fashion, showing higher foraging efficiency for seeds at higher seed abundance. Seed diversity, however, was not related to seed abundance as ants always included several species in their diet, with alternating prevalence. The proportion of the most consumed species increased logarithmically in the diet ofP. rastratusandP. pronotalisalong with their abundance in the environment probably as a consequence of diet switching (from forb and shrub seeds to grass seeds) and by an increase in foraging efficiency at higher seed densities. In contrast, foraging activity ofP. inermiswas very low at low seed abundance and its diet included only the five grasses. Among the most consumed species, proportion in the diet was not associated with relative abundance in the environment.Aristidaspp.,Pappophorumspp. andD. californicawere overall highly selected. However, the flexibility in the diet ofP. pronotalisandP. rastratusand the low foraging activity ofP. inermisduring periods of low resource abundance could attenuate potential top‐down effects in the central Monte desert. This study shows that bottom‐up effects are important in ant‐seed interactions and should be considered when predicting and evaluating ants' effects on seed resources.