Diet flexibility in three harvester ants (Pogonomyrmex spp.): effects of grazing and natural variations in the availability of seeds

The study of diet and its relationship with available resources allows us to evaluate how species that differ in their degree of ecological flexibility respond to natural and anthropogenic variations that affect food availability. In the central Monte desert, extensive cattle ranching reduces the abundance of grass seeds, the most consumed and preferred food by the harvester ants Pogonomyrmex inermis Forel, 1914, P. mendozanus (Cuezzo & Claver, 2009), and P. propinqua (Johnson, 2021). We studied the diet of these species in grazed and ungrazed habitats over four years, including a period during which the abundance of grass seeds naturally decreased. We expected that ants would modify their diet in grazed sites and that the response would be different among species depending on their ecological flexibility. The three species presented a mainly granivorous diet in both grazing conditions and during the four years, but showed differences in the breadth of their diet. Pogonomyrmex inermis and P. propinqua always had a narrow diet that consisted mainly of grass seeds, while P. mendozanus modified its diet to include higher proportions of shrub seeds and other items when the abundance of grass seed decreased. In a scenario of scarce resources, this flexibility could represent an advantage over a more strict graminivorous diet.