Environmental associations and spatial segregation among coexisting Pogonomyrmex harvester ant species in the central Monte desert (Argentina)

Abstract

Abstract Background Ant communities are widely considered to be structured by interspecific competition. Seed-harvesting ants are dominant granivores in drylands, and their colony locations influence their spatio-temporal effect on plant communities. Three closely related species in the genus Pogonomyrmex coexist locally in the central Monte desert (Argentina) with substantial overlap in natural history, diet, seed preferences, and foraging periods. We surveyed active colonies across seasons and years (including a drought) to identify structural and edaphic factors associated with intra- and interspecific spatial distributions at different scales. Methods We surveyed active colonies within ~15 ha of algarrobal (main habitat in the Biosphere Reserve of Ñacuñán) on four sampling dates during three consecutive activity seasons, and on 1.5 km segments of low-traffic dirt roads every 3 years (once after a multi-year drought). We analysed their temporal variation and distributions using one- and two-dimensional spatial functions and models for point patterns in association with structural (distance to dirt roads) and edaphic (local topography, soil permeability) features. We measured soil texture in spatial clusters for two of the species. Results Individual colonies appeared, disappeared and relocated frequently, but the number of active Pogonomyrmex spp. colonies in the algarrobal varied according to a consistent seasonal pattern (spring to early autumn) as well as between consecutive activity seasons. All three species showed intraspecific aggregation and interspecific segregation, with most colonies near low-traffic dirt roads. Along the roads, Pogonomyrmex colonies appeared homogeneously but showed strong intraspecific aggregation and interspecific segregation that was associated with subtle edaphic variations: P. inermis was associated with clay soils with lower infiltration and at lower elevations, P. mendozanus with sandy, more permeable soils and higher elevations, and P. propinquus with intermediate conditions more typical of the algarrobal. After a drought, colony abundances declined (36–60%), but edaphic associations persisted in even more heterogeneous spatial patterns (most patches shrank and some emptied). Conclusions Three sympatric, congeneric and ecologically similar harvester ant species shared temporal and spatial patterns associated with structural features (dirt roads). However, their long-term local coexistence could be facilitated by spatial segregation along edaphic-related environmental gradients minimising interspecific interactions.