Colony densities and spatial patterns of harvester ants (<i>Pogonomyrmex occidentalis</i> and <i>Pogonomyrmex rugosus</i>) in grazed and ungrazed areas of northern Arizona

Harvester ants play a crucial role as ecosystem engineers, particularly in nest-building activities that involve clearing nest-disks of vegetation. These clearings can be large and influence plant communities affecting rangeland management. In some cases, colony density may be increased by grazing but studies are sparse with conflicting results. We examined the effects of grazing on 2 harvester ant species (Pogonomyrmex occidentalis and Pogonomyrmex rugosus) at 5 sites in northern Arizona. We used preestablished grazing-exclusion treatments to compare colony density and spacing adjacent to and within grazed areas. At all sites, colony density was marginally higher in excluded treatments. Only P. occidentalis showed significant spatial patterns in response to grazing, where nests were evenly spaced on small distances (>5 m) in the grazing-excluded treatment. We observed large differences in nest size and density between ant species. Pogonomyrmex occidentalis made larger nests and cleared more land area than P. rugosus. Pogonomyrmex occidentalis peaked in density at 37 nests/ha occupying 1.87% of land area with smaller nearest neighbor distances (10.6 to 13.6 m), while P. rugosus peaked at 16 nests/ha occupying 0.20% of land area with larger nearest neighbor distances (17.9 to 24.3 m). Together our results provide limited but interesting evidence of negative effects of grazing on harvester ant nests. We discuss our findings in the context of other studies measuring colony densities in rangelands and conclude that grazing effects on nest spatial arrangements are inconsistent and differ among locations, highlighting the need for future studies.