Western Harvester Ants’ Foraging Success and Nest Densities in Relation to Grazing Intensity

Western harvester ants, Pogonomyrmex occidentalis, are seed eaters that occur in short- and mid-grass prairies. Harvester ants are efficient seed predators but they may also be seed dispersers. We examined what ants collect to address that question. We also studied how different cattle grazing intensities affected harvester ant nest densities. Items collected by western harvester ant foragers returning to their nests were categorized as non-seeds, seeds, and nothing. Harvester ants collected large amounts of non-seeds (48%), followed by seeds (33%) and nothing (19%). Western harvester ants tolerate some environmental stress caused by grazing because nest densities were highest in moderately grazed grasslands. Interestingly, other aboveground arthropods in Colorado grasslands are reported to decrease in response to grazing, especially moderate to heavy grazing regimes. Harvester ants prefer to collect seeds but do not collect them exclusively