The Influence of Vegetation Removal by Western Harvester Ants (Pogonomyrmex owyheei) in a Relict Area of Sagebrush-steppe in Central Oregon

Abstract

-We examined the influence of western harvester ants (Pogonomyrmex owyheei) as vegetation removers in a relict area of semiarid vegetation in central Oregon on The Island Research Natural Area (IRNA). Because IRNA is an almost inaccessible basaltic plateau that has been free of livestock grazing and other significant anthropogenic influences for over 70 years, it serves as an excellent natural laboratory for examining the effects of P owyheei as vegetation removers. Pogonomyrmex owyheei are able to remove plant cover around their nest sites. However, opportunities to study P owyheei in undisturbed areas of the semiarid west are rare. We randomly selected 30 active ant nest sites on IRNA and sampled the vegetation around these nests using line-intercept transects. Comparisons of mean vegetation cover between P owyheei nest site transects and a reference group of nonnest site transects show a significant reduction in total cover on the nest site transects, but no significant difference in total cover when the cleared disc areas on the P owyheei transects are excluded. Our results suggest that vegetation removal by P owyheei reduced the cover of some species (e.g., Artemisia tridentata) and, indirectly, increased the cover of other species (e.g., Stipa thurberiana). We also found significant increases in vegetation cover on the perimeter of the cleared areas surrounding the nest site, a phenomenon described as the border effect. With an estimated density of 28 ant nests per hectare, and a mean cleared area per nest site of 5.9 m2, P owyheei is an important source of vegetation removal on IRNA.