THE INFLUENCE OF VEGETATION REMOVAL BY WESTERN HARVESTER ANTS (POGONOMYRMEX OWYHEEI) IN A RELICT AREA OF SAGEBRUSH-STEPPE IN CENTRAL OREGON

We examined the influence of western harvester ants (Pogonomyrmex owyh eei) as vegetation removers in a relict area of semiarid vegetation in central Oregon on The Island Research Natural Area (IRNA). Because IR NA is an almost inaccessible basaltic plateau that has been free of li vestock grazing and other significant anthropogenic influences for ove r 70 pears, it serves as an excellent natural laboratory for examining the effects of P. owyheei as vegetation removers. Pogonomyrmex owyhee i are able to remove plant cover around their nest sites. However, opp ortunities to study P. owyheei in undisturbed areas of the semi-arid w est are rare. We randomly selected 30 active ant nest sites on IRNA an d sampled the vegetation around these nests using line-intercept trans ects. Comparisons of mean vegetation cover between P. owyheei nest sit e transects and a reference group of nonnest site transects show a sig nificant reduction in total cover on the nest sire transects, but no s ignificant difference in total cover when tile cleared disc areas on t he P. owyheei transects are excluded. Our results suggest that vegetat ion removal by P. owyheei reduced the cover of some species a (e.g., A rtemisia tridentata) and, indirectly, increased the cover of other spe cies (e.g., Stipa thurberiana). We also found significant increases in vegetation cover on the perimeter of the cleared areas surrounding th e 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 m(2), P. owyheei is an important source of veget ation removal on IRNA.