PREVALENCE OF ORYZOPSIS-HYMENOIDES NEAR HARVESTER ANT MOUNDS - INDIRECT FACILITATION BY ANTS

In semiarid, sagebrush steppe vegetation, the bunchgrass Oryzopsis hymenoides usually was the closest plant to the center of Pogonomyrmex owyheei ant mounts and often was the only plant found within the disk-shaped areas around mounds that is cleared of vegetation by the ants. Although absolute numbers of O. hymenoides seeds were greatest on mounds, proportions of its seeds in the total seedbank were not significantly different among the mound, cleared disk, and the native plant community. Mean size and density of O. hymenoides plants near mounds were significantly greater than those of plants 15-20 m from mounds at an undisturbed study area. At another study area, which 3 yr before the study was burned, mean density and size of O. hymenoides plants were not significantly different among plots located from 0-20 m from the center of ant mounds. The prevalence of O. hymenoides near ant mounds was most likely an indirect effect of reduced plant competition caused by the severe defoliation of potential competitors by the ants. -from Authors