EFFECTS OF HARVESTER ANTS ON PLANT-SPECIES DISTRIBUTION AND ABUNDANCEIN A SERPENTINE GRASSLAND

Seed harvesting ants can have important effects on the composition and structure of plant communities. We investigated two effects of Messo, andrei, the black seed-harvesting ant, on a serpentine grassland plan t community in northern California. First, to determine if selective s eed predation by ants affects plant community composition, we excluded harvester ants from 1-mediameter circular plots of grassland. Abundan ces of all species on these plots and on control plots were measured b efore and after exclosure. Second, to determine if M. andrei nest moun ds affect plant community composition, we compared plant species abund ances on and off nest mounds. M. andrei deposit large amounts of organ ic matter on their nest mounds over a foraging season, so mounds may a lter the edaphic environment. The exclusion of seed-harvesting activit y did not cause changes in the plant community. Nest mounds had a stro ng effect on plant communities: there were many more grasses and fewer forbs on ant mounds, although at least one forb, Lepidium nitidum, pr oduced twice as many seeds when it grew on nest mounds. We found that nest mounds formed islands of higher-temperature soil in the serpentin e grassland.