INFLUENCE OF HERBIVOROUS MAMMALS ON AN OLD-FIELD PLANT COMMUNITY - YEARS 1-4 AFTER DISTURBANCE

Plant community composition and change was studied in the presence and absence of different mammalian herbivores over the first four years after a disturbance that returned the field to the seed bank/colonizing stage. Four types of exclosures allowed the differential control of herbivore accessibility: ''control'' plots allowed access to all mammals (i.e., deer, woodchucks, rabbits, and small rodents); ''total exclosure'' plots excluded all mammals; ''large-gate'' plots excluded deer but allowed access to woodchucks, rabbits and small rodents; and ''small-gate'' plots allowed access to only small rodents. Results indicated that plant species richness and diversity/evenness were greater in small- and large-gate exclosures relative to controls or total exclosures. Total cover was less in control plots than in the three types of exclosures. Tests showed the abundance/occurrence of ca. one-half of the plant species varied among the four treatments: some species decreased with increasing herbivore accessibility. others increased, and some reached highest abundance under intermediate levels of herbivore access (i.e., in small or large gate plots). Treatment differences were more pronounced later in the study, and in parts of the field where herbivory was most intense. There was a tendency for Aster pilosus to dominate total exclosure plots and for Celastrus scandens to dominate controls. These results suggest that plant associations experiencing either high or low intensities of herbivory have lower diversities (in terms of both species richness and equitabilities), and different dynamics than plant associations subjected to intermediate levels of herbivory. The main effect of herbivory appears to be in altering competitive hierarchies among the plant community rather than by directly causing plant mortality.