EFFECTS OF SMALL MAMMALS AND VERTEBRATE PREDATORS ON VEGETATION IN THE CHILEAN SEMIARID ZONE

We monitored the cover and seed bank response of shrubs, perennial her bs, and ephemeral plants to experimental exclusion of both the princip al rodent herbivore, Octodon degus, and its vertebrate predators from 1989 to 1994 in a semiarid Chilean mediterranean site. Although both r ichness and species composition of the plant community at the study si te were largely determined by abiotic factors (mainly rainfall and soi l nutrients), predator and herbivore exclusion had significant effects on the relative abundance of several plant species. Experimental excl usion of herbivores was associated with increased cover of some shrubs and a perennial grass, and decreased cover and seed densities of seve ral ephemerals, especially those exotic or restricted to areas underne ath shrubs. Herbivores apparently reduced shrubs through browsing and indirectly affected herb cover and seed densities by opening up areas under shrubs and/or modifying physical and chemical conditions of the soil. Plant responses to predator exclusion were less clear. Neverthel ess, higher cover of some shrubs and ephemerals in the presence of pre dators suggests tritrophic effects through changes in small mammal den sities and/or foraging behavior.