ROLE OF BIOTIC INTERACTIONS IN A SMALL MAMMAL ASSEMBLAGE IN SEMIARID CHILE

The role of biotic interactions including vertebrate predation and int erspecific competition was studied among members of a semiarid small m ammal assemblage in a long-term field manipulation in north central Ch ile. Fences and netting selectively excluded from large, replicated gr ids. Small mammal populations were censused monthly over >4 yr, and ef fects of predator and degu exclusions on numbers and survival of small mammals were assessed with repeated measures ANOVAs. Marginally signi ficant predator exclusion effects were detected on O. degus, and highl y significant year x predation interactions in both wet and dry season s; degu survival (= persistence) was also significantly longer on excl usion grids. Predation had significant effects on numbers of Phyllotis darwini, and a significant month X predator interaction existed in th e wet season; survival was also significantly higher on predator exclu sion grids. There was a marginally significant year x competitor inter action in the dry season for this species. Akodon olivaceus had signif icant month x predator and month x competitor interactions in the wet season only; survival was significantly longer on degu exclusion grids . An irruptive species, Oligoryzomys longicaudatus, showed no signific ant predator or competitor effects in 1991-1992; however, survival was significantly longer on predator + competitor exclusion (vs. control) grids. An uncommon species, Abrothrix longipilis, had marginally nons ignificant time x competitor interactions but no treatment differences in survival. Finally, a second uncommon rodent, Abrocoma bennetti (no t analyzed numerically) had significantly longer survival on predator exclusion grids with degus. Degus and P. darwini were significantly ov errepresented (relative to availability) in diets of culpeo foxes (Pse udalopex culpaeus) and three nocturnal owls, respectively, whereas A. bennetti was significantly overrepresented in predator diets generally , Vertebrate predation has pervasive effects on some species that, how ever, may be related to their intrinsic demographic characteristics. C ompetition may be mediated by subtle behavioral interactions among ass emblage members. Long-term studies are necessary to elucidate the role of biotic and abiotic factors in semiarid environments.