Role of biotic interactions in a small mammal assemblage in semiarid Chile

The role of biotic interactions including vertebrate predation and interspecific competition was studied among members of a semiarid small mammal assemblage in a long-term field manipulation in north central Chile. Fences and netting selectively excluded from large, replicated grids. Small mammal populations were censused monthly over >4 yr, and effects of predator and degu exclusions on numbers and survival of small mammals were assessed with repeated measures ANOVAs. Marginally significant predator exclusion effects were detected on O. degus, and highly significant year x predation interactions in both wet and dry seasons; degu survival (= persistence) was also significantly longer on exclusion grids. Predation had significant effects on numbers of Phyllotis darwini, and a significant month X predator interaction existed in the wet season; survival was also significantly higher on predator exclusion grids. There was a marginally significant year x competitor interaction in the dry season for this species. Akodon olivaceus had significant 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 significant 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 nonsignificant time x competitor interactions but no treatment differences in survival. Finally, a second uncommon rodent, Abrocoma bennetti (not analyzed numerically) had significantly longer survival on predator exclusion grids with degus. Degus and P. darwini were significantly overrepresented (relative to availability) in diets of culpeo foxes (Pseudalopex 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, however, may be related to their intrinsic demographic characteristics. Competition may be mediated by subtle behavioral interactions among assemblage members. Long-term studies are necessary to elucidate the role of biotic and abiotic factors in semiarid environments.