ROLE OF BIOTIC INTERACTIONS IN A SEMIARID SCRUB COMMUNITY NORTH-CENTRAL CHILE - A LONG-TERM ECOLOGICAL EXPERIMENT

Previous studies of the Chilean mediterranean and semiarid regions hav e suggested a major role of predation, and plant-animal interactions i n structuring small mammal assemblages, and in determining trophic int eractions within the community. However, few long-term, large scale fi eld experiments have been conducted in the temperate Neotropics, and s uch studies are needed to assess the generality of patterns largely de rived from studies conducted in the Northern Hemisphere. Since 1988, w e have been investigating the role of vertebrate predation, interspeci fic competition, and small mammal herbivory in a thom scrub community located in 1 national park (Fray Jorge) in north-central Chile (Norte Chico). Large fenced enclosures are used to selectively exclude foxes and avian raptors, and the principal small mammal herbivore, Octodon d egus. Initial results have show significant effects of predator exclus ions in numbers, survival rates, population structure, and behavior of degus. Although effects of excluding degus and/or predators have been mostly nonsignificant on other small mammal and plant species thus fa r, large increases have occurred in these groups as a result of an El Nino (ENSO) event in 1991-92. The importance of long-term, large scale studies of this nature is emphasized particularly in the None Chico w here unpredictable rainfall events have dramatic effects on the commun ity, and where tho impact of desertification has been particularly sev ere.