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Previous studies of the Chilean mediterranean and semiarid regions have suggested a major role of predation, and plant-animal interactions in structuring small mammal assemblages, and in determining trophic interactions within the community. However, few long-term, large scale field experiments have been conducted in the temperate Neotropics, and such studies are needed to assess the generality of patterns largely derived from studies conducted in the Northern Hemisphere. Since 1988, we have been investigating the role of vertebrate predation, interspecific 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 degus. Initial results have show significant effects of predator exclusions 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 far, 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 where unpredictable rainfall events have dramatic effects on the community, and where tho impact of desertification has been particularly severe.