A LONG-TERM STUDY ON THE DYNAMICS OF GUILD STRUCTURE AMONG PREDATORY VERTEBRATES AT A SEMIARID NEOTROPICAL SITE

For 48 mo we monitored diets and populations of 10 species of predator y vertebrates (two foxes, four falconiforms, and four owls) in semi-de sert scrub at Auco, Chile. Pellets (birds) and feces (foxes) collected monthly contained identifiable remains of small mammals, birds, repti les, amphibians, large insects, other arthropods, and (in some fox fec es) fruits. For non-mammalian prey, seasonal variation in availability appeared to be more pronounced than longer-term variation. In contras t, availability of the eight species of small mammals varied relativel y little between predators' breeding and non-breeding seasons but decl ined by an order of magnitude through the first 3 yr of the study. Rat her than responding to these shifts in food availability by diverging in diet during ''lean'' times and converging opportunistically during ''fat'' times, predators at Auco responded in idiosyncratic ways. Pred ators of all species tended to maintain species-specific means in size of prey taken regardless of shifts in the relative availability of pr ey size classes. Two owl species maintained strict mammal diets throug hout the decline in mammal abundance, whereas two other owls plus the two fox species maintained a tight feeding guild based on arthropods a s well as mammals and other food groups. The four falconiforms failed to maintain consistent diets from season to season, and consequently e nded up temporarily in unique positions in ''diet hyperspace'' or else joined temporarily one of the two consistent guilds. The degree to wh ich falconiform diets were unique in a given season showed no clear re lation to food abundance. Changes in use of habitat did, however, rela te to changes in food abundance. None of the falconiforms resided cont inuously in the study area throughout the period, and their occurrence was particularly sporadic late in our study even for falconiform spec ies that were not particularly mammalivorous. The lack of clearcut pat terns among Auco predators in response to resource-''lean'' and resour ce-''fat'' times may result from one or more of the following: (a) fro m predators' viewpoints, food classes at Auco may be quite discrete, i nhibiting gradual diet shifts by owls and foxes at least; (b) truly op portunistic immigrants that might have increased overall diet overlap during ''fat'' times, as sometimes occurs in other consumer assemblage s, do not exist in the Auco landscape; and (c) despite the pronounced decline in small mammal densities. the predator assemblage may have re mained above the threshold of food limitation through part or all of t he study.