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 predatory vertebrates (two foxes, four falconiforms, and four owls) in semi-desert scrub at Auco, Chile. Pellets (birds) and feces (foxes) collected monthly contained identifiable remains of small mammals, birds, reptiles, amphibians, large insects, other arthropods, and (in some fox feces) fruits. For non-mammalian prey, seasonal variation in availability appeared to be more pronounced than longer-term variation. In contrast, availability of the eight species of small mammals varied relatively little between predators' breeding and non-breeding seasons but declined by an order of magnitude through the first 3 yr of the study. Rather 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. Predators of all species tended to maintain species-specific means in size of prey taken regardless of shifts in the relative availability of prey size classes. Two owl species maintained strict mammal diets throughout the decline in mammal abundance, whereas two other owls plus the two fox species maintained a tight feeding guild based on arthropods as well as mammals and other food groups. The four falconiforms failed to maintain consistent diets from season to season, and consequently ended up temporarily in unique positions in ''diet hyperspace'' or else joined temporarily one of the two consistent guilds. The degree to which falconiform diets were unique in a given season showed no clear relation to food abundance. Changes in use of habitat did, however, relate to changes in food abundance. None of the falconiforms resided continuously in the study area throughout the period, and their occurrence was particularly sporadic late in our study even for falconiform species that were not particularly mammalivorous. The lack of clearcut patterns among Auco predators in response to resource-''lean'' and resource-''fat'' times may result from one or more of the following: (a) from predators' viewpoints, food classes at Auco may be quite discrete, inhibiting gradual diet shifts by owls and foxes at least; (b) truly opportunistic immigrants that might have increased overall diet overlap during ''fat'' times, as sometimes occurs in other consumer assemblages, do not exist in the Auco landscape; and (c) despite the pronounced decline in small mammal densities. the predator assemblage may have remained above the threshold of food limitation through part or all of the study.