HETEROGENEOUS RESPONSES OF SMALL MAMMALS TO AN EL-NINO SOUTHERN-OSCILLATION EVENT IN NORTHCENTRAL SEMIARID CHILE AND THE IMPORTANCE OF ECOLOGICAL SCALE

A prolonged El Nino Southern Oscillation (ENSO) event during 1991-1992 with three times the normal annual 85-mm rainfall was accompanied by major changes in numbers of small mammals at a semiarid Mediterranean site in northcentral Chile. Several demographic patterns were evident. Akodon olivacerrs, an omnivore, had a rapid increase in population si ze of more than an order of magnitude. Phyllotis darwini, a granivore- herbivore, showed somewhat delayed, smaller increases superimposed on annual oscillations. Octodon degus, an herbivore, showed a delayed res ponse with larger increases and extended breeding in 1992-1993. Finall y, Oligorzyzomys longicaudatus, a granivore, experienced increases dur ing both a dry (1990, ppt = 32 mm) and a wet year (1992). Other specie s such as Aborthrix longipilis, Abrocoma bennetti, and Thylamys elegan s had smaller, delayed demo,oraphic responses. Reproductive rates for the first three species were higher due to the ENSO event only in O. n egus males and P. darwini as a quadratic function of time. Survival ra tes of all four principal species were significantly greater during th e 1991-1992 ENSO. Finally, average movement between captures was lower during ENSO years, suggesting behavioral changes. Explanations for th ese patterns include rainfall-related increases in food-resource level s (A. olivaceus and P. darwini), the importance of source-sink process es in vagile species from more mesic, adjacent habitats (O. longicauda tus), and delayed responses to extrinsic events in species with long g estation (O. degus, A. bennetti). The heterogeneity of species respons es suggests different capabilities for small mammals to respond to an extrinsic, large-scale event, and it emphasizes the importance of long -term studies in semiarid systems.