EFFECTS OF VERTEBRATE PREDATION ON A CAVIOMORPH RODENT, THE DEGU (OCTODON-DEGUS), IN A SEMIARID THORN SCRUB COMMUNITY IN CHILE

The effects of vertebrate predation have been monitored since 1989 on 16 replicated 0.56 ha study plots in a semiarid thorn scrub community in north-central Chile. Using fences of different heights with and wit hout holes and suspended game netting to alter principal predator (fox es and raptors) and large rodent herbivore (Octodon degus) access, fou r grids each have been assigned to the following treatments: 1) low fe ncing and holes allowing free access of predators and small mammals; 2 ) low fencing without holes to exclude degus only, 3) high fencing and netting with holes to exclude predators only; and 4) high fencing and netting without holes to exclude predators and degus. Small mammal po pulation censuses are conducted monthly using mark-recapture technique s. Degu population trends during 1989 and 1990 showed strongly but non significantly lower numbers in control plots during months when densit ies were characteristically low (September November) for this seasonal ly reproductive species; since March 1991, differences have become per sistent and increasingly significant. Predators appear to have greater numerical effects when their prey populations are low. Survival times of degus, particularly established adults, were significantly longer in predator exclusion grids during the 2 1/2 years of observation; thu s, predation also affects prey population structure.