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Since 1989, effects of biotic interactions including predation and herbivory have been examined in a replicated experimental study in a north-central Chilean semiarid thorn scrub community. Strong responses of small mammals and plants to El Nino Southern Oscillations (ENSO) have also been documented suggesting that ''bottom- up'' factors related to high rainfall are important. To simulate increased primary productivity effects on small mammals, ad lib rabbit pellet additions were initiated in mid-1997 on unfenced grids near the experimental complex. Following the 1997 El Nino event with three times normal precipitation, numbers of small mammals during pre-addition months and the first treatment year were similar on control and food addition grids. During the second year (1998 -1999), a period of severe drought, food additions had significant positive effects on numbers of two predominantly herbivo- rous ''core'' (resident) species, Octodon degus and Phyllotis darwini, and an omnivo- rous ''quasi-core'' (resident but highly fluctuating) species, Akodon oliaceus; however, all three species declined towards the end of the second treatment year. Two ''opportunistic'' (temporarily resident) species, Abrothrix longipilis (an insectivore) and Oligoryzomys longicaudatus (a granivore), showed no responses to food addi- tions. An insectivorous marsupial, Thylamys elegans (also a ''core species''), had significantly lower numbers on food addition grids. Changes in body weight distribu- tions and proportions of reproductive individuals particularly in O. degus indicate in situ responses. Whereas no differences in residency, numbers of stations visited, and trappability were observed, energy compensation ratios greater than one suggest significant immigration in the second year. Thus, food additions elicited strong responses by herbivorous/omnivorous ''core'' and ''quasi-core species'' whereas they had no effects on ''opportunistic species''. These results reinforce the view that ''bottom-up'' factors influencing food availability exert prevailing control on numer- ically important small mammal species by temporarily increasing carrying capacity, and that ''top-down'' factors (i.e., biotic interactions) become important when small mammal numbers are at or near their carrying capacity. Spatial dynamics may be important in explaining declines of species populations exhibiting initially positive responses to food additions.

Since early 1989, we have conducted a large-scale ecological manipulation in a semiarid thorn scrub community in north-central Chile. We have excluded vertebrate predators (raptors and mammalian carnivores), and larger small mammal herbivores/ competitors (i.e., degus, Octodon degus) from replicated 0.56-ha plots, and monitored small mammal population and plant responses over more than ten years. Repeated tneasures ANOVAs on minimum number known alive (MNKA) estimates of small mammals for a six-year period (1990-1996) spanning an El Nino event in 1991-1992 showed strong responses of some species to predator exclusions (e.g., O. degus; Darwin's leaf-eared mouse, Phyllotis darwini; the chinchilla-rat, Abrocoma bennetti). However, responses varied in time with significant effects during pre-El Nino (1990-1992) and El Nino (1992-1994) periods (i.e., O. degus), or pre-El Nino and post-El Nino (1994-1996) periods (P. darwini, A. bennetti). Other species showed no responses to predator exclusions (e.g., olivaceous field mouse, Akodon olivaceus; long-haired field mouse, Abrothrix longipilis; long-tailed rice rat. Oligoryzomys longi-caudatus). Some effects of competitor (degu) exclusions were detected (e.g., A. bennetti during the El Nino and post-El Nino periods; O. longicaudatus during the El Nino). Top-down' factors (i.e., biotic interactions) appear to have greater effects on core species (i.e., P. darwini, O. degus) which persist in the thorn scrub. Other species (e.g., A. longipilis, O. longicaudatus) are transitory residents or opportunistic with lesser effects of biotic interactions, and their populations may be controlled by source-sink dynamics. All species had strong responses to the 1991-1992 El Nino indicating primary control by bottom-up factors.

We analyzed the putative effects of the El Nino Southern Oscillation (ENSO) of 1991-92 in a semi-arid locality of northern Chile. We obtained 30 months of pre-ENSO data, followed by 36 months of peak and post-ENSO data (total = 5.5 yr). The rainy winter of 1991 resulted in a three-fold increase in total seed bank (perennial and ephemerals pooled) and in ephemeral (but not perennial) herb cover. Seed and herbage eaters (rodents) irrupted to population levels ca 20 times higher during the breeding season of 1991 than the preceding wintering season. Diurnal carnivorous predators (hawks, owls, and foxes) showed a delayed response to the irruption, increasing from seven individuals sighted during the wintering season of 1991 to 13 during the wintering season of 1992. A seemingly counterclockwise trajectory of predator abundance versus prey levels suggested a pattern of prey-driven dynamics, but confidence intervals were likely broad. In this semiarid locality, it appears that ENSO effects did not cascade down from higher to lower trophic levels, but rather the opposite. In this bottom-up scenario, we predict that as primary productivity varies with rainfall, so should secondary (mammal prey densities), and tertiary productivity (vertebrate predators). Long-term monitoring of this terrestrial ecosystem is needed to test this prediction.