Abstract

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 Niño 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 Niño 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 herbivorous "core" (resident) species, Octodon degus and Phyllotis darwini, and an omnivorous "quasi-core" (resident but highly fluctuating) species. Akodon olivaceus: however, all three species declined towards the end of the second treatment year. Two "opportunistic" (temporarily resident) species, Abrothrix longipilis (an insectivore) and Oligoryzomys longicoudatus (a granivore), showed no responses to food additions. An insectivorous marsupial. Thylamys elegans (also a "core species"), had significantly lower numbers on food addition grids. Changes in body weight distributions and proportions of reproductive individuals particularly in O. degus indicale 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 numerically 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.