The Small Mammal Community of a Chilean Temperate Rain Forest: Lack of Evidence of Competition between Dominant Species

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Southern Chilean temperate rain forests support a diverse small mammal fauna comprised of both rodents and marsupials. Secondary growth forests have a less diverse fauna dominated by two sigmodontines, Akodon olivaceus and Oryzomys longicaudatus , with a scarcer third species, A. longipilis , usually present. To investigate interspecific interactions among these species, a series of reciprocal removal experiments was conducted in a coastal secondary growth forest during 1981–1982. All individuals of A. olivaceus and O. longicaudatus were removed from 0.5 ha open grids during monthly censuses. The population responses and demographic attributes of these and the unmanipulated species, A. longipilis , were monitored. During the experiment (duration = 21 months), the identities of the removed species were reversed after 12 months following characteristic annual population declines to zero. An unmanipulated 1.2 ha control grid provided baseline data. Monthly removals of A. olivaceus were found to be more effective than those of O. longicaudatus due primarily to rapid rates of immigration in the latter species. No significant effect of removals on the remaining dominant species was observed either during the whole study, the months of May–December 1981–82, or months in which removals were effective in lowering experimental grid densities below control grid levels. However, the third, scarcer species, A. longipilis , showed a significant ( P < 0.05) increase in population densities in response to removal of A. olivaceus during the latter two time units. While a reciprocal response might have been predicted among the ecologically similar akodonts, the lack of significant response to removals among the numerically dominant species as reflected in population densities or demographic attributes does not support the presence of major competitive effects between A. olivaceus and O. longicaudatus in this community.