The influence of rainfall on small mammal demography: Differences among species and habitats of semiarid Chile

Background/Question/Methods

Precipitation has significant effects in the dynamics of species inhabiting arid and semiarid systems. However, the magnitude of such effects on these species depends on the life history and ecology of the species in question. At the same time, species responses may be contingent functions of the habitat in which they are found. Finally, population fluctuations generally are driven by a combination of multiple factors whose relative contributions may vary through time, space, and among species. We monitored fluctuations of small mammal species for 25 years at a semiarid community in north-central Chile. We used monthly trapping and capture-mark-recapture methods in the predominant thorn scrub habitat, and sampled seasonally over 7 years in adjacent, more mesic habitats that included fog-forest and valley bottoms. We analyzed the variation in population size observed for these species and estimated demographic parameters for the different habitats.

Results/Conclusions

The community of small mammals in the thorn scrub includes 4 resident species (Octodon degus, Phyllotis darwini, Abrothrix olivaceus, and Thylamys elegans) that are always present, although at low densities during droughts. Other small mammal species are rare or transitory in the thorn scrub, but are present in more mesic habitats. Rainfall triggers marked increases in abundance of rodents with varying lags reaching different carrying capacities depending on the duration and magnitude of the rainfall event. In all species, the influence of rainfall on population densities varied among habitats.  Although all habitats showed very different community composition between years of high vs. low rainfall, this disparity was greatest in thorn scrub. Overall, species diversity and richness was greatest in the aguadas, a habitat with year-round moisture availability that may act as a refuge during prolonged droughts. Evidence from the distribution of body weights and signs of reproductive activity indicates that some species breed primarily in the aguadas, reinforcing the importance of this habitat for biodiversity conservation in this semiarid region.