Patterns and mechanisms of population dynamics of small mammals in semiarid Chile : the role of biotic and abiotic factors

Includes bibliographical references (pages [123]-138).A long-term study of a small-mammal community in semiarid Chile has revealed large population fluctuations. These fluctuations were related to variations in rainfall generally associated with El Nino Southern Oscillation. Therefore, it appears that functioning in this system is dominated by abiotic factors. However, the presence of a high diversity of predators as well as a large herbivorous rodent species suggests that biotic interactions such as predation and competition may still be important. Therefore, the objective of my dissertation was to assess the relative contribution of biotic and abiotic factors to small-mammal population dynamics. First, I investigated temporal variation in the impact of predation and competition on numbers of three principal small mammal species, Octodon degus, Phyllotis darwini, and Abrothrix olivaceus. I used log response ratios, an index of effect size, in treatments where predators or O. degus, a putative competitor, had been excluded and removed, respectively. Predation had a greater impact on the population of O. degus than on populations of the other two species. The intensity of predation was stronger during periods of low density. Additionally, there was some evidence of competition between O. degus and A. olivaceus, particularly during prolonged droughts. Interestingly, O. degus had positive effects on P. darwini populations, but these reversed when degu numbers were high, suggesting stronger interspecific competition then. Next, I investigated the role of predation and rainfall-driven changes in resource availability on population rate of change of O. degus and P. danvini. Predation did not have a significant effect on dynamics of either species, and most of the variation of the population growth rates was explained by the interaction between density and current resources. Lagged effects of resources also played a significant role in driving population fluctuations. Finally, I examined mechanisms that triggered O. degus population changes by analyzing temporal variation in reproduction and survival parameters. These parameters were related to environmental variation depending on the persistence or change in wet vs. dry conditions. Fecundity, juvenile persistence, and adult survival changed more dramatically and, therefore, led to population increases or declines depending on wet or dry conditions respectively.Ph.D. (Doctor of Philosophy