Complex and nonlinear effects of weather and density on the demography of small herbivorous mammals

Abstract

Understanding differential and integral effects of weather and population density on vital rates (e.g., survival and recruitment rates) helps elucidate the ecological and demographic mechanisms underlying animal population dynamics. Nonlinear responses of vital rates to changes in weather conditions, such as precipitation, are important for predicting the effects of climate changes on small herbivorous mammals. We aimed to test the hypotheses: (1) that small herbivore populations increase from low to intermediate precipitation with improved habitat conditions and decline beyond the intermediate or optimum precipitation due to increased mortality in semi-arid grassland and (2) that increases in population size would result in stronger negative effects on recruitment than on survival of small mammals. We live-trapped a population of the Daurian pika (Ochotona dauurica), a small herbivorous mammal, in north central Inner Mongolia, China, biweekly between May and November from 2010 to 2012. We estimated the effects of temperature, precipitation, and population size on the survival probabilities and recruitment rates of O. dauurica using mark-recapture methods. Increases in temperature improved the recruitment but reduced the survival of O. dauurica, resulting in negative net effects on population growth rates. Increased precipitation initially resulted in positive effects and then had negative effects on population growth rates primarily through nonlinear effects on survival probabilities, supporting the optimum habitat hypothesis. Changes in population size had stronger effects on recruitment than on survival of O. dauurica, suggesting that density-dependent feedback to recruitment may be a primary regulatory mechanism of small mammal populations.