Abstract. Predation is a strong selective force that affects animal population dynamics either through direct killing or predation risk effects. Although recent empirical research has shown that the nonconsumptive effect of predator risk on prey dynamics can be as large (or even larger) as direct effects, little is known about the strength of predation risk effects in wild populations or the physiological mechanisms that mediate them. Here, we test both the predator-sensitive food hypothesis and predation-stress hypothesis in a single system by investigating activity budgets, stress/sex hormone levels, and demography of plateau pikas (Ochotona curzoniae) and their predators in Haibei Alpine Meadow Ecosystem Research Station of the Chinese Academy of Science. During the study period (2009 and 2010), plateau pikas experienced various predation pressures due to many predators being poisoned in 2010. In the year of high predator numbers, pikas spent more time on vigilant duty and less time foraging and they also showed higher plasma corticosterone levels and lower plasma estradiol and testosterone levels. Reproductive success and male with developed testes also reduced in the year of higher predation risk. In general, our results support both the predator-sensitive food and predation-stress hypothesis. Predator-induced risk affects prey reproduction by changes in feeding patterns and stress physiology.

Individuals in poor physiological condition are known to be more susceptible to infection that, once it occurs, further deteriorates the individuals' conditions, making them even more susceptible to infection and leading to death. This vicious synergy between the host condition and infection has recently been proposed to be among the key factors determining the oscillation characteristics in natural populations. Field studies that directly test such a hypothesis, however, are currently scarce, and the physiological traits involved remain largely unclear. In this field study, we systematically examined the synergistic effects of maternal stress and coccidian parasitic interaction on the physiological conditions and overwinter survival of offspring in the root vole (Microtus oeconomus) via repeated live trapping and measurements of faecal corticosterone metabolite (FCM) levels and immunocompetence in captured animals. We found that maternal density stress significantly increased FCM levels and both the prevalence and intensity of coccidian infection in offspring, significantly suppressed offspring immunocompetence and reduced offspring overwinter survival. Coccidian infection significantly potentiated increases in FCM levels and exacerbated the immune suppression induced by maternal density stress. Coccidian infection also significantly accelerated a decrease in overwinter survival and a population decline in maternal‐density‐stressed populations. Our findings indicate that both stress and immune systems are involved in the synergistic process through which stressors and parasites interact with the host to influence the fitness of individuals and cause a population decline in the natural environment. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13129/suppinfo is available for this article.

Although food supplementation is well known to increase population density, there is still debate on the causative effects of food supplementation on reproduction, survival, and immigration. Large manipulative experiments, which exclude any confounding effects of dispersal and predation, are essential for clarifying the debate. In this study, we investigated the effects of food supplementation on Brandt's vole population dynamics and plant community in eight large enclosures (0.48 ha each) from 2010 to 2014. Food supplementation showed significant positive effects on population density due to increases in recruitment; however, it showed a complex effect on survival of voles: positive in non-breeding seasons, but negative in breeding seasons. In addition, food supplementation increased the quality of plants (as reflected by increased crude protein content), but decreased the quantity of less preferred plants in experimental enclosures. Thus, food seems to have direct positive effects on small rodents through improvement of food supply and indirect negative effects through food-induced density-dependent effects, and may have long-term effects on rodents through altering plant community composition and abundance.