REPRODUCTIVE STRATEGIES UNDER A CYCLIC ENVIRONMENT

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Understanding how animal populations are regulated or limited in the wild still remains an outstanding issue in ecology, and especially in a conservation context. Environmental stressors such as limitations for food or other resources determine individual birth and death rates and ultimately make populations decline or recover. Population ecologists have been studying the interactions between climatic conditions and population density as surrogates for resource limitation. Studies with model species for which the life history response can be linked to predictable changes in resource availability serve an important role in improving our ability to predict population trajectories. Predators relying on cyclic prey species provide a useful tool in this context as they usually experience dramatic variation in demographic traits according to variation in prey densities. This variation is moreover relatively easy to quantify. Here we propose a pilot study on the tawny owl population of the Kielder Forest, northern England. This nocturnal raptor is specialist predator of field voles. Our objective is to fully understand the consequences of the variation in prey densities over individual lifetimes. Paradoxically, this analysis has only been seldom conducted on species experiencing cyclic demographic variation, while it has recently been estimated that about one third of animal populations, from mites to mammals and birds, display such cyclic patterns. Life history strategy of organisms living in cyclic environment may include phenotypic plasticity in some traits such as age of first reproduction or reproductive investment, so as to allow them to deal with changing prey availability. Plasticity thus has the potential to buffer partially or completely the impact of changes in prey abundance. First, we will consider whether environmental conditions experienced at birth by owlets remain detectable throughout their entire lifetime. In other words, we will investigate whether there is a 'cohort effect' on owl survival, age of first reproduction, breeding parameters (such as clutch size or laying date) and breeding frequency. Second, we will measure the covariation between these different life-history traits as this pattern greatly influences population dynamics. Third, we are interested in determining whether phenotypically different individuals adopt different reproductive strategies to cope with vole cyclicity, a source of variation that is at least partly predictable in time. Finally, this project represents an initial step toward the global understanding of how populations are regulated under cyclic environmental regime. Such knowledge appears critical in face of the current disappearance of cyclic dynamics in rodent species throughout Europe. Those species indeed constitute key stone species in many ecosystems and food chains, and numerous predators, including species of high conservation concern, are directly and highly dependent on them.