Central to nearly all theories of population regulation is that ultimately, numerical fluctuations in animal populations occur as a result of individuals responding to environmental factors via altered mortality and natality. Although considerable interest has been devoted to understanding the environmental forces influencing reproductive attributes of populations, similar efforts to understand the physiology of mortality and survival have been slow to develop. Because survival and immunity are inescapably linked, we explore the role of host immunocompetence as a mechanism by which intrinsic and extrinsic environmental selective forces influence fitness or survival of individuals. We review the wild mammalian and avian literature for evidence that environmental stressors such as season-climatic change, nutrition, and social interactions result in alterations in host immunocompetence, and ultimately, fluctuations in animal populations. Because immune responsiveness is largely genetically based, we specifically examine how environment can influence the frequency of immunocompetent genotypes within animal populations through selection and propose a model of population regulation incorporating the environment-genetic link. Immunological techniques for studying the complex interrelationships among environmental stressors and immunity in wild animal species is also reviewed.
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