This book examines the effects of environmental heterogeneity, or patchiness, on populations of plants and animals. The factors explored include variations in space, time, climactic conditions, food and other resources, and exposure to predators and parasites. In contrast to the once-prevailing view that environmental variation can be averaged-out without losing essential dynamics, the contributors to this volume find such heterogeneities often play a significant role in structuring large populations, especially in lessening the risk of extinction. Topics include the ways animals choose between patches that will expose them to different probabilities of starvation and predation, conservation in a variable environment and the optimal size of reserves, sex determination and sex ratios, patchiness and community structure, and extinctions of populations in correlated environments. The book will be of interest to ecologists, entomologists, environmental scientists, population geneticists, and biologists specializing in evolution, population, or conservation.