It is obvious that nature is undergoing rapid changes as a result of human activities such as industry, agriculture, travel, fisheries, urbanization, etc. What effects do these activities have? Are they disturbing equilibria in ecological populations and communities, i.e., are they upsetting the balance of nature, or are they enhancing naturally occurring disequilibria, perhaps with even worse consequences? This book examines these questions, first by providing evidence for equilibrium and nonequilibrium (= disequilibrium) conditions in natural systems, and second by examining human-induced effects, among them those due to climate change, habitat destruction and introduction of alien species. One often hears the argument, not only from non-scientists but also from some scientists, that large-scale fluctuations in climate, sea levels etc. have occurred over and over again in the geological past, long before human activities could possibly have had any impact, that human effects are very small compared to those naturally occurring anyway, and that they cannot significantly affect the environment. Is this indeed so? Or is it possible that naturally occurring fluctuations are being dangerously enhanced by humans?