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habitats. Although both of these categories of pollutants pose significant threats to wildlife, this chapter focuses on toxic pollutants because of their specific effects on wildlife. Different species vary in their sensitivity to toxic pollution. For example, populations of fish living in lakes in the northeastern U.S. have proven to be extremely intolerant of the increased acidity caused by acid rain. On the other hand, fish populations in naturally acidic Florida lakes thrive under conditions that would kill fish from northeastern lakes. Why are some fish populations sensitive to the
effects of acidity while others are tolerant? The process of evolution allows species to optimize their chances of survival by adapting to the biological, chemical, and physical characteristics of their environment. Evolution, however, occurs over the course of many generations. Over thousands of years fish populations have evolved a tolerance of the conditions in the naturally acidic Florida lakes. Fish in northeastern lakes have evolved to survive under very different conditions, and have no mechanism for coping with high levels of acidity. In recent times humans have