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The diversity of the earth's climates superimposed upon a complex configuration of physical features has provided the conditions for the evolution of a remarkable array of living things which are linked together into complex ecosystems. The kinds of organisms comprising the ecosystems of the world, and the nature of their interactions, have constantly changed through time due to coevolutionary interactions along with the effects of a continually changing physical environ- ment. In recent evolutionary time there has been a dramatic and ever-accelerating rate of change in the configuration of these ecosystems because of the increasing influence of human beings. These changes range from subtle modifications caused by anthropogenically induced alterations in atmospheric properties to the total destruction of ecosystems. Many of these modifications have provided the fuel, food, and fiber which have allowed the expansion of human populations. Unfortunately, there have been many unanticipated changes which accompanied these modifications which have had effects detrimental to human welfare in- cluding substantial changes in water and air quality. For example, the use of high-sulfur coal to produce energy in parts of North America is altering the properties of freshwater lakes and forests because of acidification.