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Pollution is one of the primary ways in which humans have caused drastic modifications of wildlife habitat. Historically we have regarded the air, water, and soil that surround us as waste receptacles and have given little consideration to the ecological consequences of our actions. As a result, wildlife populations are confronted with a bewildering array of pollutants that we release into the environment either by intent or accident.

In some cases wildlife populations have suffered severe losses or even faced extinction due to pollution. For example, the bald eagle, peregrine falcon, and brown pelican all nearly became extinct before scientists discovered that the synthetic chemical DDT was the cause of devastating reproductive failure in these species. Oil spills, such as the fouling of the coast of southern Alaska by the grounding of the Exxon Valdez, take an immediate toll on many species with the misfortune of living near such blunders. Toxic metals can kill adult members of wildlife populations and cause the production of deformed offspring, as seen at Kesterson Reservoir in the San Joaquin Valley. Acid rain has caused hundreds of fish populations to disappear from lakes in the northeastern U.S. and Scandinavia. In this chapter each of these notorious instances of the impacts of pollution on wildlife are described. The chapter also provides a general discussion of the origins and effects of synthetic chemicals, oil spills, toxic metals, and acid rain.

Pollution can be defined as the human alteration of chemical or physical characteristics of the environment to a degree that is harmful to living organisms. Some forms of pollution exert a destructive influence on wildlife by killing or impairing the health of individuals. Synthetic chemicals, oil, toxic metals, and acid rain are included in this category of toxic pollutants. Other forms of pollution affect wildlife in a more indirect manner by altering or destroying wildlife habitat. Examples include the obliteration of canyons, marshes, and grasslands with solid waste landfills; the destruction of the ozone layer by chlorofluorocarbons, which may lead to widespread damage due to the effects of excessive ultraviolet radiation on wildlife and their food sources; and carbon dioxide accumulation in the atmosphere, which may lead to global changes in climate and the distribution of wildlife 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 released thousands of synthetic chemicals into the environment and altered the distribution of many naturally occurring substances, thereby creating conditions that wildlife species had never experienced before. In many instances these new conditions have disrupted the delicate biological machinery evolved by organisms over thousands of years.

The use of synthetic chemicals to control pests, principally insects, weeds, and fungi, became an integral part of agriculture and disease control after World War II. These chemicals were credited with providing an inexpensive means of increasing crop production, preventing spoilage of stored foods, and saving many millions of human lives by the prevention of certain insect-borne diseases.

The history of DDT use in the U.S. is symbolic of the gradual development of an awareness of the ecological consequences of pesticide application. DDT was one of the most widely used pesticides in the post-War era. The first significant applications of DDT in the 1940s saved millions of human lives from malaria, typhus, and other deadly diseases. DDT was considered such an extraordinarily valuable substance that in 1948 the Nobel Prize in medicine was awarded to Paul Mueller, the Swiss chemist who discovered the compound's insecticidal properties. By 1964, DDT was so broadly applied that annual production in the U.S. reached 90 million kilograms. By the late 1960s, however, wildlife biologists realized that DDT was producing disastrous side effects in wildlife species. In the 1970s most industrialized countries banned the use of DDT because of its unacceptable effects on wildlife and, ultimately, humans.

DDT is classified as an "organochlorine" chemical, a descriptive label that reflects its chemical structure, consisting of a combination of carbon (organic molecules are defined as those comprised of at least some amount of carbon) and chlorine atoms. Other organochlorines that are important environmental pollutants include polychlorinated biphenyls (PCBs) and dioxins. PCBs were used as insulators in the electrical industry until the environmental threat posed by their toxicity was realized in the mid-1970s. Dioxins are the most potent chemical carcinogens known, and are present in the environment largely as a byproduct of various industrial activities (e.g., bleaching of paper).

The properties that make DDT and other organochlorine pesticides toxic to insect pests also make them hazardous to wildlife. The most important property of a pesticide, of course, is that it has a deleterious or toxic effect on pests. Unfortunately, it is difficult to formulate chemicals that exert toxic effects in pests alone. Another important property of a pesticide is that it persist long enough after application for pests to encounter it. Organochlorine pesticides are extremely persistent. For example, some of the DDT applied in this country in the early 1970s is still present in the environment today. An additional important characteristic of organochlorines is their tendency to be accumulated by living organisms. Organochlorines are strongly attracted to fats present in cells and tissues of living organisms. Since organochlorines resist degradation, these compounds can gradually accumulate to high concentrations in tissues of vertebrates.

The chemical characteristics of organochlorines have led to their distribution across the entire globe (Risebrough et al. 1968a,b). Organochlorines have a slight tendency to vaporize and become suspended in the atmosphere. Once this occurs, organochlorine molecules are subject to air movements that may transport them to any part of the earth's surface, including remote oceanic and polar regions. Due to their broad distribution via atmospheric transport, trace amounts of organochlorines are present in all vertebrates, including humans. It is thought that virtually every person in the U.S. is exposed to dioxin on a daily basis (Travis et al. 1989). Trace quantities of DDT, PCBs, and other organochlorines are commonly present in human tissues.

While the use of DDT and other organochlorine compounds has been curtailed in the U.S. and other developed countries, their use in developing nations continues, particularly in efforts to control the spread of human disease. However, the evolution of resistance to DDT by many insect pests has sharply reduced its effectiveness and eventually its complete replacement by other pesticides is expected to occur.

Other types of synthetic insecticides have been developed that pose lesser environmental threats than the organochlorines. Organophosphates, with a chemical structure consisting of carbon and phosphorus atoms, are now widely used. These compounds degrade much more readily than the organochlorines, and therefore have less of an impact on nontarget species. Malathion, large quantities of which have been used in attempts to control the Mediterranean fruit fly in California, is a well known organophosphate insecticide. Other synthetic pesticides are used to control weeds, fungi, and other pests. Although these more recent generations of chemicals pose less of an ecological threat than the organochlorines, they still have been shown to produce adverse effects in wildlife populations. Because annual rates of overall pesticide application show no sign of decreasing wildlife populations will continue to be affected by exposure to pesticides.


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