The degradation of air, land, and water as a result of the release of chemical and biological wastes has wide-ranging effects on animals. On a large scale, pollution destroys habitats and produces population crashes and even the extinction of species. Hazardous chemicals introduced into an environment sometime render it unfit for life (as at Love Canal, New York, or Times Beach, Missouri). At the individual level, pollution causes abnormalities in growth, development, and reproduction. Hazardous chemicals, introduced either intentionally (such as fertilizers, herbicides, and pesticides) or through neglect (as with industrial wastes), have a variety of detrimental, sometimes devastating effects on animals. They affect the metabolism, growth and development, reproduction, and average life span of many species.
A few examples will illustrate the effects of chemical pollution on animals. In the 1940's, the new insecticide dichloro-diphenyl-trichloroethane (DDT) was regarded as a miracle. It saved millions of lives in the tropics by killing the mosquitoes that spread deadly malaria. DDT saved millions more lives with increased crop yields resulting from DDT's destruction of insect pests. This miraculous pesticide, however, turned out to be a long-lasting nemesis to many species of wildlife and to the environment. In the United States, ecol-ogists and wildlife biologists during the 1950's and 1960's witnessed a stunning decline in the populations of several predatory birds, especially fish-eaters, such as bald eagles, cormorants, os-preys, and brown pelicans. The population de-
cline drove the brown pelican and bald eagle close to extinction. In 1973, the U.S. Congress passed the Endangered Species Act, which banned the use of DDT. The once-threatened species have somewhat recovered since. In the mid 1950's, the World Health Organization used DDT on the island of Borneo to control malaria. DDT entered food webs through a caterpillar. Wasps that fed on the caterpillar were first destroyed. Gecko lizards that ate the poisoned insects accumulated high levels of DDT in their bodies. Both geckos and the village cats that ate the geckos died of DDT poisoning. The rat population exploded with its natural enemy, cats, eliminated. The village was then threatened with an outbreak of plague, carried by the uncontrolled rats.
Although DDT has been banned in much of the world, there is a growing concern over the effects of a number of chlorinated compounds. These chemicals, described as "environmental estrogens," interfere with normal sex hormone functions by mimicking the effects of the hormone estrogen or enhancing estrogen's potency. High levels of chlorinated compounds, such as dioxin and polychlorinated biphenyls (PCBs), in the Great Lakes have led to a sharp decline in populations of river otters and a variety of fish-eating birds, including the newly returned bald eagles. These chemicals are also the cause of deformed offspring, or eggs that never hatch. In Florida's Lake Apopka, a spill of chlorinated chemicals in 1980 led to a 90 percent drop in the birth rate of the lake's alligators. These are only a few examples of the detrimental effects of synthetic chemicals on various animals.
Air pollution leads to acid rain and the greenhouse effect, as well as damage to the ozone layer. Acid rain drops out of the skies onto areas at great distances from the source of the acids, and destroys forests and lakes in sensitive regions. As a result, fish populations are dwindling or being eliminated from lakes and streams by a lower pH caused by acid deposition. The strongest evidence comes from data collected from the past twenty-
Frogs and toads have been amazingly resilient throughout their evolution and hence have a wide distribution in ponds and swamps all over the planet. Their evolutionary longevity, however, appears to offer inadequate defense against the pollution to the environment brought about by human activities. During the 1990's, biologists around the world have documented an alarming decline in amphibian populations. Thousands of species of frogs, toads, and salamanders are experiencing dramatic decreases in numbers, and many have gone extinct or become endangered.
The specific causes of the worldwide decline in amphibians are not fully understood. All the likely factors, however, point at human modification of the biosphere, the portion of the earth that sustains life. The draining of wetland habitats, one type of habitat destruction, is a major cause of the decline. Amphibian bodies at all developmental stages are protected only by thin, permeable skins through which pollutants can easily penetrate. The double life on land and in water exposes their permeable skin to a wide range of aquatic and terrestrial habitats and hence a correspondingly wide range of environmental toxins.
Many amphibians' eggs and larvae develop in ponds and streams during spring, a time when acid rain causes an acidity increase in freshwater ecosystems. Increased ultraviolet (UV) light damages eggs and causes deformity among offspring. In addition, many amphibians are herbivores as larvae and insectivores as adults. Thus, they are vulnerable to both herbicides and insecticides in their food. Dramatic decline of amphibian populations provides an early warning of environmental degradation. It also depletes the food source for large carnivores that feed on them and may risk keeping insect populations in check.
five to forty-five years in Adirondack lakes and in Nova Scotia rivers. Studies during this period clearly show declines in acid-sensitive species. Similar results were obtained from analyzing fish population and water acidity in Maine, Massachusetts, Pennsylvania, and Vermont. The consensus is that fish populations would be eliminated if the surface waters were to acidify to between pH 5.0 and pH 5.5. The effects of acid rain on other animals are indirect, either through the dwindling fish population (as a food source for other animals) or stunted forest growth (disturbance to habitats).
The effect of global warming on the animal kingdom is also a serious and complex issue. As global temperature rises, ice caps in polar regions and glaciers melt, ocean waters expand in response to atmospheric warming, and thus the sea level elevates. The expected sea level rise will flood coastal cities and coastal wetlands. These threatened ecosystems are habitats and breeding grounds for numerous species of birds, fish, shrimp, and crabs, whose populations could be severely diminished. The Florida Everglades will virtually disappear if the sea level rises two feet. The impact of global warming on forests could be profound. The distribution of tree species is exquisitely sensitive to average annual temperature, and small changes could dramatically alter the extent and species composition of forests. This in turn could dramatically alter the population distribution of animals. A rise may make temperatures unsuitable for some species, hence reducing biodiversity.
The effect of the punctuated ozone layer on animals is yet to be fully understood. It is known that the high energy level of UV radiation can damage biological molecules, including the genetic material deoxyribonucleic acid (DNA), causing mutation. In small quantities, UV light helps the skin of humans and many animals produce vitamin D, and causes tanning. However, in large doses, UV light causes sunburn and premature aging of skin, skin cancer, and cataracts, a condition in which the lens of the eye becomes cloudy. Due to UV radiation's ability to penetrate, even animals covered by hair and thick fur cannot escape from these detrimental effects. Ozone damage costs U.S. farmers over $2 billion annually in reduced crop yields. All who depend on forestry and agriculture may bear a much higher cost if the emission of pollutants that destroy ozone is not regulated soon.
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