Extinction is actually a normal process in the course of evolution. Throughout geological time, many more species have become extinct than exist today. These species slowly disappeared because of climatic changes and the inability to adapt to such conditions as competition and predation. Since the 1600s, however, the process of extinction has accelerated rapidly through the impact of both human population growth and technological advances on natural ecosystems. Today the majority of the world's environments are changing faster than the ability of most species to adapt to such changes through natural selection.
Causes. Species become extinct or endangered for a number of reasons, but the primary cause is the destruction of habitat. Drainage of wetlands, conversion of shrub lands to grazing lands, cutting and clearing of forests (especially in the Tropics, where the rain forests will be gone by AD 2000 if destruction continues at its present rate), urbanization and suburbanization, and highway and dam construction have seriously reduced available habitats. As the various habitats become fragmented into "islands," the remaining animal populations crowd into smaller areas, causing further habitat destruction. Species in these small islands lose contact with other populations of their own kind, thereby reducing their genetic variation and making them less adaptable to environmental change. These small populations are highly vulnerable to extinction; for some species, the fragmented habitats become too small to support a viable population.
Since the 1600s, commercial exploitation of animals for food and other products has caused many species to become extinct or endangered. The slaughter of great whales for oil and meat, for example, has brought them to the brink of extinction; the African rhinoceros, killed for its horn, is also critically endangered. The great auk became extinct in the 19th century because of overhunting, and the Carolina parakeet perished as a species because of a combination of overhunting and habitat destruction.
Introduced diseases, parasites, and predators against which native flora and fauna have no defenses have also exterminated or greatly reduced some species. The accidental introduction of a blight, for example, eliminated the chestnut tree from North American hardwood forests. Predator and pest control also have adverse effects. Excessive control of prairie dogs, for example, has nearly eliminated one of their natural predators, the black-footed ferret.
Pollution is another important cause of extinctions. Toxic chemicals especially chlorinated hydrocarbons such asdichloro-diphenyl-trichloroethane (DDT) and polychlorinated biphenyls (PCBs)have become concentrated in food webs, affecting most strongly those species at the end of the chain . Both DDT and the PCBs, for example, interfere with the calcium metabolism of birds, causing soft-shelled eggs and malformed young. PCBs also impair reproduction in some carnivorous animals. Water pollution and increased water temperatures have wiped out endemic races of fish in several habitats.
Preservation Efforts. Some private and governmental efforts have been mobilized to save declining species. One immediate approach is to protect a species by legislation. Laws were enacted in the U.S. in the early 1900s, for example, to protect wildlife from commercial trade and overhunting. In 1973 the Endangered Species Act provided mechanisms for the conservation of ecosystems on which endangered species depend; it also discouraged the exploitation of endangered species in other countries by banning the importation and trade of any product made from such species. The U.S. also has various agreements with other nations for example, with Canada and Mexico for the legal protection of migratory birds.
International efforts center on the Convention on International Trade in Endangered Species of Wild Fauna and Flora, ratified by 96 nations and signed by the U.S. in 1973. Its purposeis to restrict exploitation of wildlife and plants by regulating and restricting trade in species. The effectiveness of such laws in various countries, however, depends on enforcement and support by people and the courts. Because of a lack of law enforcement, the willingness of some segments of society to trade in endangered species, and the activities of poachers and dealers who supply the trade, the future of many species is in doubt in spite of legal protection.
Efforts to save endangered species also include the propagation of breeding stock for release in the wild, either to restore a breeding population (as in the case of the peregrine falcon) or to augment a natural population (as in the case of the whooping crane). Another approach involves the determination of critical habitats that must be preserved for endangered species. These habitats may be protected by the establishment of reserves; the value of these may be limited, however, because of the island effect.
The problem of acid rain may be said to have originated with the Industrial Revolution, and it has been growing ever since. The severity of its effects has long been recognized in local settings, as exemplified by the spells of acid smog in heavily industrialized areas. The wide destructiveness of acid rain, however, has come to be realized only in recent decades. One large area that has been studied extensively is northern Europe, where acid rain has eroded structures, injured crops and forests, and threatened or depleted life in freshwater lakes. In 1983, for example, published reports indicated that 34 percent of the forested areas of West Germany had been damaged by acid rain. The northeastern U.S. and eastern Canada have also been affected by this form of pollution, as well as other areas of these countries and other regions of the world.
Industrial emissions have been blamed as the major cause of acid rain. Because the chemical reactions involved in the production of acid rain in the atmosphere are complex and as yet little understood, industries have challenged such assessments and stressed the need for further studies; and because of the cost of pollution reduction, governments have tended to support this attitude. U.S. government studies released in the early 1980s, however, strongly implicated industries as the main source of acid rain in the eastern U.S. and Canada. In 1988, as part of the UN-sponsored Long-Range Transboundary Air Pollution Agreement, the U.S., along with 24 other nations, ratified a protocol freezing the rate of nitrogen oxides emissions at 1987 levels. The 1990 amendments to the Clean Air Act of 1967 put in place regulations to reduce the release of sulfur dioxide from power plants to 10 million tons per year by Jan. 1, 2000. This amount is about one-half the emissions of 1990.
Periods of poor atmospheric mixing of only a few days, and sometimes only a few hours, can lead to high concentrations of hazardous materials in high-pollution areas and, under severe conditions, can result in illness and even death. An inversion in Donora, Pa., in 1948 caused respiratory illness in over 6000 persons and led to the death of 20. Severe pollution in London took 3500 to 4000 lives in 1952 and another 700 in 1962. Release of methyl isocyanate into the air during a temperature inversion caused the disaster at Bhopal, India, in December 1984, with at least 3300 deaths and more than 20,000 illnesses. The effects of long-term exposure to low concentrations are not well defined; however, those most at risk are the very young, the elderly, smokers, workers whose jobs expose them to toxic materials, and persons with heart or lung disease. Other adverse effects of air pollution are potential injury to livestock and crops.
Often, the first noticeable effects of pollution are aesthetic and may not necessarily be dangerous. These include visibility reduction due to tiny particles suspended in air, or bad odors, such as the rotten egg smell produced by hydrogen sulfide emitted from pulp and paper mills.Sources and Control. The combustion of coal, oil, and gasoline accounts for much of air pollution. More than 80 percent of the sulfur dioxide, 50 percent of the nitrogen oxides, and 30 to 40 percent of the particulate matter emitted to the atmosphere in the U.S. are produced by fossil-fuel-fired electric power plants, industrial boilers, and residential furnaces. Ninety percent of the carbon monoxide and almost 50 percent of the nitrogen oxides and hydrocarbons come from burning gasoline and diesel fuels in cars and trucks. Other major pollution sources include iron and steel mills; coke ovens; zinc, lead, and copper smelters; municipal incinerators; petroleum refineries; cement plants; large solvent users; and nitric and sulfuric acid plants.
Potential pollutants may exist in the materials entering a chemical or combustion process (such as lead in gasoline), or they may be produced as a result of the process itself. Carbon monoxide, for example, is a typical product of internal-combustion engines. Methods for controlling air pollution include removing the hazardous material before it is used, removing the pollutant after it is formed, or altering the process so that the pollutant is not formed or occurs only at very low levels. Automobile pollutants can be controlled by burning the gasoline as completely as possible, by recirculating fumes from fuel tank, carburetor, and crankcase, and by changing the engine exhaust to harmless substances in catalytic converters. Industrially emitted particulates may be trapped in cyclones, electrostatic precipitators, and filters. Pollutant gases can be collected in liquids or on solids, or incinerated into harmless substances.Large-Scale Effects. The tall smokestacks used by industries and utilities do not remove pollutants but simply boost them higher into the atmosphere, thereby reducing their concentration at the site. These pollutants may then be transported over large distances and produce adverse effects in areas far from the site of the original emission. Sulfur dioxide and nitrogen oxide emissions from the central and eastern U.S. are causing acid rain in New York State, New England, and eastern Canada. The pH level, or relative acidity, of many freshwater lakes in that region has been altered so dramatically by this rain that entire fish populations have been destroyed. Similar effects have been observed in Europe. Sulfur dioxide emissions and the subsequent formation of sulfuric acid can also be responsible for the attack on limestone and marble at large distances from the source.
The worldwide increase in the burning of coal and oil since the late 1940s has led to ever increasing concentrations of carbon dioxide. The resulting "greenhouse effect", which allows solar energy to enter the atmosphere but reduces the reemission of infrared radiation from the earth, could lead to a warming trend that might affect the global climate and lead to a partial melting of the polar ice caps. Possibly an increase in cloud cover or absorption of excess carbon dioxide by the oceans would check the greenhouse effect before it reached the stage of polar melting. Never-the-less, research reports released in the U.S. in the 1980s indicate that the greenhouse effect is definitely under way and that the nations of the world should be taking immediate steps to deal with it.Government Action. In the U.S., the Clean Air Act of 1967 as amended in 1970, 1977, and 1990 is the legal basis for air-pollution control throughout the U.S. The Environmental Protection Agency (EPA) has primary responsibility for carrying out the requirements of the act, which specifies that air-quality standards be established for hazardous substances. These standards are in the form of concentration levels that are believed to be low enough to protect public health. Source emission standards are also specified to limit the discharge of pollutants into the air so that air-quality standards will be achieved. The act was also designed to prevent significant deterioration of air quality in areas where the air is currently cleaner than the standards require. The amendments of 1990 identified ozone, carbon monoxide, particulate matter, acid rain, and air toxins as major air pollution problems. On the international scene, 49 countries agreed in March 1985 on a UN convention to protect the ozone layer. This "Montr�al Protocol," which was renegotiated in 1990, calls for the phaseout of certain chlorocarbons and fluorocarbons by the year 2000 and provides aid to developing countries in making this transition.
Heat may also be considered a pollutant when increased temperatures in bodies of water result from the discharge of cooling water by factories and power plants.Effects of Water Pollution. Notable effects of water pollution include those involved in human health. Nitrates in drinking water can cause a disease in infants that sometimes results in death. Cadmium in sludge-derived fertilizer can be absorbed by crops; if ingested in sufficient amounts, the metal can cause an acute diarrheal disorder and liver and kidney damage. The hazardous nature of mercury, arsenic, and lead has long been known or strongly suspected.
Lakes are especially vulnerable to pollution. One problem, eutrophication, occurs when lake water becomes artificially enriched with nutrients, causing abnormal plant growth. Runoff of chemical fertilizer from cultivated fields may trigger this. The process of eutrophication can produce aesthetic problems such as bad tastes and odors and unsightly green scums of algae, as well as dense growth of rooted plants, oxygen depletion in the deeper waters and bottom sedimentsof lakes, and other chemical changes such as precipitation of calcium carbonate in hard waters. Another problem, of growing concern in recent years, is acid rain, which has left many lakes in the northeastern U.S. and Canada totally devoid of life.Sources and Control. The major sources of water pollution can be classified as municipal, industrial, and agricultural. Municipal water pollution consists of wastewater from homes and commercial establishments. For many years, the main goal of treating municipal wastewater was simply to reduce its content of suspended solids, oxygen-demand ing materials, dissolved inorganic compounds (particularly compounds of phosphorus and nitrogen), and harmful bacteria. In recent years, however, more stress has been placed on improving the means of disposal of the solid residues from municipal treatment processes. The basic treatment of municipal wastewater falls into three stages: primary treatment, including grit removal, screening, grinding, flocculation, and sedimentation; secondary treatment, which entails oxidation of dissolved organic matter, using biologically active sludge, which is then filtered off; and tertiary treatment, using advanced biological methods of nitrogen removal and chemical and physical methods such as granular filtration and activated carbon adsorption. The handling and disposal of solid residues can account for 25 to 50 percent of the capital and operational costs of a treatment plant.
The characteristics of industrial wastewaters can differ markedly both within and among industries. The impact of industrial discharges depends not only on their collective characteristics, such as biochemical oxygen demand and the amount of suspended solids, but also on their content of specific inorganic and organic substances. Three options (which are not mutually exclusive) are available in controlling industrial wastewater: Control can take place at the point of generation within the plant; wastewater can be pretreated for discharge to municipal treatment systems; or wastewater can be treated completely at the plant and either reused or discharged directly into receiving waters.
Agriculture, including commercial livestock and poultry farming, is the source of many organic and inorganic pollutants in surface waters and groundwater. These contaminants include both sediment from the erosion of cropland and compounds of phosphorus and nitrogen that partly originate in animal wastes and commercial fertilizers. Animal wastes are high in oxygen-demand ing material, nitrogen, and phosphorus, and they often harbor pathogenic organisms. Wastes from commercial feeders are contained and disposed of on land; their main threat to natural waters, therefore, is via runoff and leaching. Control may involve settling basins for liquids, limited biological treatment in aerobic or anaerobic lagoons, and a variety of other methods.Marine Pollution. Wastes that are discharged directly into U.S. marine waters are estimated conservatively to exceed 45 million metric tons per year. About 80 percent of this amount is waste produced by dredging, 10 percent is industrial waste, and 9 percent is sewage sludge. The presence of toxic substances, the rapid uptake of contaminants by marine organisms, heavy deposits of materials on the bottom environment near the shore, and excessive growth of undesirable organisms the combination of all these aspects has very serious consequences.Oil Spills. These large-scale accidental discharges of liquid petroleum products are an important cause of pollution along shore lines. The most spectacular involve the supertankers used for oil transport, but many other ships also spill oil, and offshore drilling operations contribute a large share of the pollution. One estimate is that of every million tons of oil shipped, one ton is spilled. Some of the largest spills thus far recorded involve the tanker Amoco Cadiz off the French coast in 1978 (1.6 million barrels of crude oil) and the Ixtoc I oil well in the Gulf of Mexico in 1979 (3.3 million barrels). The largest spill in the U.S. (240,000 barrels) was that of the tanker Exxon Valdez in Prince William Sound, Gulf of Alaska, in March 1989. Within a week, under high winds, this spill had become a 6700-sq-km (2600-sq-mi) slick that endangered wildlife and fisheries in the entire gulf area.
The oil spills in the Persian Gulf in 1983, during the Iran-Iraq conflict, and in 1991, during the Persian Gulf War, resulted in enormous damage to the entire area, especially to the marine life.Legislation. The primary legislative basis for managing water pollution is the Federal Water Pollution Control Act of 1956, as amended by the Water Quality Act of 1965, the Federal Water Pollution Control Act amendments of 1972, and the Clean Water Act of 1977. In addition, ocean dumping is controlled by the Marine Protection, Research and Sanctuaries Act of 1972.
The 1972 amendments established stringent controls and cleanup deadlines for industrial and municipal pollution, shifting the emphasis from the regulation of general water quality to the setting of rigorous standards for wastewater in particular. A system of issuing permits for discharges into U.S. waters was also created. The Clean Water Act of 1977 relaxed the deadlines of 1972 but added provisions to strengthen the suppression of toxic water pollutants.
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