Pollution is the release or discharge of a substance, chemical, or heat energy into the natural environment, including the air, water, or soil, as a result of human activities. Pollution can be poisonous to living organisms and can modify and degrade habitats. It affects every ecosystem on earth and, therefore, changes the way in which the environment can provide living organisms with the habitat and condi tions they need to survive. In this way, pollution threatens biodiversity.

Toxification and chemical contamination from industrial, agricultural, and residential sources can poison an ecosystem and render it unfit for life. Common chemicals, such as pesticides and herbicides, can persist in the environment and affect every level of the food web, including nontarget organisms. In particular, persistent organic pollutants (POPs), a group of chemicals including pesticides, dioxins, furans, and polychlorinated biphenols (PCBs), do not break down once they are released into the environment, instead bioac-cumulating in the tissues of animals. Bioaccumulation is the process by which these fat-soluble chemicals build up in the fatty tissues of animals after they ingest them by consuming contaminated food or water. The chemicals are passed up the food chain, and the level of contamination is magnified higher up the food chain in a process called biomag-nification. For example, a polar bear that eats a seal that ate contaminated fish will ingest the chemicals that accumulated in the tissues of both the seal and the fish, and it will further concentrate the chemicals in its own tissues. These chemicals can cause cancer, and, if they are ingested at high enough doses, they can kill an organism. Continual exposure at lower doses can, over time, cause the chemicals to reach lethal levels.

Chemical contamination, particularly by POPs and other organochlorine compounds, can also disrupt the endocrine system by mimicking estrogen and other hormones in the body. Moreover, they can lead to reproductive failure or dysfunction, developmental problems, decreased sperm counts, compromised immune systems, and birth defects and deformities. Some organochlorines can even cause genetic mutations. The presence of organo-chlorine chemicals in the food web has caused

Congested traffic in Cairo, Egypt. Emissions of nitrogen oxides and sulfur oxides from automobiles contribute to smog, which causes repiratory problems for humans and declines in plant populations. (UN photo)

population declines by altering or hindering the physiological processes involved in reproduction, development, and immune response. For example, in the 1950s and 1960s, the populations of brown pelicans, ospreys, cormorants, bald eagles, and other raptors declined in the United States as a result of DDT contamination. This pesticide was passed up the food chain and accumulated in the tissues of the birds. One of the breakdown products of DDT is DDE, which reduced the calcium content of their eggshells, decreasing their reproductive success rate and leading to a decline in populations (Miller, 1994).

Global climate change and stratospheric ozone depletion also affect biodiversity and can lead to its loss. Anthropogenic emissions of greenhouse gases such as carbon dioxide and methane, from factories, power plants, defor estation, and automobiles, increase the earth's average temperature on a global scale in a process called global warming. As the troposphere warms, climates, including temperature and precipitation patterns, will be altered and may no longer be suitable for the plants and animals inhabiting them. This loss of habitat will cause populations to decline unless organisms are able to migrate to suitable new habitats. The release of ozone-depleting substances such as chlorofluorocarbons (CFCs) contributes to the depletion of the stratospheric ozone layer, as well as to global warming. As stratospheric ozone levels decline, more ultraviolet (UV) radiation can reach the earth's surface. The increased UV radiation is harmful to plants and animals, and it has been linked to a decrease in phytoplankton populations, which form the base of marine food webs. It has also been linked to an increased incidence of cataracts and skin cancer in humans and wildlife. Some plants, such as loblolly pine, are sensitive to UV radiation; a drop of approximately 1 percent in the total yield for some food crops, including corn, rice, wheat, and soybeans, has been associated with each 3 percent drop in stratospheric ozone (ibid.).

The emission of nitrogen oxides (NOx) and sulfur oxides (SOx) also threatens biodiversity. NOx emissions, primarily from automobiles, factories, and power plants, contribute to smog formation, which can cause respiratory and other health problems in humans and declines in plant populations near pollution sources. NOx and SOx emissions, primarily sulfur dioxide from industrial sources, combine with water vapor in the atmosphere to cause acid precipitation. Acid precipitation, including acid rain, acidifies soils and surface water (decreasing its pH). Acid rain can lower the pH of lakes and streams, making them unsuitable for aquatic species. Furthermore, acid rain has been shown to impair the growth of some vegetation.

Thermal pollution is caused by the discharge of cooling water from industrial sources and power plants into bodies of water. The cooling water absorbs heat generated during industrial processes, making the cooling water effluent warmer than the surface water that receives it. Thermal pollution raises the water temperature, thereby lowering the dissolved oxygen level in the water and reducing the amount of oxygen available for aquatic organisms. At the same time, higher water temperatures can increase the biological oxygen demand (BOD) of the aquatic ecosystem. BOD refers to the amount of oxygen required by the organisms living in the ecosystem. As water temperature increases, the bacterial metabolic rate can also increase, raising the BOD of the ecosystem. If the thermal pollution is great enough, the decreased dissolved oxygen content and the increased BOD may be sufficient to create a dead zone, a region in which no aquatic organisms can survive.

The release of effluent rich in organic wastes and nutrients can also raise the BOD, through a process caused eutrophication. The discharge from sewage treatment plants (a point source), and agricultural runoff (a nonpoint source) containing fertilizers and animal wastes, are common human sources of nutrients that can alter aquatic ecosystems. The organic wastes in the effluent provide a food source for aerobic bacteria, which demand more oxygen as they digest the organic matter. Nutrients, including phosphates (compounds containing phosphorus) and nitrates (compounds containing nitrogen), that are released by the sewage treatment plant, in the runoff, or by bacterial decomposition make possible the rapid growth of algae, phytoplankton, and aquatic plants. These algal blooms on the surface of bodies of water can prevent sunlight from reaching the plants growing below them. Because other organisms depend on those submerged plants for food, the food chain can be upset and the lake or stream can be rendered unsuitable habitat. Algal blooms can also reduce the dissolved oxygen content of water below the surface.

Waste disposal also alters and degrades the environment and impacts biodiversity. Hazardous waste, radioactive or nuclear waste, municipal and industrial solid waste, mine tailings, and household garbage all require land for disposal. Wastes disposed of in landfills produce leachate that can contaminate soils, groundwater, and surface water. Leachate forms as rainfall percolates through a landfill and leaches out toxic compounds and heavy metals from the waste. Landfills also con tribute to air pollution by releasing hydrogen sulfide gas, methane, and volatile organic compounds produced by anaerobic decomposition of organic wastes. As a result, waste disposal can cause the loss of biodiversity by taking land and polluting air, water, and soils. For example, many landfills in the United States were created by filling in wetlands, displacing plant and animal life. Waste incinerators also pollute the environment by producing toxic ash and by emitting dioxins, toxins, and volatile organic compounds into the air.

Improper disposal of waste, such as littering and dumping in lakes, rivers, or the ocean also harms biodiversity. For instance, plastic six-pack rings and fishing nets in the ocean can choke or strangle marine mammals and other wildlife. In addition to solid waste disposal, marine pollution includes spills and discharges of chemicals, oil, and oily wastes. These pollutants poison aquatic life and sea birds and degrade aquatic habitats, coastal habitats, and wetlands. Moreover, many of these chemicals persist in the environment and can bioaccu-mulate in wildlife.

Pollution is often an unintentional byproduct of beneficial industrial processes and sometimes involves substances that are not harmful in other contexts. Pollution impacts and threatens biodiversity because it can poison plants and animals, interfere with physiological processes, degrade habitats, and even alter animal behavior. Noise and light pollution do not usually harm wildlife directly, but they can cause animals to alter their ranges or to change their behavior. For example, lights can disrupt the nesting behavior of sea turtles (Meffe and Carroll, 1997). The effects of pollution are long lasting and may change in unknown ways over time. As a result, pollution will continue to affect biodiversity in known and unforeseen ways. In particular, the combination and accumulation of chemicals in the environment will continue to threaten biodiversity by contaminating environments and by inhibiting the basic physiological processes—especially reproductive success—that make all life possible.

Advances in technology have decreased pollution, but much remains to be done. In some cases pollution can be prevented by installing control technology and altering industrial processes. In other cases, switching to cleaner alternatives can eliminate the toxic by-products of industrial processes. Laws and regulations, new technology, new components, and economic incentives can increase efforts to control pollution.

Toxification and chemical contamination can be prevented by modifications to machinery and industrial equipment and processes. For example, closed-circuit manufacturing systems can limit the discharge of chlorine and other chemicals used to bleach paper. Unbleached and oxygen-bleached paper are alternatives to chlorine bleached paper that create less pollution. Alternative farming practices, such as integrated pest management, utilize natural methods instead of chemical pesticides to control insects and other pests. By using nontoxic alternatives to poisonous household chemicals, such as some cleaning products, and by decreasing the amount of chemicals and harmful products we use, we can reduce the amount of chemicals released into the environment. The Stockholm Convention on Persistent Organic Pollutants (POPs) is an international agreement that aims to prevent the use, production, and trade of POPs.

Global climate change can be mitigated by increasing energy efficiency and by controlling carbon emissions. By consuming less electricity and fossil fuel we can decrease carbon emissions. Renewable energy sources, such as solar, wind, and geothermal energy, produce electricity without the carbon emissions created by burning fossil fuels such as oil, gas, and coal. Hybrid automobiles use alternative fuel sources, such as hydrogen fuel cells, to power automobiles, along with varying amounts of gasoline or electricity. These automobiles emit less carbon dioxide than traditional automobiles. The UN Framework Convention on Climate Change and the Kyoto Protocol are international agreements that aim to reduce greenhouse gas emissions globally.

Stratospheric ozone depletion can be mitigated by controlling emissions of CFCs and other ozone-depleting substances (ODS). Alternatives to CFCs and other ODS have been developed, and using such alternatives can minimize the loss of stratospheric ozone caused by anthropogenic sources. For example, in the United States, CFCs are no longer used in spray cans. The Montreal Protocol on Ozone Depleting Substances is an international agreement to phase out ODS globally.

The emission of nitrogen oxides (NOx) and sulfur oxides (SOx) is controlled by installing control technology, such as scrubbers in smokestacks, that removes such pollutants from the discharge. Changing fuel sources— from those with high sulfur or nitrogen content, such as low-grade coal, to those with lower sulfur or nitrogen content, such as higher-grade coal or natural gas—also reduces NOx and SOx emissions. Increasing energy efficiency, using less electricity, and driving automobiles less also reduce the amount of NOx and SOx emissions.

Thermal pollution can be reduced by decreasing the amount of heated water discharged from power plants and other industrial sources. This decrease can be achieved by lowering the amount of electricity used, decreasing the amount of water used for cooling, and increasing energy efficiency. Allow ing the heated water to cool in cooling ponds before reusing it or discharging it can also decrease thermal pollution. Heat from the cooling water can be transferred to the atmosphere by using wet or dry cooling towers (Miller, 1994).

Eutrophication can be controlled by using compost and other natural forms of fertilizer instead of artificial fertilizers. Runoff can be reduced by soil conservation and erosion control. Moreover, preserving wetlands and coastal areas can mitigate eutrophication, because they filter out and retain nutrients. Tertiary treatment at sewage treatment plants and industrial facilities is a process that removes nutrients, including phosphates, before effluent is discharged. Limiting the use of phosphates in household chemicals, such as laundry detergents, can also reduce the amount of nutrients discharged in sewage treatment plant effluent (ibid.).

Waste disposal and its effects on biodiversity can be reduced by composting, reusing and recycling products and wastes, and altering industrial processes so that they use fewer toxics and produce less waste. To minimize the amount of garbage we produce, we can purchase products with less packaging or with recyclable packaging. We can also buy products that last longer and repair broken items, instead of throwing them away and buying new ones. By decreasing the use of nuclear power, we decrease the creation of nuclear waste. Proper waste management, including the prevention of littering, ocean dumping, and other forms of improper waste disposal, is also important to minimize the pollution and other harmful effects of waste disposal. (ibid.)

To protect biodiversity, our goals should be to prevent pollution and reduce waste before it is created, rather than to clean up pollution and manage wastes after they are a problem. Once they are produced, pollution and wastes do not break down quickly; they have lasting effects on the health of the environment, wildlife, vegetation, and humans. Increasing energy efficiency, reducing consumption of harmful chemicals, reducing the quantity of garbage and other wastes, switching to nontoxic alternatives, and adopting environmentally sustainable manufacturing processes offer the greatest benefit for biodiversity and the environment.

See also: Agriculture and Biodiversity Loss: Genetic Engineering and the Second Agricultural Revolution; Food Webs and Food Pyramids; Global Climate Change; Hole in Ozone Layer; Nitrogen Cycle; Pro-toctists


Carson, Rachel. 1962. Silent Spring. Boston: Houghton Mifflin; Colborn, Theo, Dianne Dumanoski, and John Peterson Myers. 1997. Our Stolen Future. New York: Plume/Penguin; Meffe, Gary, and C. Ronald Carroll. 1977. Principles of Conservation Ecology. Sunderland, MA: Sinauer; Miller, G. Tyler, Jr. 1994. Liv-ing in the Environment, 8th ed. Belmont, CA: Wadsworth; Nebel, Bernard J., and Richard T. Wright. 1993. Environmental Science—The Way the World-Works, 4th ed. Englewood Cliffs, NJ: Prentice Hall; Thornton, Joe. 2000. Pandora's Poison. Cambridge, MA: MIT Press.

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