DDT Dichlorodiphenyltrichloroethane, a chlorinated hydrocarbon that has been widely used as a pesticide but is now banned in some countries. 

debt-for-nature swap Agreement in which a certain amount of foreign debt is canceled in exchange for local currency investments that will improve natural resource management or protect certain areas in the debtor country from harmful development. 

deciduous plants Trees, such as oaks and maples, and other plants that survive during dry seasons or cold seasons by shedding their leaves. Compare coniferous trees, succulent plants. 

Decomposer Organism that digests parts of dead organisms and cast-off fragments and wastes of living organisms by breaking down the complex organic molecules in those materials into simpler inorganic compounds and then absorbing the soluble nutrients. Producers return most of these chemicals to the soil and water for reuse. Decomposers consist of various bacteria and fungi. Compare consumer, detritivore, producer. 

deductive reasoning Using logic to arrive at a specific conclusion based on a generalization or premise. It goes from the general to the specific. Compare inductive reasoning. 

deforestation Removal of trees from a forested area without adequate replanting.

degradable pollutant Potentially polluting chemical that is broken down completely or reduced to acceptable levels by natural physical, chemical, and biological processes. Compare biodegradable pollutant, nondegradable pollutant, slowly degradable pollutant.

degree of urbanization Percentage of the population in the world, or a country, living in areas with a population of more than 2,500 people (higher in some countries). Also urbanization. Compare urban growth. 

Democracy Government by the people through their elected officials and appointed representatives. In a constitutional democracy, a constitution provides the basis of government authority and puts restraints on government power through free elections and freely expressed public opinion. 

demographic transition Hypothesis that countries, as they become industrialized, have declines in death rates followed by declines in birth rates. 

depletion time Time it takes to use a certain fraction, usually 80%, of the known or estimated supply of a nonrenewable resource at an assumed rate of use. Finding and extracting the remaining 20% usually costs more than it is worth. 

desalination Purification of salt water or brackish (slightly salty) water by removal of dissolved salts.

Desert Biome in which evaporation exceeds precipitation and the average amount of precipitation is less than 25 centimeters (10 inches) a year. Such areas have little vegetation or have widely spaced, mostly low vegetation. Compare forest, grassland. 

Desertification Conversion of rangeland, rain-fed cropland, or irrigated cropland to desertlike land, with a drop in agricultural productivity of 10% or more. It usually is caused by a combination of overgrazing, soil erosion, prolonged drought, and climate change.

Detritivore Consumer organism that feeds on detritus, parts of dead organisms, and cast-off fragments and wastes of living organisms. The two principal types are detritus feeders and decomposers. 

detritus Parts of dead organisms and cast-off fragments and wastes of living organisms. 

detritus feeder Organism that extracts nutrients from fragments of dead organisms and their castoff parts and organic wastes. Examples are earthworms, termites, and crabs. Compare decomposer. 

deuterium (D; hydrogen-2) Isotope of the element hydrogen, with a nucleus containing one proton and one neutron and a mass number of 2. 

developed country Country that is highly industrialized and has a high per capita GNP. Compare developing country. Formerly called more 

developed country (MDC). 

developing country Country that has low to moderate industrialization and low to moderate per capita GNP. Most are located in Africa, Asia, and Latin America. Compare developed country. Formerly called less developed country (LDC). 

dew point Temperature at which condensation occurs for a given amount of water vapor. 

dieback Sharp reduction in the population of a species when its numbers exceed the carrying capacity of its habitat. See carrying capacity.

differential reproduction Phenomenon in which individuals with adaptive genetic traits produce more living offspring than do individuals without such traits. See natural selection.

Dioxins Family of 75 different chlorinated hydrocarbon compounds formed as unwanted by-products in chemical reactions involving chlorine and hydrocarbons, usually at high temperatures. 

dissolved oxygen (DO) content Amount of oxygen gas (O2) dissolved in a given volume of water at a particular temperature and pressure, often expressed as a concentration in parts of oxygen per million parts of water. See biological oxygen demand. 

Disturbance A discrete event that disrupts an ecosystem or community. Examples of natural disturbances include fires, hurricanes, tornadoes, droughts, and floods. Examples of human-caused disturbances include deforestation, overgrazing, and plowing. 

divergent plate boundary Area where earth's lithospheric plates move apart in opposite directions. Compare convergent plate boundary, transform fault. 

DNA (deoxyribonucleic acid) Large molecules in the cells of organisms that carry genetic information in living organisms.

domesticated species Wild species tamed or genetically altered by crossbreeding for use by humans for food (cattle, sheep, and food crops), pets (dogs and cats), or enjoyment (animals in zoos and plants in gardens). Compare wild species. 

Dose Amount of a potentially harmful substance an individual ingests, inhales, or absorbs through the skin. See dose-response curve, median lethal dose. Compare response. 

dose-response curve Plot of data showing effects of various doses of a toxic agent on a group of test organisms. See dose, median lethal dose, response. 

doubling time Time it takes (usually in years) for the quantity of something growing exponentially to double. It can be calculated by dividing the annual percentage growth rate into 70. See rule of 70. 

dredge spoils Materials scraped from the bottoms of harbors and streams to maintain shipping channels. High levels of toxic substances that have settled out of the water often contaminate these materials. See dredging.

Dredging Type of surface mining in which chain buckets and draglines scrape up sand, gravel, and other surface deposits covered with water. It is also used to remove sediment from streams and harbors to maintain shipping channels. See dredge spoils. Compare area strip mining, contour strip mining, mountaintop removal, open-pit mining, subsurface.

drift-net fishing Catching fish in huge nets that drift in the water.

Drought Condition in which an area does not get enough water because of (1) lower-thannormal precipitation or (2) higher-than-normal temperatures that increase evaporation. 

dust dome Dome of heated air that surrounds an urban area and traps pollutants, especially suspended particulate matter. See also urban heat island. 

dust plume Elongation of a dust dome by winds that can spread a city's pollutants hundreds of kilometers downwind.