tailings Rock and other waste materials removed as impurities when waste mineral material is separated from the metal in an ore.

tar sand Deposit of a mixture of clay, sand, water, and varying amounts of a tarlike heavy oil known as bitumen. Bitumen can be extracted from tar sand by heating. It is then purified and upgraded to synthetic crude oil. See bitumen.

tectonic plates Various-sized areas of the earth's lithosphere that move slowly around the mantle's flowing asthenosphere. Most earthquakes and volcanoes occur around the boundaries of these plates. See lithosphere, plate tectonics.

temperature inversion Layer of dense, cool air trapped under a layer of less dense, warm air. This prevents upward-flowing air currents from developing. In a prolonged inversion, air pollution in the trapped layer may build up to harmful levels. Also thermal

inversion. See radiation temperature inversion, subsidence temperature inversion.

teratogens Chemical, ionizing agent, or virus that causes birth defects. Compare carcinogen, mutagen.

Terracing Planting crops on a long, steep slope that has been converted into a series of broad, nearly level terraces with short vertical drops from one to another that run along the contour of the land to retain water and reduce soil erosion.

terrestrial Pertaining to land. Compare aquatic.

tertiary (higher-level) consumers Animals that feed on animal-eating animals. They feed at high trophic levels in food chains and webs. Examples are hawks, lions, bass, and sharks. Compare detritivore, primary consumer, secondary consumer.

theory of evolution Widely accepted scientific idea that all life-forms developed from earlier lifeforms. Although this theory conflicts with the creation stories of many religions, it is the way biologists explain how life has changed over the past 3.6-3.8 billion years and why it is so diverse today.

theory of island biogeography The number of species found on an island is determined by a balance between two factors: the (1) immigration rate (of species new to the island) from other inhabited areas and (2) extinction rate (of species established on the island). The model predicts that at some point the rates of immigration and extinction will reach an equilibrium point that determines the island's average number of different species (species diversity). Also species equilibrium model.

thermocline Zone of gradual temperature decrease between warm surface water and colder deep water in a lake, reservoir, or ocean.

threatened species Wild species that is still abundant in its natural range but likely to become endangered because of a decline in numbers. Compare endangered species.

threshold effect Harmful or fatal effect of a small change in environmental conditions that exceeds the limit of tolerance of an organism or population of a species. See law of tolerance.

throughput Rate of flow of matter, energy, or information through a system. Also used is flows. Compare input, output.

tolerance limits Minimum and maximum limits for physical conditions (such as temperature) and concentrations of chemical substances beyond which no members of a particular species can survive. See law of tolerance.

total fertility rate (TFR) Estimate of the average number of children who will be born alive to a woman during her lifetime if she passes through all her childbearing years (ages 15-44) conforming to age-specific fertility rates of a given year. In simpler terms, it is an estimate of the average number of children a woman will have during her childbearing years.

toxic chemical Chemical that is fatal to humans in low doses or fatal to more than 50% of test animals at stated concentrations. Most are neurotoxins, which attack nerve cells. See carcinogen, hazardous chemical, mutagen, teratogen.

toxic waste Form of hazardous waste that causes death or serious injury (such as burns, respiratory diseases, cancers, or genetic mutations). See hazardous waste.

toxicity Measure of how harmful a substance is.

toxicology Study of the adverse effects of chemicals on health.

traditional intensive agriculture Producing enough food for a farm family's survival and perhaps a surplus that can be sold. This type of agriculture uses higher inputs of labor, fertilizer, and water than traditional subsistence agriculture. See traditional subsistence agriculture. Compare industrialized agriculture.

traditional subsistence agriculture Production of enough crops or livestock for a farm family's survival and, in good years, a surplus to sell or put aside for hard times. Compare industrialized agriculture, traditional intensive agriculture.

tragedy of the commons Depletion or degradation of a potentially renewable resource to which people have free and unmanaged access. An example is the depletion of commercially desirable fish species in the open ocean beyond areas controlled by coastal countries. See common-property resource.

transform fault Area where the earth's lithospheric plates move in opposite but parallel directions along a fracture (fault) in the lithosphere. Compare convergent plate boundary, divergent plate boundary.

transmissible disease Disease caused by living organisms (such as bacteria, viruses, and parasitic worms) that can spread from one person to another by air, water, food, or body fluids (or in some cases by insects or other organisms). Compare nontransmissible disease.

Transpiration Process in which water (1) is absorbed by the root systems of plants, (2) moves up through the plants, (3) passes through pores (stomata) in their leaves or other parts, and (4) evaporates into the atmosphere as water vapor.

tree plantation Site planted with one or only a few tree species in an even-aged stand. When the stand matures it is usually harvested by clear-cutting and then replanted. These plantations normally are used to grow rapidly growing tree species for fuelwood, timber, or pulpwood. Also tree farm. See even-aged management. Compare oldgrowth forest, second-growth forest, uneven-aged management.

trophic level All organisms that are the same number of energy transfers away from the original source of energy (for example, sunlight) that enters an ecosystem. For example, all producers belong to the first trophic level, and all herbivores belong to the second trophic level in a food chain or a food web.