radiation Fast-moving particles (particulate radiation) or waves of energy (electromagnetic radiation). See alpha particle, beta particle, gamma rays.

radiation temperature inversion Temperature inversion that typically occurs at night in which a layer of warm air lies atop a layer of cooler air nearer the ground as the air near the ground cools faster than the air above it. As the sun rises and warms the earth's surface, the inversion normally disappears by noon and disperses the pollutants built up during the night. See temperature inversion. Compare subsidence temperature inversion.

radioactive decay Change of a radioisotope to a different isotope by the emission of radioactivity.

radioactive waste Waste products of nuclear power plants, research, medicine, weapon production, or other processes involving nuclear reactions. See radioactivity.

Radioactivity Nuclear change in which unstable nuclei of atoms spontaneously shoot out "chunks" of mass, energy, or both at a fixed rate. The three principal types of radioactivity are gamma rays and fast-moving alpha particles and beta particles.

radioisotope Isotope of an atom that spontaneously emits one or more types of radioactivity (alpha particles, beta particles, gamma rays).

rain shadow effect Low precipitation on the far side (leeward side) of a mountain when prevailing winds flow up and over a high mountain or range of high mountains. This creates semiarid and arid conditions on the leeward side of a high mountain range.

range of tolerance Range of chemical and physical conditions that must be maintained for populations of a particular species to stay alive and grow, develop, and function normally. See law of tolerance.

Rangeland Land that supplies forage or vegetation (grasses, grasslike plants, and shrubs) for grazing and browsing animals and is not intensively managed. Compare feedlot, pasture.

rare species Species that (1) has naturally small numbers of individuals, often because of limited geographic ranges or low population densities, or (2) has been locally depleted by human activities.

realized niche Parts of the fundamental niche of a species that are actually used by that species. See ecological niche, fundamental niche.

recharge area Any area of land allowing water to pass through it and into an aquifer. See aquifer, natural recharge.

Recycling Collecting and reprocessing a resource so it can be made into new products. An example is collecting aluminum cans, melting them down, and using the aluminum to make new cans or other aluminum products. Compare reuse.

Reforestation Renewal of trees and other types of vegetation on land where trees have been removed; can be done naturally by seeds from nearby trees or artificially by planting seeds or seedlings.

relative humidity Amount of water vapor in a certain mass of air, expressed as a percentage of the maximum amount it could hold at that temperature. Compare absolute humidity.

reliable runoff Surface runoff of water that generally can be counted on as a stable source of water from year to year. See runoff.

renewable resource Resource that can be replenished rapidly (hours to several decades) through natural processes. Examples are trees in forests, grasses in grasslands, wild animals, fresh surface water in lakes and streams, most groundwater, fresh air, and fertile soil. If such a resource is used faster than it is replenished, it can be depleted and converted into a nonrenewable resource. See also environmental degradation. Compare nonrenewable resource and perpetual resource.

replacement-level fertility Number of children a couple must have to replace them. The average for a country or the world usually is slightly higher than 2 children per couple (2.1 in the United States and 2.5 in some developing countries) because some children die before reaching their reproductive years. See also total fertility rate.

reproductive isolation Long-term geographic separation of members of a particular sexually reproducing species.

reserves Resources that have been identified and from which a usable mineral can be extracted profitably at present prices with current mining technology. See identified resources, undiscovered resources.

reserve-to-production ratio Number of years reserves of a particular nonrenewable mineral will last at current annual production rates. See reserves.

resilience Ability of a living system to restore itself to original condition after being exposed to an outside disturbance that is not too drastic. See constancy, inertia.

resource Anything obtained from the living and nonliving environment to meet human needs and wants. The term can also be applied to other species.

resource partitioning Process of dividing up resources in an ecosystem so species with similar needs (overlapping ecological niches) use the same scarce resources at different times, in different ways, or in different places. See ecological niche, fundamental niche, realized niche.

restoration ecology Research and scientific study devoted to restoring, repairing, and reconstructing damaged ecosystems.

reuse Using a product over and over again in the same form. An example is collecting, washing, and refilling glass beverage bottles. Compare recycling.

riparian rights System of water law that gives anyone whose land adjoins a flowing stream the right to use water from the stream, as long as some is left for downstream users. Compare prior appropriation.

riparian zones Thin strips and patches of vegetation that surround streams. They are very important habitats and resources for wildlife.

Risk Probability that something undesirable will result from deliberate or accidental exposure to a hazard. See risk analysis, risk assessment, risk-benefit analysis, risk management.

risk analysis Identifying hazards, evaluating the nature and severity of risks (risk assessment), using this and other information to determine options and make decisions about reducing or eliminating risks (risk management), and communicating information about risks to decision makers and the public (risk communication).

risk management Using risk assessment and other information to determine options and make decisions about reducing or eliminating risks. See risk, risk analysis, risk-benefit analysis, risk communication.

risk-benefit analysis Estimate of the short- and long-term risks and benefits of using a particular product or technology. See risk assessment.

rock Any material that makes up a large, natural, continuous part of earth's crust. See igneous rock, metamorphic rock, mineral, sedimentary rock.

rock cycle Largest and slowest of the earth's cycles, consisting of geologic, physical, and chemical processes that form and modify rocks and soil in the earth's crust over millions of years.

r-selected species Species that reproduce early in their life span and produce large numbers of usually small and short-lived offspring in a short period of time. Also rstrategists. Compare K-selected species.

rule of 70 Doubling time (in years) = 70/percentage growth rate. See doubling time, exponential growth.

ruminants Grazing animals with complex digestive systems that enable them to convert grass and other roughage into meat and milk.

Runoff Fresh water from precipitation and melting ice that flows on the earth's surface into nearby streams, lakes, wetlands, and reservoirs. See reliable runoff, surface runoff, surface water. Compare groundwater.