background extinction Normal extinction of various species as a result of changes in local environmental conditions. Also used natural rate of extinction. Compare mass depletion, mass extinction.

Bacteria Prokaryotic, one-celled organisms. Some transmit diseases. Most act as decomposers and get the nutrients they need by breaking down complex organic compounds in the tissues of living or dead organisms into simpler inorganic nutrient compounds. 

Classification Monera barrier islands Long, thin, low offshore islands of sediment that generally run parallel to the shore along some coasts.

basic solution Water solution with more hydroxide ions (OH2) than hydrogen ions (H1); water solution with a pH greater than 7. Compare acid solution, neutral solution. 

benefit-cost analysis Estimates and comparison of short-term and long-term benefits (gains) and costs (losses) from an economic decision. Also can use cost-benefit analysis. 

benthos Bottom-dwelling organisms. Compare decomposer, nekton, plankton. 

bioaccumulation An increase in the concentration of a chemical in specific organs or tissues at a level higher than would normally be expected. Compare biomagnification. 

biocentric Life centered. Compare anthropocentric.

biodegradable Capable of being broken down by decomposers. 

biodegradable pollutant Material that can be broken down into simpler substances (elements and compounds) by bacteria or other decomposers. Paper and most organic wastes such as animal manure are biodegradable but can take decades to biodegrade in modern landfills. Compare degradable pollutant, nondegradable pollutant, slowly degradable pollutant. 

Biodiversity Variety of different species (species diversity), genetic variability among individuals within each species (genetic diversity), variety of ecosystems (ecological diversity), and functions such as energy flow and matter cycling needed for the survival of species and biological communities (functional diversity).

biofuel Gas or liquid fuel (such as ethyl alcohol) made from plant material (biomass). 

biogeochemical cycle Natural processes that recycle nutrients in various chemical forms from the nonliving environment to living organisms and then back to the nonliving environment. Examples are the carbon, oxygen, nitrogen, phosphorus, sulfur, and hydrologic cycles. Also nutrient cycle. 

bioinformatics Applied science of managing, analyzing, and communicating biological information. 

biological evolution Change in the genetic makeup of a population of a species in successive generations. If continued long enough, it can lead to the formation of a new species. Note that populations--not individuals--evolve. Also used is evolution. See also adaptation, differential reproduction, natural selection, theory of evolution.

biological oxygen demand (BOD) Amount of dissolved oxygen needed by aerobic decomposers to break down the organic materials in a given volume of water at a certain temperature over a specified time period. See dissolved oxygen content. 

biological pest control Control of pest populations by natural predators, parasites, or disease-causing bacteria and viruses (pathogens). 

Biomagnification Increase in concentration of DDT, PCBs, and other slowly degradable, fat-soluble chemicals in organisms at successively higher trophic levels of a food chain or web. Also biological amplification can be used. Compare bioaccumulation. 

Biomass Organic matter produced by plants and other photosynthetic producers; total dry weight of all living organisms that can be supported at each trophic level in a food chain or web; dry weight of all organic matter in plants and animals in an ecosystem; plant materials and animal wastes used as fuel. 

biome Terrestrial regions inhabited by certain types of life, especially vegetation. Examples are various types of deserts, grasslands, and forests. 

Biosphere Zone of earth where life is found. It consists of parts of the atmosphere (the troposphere), hydrosphere (mostly surface water and groundwater), and lithosphere (mostly soil and surface rocks and sediments on the bottoms of oceans and other bodies of water) where life is found. Sometimes called the ecosphere. 

biotic Living organisms. Compare abiotic. 

biotic potential Maximum rate at which the population of a given species can increase when there are no limits on its rate of growth. See environmental resistance.

bitumen Gooey, black, high-sulfur, heavy oil extracted from tar sand and then upgraded to synthetic fuel oil. See tar sand.

broadleaf deciduous plants Plants such as oak and maple trees that survive drought and cold by shedding their leaves and becoming dormant. Compare broadleaf evergreen plants, coniferous evergreen plants.

broadleaf evergreen plants Plants that keep most of their broad leaves year round. Examples are the trees found in the canopies of tropical rain forests. Compare broadleaf deciduous plants, coniferous evergreen plants. 

buffer Substance that can react with hydrogen ions in a solution and thus hold the acidity or pH of a solution fairly constant. See pH.