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Backgraund extinction - The continuous, low-level extinction of species that has occurred throughout much of the history of life. Compare with the mass extinction.

Base - A compound that releases hydroxyl ions (OH-) when dissolved in water. A base turns red litmus paper blue. Compare acid.

B-horizon - The light-colored, partially weathered soil layer underneath the A-horizon; subsoil. The B-horizon contains much less organic material than the A-horizon.

Biennial - A plant that requires two years to complete its life cycle. Compare annual and perennial.

Biochannel - The paths made by roots, animals, insects, and other soil biota that act as conduits for water and air through the soil.

Biocontrol (Biological control) - Controlling crop pests by using living organisms harmless to the plants but which destroy or reduce the number of harmful pests.

Biodegradable - A chemical pollutant decomposed (broken down) by living organisms or other natural processes. Compare nondegradable.

Biodiversity (biological diversity) - The variety of life forms: the different plants, animals and microorganisms, the genes they contain, and the ecosystems they form. It is usually considered at three levels: genetic diversity, species diversity and ecosystem diversity.

Biogeography - The study of the origin, geography, and distribution of organisms.

Biological water-use efficiency - The total dry weight of plant material produced per total water lost by transpiration.

Biomass- The total weight of organic material present per unit area.

Biome - A major ecological community type (for example, grassland); a major biotic unit consisting of plant and animal communities having similarities in form and environmental conditions.

Biosphere - The largest, all-encompassing ecosystem that includes soil, water and the atmosphere.

Biota - The living organisms of a region.

Biotic - Of or relating to life; caused or produced by living things.

Boundry planting - The method of planting trees, specifically to function as boundary markers, live fences, windbreaks, or firebreaks. Additional benefits include microclimate regulation and protection and the production of green manure, fodder, or fuel-wood.

Buffer zones - Areas on the edge of protected areas that have land use controls and allow only those activities (such as research, recreation, and tourism) that are compatible with protecting the core area.

Bund - An embankment used to control the flow of water.