Biomass (ecology)

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Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms, plants or animals.[4] The mass can be expressed as the average mass per unit area, or as the total mass in the community.

How biomass is measured depends on why it is being measured. Sometimes the biomass is regarded as the natural mass of organisms in situ, just as they are. For example, in a salmon fishery, the salmon biomass might be regarded as the total wet weight the salmon would have if they were taken out of the water. In other contexts, biomass can be measured in terms of the dried organic mass, so perhaps only 30% of the actual weight might count, the rest being water. For other purposes, only biological tissues count, and teeth, bones and shells are excluded.

In stricter scientific applications, biomass is measured as the mass of organically bound carbon (C) that is present. The total live biomass on earth is about 560 billion tonnes C,[1] and the total annual primary production of biomass is just over 100 billion tonnes C/yr.[5]

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Ecological pyramids

An ecological pyramid is a graphical representation which shows, for a given ecosystem, the relationship between biomass or biological productivity and trophic levels.

  • A biomass pyramid shows the amount of biomass at each trophic level.
  • A productivity pyramid shows the production or turn-over in biomass at each trophic level.

An ecological pyramid provides a snapshot in time of an ecological community.

The bottom of the pyramid represents the primary producers (autotrophs). The primary producers take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates. This mechanism is called primary production. The pyramid then proceeds through the various trophic levels to the apex predators at the top.

When energy is transferred from one trophic level to the next, typically only ten percent is used to build new biomass. The remaining ninety percent goes to metabolic processes or is dissipated as heat. This energy loss means that productivity pyramids are never inverted, and generally limits food chains to about six levels. However, in oceans, biomass pyramids can be wholly or partially inverted, with more biomass at higher levels.

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