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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. The mass can be expressed as the average mass per unit area, or as the total mass in the community.
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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. The mass can be expressed as the average mass per unit area, or as the total mass in the community. It might be measured in grams per square metre or tonnes per square kilometre, or it might be measured as the total mass present in a system such as a lake.
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.
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.
Ecological pyramids provide a snapshot in time of an ecological community. They begin with producers on the bottom and proceed through the various trophic levels to the highest 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.
Terrestrial biomass
Terrestrial biomass generally decreases markedly at each higher trophic level. Examples of terrestrial producers are grasses, trees and scrubs. These have a much higher biomass than the animals that consume them, such as deer, zebras and insects. The level with the least biomass are the highest predators in the food chain, such as foxes and eagles.
In a temperate grassland, grasses and other plants are the primary producers at the bottom of the pyramid. Then come the primary consumers, grasshoppers, voles and bison, followed by the secondary consumers, shrews, hawks and small cats, and finally the tertiary consumers, large cats and wolves. The biomass pyramid is not inverted, and decreases markedly at each higher level.
Ocean biomass
Ocean biomass, in a reversal of terrestrial biomass, often increases at higher trophic levels. In the ocean, the food chain typically starts with phytoplankton, and follows the course:
- Phytoplankton ? zooplankton ? predatory zooplankton ? filter feeders ? predatory fish
Phytoplankton is the main primary producer at the bottom of the marine food chain. It uses photosynthesis to convert inorganic carbon into protoplasm. Phytoplankton is then consumed by microscopic animals called zooplankton.
Zooplankton comprise the second level in the food chain, and includes the larva of fish, squid, lobsters and crabs, small crustaceans such as copepods and krill, and many other types.
In turn, the smaller zooplankton is consumed both by larger predatory zooplankters and by fish, the third level in the food chain. The fish that eat zooplankton are usually forage fish, which are small schooling filter feeding fish. These constitute a fourth trophic level. Seals consuming forage fish are a fifth level, and orcas consuming the seals a sixth level. Baleen whales can consume zooplankton directly, leading to a food chain with only three or four trophic levels.
Marine environments can have inverted biomass pyramids. In particular, the biomass of consumers (small fish, shrimp) is larger than the biomass of primary producers. This happens because these primary producers are tiny phytoplankton that rapidly grow and reproduce, instead of plants which grow and reproduce slowly.
Global biomass
The most successful animal species, in terms of biomass, is probably the Antarctic krill, Euphausia superba, with a biomass of about 500 million tonnes. However, as a group, the small aquatic crustaceans called copepods form the largest animal biomass on earth.
| Species | individual count | average living weight
of individual in kg | % biomass | Number of
Total Carbon Atoms | biomass in million tons |
|---|
| crops | | | | | 2,000 | | Antarctic krill (wet) | | | wet | | 500 | | humans | 6.6 billion | 50kg | 30% | 6.6 x 109 x 5x1026 | 100 | | copepod (plankton) | | 10-6 - 10-9 kg
| | | | cattle | 1.3 billion | 400kg | 30% | | 156 | | sheep and goats | 1.75 billion | 60kg | 30% | | 31.5 | | chickens | 24 billion | 2kg | 30% | | 14.4 | | ants | 107 - 108 billion | 3 x 10-7kg
(0.3 milligrams) | 30% | | 900 - 9,000 |
Humans comprise about 100 million tonnes of the Earth's biomass, domesticated animals about 700 million tonnes (1.0%), and crops about 2 billion tonnes. The total biomass of bacteria is estimated to equal that of plants .
Net primary production is the rate at which biomass is generated in a given area, mainly due to photosynthesis. Some global producers of biomass in order of productivity rates are:
Global primary production can be estimated from satellite observations. Satellites scan the normalised difference vegetation index (NDVI) over terrestrial habitats, and scan sea-surface chlorophyll levels over oceans. This results in 56.4 billion tonnes C/yr (53.8%), for terrestrial primary production, and 48.5 billion tonnes C/yr for oceanic primary production.. Thus, the total photoautotrophic primary production for the Earth is about 104.9 billion tonnes C/yr. This translates to about 426 gC/m²/yr for land production (excluding areas with permanent ice cover), and 140 gC/m²/yr for the oceans.
However, there is a much more significant difference in standing stocks - while accounting for almost half of total annual production, oceanic autotrophs account for only about 0.2% of the total biomass.
See also
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