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.
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. Apart from bacteria, the total live biomass on earth is about 560 billion tonnes C, and the total annual primary production of biomass is just over 100 billion tonnes C/yr. However, the total live biomass of bacteria may exceed that of plants and animals.
- 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.
Terrestrial biomass generally decreases markedly at each higher trophic level (plants, herbivores, carnivores). Examples of terrestrial producers are grasses, trees and shrubs. 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, but decreases markedly at each higher level.
There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. In all, it has been estimated that there are about five million trillion trillion, or 5 × 1030 (5 nonillion) bacteria on Earth with a total biomass equaling that of plants. Some researchers believe the total biomass of bacteria exceeds that of all plants and animals.
Estimates for the global biomass of species and specie groups are not always consistent across the literature. Apart from bacteria, the total global biomass has been estimated at about 560 billion tonnes C. Most of this biomass is found on land, with only 5 to 10 billion tonnes C found in the oceans. On land there is about 1,000 times more plant biomass (phytomass) than animal biomass (zoomass). About 18% of this plant biomass is eaten by the land animals. However in the ocean the animal biomass is nearly 30 times larger than the plant biomass. Most ocean plant biomass is eaten by the ocean animals.
Humans comprise about 100 million tonnes of the Earth’s dry biomass, domesticated animals about 700 million tonnes, and crops about 2 billion tonnes. The most successfulanimal species, in terms of biomass, may well be Antarctic krill, Euphausia superba, with a fresh biomass approaching 500 million tonnes, although domestic cattle may also reach these immense figures. However, as a group, the small aquatic crustaceans called copepods may form the largest animal biomass on earth. A 2009 paper in Science estimates, for the first time, the total world fish biomass as somewhere between 0.8 and 2.0 billion tonnes. It has been estimated that about about 1% of the global biomass is due to phytoplankton,and a staggering 25% is due to fungi.