Biomass (ecology)

The biomass 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.

Apart from bacteria, the total global live biomass has been estimated as 550 or 560 billion tonnes C,[1] most of which is found in forests.[2]

Shallow aquatic environments, such as wetlands, estuaries and coral reefs, can be as productive as forests, generating similar amounts of new biomass each year on a given area.[3]

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 some applications, biomass is measured as the mass of organically bound carbon (C) that is present.

The total live biomass on Earth is about 550–560 billion tonnes C,[1][5] and the total annual primary production of biomass is just over 100 billion tonnes C/yr.[6] The total live biomass of bacteria may be as much as that of plants and animals[7] or may be much less.[1][8][9][10][11] The total number of DNA base pairs on Earth, as a possible approximation of global biodiversity, is estimated at (5.3±3.6)×10³⁷, and weighs 50 billion tonnes.[12][13]

Ecological pyramids

An energy pyramid illustrates how much energy is needed as it flows upward to support the next trophic level. Only about 10% of the energy transferred between each trophic level is converted to biomass.

Main article: Ecological pyramid

An ecological pyramid is a graphical representation that 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.

Terrestrial biomass

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, such as grasshoppers, voles and bison, followed by the secondary consumers, shrews, hawks and small cats. Finally the tertiary consumers, large cats and wolves. The biomass pyramid decreases markedly at each higher level.

Ocean biomass

See also: Marine life

Ocean or marine biomass, in a reversal of terrestrial biomass, can increase 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

Ocean food web showing a network of food chains

Biomass pyramids
Compared to terrestrial biomass pyramids, aquatic pyramids are inverted at the base

Prochlorococcus, an influential bacterium

Phytoplankton are the main primary producers at the bottom of the marine food chain. Phytoplankton use photosynthesis to convert inorganic carbon into protoplasm. They are then consumed by microscopic animals called zooplankton.

Zooplankton comprise the second level in the food chain, and includes small crustaceans, such as copepods and krill, and the larva of fish, squid, lobsters and crabs.

In turn, small zooplankton are consumed by both larger predatory zooplankters, such as krill, and by forage fish, which are small, schooling, filter-feeding fish. This makes up the third level in the food chain.

A fourth trophic level can consist of predatory fish, marine mammals and seabirds that consume forage fish. Examples are swordfish, seals and gannets.

Apex predators, such as orcas, which can consume seals, and shortfin mako sharks, which can consume swordfish, make up a fifth trophic level. Baleen whales can consume zooplankton and krill 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 (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which are r-strategists that grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers, such as forests, are K-strategists that grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production.

Among the phytoplankton at the base of the marine food web are members from a phylum of bacteria called cyanobacteria. Marine cyanobacteria include the smallest known photosynthetic organisms. The smallest of all, Prochlorococcus, is just 0.5 to 0.8 micrometres across.[14] In terms of individual numbers, Prochlorococcus is possibly the most plentiful species on Earth: a single millilitre of surface seawater can contain 100,000 cells or more. Worldwide, there are estimated to be several octillion (10²⁷) individuals.[15] Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans.[16] The bacterium accounts for an estimated 20% of the oxygen in the Earth's atmosphere, and forms part of the base of the ocean food chain.[17]

Bacterial biomass

There are typically 50 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. In a much-cited study from 1998,[7] the world bacterial biomass had been mistakenly calculated to be 350 to 550 billions of tonnes of carbon, equal to between 60% and 100% of the carbon in plants. More recent studies of seafloor microbes cast considerable doubt on that; one study in 2012[8] reduced the calculated microbial biomass on the seafloor from the original 303 billions of tonnes of C to just 4.1 billions of tonnes of C, reducing the global biomass of prokaryotes to 50 to 250 billions of tonnes of C. Further, if the average per-cell biomass of prokaryotes is reduced from 86 to 14 femtograms C,[8] then the global biomass of prokaryotes was reduced to 13 to 44.5 billions of tonnes of C, equal to between 2.4% and 8.1% of the carbon in plants.

As of 2018, there continues to be some controversy over what the global bacterial biomass is. A census published by the PNAS in May 2018 gives for bacterial biomass ~70 billions of tonnes of carbon, equal to 15% of the whole biomass.[1] A census by the Deep Carbon Observatory project published in December 2018 gives a smaller figure of up to 23 billion tonnes of carbon.[9][10][11]

Geographic location	Number of cells (× 10^29)	Billions of tonnes of carbon
Ocean floor	2.9[8] to 50[18]	4.1[8] to 303[7]
Open ocean	1.2[7]	1.7[7][8] to 10[7]
Terrestrial soil	2.6[7]	3.7[7][8] to 22[7]
Subsurface terrestrial	2.5 to 25[7]	3.5[7][8] to 215[7]

Global biomass

Biomass by life form

Estimates for the global biomass of species and higher level groups are not always consistent across the literature. The total global biomass has been estimated at about 550 billion tonnes C.[5][1] Most of this biomass is found on land, with only 5 to 10 billion tonnes C found in the oceans.[5] 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.[19] However, in the ocean, the animal biomass is nearly 30 times larger than the plant biomass.[20] Most ocean plant biomass is eaten by the ocean animals.[19]

	name	number of species	date of estimate	individual count	mean living mass of individual	percent biomass (dried)	total number of carbon atoms	global dry biomass in million tonnes	global wet (fresh) biomass in million tonnes
Terrestrial	Humans	1	2019	7.7 billion[21]	50 kg (incl children)	30%	4.015×10^36[22]	105	385
			2005	4.63 billion	62 kg (excl. children)[23]				287[23]
	Cattle	1		1.3 billion[24]	400 kg	30%		156	520
	Sheep and goats	2	2002	1.75 billion[25]	60 kg	30%		31.5	105
	Chickens	1		24 billion	2 kg	30%		14.4	48
	Ants	12,649[26]		10^7–10^8 billion[27]	3×10^−6 kg (0.003 grams)	30%		10–100	30-300
	Earthworms	>7,000	1881 Darwin	1.3×10^6 billion [28]	3 g	30% [29]		1,140–2,280[28]	3,800–7,600[28]
	Termites	>2,800	1996						445[30]
Marine	Blue whales[31]	1	Pre-whaling	340,000		40%[32]			36
			2001	4,700		40%[32]			0.5
	Fish	>10,000	2009						800-2,000[33]
	Antarctic krill	1	1924–2004	7.8×10^14	0.486 g				379[34]
	Copepods (a zooplankton)	13,000			10^−6–10^−9 kg		1×10^37[35]
	Cyanobacteria (a picoplankton)	?	2003						1,000[36]
Global	Prokaryotes (bacteria)	?	2018	1×10^31 cells[1]				23,000[9] – 70,000[1]

Humans comprise about 100 million tonnes of the Earth's dry biomass,[37] domesticated animals about 700 million tonnes, earthworms over 1,100 million tonnes,[28] and annual cereal crops about 2.3 billion tonnes.[38]

The most successful animal species, in terms of biomass, may well be Antarctic krill, Euphausia superba, with a fresh biomass approaching 500 million tonnes,[34][39][40] 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.[41] 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.[42][43] It has been estimated that about 1% of the global biomass is due to phytoplankton,[44] and 25% is due to fungi.[45][46]

• 
  Grasses, trees and shrubs have a much higher biomass than the animals that consume them
• 
  The total biomass of bacteria may equal that of plants.[7]
• 
  Copepods may form the largest biomass of any animal species group.[41]
• 
  Antarctic krill form one of the largest biomasses of any individual animal species.[39]
• 
  It has been claimed that fungi make up 25% of the global biomass

Global rate of production

Globally, terrestrial and oceanic habitats produce a similar amount of new biomass each year (56.4 billion tonnes C terrestrial and 48.5 billion tonnes C oceanic).

Net primary production is the rate at which new biomass is generated, mainly due to photosynthesis. 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.[6] 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. Autotrophs may have the highest global proportion of biomass, but they are closely rivaled or surpassed by microbes.[47][48]

Terrestrial freshwater ecosystems generate about 1.5% of the global net primary production.[49]

Some global producers of biomass in order of productivity rates are

Producer	Biomass productivity (gC/m²/yr)	Ref	Total area (million km²)	Ref	Total production (billion tonnes C/yr)
Swamps and marshes	2,500	[3]
Tropical rainforests	2,000	[50]	8		16
Coral reefs	2,000	[3]	0.28	[51]	0.56
Algal beds	2,000	[3]
River estuaries	1,800	[3]
Temperate forests	1,250	[3]	19		24
Cultivated lands	650	[3][52]	17		11
Tundras	140	[3][52]
Open ocean	125	[3][52]	311		39
Deserts	3	[52]	50		0.15

See also

• Biomass – Biological material used as a renewable energy source (as in bioproducts)
• Organic matter – Matter composed of organic compounds
• Productivity (ecology) – The rate of generation of biomass in an ecosystem
• Primary nutritional groups
• Standing stock – A measurement of population numbers per unit area or volume
• Lake Pohjalampi - a biomass manipulation study
• List of commercially important fish species – The aquatic animals that are harvested commercially in the greatest amounts

References

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Further reading

• Foley JA, Monfreda C, Ramankutty N, Zaks D (July 2007). "Our share of the planetary pie". Proceedings of the National Academy of Sciences of the United States of America. 104 (31): 12585–6. Bibcode:2007PNAS..10412585F. doi:10.1073/pnas.0705190104. PMC 1937509. PMID 17646656.
• Haberl H, Erb KH, Krausmann F, Gaube V, Bondeau A, Plutzar C, Gingrich S, Lucht W, Fischer-Kowalski M (July 2007). "Quantifying and mapping the human appropriation of net primary production in earth's terrestrial ecosystems". Proceedings of the National Academy of Sciences of the United States of America. 104 (31): 12942–7. Bibcode:2007PNAS..10412942H. doi:10.1073/pnas.0704243104. PMC 1911196. PMID 17616580.
• Purves WK, Orians GH (2007). Life: The Science of Biology (8th ed.). W. H. Freeman. ISBN 978-1-4292-0877-2.

External links

• Media related to Biomass at Wikimedia Commons
• The mass of all life on Earth is staggering — until you consider how much we’ve lost
• Counting bacteria
• Trophic levels
• Biomass distributions for high trophic-level fishes in the North Atlantic, 1900–2000