Biomass ecology

Biomass refers to the total mass of living biological organisms present in a particular area or ecosystem at a specific time. It encompasses all forms of life, including microorganisms, plants and animals, and provides a fundamental indicator of ecosystem productivity, energy flow and ecological balance.

Definition and Measurement of Biomass

Biomass may be expressed either as the mass per unit area or as the total mass within a community. It can be categorised as species biomass, which relates to one or more specific species, or community biomass, which includes all species in a given ecological community.
Measurements of biomass vary depending on the purpose of investigation. In some contexts, biomass represents the natural in situ mass of organisms, such as the total wet weight of salmon in a fishery. Alternatively, biomass may be measured as dried organic mass, excluding water content, which can represent a significant proportion of the organism’s natural mass. In ecological and biogeochemical studies, the focus may be limited to biological tissues, excluding teeth, bones and shells. For certain analytical purposes, biomass is measured in terms of the mass of organic carbon contained within organisms.
Global estimates highlight the immense scale and distribution of biomass. In 2018, research estimated the total live biomass on Earth at approximately 550 billion tonnes of carbon, most of which is stored in plants. Global net primary production, representing the annual accumulation of biomass by primary producers, has been estimated to exceed 100 billion tonnes per year. Earlier assumptions that bacterial biomass was comparable to plant biomass have been revised, with modern analyses suggesting substantially lower total biomass for bacteria and archaea.

Ecological Pyramids and Energy Flow

Ecological pyramids provide a graphical representation of the distribution of biomass or primary productivity across trophic levels within an ecosystem. They offer a snapshot of the structure and functioning of a biological community.
A biomass pyramid shows the amount of living matter present at each trophic level, from primary producers to apex predators. A productivity pyramid illustrates the rate of energy production at each level. At the base of these pyramids lie the autotrophic primary producers, which capture energy from sunlight or inorganic chemicals through primary production and convert it into organic molecules.
Energy transfer between trophic levels is notoriously inefficient. Typically, only around ten per cent of energy is incorporated into new biomass as organisms feed; the remainder is expended in metabolic processes or lost as heat. This energy loss constrains productivity pyramids, which are never inverted, and generally limits food chains to about six trophic levels.
However, biomass pyramids can differ across ecosystems. In marine environments, biomass pyramids may be inverted, with higher trophic levels containing more biomass than primary producers. This occurs because marine phytoplankton reproduce rapidly and require only a small standing stock to sustain high productivity rates, in contrast to larger and slower-growing terrestrial plants.

Terrestrial Biomass Patterns

In terrestrial ecosystems, biomass generally decreases markedly at successive trophic levels. Primary producers such as grasses, shrubs and trees possess far greater biomass than herbivores, while carnivores and top predators hold the smallest proportion.
A temperate grassland illustrates a typical structure: grasses form the base of the biomass pyramid, followed by primary consumers such as bison, voles and grasshoppers. Secondary consumers include shrews, hawks and small cats, while tertiary consumers such as wolves and large felids occupy the apex. Each increase in trophic level brings a significant reduction in biomass due to energy dissipation.
Alterations in plant species composition can influence soil decomposer communities, illustrating the sensitivity of biomass distributions to ecological change. Plant species employing the C3 carbon fixation pathway have been observed to increase in biomass in response to elevated carbon dioxide concentrations, with responses noted up to 900 ppm.

Ocean Biomass and Marine Trophic Structures

Marine ecosystems display distinct biomass patterns compared with terrestrial environments. The marine food chain typically begins with phytoplankton, minute autotrophic organisms responsible for substantial global primary production. These are consumed by zooplankton, which include an extraordinary diversity of organisms such as copepods, krill and the larvae of numerous marine animals.
Predatory zooplankton and forage fish form higher trophic levels, followed by predatory fish, marine mammals and seabirds. Apex predators such as orcas and shortfin mako sharks may occupy fifth trophic levels. Baleen whales, which feed directly on zooplankton and krill, represent an alternative three- or four-level chain, illustrating the variability of marine trophic structures.
Marine biomass pyramids are often inverted, with consumers such as copepods and forage fish maintaining greater biomass than primary producers. This is attributed to the extremely high reproductive rates of phytoplankton, which can sustain rapid turnover despite possessing low standing biomass.
Cyanobacteria, especially Prochlorococcus, form a crucial component of marine biomass. Extremely small at only 0.5–0.8 micrometres, this species is among the most abundant organisms on Earth, with a single millilitre of seawater containing up to 100,000 cells. Globally, it is estimated that several octillion individuals exist, contributing as much as 20 per cent of atmospheric oxygen and forming a significant foundation of the marine food web.

Bacterial and Archaeal Biomass

Prokaryotes, including bacteria and archaea, represent a substantial part of global biomass. Early studies estimated that global prokaryotic biomass ranged between 350 and 550 billion tonnes of carbon, comparable to plant biomass. Later research significantly revised these estimates, particularly for deep-sea sediments, reducing values for subseafloor prokaryotic biomass from around 300 billion tonnes to approximately 4 billion tonnes.
More recent global assessments place bacterial biomass at around 70 billion tonnes of carbon, with the majority located in the terrestrial deep subsurface. Archaeal biomass is estimated at around 7 billion tonnes of carbon. Combined estimates suggest that between 23 and 31 billion tonnes of carbon are stored in prokaryotes, with roughly 70 per cent of this mass located in deep subsurface environments. These figures highlight ongoing advancements in the ability to assess microbial populations, particularly in difficult-to-sample regions such as deep ocean sediments and subsurface aquifers.