Biomass is a core concept in biology and ecology, representing the total mass of living organisms within a specific area or ecosystem. This measurement provides a quantitative understanding of the amount of life an environment supports. It includes the entire spectrum of life, from microscopic bacteria and fungi to massive plants and animals. Understanding the distribution and flow of this mass is important for assessing the health and capacity of ecological systems.
Defining Biological Mass
Biomass is a measure of the organic material that makes up all living things in a given area at a single point in time. This snapshot of living material is formally referred to as standing crop biomass. The rate of biomass accumulation is known as net primary productivity (NPP), which is the total amount of carbon fixed by producers minus the energy they use for respiration. NPP represents the portion of biomass available to the next trophic level in the ecosystem.
When scientists measure biomass, they use the dry weight of the organisms to ensure consistency. Water content varies dramatically between species and within the same organism, making wet weight an unreliable metric for comparison. Removing the water by drying a sample in an oven provides a standardized measure of the organic compounds—such as carbohydrates, proteins, and lipids—that make up the living tissue. This mass includes producers (plants and algae), consumers (herbivores and carnivores), and decomposers (bacteria and fungi).
Methods for Quantifying Biomass
Calculating biomass involves a combination of direct measurement and estimation techniques, often expressed in units like grams per square meter or tons per hectare. The most direct method is destructive sampling, where organisms are collected from a defined area, dried in an oven, and then weighed to determine the dry mass. This technique yields a highly accurate reading of the standing crop for that particular sample.
Because destructive sampling is impractical for large areas or protected species, scientists often rely on estimation methods. Techniques like allometric equations estimate the mass of large organisms, such as trees, by measuring dimensions like trunk diameter and height. These measurements are entered into mathematical formulas that correlate size to total dry mass. For vast areas, remote sensing using satellites measures features like the Normalized Difference Vegetation Index (NDVI) or sea-surface chlorophyll levels, providing broad estimates of producer biomass and productivity across entire biomes.
Biomass and Trophic Structure
Biomass is central to an ecosystem’s trophic structure, which describes the flow of energy through different feeding levels. The ecological pyramid of biomass illustrates this flow, typically showing a large mass of producers at the base and progressively smaller masses of consumers at higher levels. Producers, such as plants, convert light energy into chemical energy, forming the largest reservoir of biomass.
The total biomass decreases significantly as energy moves up the food chain from one trophic level to the next. This reduction is a consequence of the 10% rule of energy transfer, which posits that only about 10% of the energy stored in one level is converted into biomass at the next. The remaining 90% is lost primarily as heat during metabolic processes, such as respiration, or as waste and uneaten material. This loss means that a large mass of primary producers (e.g., phytoplankton) is necessary to support a much smaller mass of primary consumers (e.g., zooplankton) and even less mass of secondary consumers.
The diminishing energy transfer limits the number of trophic levels an ecosystem can support. Top predators, which occupy the highest level, always have the least total biomass in the system. While this pyramid shape is characteristic of most terrestrial ecosystems, some aquatic systems can exhibit an inverted pyramid of biomass. In these cases, producers, like fast-reproducing phytoplankton, have a small standing crop biomass but a very high turnover rate that is constantly consumed by a larger biomass of long-lived primary consumers.
Global Distribution of Biomass
The distribution of the planet’s total living biomass, estimated at around 550 billion tons of carbon, is heavily skewed toward terrestrial environments. Land-based ecosystems hold approximately 86% of the world’s biomass. Plant life accounts for about 82% of the planet’s total biomass, meaning biomes like tropical rainforests and temperate forests are the largest reservoirs of life on Earth.
In contrast, the oceans, which cover over 70% of the globe, contain only about 1% of the total global biomass, largely because primary producers, like phytoplankton, have a very small standing crop. Animal biomass is more prevalent in marine environments, with about 78% of all animal mass found in the ocean, largely comprising arthropods and fish. Another significant portion of global biomass, particularly bacteria and archaea, is concentrated in deep subsurface environments, extending thousands of meters below the surface of both land and ocean floors.