2 Productivity and Trophic Structure
Learn how primary productivity, feeding relationships, and energy transfer shape trophic structure and ecological pyramids.
Primary producers, including plants, algae, and some bacteria, capture energy and store it as organic matter, or biomass. Most producers use photosynthesis, while some use chemical energy. is the rate of this energy capture and biomass formation, usually expressed per unit area per unit time, such as grams of carbon per square meter per year or energy per square meter per year.
is the total energy or organic matter producers generate. Producers use some of that energy for respiration and other life processes, so only the remainder is available for growth. is calculated as:
NPP indicates how much new producer biomass is potentially available to consumers. It is a rate, not a measurement of the total producer biomass present at a particular moment.
Trophic levels and feeding relationships
A is an organism’s feeding position in the flow of energy through an ecosystem. The first consists of primary producers. Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. An organism can feed at more than one ; for example, an omnivore that eats both plants and animals may occupy multiple feeding positions.
A shows one pathway of energy transfer. In the example , each arrow points from food to consumer and shows the direction energy moves.
In nature, organisms often eat—and are eaten by—multiple species. Interconnected food chains form a , which represents feeding relationships in a community more realistically than a single pathway. Decomposers, including fungi and bacteria, break down dead organisms and wastes from every . Their activity returns nutrients to the environment, while energy continues to dissipate as heat.
Energy transfer
Energy enters most ecosystems as sunlight, is captured by producers, and then passes to consumers when organisms eat. Energy flows through ecosystems, while nutrients cycle. At each transfer, only some of the energy in one becomes biomass available to the next. Organisms use much of the energy they take in for respiration, movement, growth, and other processes; some energy also remains in uneaten material or waste. Energy used in metabolism is ultimately released as heat.
is the fraction transferred between trophic levels. A useful rule of thumb is that roughly of the energy at one level becomes available as production at the next, but the actual percentage varies among ecosystems and organisms. For example, if producers store units of energy, the next level might store about units, and the level after that about units.
The declining energy supply helps explain why food chains usually have relatively few levels and why ecosystems need sufficient to support consumers.
Ecological pyramids
Ecological pyramids compare a feature of organisms across trophic levels, with producers at the base and consumers above them. Each type represents a different feature and answers a different question.
Pyramid of energy: Shows energy available or transferred at each level over a period of time. It is always upright because energy is lost at each transfer.
Pyramid of biomass: Shows the biomass present at each level at a particular time. It is often upright on land but can be inverted in aquatic ecosystems. For example, phytoplankton may have less standing biomass than the zooplankton that consume them because phytoplankton reproduce and are consumed rapidly.
Pyramid of numbers: Shows the number of organisms at each level. It can be upright or inverted; a single tree, for instance, may support many insects.
Energy pyramids show energy flow over time. Biomass and numbers pyramids show, at a snapshot in time, how much living material or how many organisms are present.