Trophic Levels
The hierarchical positions organisms occupy in a food chain based on their feeding relationships. Each level represents a step in the transfer of energy, from producers at the base to apex predators at the top. The number of trophic levels in an ecosystem is limited by the energy available at each successive step.
Example: In a lake ecosystem, phytoplankton are the producers (trophic level 1), zooplankton are primary consumers (level 2), small fish are secondary consumers (level 3), and large bass are tertiary consumers (level 4).
Producers (Autotrophs)
Organisms that synthesize their own food from inorganic molecules using energy from sunlight or chemical reactions. Photoautotrophs such as plants and algae use photosynthesis, while chemoautotrophs such as deep-sea vent bacteria use chemical energy. Producers form the foundation of all ecosystem energy flow.
Example: Grasses in a savanna capture sunlight and convert carbon dioxide and water into glucose through photosynthesis, producing the chemical energy that sustains zebras, wildebeest, and all other consumers in the ecosystem.
Primary Consumers
Herbivorous organisms that feed directly on producers, occupying the second trophic level. They convert plant biomass into animal biomass and serve as the critical link between autotrophic production and the rest of the consumer food web.
Example: Caterpillars feeding on oak leaves are primary consumers. They digest plant tissue to obtain energy and nutrients, which then become available to secondary consumers like songbirds that eat the caterpillars.
Secondary and Tertiary Consumers
Organisms that feed on other consumers. Secondary consumers eat primary consumers and occupy the third trophic level, while tertiary consumers eat secondary consumers at the fourth level. Because each transfer loses approximately 90 percent of energy, these higher-level consumers require large foraging ranges to meet their energy needs.
Example: In an ocean food web, krill (primary consumer) are eaten by mackerel (secondary consumer), which are eaten by tuna (tertiary consumer). A single tuna must consume enormous quantities of prey because only a fraction of the energy at each level is passed upward.
Decomposers
Organisms such as bacteria, fungi, and detritivores that break down dead organic matter and waste products, releasing nutrients back into the soil and atmosphere. While they recycle matter for reuse by producers, the energy they extract is ultimately lost as heat through respiration.
Example: Fungi growing on a fallen log in a forest decompose the wood, releasing carbon dioxide and returning nitrogen and phosphorus to the soil where tree roots can absorb them again.
Ten Percent Rule
The ecological generalization that approximately 10 percent of the energy at one trophic level is converted into biomass at the next trophic level. The remaining 90 percent is lost primarily as metabolic heat during cellular respiration, with smaller amounts lost through incomplete digestion and excretion.
Example: If a field of grass fixes 10,000 kcal of energy from sunlight, approximately 1,000 kcal is available to rabbits that eat the grass, 100 kcal to foxes that eat the rabbits, and only 10 kcal to an eagle that eats the foxes.
Energy Pyramid
A graphical model showing the relative amount of energy available at each trophic level of an ecosystem. The pyramid shape results from energy loss at each transfer, with the broadest bar at the producer base and progressively narrower bars for each consumer level. Unlike biomass or numbers pyramids, energy pyramids are never inverted.
Example: An energy pyramid for a temperate forest might show 20,000 kcal/m2/year at the producer level, 2,000 at the primary consumer level, 200 at the secondary consumer level, and 20 at the tertiary consumer level.
Primary Productivity (GPP and NPP)
Gross primary productivity (GPP) is the total rate at which producers capture and store chemical energy through photosynthesis. Net primary productivity (NPP) is GPP minus the energy producers use for their own respiration. NPP represents the energy actually available to consumers and decomposers in the ecosystem.
Example: A tropical rainforest has a high GPP of about 8,000 kcal/m2/year. After subtracting the roughly 3,500 kcal/m2/year the plants use for their own cellular respiration, the NPP available to herbivores and decomposers is approximately 4,500 kcal/m2/year.
