Ecology and Classification

Energy Flow, Trophic Levels, and Food Webs

Every ecosystem on Earth relies on a continuous supply of energy. The sun is the primary source of energy for nearly all ecosystems on Earth. Producers, which are also known as autotrophs, capture this solar energy or use chemical energy to make their own food. Because producers form the foundation of living matter, they determine the amount of biomass, the total mass of living matter within a given trophic level or area, available to support other living things.

Energy moves through an ecosystem across distinct feeding stages called trophic levels. Primary consumers are herbivores that feed directly on producers. Secondary consumers feed on primary consumers and can be carnivores or omnivores. Tertiary consumers feed on secondary consumers and occupy a higher trophic level. While a single feeding sequence is called a food chain, feeding relationships in nature are rarely linear. A food web consists of multiple interconnected food chains within an ecosystem, reflecting the complex feeding interactions among organisms. Along with active predators, scavengers feed on dead organisms, although some scavengers also hunt live prey.

A critical principle governing ecosystems is that energy flows in one direction through an ecosystem, whereas chemical nutrients cycle continuously. Most energy lost between trophic levels is released into the environment as heat. According to the 10 percent rule, about 10 percent of the energy stored at one trophic level becomes stored energy at the next level. For example, 1,000 energy units in producers would support about 100 units in primary consumers. Following this same pattern, those 100 units in primary consumers would support only about 10 units in secondary consumers, and just 1 unit in tertiary consumers. Decomposers break down dead organic matter and return nutrients to the environment, where producers can use them again; nutrients cycle, but they can also move into or out of a particular ecosystem.