Living things require energy to grow, breathe, and reproduce. This energy must be available within an ecosystem, or a community that consists of all the living and nonliving things in an area, including soil, plants, and animals.

The sun is the first source of energy for almost every ecosystem. Plants and other living things, or organisms, convert the sun's energy, or solar energy, into biomass, which is the energy that is part of organisms. As organisms eat other organisms, the biomass, or energy, moves all the way up the food chain. A food chain is a path that energy takes through a certain ecosystem.

Producers and Consumers

Each organism in an ecosystem is part of many food chains. Together, these food chains make a food web. Within this ecosystem structure, organisms are grouped into categories called trophic levels.

The first trophic level includes organisms called primary producers. They make their own food, and plants and algae are a few examples. Primary producers use a process called photosynthesis to create nutrients from sunlight, carbon dioxide, and water. This process is how plants convert solar energy into biomass.

The second trophic level includes organisms called primary consumers. They can't produce their own food and therefore rely on eating other organisms to get energy. An example might be a cow (a primary consumer) eating grass (a producer">primary producer).

Primary consumers include herbivores and omnivores. Herbivores are organisms that only eat plants, and omnivores are organisms that eat both plants and animals.

This third trophic level includes secondary consumers. They eat primary consumers. They can either omnivores or carnivores, which are animals that only eat other animals. An example might be a snake that eats an insect.

The fourth trophic level contains tertiary consumers. These organisms are carnivores or omnivores that eat secondary consumers. An example might be an owl that eats a mouse.

Trophic levels are best seen in a model of a food chain. In reality, primary producers would be eaten by many different organisms, not just grasshoppers. Tertiary consumers like the hawk would need to hunt many organisms to survive. Most ecosystems are more complex and would be better represented by an interlinked series of food chains called a food web.

Moving Energy from One Level to the Next

Only so much biomass, or energy, can move from one trophic level to the next. Energy is lost at each step along the food chain.

An energy pyramid is a good way to show energy loss between trophic levels. Each step of the pyramid represents a different trophic level. The primary producers are at the bottom level, and the tertiary consumers are at the top level.

The size of each level in the pyramid represents the rate of energy flow, or how much energy passes through each trophic level. The steps decrease in size as you travel up the pyramid because energy is lost at every level in the food chain. Eventually, the step can't get any smaller, because there is no energy left to support another trophic level.

Only a small amount, or 10 percent, of energy moves from one trophic level to the next. This is known as the 10 percent rule. It limits the number of trophic levels an ecosystem can support. For example, when a primary consumer eats a primary producer, the consumer only gets 10 percent of the producer's energy. So, if an insect eats a plant, it only gets 10 percent of the energy from the plant. The next consumer would only receive 10 percent of the energy from the insect. This continues all the way up the food chain.