A food web consists of all the food chains in a single ecosystem. Each living thing in an ecosystem is part of multiple food chains. Each food chain is one possible path that energy and nutrients may take as they move through the ecosystem. Not all energy is transferred from one trophic level to another. Energy is used by organisms at each trophic level, meaning that only part of the energy available at one trophic level is passed on to the next level. All of the interconnected and overlapping food chains in an ecosystem make up a food web. Similarly, a single organism can serve more than one role in a food web. For example, a queen conch can be both a consumer and a detritivore, or decomposer.

Food webs consist of different organism groupings called trophic levels. In this example of a coral reef, there are producers, consumers, and decomposers.

• Producers make up the first trophic level. A producer, or autotroph, is an organism that can produce its own energy and nutrients, usually through photosynthesis or chemosynthesis.
• Consumers are organisms that depend on producers or other consumers to get their food, energy, and nutrition. There are many different types of consumers. First-order consumers, or primary consumers, are usually herbivores. They eat producers. Secondary consumers prey on primary consumers. They are usually carnivores, but can be omnivores as well. Tertiary consumers are carnivores that mostly eat other carnivores. They prey on secondary consumers. These predator-prey relationships make up the food web. Different predators eat different kinds of prey until a top predator is reached. Top predators are at the top of the food chain and have no predators of their own.
• Detritivores and decomposers complete the cycling of energy through the food web. Detritivores are organisms that consume dead organic material. Decomposers are organisms that break down dead organic material and return nutrients to the sediment. These nutrients are used by the producers during photosynthesis to create energy, thus completing the cycle.