A food web is all of the interactions between the species within a community that involve the transfer of energy through consumption. A food web incorporates different food chains within an environment. These types of interactions occur between producer and consumer, and between predator and prey.

The transfer of energy starts with plants. Plants are able to convert sunlight into a chemical form of energy through the process of photosynthesis. One of the end products of photosynthesis is glucose, a sugar in which energy is stored. Plants are called producers because they produce their own energy without consuming another organism.

Animals are called consumers; they must consume plants and other animals to obtain energy. Animals that feed only on plants are called herbivores, or primary consumers since they eat producers. Animals that feed on other animals are called carnivores. They are called secondary consumers if they eat primary consumers. Another kind of secondary consumer is an omnivore, which is an animal that feeds on both plants and other animals. Any animal that hunts and feeds on other animals is called a predator, and the animals that predators feed on are called prey. All carnivores are predators, and herbivores—and sometimes omnivores or other carnivores—are their prey. 

Energy moves up a food chain, starting with plants, then moving up through herbivores, and then carnivores. A fourth group of organisms called decomposers breaks down the organic matter left behind by these other organisms, like dead animals or plants. Organisms within each group gain energy by feeding on the organisms in the group below them on the food chain. The top predator of a food chain is the carnivore within the community that does not have any predators of its own. When the top predator dies, their remains are broken down by decomposers—bacteria and fungi. Decomposers return nutrients back to soil to help restart the food chain.

An example of a marine food chain starts with seaweed as the producer, which is then fed on by zooplankton. From there, crustaceans will consume the zooplankton, and larger fish will consume the crustaceans. At this point, the fish will be eaten by squids, and finally, the squids will be devoured by sharks. Sharks are a top predator with no predators of their own, so they are at the top of the food chain.

The amount of available energy decreases as it moves up the food chain. This is because only plants are able to produce energy in the first place, and at each step of the food chain energy is lost in the form of heat as the organism maintains their body and performs daily functions.  

Since a food web is representative of an entire community, it will consist of multiple food chains that are intertwined. There are usually multiple predator species that feed on a particular prey species, and one predator species usually feeds on multiple prey species. The same goes for the relationship between plants and herbivores.

A food web describes the many species and interactions within an ecosystem. Mapping these interactions can show us how an entire ecosystem could fall apart if it loses even one species.