food web, a complex network of interconnecting and overlapping food chains showing feeding relationships within a community. A food chain shows how matter and energy from food are transferred from one organism to another, whereas a food web illustrates how food chains intertwine in an ecosystem. Food webs also demonstrate that most organisms consume or are consumed by more than one species, which food chains often do not show.

Structure

All food webs, except those centred deep within caves or near hydrothermal vents on the ocean floor, are powered by the Sun. Organisms within food webs are divided into two main categories: producers (also called autotrophs), which make their own food, and consumers (also called heterotrophs), which depend on producers or other consumers for nourishment.

In general, food energy in an ecosystem can be thought of as being structured like a pyramid, with energy moving upward, and each level in this energy pyramid corresponds to a trophic level (or feeding level) within the ecosystem. Producers form the pyramid’s base; plants are the most recognizable producers, but algae, phytoplankton, and other organisms are also included in this category. Most producers use photosynthesis to create food for other organisms. An oak tree is an example of a producer: it produces leaves that are eaten by insects and birds and acorns that are consumed by squirrels and other mammals.

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community ecology: Food chains and food webs

Primary consumers, which form the pyramid’s second level, are herbivores (such as leaf-eating insects) that dine on producers; however, omnivores (animals that can eat both plants and other animals), such as opossums or raccoons, might also qualify as primary consumers if they feed exclusively on plant material. Secondary consumers, which make up the third level, are carnivores or omnivores (such as snakes, spiders, and small predatory fishes) that prey on primary consumers, whereas tertiary consumers are very often large carnivores (such as wolves, large felines, birds of prey, and sharks and other large predatory fishes) that prey on secondary consumers (see also apex predator).

Other important members of food webs include detritivores and decomposers, whose activities remove dead material from the ecosystem, converting it to basic materials that can be used by producers again. Detritivores are scavengers (such as vultures or beetles) whose diet largely consists of the remains of dead organisms. Decomposers (such as fungi and bacteria) break down organic materials into basic organic and inorganic compounds made up of nitrogen, carbon, calcium, phosphorus, and other chemical elements, which plants and other producers use for growth.

Food web interactions

Although depictions of food webs often show direct single-line paths of consumption from producers to consumers on various trophic levels like food chains do, they can also show the ways in which some organisms diverge from these patterns. For example, larger carnivores and omnivores whose diets are not limited to a few types of animals may also eat primary consumers if given the opportunity. In addition, many organisms within a food web may be part of several food chains within that ecosystem. For example, squirrels eat a variety of foods, including nuts, fruits, seeds, fungi, and insects. Similarly, squirrels are prey for not only foxes but also hawks, owls, and other predators.