food chain

The complex interrelationships between animals and plants in a food web. A food web shows how different food chains are linked in an ecosystem. Note that the arrows indicate movement of energy through the web. For example, an arrow shows that energy moves from plants to the grasshopper, which eats the plants.

In ecology, a sequence showing the feeding relationships between organisms in a habitat or ecosystem. It shows who eats whom. An organism in one food chain can belong to other food chains. This can be shown in a diagram called a food web.

One of the most important aspects of food is that it provides energy for an organism. So a food chain shows where each organism gets its energy. The arrow in a food chain represents the direction of energy flow. Not all of the energy in all of the organisms at one step of a food chain is available to the organisms later in the chain. In general, fewer organisms are found at each step, or trophic level, of the chain. A pyramid of numbers shows this clearly. Some organisms may be small but very numerous, so population size may not be a good measure of how much of an organism there is in a habitat. Biomass – the total mass of organisms in an area – may be a more useful measure.

Some food chains start with organisms that decompose the remains of dead plants and animals. Decomposers can be fungi and bacteria and they have an important role in a habitat or ecosystem. Nutrients inside the dead organisms are released by the decomposers and are made available to other organisms. The carbon cycle shows how carbon compounds are cycled.

Energy in the form of food is shown to be transferred from autotrophs, or producers, which are principally plants and photosynthetic micro-organisms, to a series of heterotrophs, or consumers. The heterotrophs comprise the herbivores, which feed on the producers; carnivores, which feed on the herbivores; and decomposers, which break down the dead bodies and waste products of all four groups (including their own), ready for recycling.

Consider a food chain starting with grass, then caterpillar, then blue tit, then sparrowhawk. This chain starts, as do most others, with a green plant. The reason for this is that the plant makes high-energy food (carbohydrate) using the energy of sunlight through photosynthesis. The plant is known as a producer, because it produces food. All the other members are consumers, because they consume the food made by the first organism. The second member, in this case a caterpillar, is a herbivore, because it eats plants. The third and later members are carnivores. In reality, however, organisms have varied diets, relying on different kinds of food, so the food chain is an oversimplification. The more complex food web shows a greater variety of relationships, but again emphasizes that energy passes from plants to herbivores to carnivores.

Environmentalists have used the concept of the food chain to show how poisons and other forms of pollution can pass from one animal to another, threatening rare species. For example, the pesticide DDT, which is now banned in the UK, has been found in lethal concentrations in the bodies of animals at the top of the food chain, such as the golden eagle Aquila chrysaetos. In the last organism it may have risen to a level that harms the animal. This is known as bioaccumulation.