Food Chains and Webs

About this topic

A food chain shows the transfer of energy from one organism to the next: producers (plants making food by photosynthesis) are eaten by primary consumers (herbivores), which are eaten by secondary and tertiary consumers (carnivores). Arrows point in the direction of energy flow. Decomposers break down dead remains at every stage.

Real ecosystems are rarely simple lines. Most animals eat several foods and are eaten by several predators, so the chains link together into a food web. A web is more stable than a single chain because if one food source disappears, consumers can switch to another. Removing a species can still cause knock-on effects, such as a rise in its prey or a fall in its competitors.

A grassland example is grass, then grasshopper, then frog, then snake, then hawk. If the frog population collapsed through disease, grasshoppers would increase and eat more grass, while snakes would have to hunt other prey. Ecologists use webs to predict the effects of pollution, hunting or introduced species.

A frequent error is drawing arrows from the eater to the eaten. The arrow means energy goes to, so it points from the grass to the grasshopper. Also remember that the first link is always a producer, and that every organism, including top predators, eventually feeds decomposers.