Why food chains rarely stretch beyond five links
Each step up a food chain loses most of its energy. Generally only about a tenth of what one feeding level holds reaches the next; the rest is burned in the business of staying alive. That steep tax explains why chains seldom run past five levels, and why a salad feeds you more efficiently than lettuce-fed meat.
A food chain is a single line traced through the far messier web of who eats whom. It usually begins with a producer, an organism such as grass or algae that makes its own food, and ends with a top predator like a grizzly bear or killer whale, or with the earthworms, fungi and bacteria that recycle the dead. A classic example runs from a green plant to a snail, a frog, a snake and finally an eagle. Ecologists measure a chain by its length, the number of links from the base to a given consumer.
Most chains ultimately run on sunlight, captured by photosynthesis. Yet some life gets by without it. Around hydrothermal vents and cold seeps, bacteria and archaea draw energy from hydrogen sulfide and methane, building carbohydrates much as plants do and anchoring food chains in near-total darkness.
At the other end, decomposers and detritivores break down every level once it dies, returning simple nutrients to the soil for plants to use again; there are estimated to be more than 100,000 different decomposers. Removing a single link can be devastating, especially a keystone species. On the Pacific coast, sea otters eat sea urchins; without the otters, urchins graze kelp forests bare, and the many species that depend on the kelp suffer.
The idea is old. The 10th-century Arab philosopher al-Jahiz discussed feeding chains, though the modern concepts of food chains and webs were introduced by Charles Elton. Today they help ecotoxicologists follow how pollutants concentrate as they climb from prey to predator, and they give theorists a simple model whose mathematics still turns out to be surprisingly complicated.
Source: Food chain
