Food Chains and Food Webs

A food chain is a single line: grass gets eaten by a rabbit, the rabbit gets eaten by a fox. Real ecosystems never run on just one line. Rabbits also eat clover, foxes also eat mice, hawks eat both rabbits and mice. Draw every one of those feeding relationships on the same diagram and the separate chains merge into a food web, a tangle of arrows showing who eats whom across the whole community. Each arrow points from the organism being eaten toward the organism eating it, because that is the direction the energy is actually traveling, not the direction of the attack.

That tangle of connections is exactly why a food web reacts in places you would not expect. Species are rarely linked to just one other species, so a change can travel sideways through predator-prey links that have nothing to do with the original organism. Hunt out sea otters along a Pacific coast and nothing touches the kelp directly, but otters eat sea urchins, urchins eat kelp. Remove the otters and urchin populations explode, the urchins graze the kelp forest down to bare rock, and species that sheltered in the kelp lose their habitat. The damage traveled through two indirect links to reach a part of the ecosystem the otters never touched.

A food web is the map of who eats whom; an energy pyramid is what happens to the energy as it moves along those same arrows. Every step up a chain embedded in the web, only about ten percent of the energy stored in one organism's tissue gets converted into the tissue of whatever eats it next, the rest is lost as heat or used for the eater's own living. So the food web shows the paths energy can take, and the energy pyramid shows why those paths support fewer and fewer organisms the higher you climb.