food chain

A food chain is the path that energy follows through an ecosystem, one organism eating another. It always starts with a producer — a green plant or an alga that makes its own food from sunlight through photosynthesis — and continues with consumers: a herbivore that eats the plant, then a predator that eats the herbivore, and often a second predator above that. Decomposers such as bacteria and fungi finish the job, breaking down dead material and returning nutrients to the soil. A food web is simply many food chains woven together in the same habitat.

The arrows in a food chain show the direction the energy flows, from the organism being eaten to the one that eats it, and the Sun sits at the start of every one as the original source of that energy. Energy shrinks at each step: most of it is used up as the animal moves, grows and gives off heat, so only a small fraction is passed along. That is why food chains are short — there is not enough energy left to support many levels of predators.

Questions here usually ask you to name the producer, to put organisms in order, or to identify the Sun as the ultimate energy source.