Ecosystems and interdependence
Biology · Key Stage 3 · The School
Chains and webs
A FOOD CHAIN shows one route energy takes: producer, primary consumer, secondary consumer. Real ecosystems are FOOD WEBS, because most organisms eat and are eaten by several others. Webs are more useful precisely because they show the alternatives available when one link changes.
Energy runs out going up
Only about ten per cent of the energy at one level reaches the next. The rest is used in movement and respiration or lost as heat, and some parts are never eaten. That is why food chains are rarely more than four or five links long, and why there are far fewer top predators than there are plants beneath them.
Pull one thread
Remove a species and the effect travels both ways. Removing a predator can let its prey increase until it strips the vegetation, which then harms everything depending on that plant. Removing a plant starves whatever ate it. Because a web has alternative routes, the effect is not always disastrous — which is exactly why you have to trace it rather than assume.
10,000 kJ of light energy reaches a field of grass. About 1,000 kJ ends up in the grass as new growth. Rabbits eating it store about 100 kJ. A fox eating the rabbits stores about 10 kJ. Each level passes on roughly a tenth, because most of the energy went into movement, warmth and waste. That is why there is no fifth level — there would be about 1 kJ left, not enough to keep anything alive.