12.4 Food Chains & Webs

Producers, consumers and decomposers are linked by who eats whom. Drawing those links reveals the structure that holds an ecosystem together.

The Food Chain A food chain is a simple line showing “who eats whom” in an ecosystem, with each arrow pointing from the eaten to the eater (the direction food energy flows). For example, in a grassland:

\text{Grass} \rightarrow \text{Grasshopper} \rightarrow \text{Frog} \rightarrow \text{Snake} \rightarrow \text{Eagle}

It always starts with a producer.

Trophic Levels Each position in a food chain is a trophic level (troph = food):

  1. Producers (green plants) — first level.
  2. Herbivores (hare, deer) — second level.
  3. Small carnivores (frog) — third level.
  4. Large carnivores (tiger, eagle, vulture) — next level.

If you count the numbers of organisms at each level, you usually get a pyramid: many plants at the base, fewer herbivores, fewer carnivores still at the top.

The Food Web In reality, no organism eats just one thing, and most are eaten by several. So food chains cross and interlink into a network called a food web. A single grasshopper might be eaten by a frog, a bird and a mouse — and each of those by others. The food web is what makes an ecosystem resilient, but also means a change to one species touches many.

Nature wastes nothing

Closing the Loop After the plants and animals of a food web die, decomposers (fungi like the rainy-season mushrooms on dead logs, and bacteria) break them down, and tiny insects like beetles and flies break down dung (even elephant dung!). This decomposition returns nutrients to the soil, where producers take them up again to start a new chain. So the food web is really a loop — nutrients cycle round and round, and nothing is truly wasted.

Science Talk Look at the chain Grass → Grasshopper → Frog → Snake → Eagle. If frogs vanished, what would happen to the grasshoppers (the frogs’ food) and to the snakes (which ate the frogs)? Talk it through — then notice how a single missing link disturbs both directions of the chain. That’s the idea behind the next topic.