Food chains, trophic levels and food webs

Feeding links can be joined up. Activity 12.6 takes a grassland with grass, frog, hare, grasshopper, snake and eagle, and asks you to draw arrows for who eats whom.

Read a chain like a story. One is short: grass is eaten by the hare, and the hare by the leopard. Another is longer:

Grass → Grasshopper → Frog → Snake → Eagle

The arrow points from the food to the one that eats it.

A straight line like this is a food chain: a simple sequence showing who eats whom in an ecosystem.

Count the numbers (Activity 12.7). In a crop field of millets, mice and eagles, count each kind and stack the numbers with the largest at the base and the smallest at the top. The shape is a pyramid: lots of millet plants, fewer mice, and fewer eagles still.

Each organism in a food chain has a position called its trophic level (troph = food):

But nature isn't made of separate straight lines. Activity 12.8 asks you to add the missing arrows to a diagram of grasses, grasshopper, bird, frog, snake, hawk, fox, hare, mouse and owl. A mouse isn't eaten only by a snake: an owl or a fox may eat it too.

So the food chains of an ecosystem are interlinked, because each organism may be eaten by two or more kinds of organism. Together the chains form a network called a food web.