Biology · Introductory biology · Worked example

Assign trophic levels in a food web

In a meadow, grasshoppers and mice eat grass, mice also eat grasshoppers, sparrows eat grasshoppers, snakes eat mice, and hawks eat mice, sparrows and snakes. Assign each species its trophic level or levels, and find the longest food chain.

Start with the producer

Grass makes its own food by photosynthesis: trophic level 1.

Place the grasshopper and the mouse

The grasshopper eats only grass, so it is a primary consumer at level 2. The mouse eats grass, which puts it at level 2, and grasshoppers, which puts it at level 3: it is an omnivore feeding at two levels.

Place the sparrow and the snake

The sparrow eats grasshoppers, so it feeds at level 3. The snake eats mice, which sit at level 2 or 3, so the snake feeds at level 3 or 4.

Place the hawk

The hawk eats mice, which puts it at level 3 or 4, sparrows (level 4) and snakes (level 4 or 5). It feeds at levels 3, 4 and 5.

Find the longest chain

Grass → grasshopper → mouse → snake → hawk has five levels. At 10% efficiency per step, the hawk’s share along that chain is about 0.01% of the energy the grass captured.

Result

Grass is level 1 and the grasshopper level 2; the mouse feeds at levels 2 and 3, the sparrow at 3, the snake at 3 and 4, and the hawk at 3, 4 and 5. The longest chain has five levels.