Habitats, food webs and human impact

Reading a food web

Arrows show the direction that energy travels — always from the eaten to the eater. Grass $\to$ rabbit means the rabbit eats the grass. Drawing the arrow the other way round is the single most common mistake, and it reverses the whole meaning.

Producers and consumers

Producers (green plants) make their own food by photosynthesis. Primary consumers eat producers; secondary and tertiary consumers eat other consumers. Decomposers break down dead material and return nutrients to the soil.

What happens when one species falls

Removing one species affects the whole web, not just its neighbours. If foxes disappear, rabbits increase, so grass decreases, so other grass-eaters fall. A full answer traces the chain at least two steps.

Human impact

Deforestation, pollution and overfishing all reduce biodiversity. Naming the impact is worth little; explaining the mechanism — “fewer trees means less habitat, so fewer species can live there” — is what earns marks.