Drawing a food web

A food web is a network of interconnected food chains. Place producers (plants) at the bottom. Draw arrows pointing from the organism being eaten to the organism eating it: the arrow shows the direction of energy transfer. Primary consumers (herbivores) feed on producers. Secondary consumers (carnivores) feed on primary consumers. Tertiary consumers feed on secondary consumers. Decomposers break down dead organisms at every level.

Pyramids of numbers and biomass

A pyramid of numbers shows the number of organisms at each trophic level. It can be inverted (e.g. One tree supporting many insects). A pyramid of biomass shows the total dry mass of organisms at each level: this is almost always a true pyramid shape because biomass decreases at each level. Draw the producer level at the bottom as the widest bar, with each higher level narrower.

Energy transfer and loss

Energy enters the ecosystem as light, which producers convert to chemical energy by photosynthesis. At each trophic level, energy is lost: (1) through respiration (as heat), (2) in excretory products (urine, CO₂), (3) in parts not eaten (bones, fur, roots). Typically only about 10% of energy is transferred from one trophic level to the next. This explains why food chains rarely have more than four or five links.

Predicting effects of population changes

If a species is removed from a food web, trace the effect both up and down. If a predator is removed, its prey population increases. If a prey species is removed, its predators decrease and may switch to alternative prey. These knock-on effects can cascade through the web. When answering, always follow the arrows and name the species and the direction of change.