GCSE Biology: Ecology and Ecosystems Guide

Food chains and webs

A food chain shows the flow of energy from producers (which make their own food) through a series of consumers. A food web shows how multiple food chains interconnect within an ecosystem.

Energy transfer

Energy is lost at each stage of a food chain — through movement, heat and waste — which is why food chains rarely have more than four or five trophic levels, and why there are usually far fewer top predators than producers.

Population and competition

Organisms in an ecosystem compete for limited resources such as food, water, light and space, and are also interdependent — the loss of one species can affect others throughout the food web.

Human impact

Human activity affects ecosystems through deforestation, pollution and climate change, which can reduce biodiversity by destroying habitats and disrupting food chains.

Common mistakes

• Drawing food chain arrows the wrong way — they should point in the direction energy flows (towards the consumer).
• Forgetting energy is lost, not "used up completely," at each stage.
• Assuming removing one species only affects that species, rather than the wider food web.