Pearson Edexcel International GCSE Science Single Award 4: Biology: Ecology and the environment

Ecology explains where organisms live, how feeding transfers substances and energy, and how carbon returns between organisms and the environment. These notes follow only Topic 4 of Pearson Science Single Award 4SS0 Issue 4. Pollution, greenhouse gases and other human-influence points found in Double Award or separate Biology are outside this note.

The specification names a quadrat investigation. This page explains its ecological purpose and interpretation, while the separate practical-skills hub owns the full method, variable control and evaluation framework.

Organisms and their environment

A population is all organisms of one species living in an area at a particular time. A community consists of populations of different species living and interacting in that area. A habitat is the place where an organism lives. An ecosystem includes a community and the non-living environment with which it interacts.

These terms describe different scales. A pond is a habitat for a water beetle, all water beetles in the pond form a population, the beetles and all other populations form a community, and the community together with water, light, temperature and dissolved substances forms the pond ecosystem.

An organism's population size and distribution depend on abiotic factors, such as temperature, light intensity, water availability and pH, and biotic factors, such as food, competition, predation and disease. A factor can change both where organisms occur and how many survive there. However, a correlation does not by itself prove causation because several factors may change together.

Using quadrats

A quadrat samples organisms in a known area. It is suited to plants or slow-moving organisms, not fast animals that enter and leave while counting. To compare two areas, use the same quadrat size, a consistent counting rule and enough replicates in each area. Random coordinates can reduce selection bias.

For an estimate of abundance, calculate mean number per quadrat and scale by the ratio between total habitat area and quadrat area. For percentage cover, estimate how much of the quadrat is occupied. The result is an estimate, so reliability depends on sample size, representative positioning and patchiness.

If Area A has more daisies and also receives more light, the evidence shows an association. A causal explanation might be that greater light raises photosynthesis, but soil moisture, trampling or competition could also differ. A strong conclusion distinguishes the measured pattern from the proposed mechanism.

Feeding relationships and trophic levels

Producers make organic material using an external energy source, usually light. Primary consumers feed on producers. Secondary consumers feed on primary consumers, and tertiary consumers feed at a later level.