What biodiversity means and how it is measured revision notes

Edexcel International GCSE Biology (4BI1), chapter 14: Ecosystems, specification point 4.3B, 4.4B. Free lesson.

Watch the video lesson · IGCSE Biology past papers

Biodiversity is the number of different species living in an area, not the total number of organisms. You can compare biodiversity by counting species in randomly placed quadrats, and greater biodiversity usually makes an ecosystem more stable.

What biodiversity counts

Biodiversity means the number of different species living in an area. It counts kinds of organisms, not how many individuals there are. Two fields could each contain 200 plants but have very different biodiversity if one has 2 species and the other has 18. A field with 10,000 wheat plants and no other species has high plant abundance but very low biodiversity. A native woodland or coral reef usually has many different species. A field growing one crop is a monoculture, meaning an area dominated by a single crop species.

Why biodiversity makes an ecosystem more stable

A community with many species is generally more stable. If one species disappears, another may be able to take over some of its role, so the community can continue. In a community with only two or three species, losing one can cause the community to collapse. Low biodiversity therefore makes an ecosystem fragile. High biodiversity: many species and greater stability. Low biodiversity: few species and greater risk if one is lost.

How to compare biodiversity using quadrats

To compare two areas, such as a mown lawn and an uncut area, record the number of different species in each quadrat. This differs from a survey that counts individuals. Random placement matters because choosing places that look interesting could give a biased result.

What a quadrat comparison can show

An uncut area almost always has a higher mean number of species than a mown area. Mowing removes taller plants before they can flower and set seed, leaving species that grow flat. The comparison is only useful if quadrats are placed randomly, not in places selected because they look species-rich. A higher mean number of species per quadrat indicates higher biodiversity in the area surveyed.

Why replacing a hedgerow with wheat reduces biodiversity

A mixed hedgerow and its field margins contain many species. Replacing them with one wheat variety can increase the number of crop plants, but it reduces the number of species. The resulting monoculture is fragile: a pest or disease that attacks wheat could destroy the whole field because there are no other species to take over. Compare the number of species before and after the change, not the total number of plants.