Biodiversity and its measurementEdexcel International A Level Biology: Revision notes

Biodiversity is the variety of living organisms in a habitat, and it can be considered at three levels: the number of different species, the genetic diversity within each species (the variety of alleles in its gene pool) and the variety of habitats and ecosystems.

Over geological time the variety of life has become extensive, but it is now being threatened by human activity. Habitat destruction (for farming, plantations and building), pollution, over-harvesting, climate change and introduced species all reduce the number of species and the genetic diversity within them.

A species that is endemic is found naturally in only one geographical area and nowhere else. Endemic species, such as those on islands and mountains, are very vulnerable: if the habitat is lost, the whole species is lost.

Species richness is the number of different species in a habitat. It is measured by sampling the habitat (for example random quadrats for plants, or pitfall traps for ground invertebrates), identifying every organism and counting the species.

It is simple, but it has a weakness: it ignores abundance. A field with 5 species, one of which makes up 95% of the individuals, has the same richness as a field with 5 species in similar numbers, although the second is clearly more diverse.

Biodiversity within a species is measured by its genetic diversity. One measure is the heterozygosity index:

heterozygosity index = number of heterozygotes ÷ number of individuals in the population

The value lies between 0 and 1. A high value means many individuals carry two different alleles, so the population has high genetic diversity.

Worked example: a population of 60 lizards contains 15 heterozygotes for a gene. Heterozygosity index = 15 ÷ 60 = 0.25. A mainland population with 82 heterozygotes among 200 lizards has an index of 0.41, so the island population is less genetically diverse.

Small, isolated populations (for example after habitat loss) tend to have a lower heterozygosity index because they have fewer alleles and more inbreeding. Lower diversity gives natural selection less variation to act on.

To compare biodiversity in different habitats, use the index of diversity, D, which takes account of both the number of species and their relative abundance:

D = N(N − 1) ÷ Σn(n − 1)

N is the total number of individuals of all species; n is the number of individuals of each species; Σ means 'sum of'.

Worked example: a sample has 3 species with 10, 5 and 1 individuals. N = 16, so N(N − 1) = 16 × 15 = 240. Σn(n − 1) = (10×9) + (5×4) + (1×0) = 110. D = 240 ÷ 110 = 2.2.

A high D means many species with similar numbers of individuals (a diverse habitat). A low D (close to 1) means few species, or one species dominating. Results are only reliable if sampling is random, sufficiently large and repeated, and comparisons need similar sampling effort in each habitat.

• Biodiversity can be species richness (habitat) or genetic diversity (within a species).
• Endemic = found in one place only, so highly vulnerable to habitat loss.
• Heterozygosity index = heterozygotes ÷ individuals.
• D = N(N − 1) ÷ Σn(n − 1); higher D = more diverse.
• Evaluate conclusions: sample size, randomness, repeats and other variables.