Biodiversity and its measurementEdexcel A-Level Biology A: Revision notes

Over billions of years evolution has produced an extensive variety of life, from bacteria to plants and animals. This variety is called biodiversity. Biodiversity is now being threatened by human activity, so we need ways to measure it and monitor change.

Main human threats include:

• habitat destruction, for example clearing forest for agriculture or building
• fragmentation of habitats into small, isolated patches
• over-exploitation, such as overhunting and overfishing
• pollution and climate change
• the introduction of invasive species

Extinction is permanent, so species, and the alleles they carry, are lost for ever.

Biodiversity can be considered at different levels:

• habitat diversity: the range of different habitats in an area
• species diversity: the number of species and how many individuals there are of each
• genetic diversity: the variety of alleles within the gene pool of a species

A species that is found naturally in only one geographical area, and nowhere else, is endemic to that area. Endemism is the state of being endemic. Endemic species are often especially vulnerable, because if their one habitat is destroyed, the whole species is lost.

Within a habitat, biodiversity can be measured using species richness: the number of different species in the habitat. It is found by sampling the habitat, identifying the organisms and counting the number of species.

Species richness is quick and simple, but it has a limitation: it counts every species equally and takes no account of abundance. A field with five species, of which one makes up 95% of individuals, has the same species richness as a field with five species in equal numbers, yet it is less diverse.

Sampling must be representative (random sampling, repeated, with enough samples), or the count may miss species.

Biodiversity within a species is its genetic diversity: the variety of alleles in the population. A population with many different alleles is better able to adapt to a changing environment. Small, isolated populations tend to lose alleles and become less diverse.

Genetic diversity at a gene locus can be measured using the heterozygosity index:

$H = \frac{\text{number of heterozygotes}}{\text{number of individuals in the population}}$

Worked example. A sample of 40 individuals is screened at one locus and 12 are heterozygous.

$H = 12 \div 40 = 0.30$

H ranges from 0 (no heterozygotes) to 1 (all individuals heterozygous). A higher value means greater genetic diversity at that locus.

To compare biodiversity in different habitats, we need a measure that includes both the number of species and the number of individuals in each. The index of diversity, D, does this:

$D = \frac{N(N-1)}{\sum n(n-1)}$

where $N$ is the total number of individuals of all species and $n$ is the number of individuals of each species.

Worked example. A sample has four species, with 10, 5, 3 and 2 individuals.

• $N = 20$, so $N(N-1) = 20 \times 19 = 380$
• $\sum n(n-1) = 90 + 20 + 6 + 2 = 118$
• $D = 380 \div 118 = 3.2$

The higher the value of D, the greater the diversity. A habitat dominated by one species gives a large $\sum n(n-1)$ and so a low D.

• Species richness is simple but ignores abundance.
• The index of diversity is better for comparing habitats because it includes abundance and evenness.
• The heterozygosity index measures diversity within a species, which species counts cannot show.

A reliable comparison needs large, random samples taken in the same way in each habitat, repeated at different times. A single small sample may not be representative of the habitat.