How Is Species Diversity Investigated?

Species diversity is investigated by sampling the species in a habitat and recording how many individuals of each species are present. Quadrats are useful for plants and other organisms that do not move much; transects show how a community changes across an environmental gradient.

Using quadrats

A quadrat is a frame that marks out a fixed area. Place quadrats at randomly chosen coordinates, identify the species inside each one and record their abundance (the number of individuals). Repeat this across the habitat so the results are more representative. You can then calculate the mean abundance of each species per quadrat.

Using transects

A transect is a line laid across a habitat, such as from a shaded area into an open field. Place quadrats at set intervals along it and record the species and their abundance at each point. This systematic approach helps you see whether the community changes with conditions along the line. Unlike quadrats placed at random coordinates, a transect is chosen to investigate a pattern across the habitat.

Comparing diversity

Species richness is the number of different species present. Diversity also takes account of how evenly individuals are distributed between those species. For AQA Biology, you may calculate the index of diversity:

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

Here, $N$ is the total number of individuals of all species, and $n$ is the number of individuals of each species. The $n(n-1)$ values are added for all species. A higher index indicates greater diversity. For example, samples with counts of $5$ and $5$, or $9$ and $1$, both have two species, but the more even $5$-and-$5$ sample has the higher index.

Exam mistake to avoid: counting species alone gives species richness, not the full picture of species diversity. Keep quadrat size and sampling effort the same when comparing habitats.