Each level describes variation at a different scale: genetic biodiversity refers to allelic variation within a single species, species biodiversity describes the variety of species in an area, and habitat biodiversity counts the number of distinct habitats or ecosystems in a larger region.
Exam tip: If asked to define biodiversity, always mention all three levels to get full marks
2. Sampling Methods for Biodiversity★★☆☆☆⏱ 7 min
It is almost always impractical to count every individual organism in a large study area, so ecologists use sampling to estimate biodiversity from a smaller representative subset. The choice of sampling method depends on the research question being asked.
**Systematic sampling**: Samples are taken at fixed intervals along a transect, used to investigate how biodiversity changes along an environmental gradient (e.g. altitude, distance from pollution)
**Stratified sampling**: The study area is divided into distinct habitat strata, and samples are taken proportionally from each to avoid under-sampling small habitats
Opportunistic sampling: Sampling easily accessible areas, rarely used for formal measurements due to severe bias
Exam tip: You will often be asked to justify the choice of sampling method, so link the method to the research question
3. Simpson's Index of Diversity Calculation★★★☆☆⏱ 8 min
Simpson's Index of Diversity ($D$) is the standard quantitative measure of biodiversity used in CIE A-Level Biology. Unlike species richness alone, it accounts for both richness (number of species) and evenness (relative abundance of each species), giving a more accurate measure of overall diversity.
D = 1 -
Left( \sum \left( \frac{n}{N} \right)^2 \right)
Where $n$ = total number of individuals of a single species, $N$ = total number of individuals of all species in the sample, and the sum is calculated across all species. $D$ ranges from 0 to 1, with values closer to 1 meaning higher diversity.
Common Pitfalls
Why: Richness ignores the relative abundance of species, so two sites with the same richness can have very different actual diversity
Why: Students often mix up the scale of Simpson's Index, which runs from 0 (low) to 1 (high)
Why: This is a common arithmetic error that leads to an incorrect final D value
Why: Systematic sampling is designed to detect gradients, not produce an unbiased estimate of whole-site diversity