Module 4: Measuring Biodiversity
These free OCR A Level Biology Measuring Biodiversity revision notes have been written for specification points 4.2.1(b.i), 4.2.1(c) and 4.2.1(d).
Measuring Biodiversity
Sampling techniques allow for a reliable estimation of biodiversity to be made by collecting data on which species are present in an area and their abundance.
The table below outlines the different sampling methods:
| Sampling Method | Description | Advantage | Disadvantage |
|---|---|---|---|
| Random | Sites are chosen randomly. | Removes bias. | Species may be missed. |
| Opportunistic | Locations are chosen based on availability, accessibility or knowledge. | Produces data quickly. | Inherently biased, making the data highly unreliable. |
| Stratified | The habitat is divided into areas (strata) based on their differences, and each is sampled. | All different areas of a habitat are sampled. | Some areas may be smaller, overrepresenting some species. |
| Systematic | Samples are taken at regular intervals along a measurable abiotic gradient. | Provides paired data: The location and abundance of species sampled is linked with measured abiotic factors. | Species outside of the transect are missed. |
Field equipment:
Several standard techniques are used to collect data on different organisms:
- Pitfall traps: Containers sunk into the ground to catch small ground-dwelling animals.
- Pooters: Suck up small invertebrates into a container without harm.
- Quadrats: Square frames (e.g. 0.5 m × 0.5 m or 1 m × 1 m) used to estimate plant abundance. Quadrats are laid randomly or at intervals along a transect. Species may be counted directly or estimated by percentage cover.
- Sweep nets: Used for catching insects in tall vegetation.
- Transects: Lines across the habitat. A line transect records species touching the line; a belt transect involves placing quadrats along the line.
Measuring Species Richness and Species Evenness
Biodiversity within a habitat depends not only on the number of different species present, but also on how evenly the individuals are distributed among those species.
For example, the table below compares the abundance of 20 species in two habitats:
| Habitat | Species Present | Richness | Evenness | Interpretation |
|---|---|---|---|---|
| Wildflower meadow | 20 species, each with similar abundance. | High | High | High biodiversity: many species with balanced populations. |
| Managed lawn | 20 species, dominated by 2 species. | High | Low | Lower biodiversity: same richness but reduced evenness. |
Simpson’s Index of Diversity
Simpson’s Index of Diversity (D) provides a quantitative measure of biodiversity that takes both species richness and evenness into account.
It can be used for comparing different habitats or monitoring changes over time.
The Formula
D = 1 – ∑ (n / N)²
Where:
- n = number of individuals of a particular species
- N = total number of individuals of all species
- ∑ = the sum of
- D = a value between 0 and 1.
Interpreting D
A high D (closer to 1) means high biodiversity; many species and/or even populations.
A low D (closer to 0) means low biodiversity; few species and/or dominated by one.
Example Calculation
Suppose a habitat has the following species counts:
| Species | Number of Individuals (n) |
|---|---|
| Buttercup | 25 |
| Dandelion | 25 |
| Daisy | 25 |
| Dock | 25 |
| Total (N) | 100 |
Using the formula:
D = 1 − ∑ (n / N)²
For convenience, data is typically processed in the following format:
| Species | n | n/N | (n/N)² |
|---|---|---|---|
| Dandelion | 85 | 0.85 | 0.7225 |
| Buttercup | 5 | 0.05 | 0.0025 |
| Daisy | 5 | 0.05 | 0.0025 |
| Dock | 5 | 0.05 | 0.0025 |
| Sum (∑) | 0.73 | ||
| D = 1 − ∑ | 0.27 |
This low Simpsons Index of Diversity value (D) indicates low species evenness and biodiversity.