# Biodiversity and measurement

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u19-biodiversity-and-measurement/

This sub-topic explores the definition of biodiversity across habitat, species and genetic levels, common sampling techniques to survey wild populations, and how to calculate and interpret Simpson's Index of Diversity for comparing sites.

**Prerequisites:** [Basic ecological sampling techniques](https://www.owlsprep.com/study/cie-9700-u18-ecological-sampling/); [Species concepts](https://www.owlsprep.com/study/cie-9700-u19-classification-of-species/)

## Learning objectives

- Define biodiversity across genetic, species and habitat levels
- Compare appropriate sampling methods for different biodiversity research goals
- Calculate and interpret Simpson's Index of Diversity
- Explain why measuring biodiversity is important for ecological research

## Levels of Biodiversity

**Biodiversity** — The variety of living organisms present in an area, measured at three nested levels of biological organization

*Example:* A tropical rainforest has far higher biodiversity than a single-species pine plantation

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.

> **info**
>
> High biodiversity generally corresponds to a more stable, healthy ecosystem, while low biodiversity often signals environmental disturbance or degradation.

**Worked example:** State which level of biodiversity each example describes: (a) Variation in height alleles in a wild wheat population, (b) Number of different species of trees in a single forest, (c) Number of wetland, dune and heath habitats in a coastal reserve

1. Recall the definition of each level of biodiversity:
2. - Genetic biodiversity: variation within a single species
- Species biodiversity: number of different species
- Habitat biodiversity: number of distinct habitat types
3. Match each example to the correct level:
4. (a) Genetic biodiversity: describes allelic variation within wild wheat
(b) Species biodiversity: counts number of tree species
(c) Habitat biodiversity: counts distinct habitat types in the reserve

> **Exam tip:** If asked to define biodiversity, always mention all three levels to get full marks

## Sampling Methods for Biodiversity

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.

**Random sampling** — A sampling method where every location in the study area has an equal chance of being selected, eliminating selection bias

- **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

**Worked example:** An ecologist wants to estimate the overall biodiversity of a large, relatively uniform heathland site. Which sampling method should they use, and why?

1. Identify the research goal: produce an unbiased estimate of overall biodiversity for the entire heathland site, with no gradient to investigate
2. Simple random sampling is the most appropriate method here
3. Random sampling eliminates selection bias, giving the best unbiased estimate of overall diversity for a uniform site. Systematic sampling is unnecessary here, as there is no gradient to measure.

> **Exam tip:** You will often be asked to justify the choice of sampling method, so link the method to the research question

## Simpson's Index of Diversity Calculation

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.

**Worked example:** A student samples 50 individuals from a pond, counting: 24 of species A, 16 of species B, 8 of species C, 2 of species D. Calculate Simpson's Index of Diversity for this pond.

1. Step 1: Calculate total number of individuals $N$
2. $$N = 24 + 16 + 8 + 2 = 50$$
3. Step 2: Calculate $(n/N)^2$ for each species
4. $$\left(\frac{24}{50}\right)^2 = 0.2304, \quad \left(\frac{16}{50}\right)^2 = 0.1024, \quad \left(\frac{8}{50}\right)^2 = 0.0256, \quad \left(\frac{2}{50}\right)^2 = 0.0016$$
5. Step 3: Sum all squared values
6. $$\sum (n/N)^2 = 0.2304 + 0.1024 + 0.0256 + 0.0016 = 0.36$$
7. Step 4: Calculate $D$ using the formula
8. $$D = 1 - 0.36 = 0.64$$
9. Step 5: Interpret the result: A value of 0.64 indicates moderate biodiversity in the pond.

> **tip**
>
> Always show all working for Simpson's Index calculations. Even if you make an arithmetic error, you can still earn most of the marks for correct method.

## Common pitfalls

- **Wrong:** Comparing biodiversity using only species richness
  - Why it fails: Richness ignores the relative abundance of species, so two sites with the same richness can have very different actual diversity
  - Correct: Always use Simpson's Index to compare biodiversity between sites, as it accounts for both richness and evenness
- **Wrong:** Interpreting a low Simpson's D value as high diversity
  - Why it fails: Students often mix up the scale of Simpson's Index, which runs from 0 (low) to 1 (high)
  - Correct: Remember: higher D = higher diversity, values closer to 1 mean more diverse ecosystems
- **Wrong:** Forgetting to square each $(n/N)$ term during calculation
  - Why it fails: This is a common arithmetic error that leads to an incorrect final D value
  - Correct: Check that every term is squared before summing, and show all working to get method marks
- **Wrong:** Using systematic sampling to estimate overall site diversity
  - Why it fails: Systematic sampling is designed to detect gradients, not produce an unbiased estimate of whole-site diversity
  - Correct: Use random sampling for estimating overall diversity, and systematic only for investigating changes along environmental gradients

## Cheatsheet

| Concept | Key Information | Exam Note |
| --- | --- | --- |
| Levels of biodiversity | Genetic = variation within species; Species = number of species; Habitat = number of habitats | Always name all three when asked for a definition |
| Random sampling | Unbiased site selection | Best for estimating overall site diversity |
| Systematic sampling | Fixed-interval samples along a transect | Best for measuring change along environmental gradients |
| Simpson's D formula | $D = 1 - \sum (n/N)^2$ | 0 = low diversity, 1 = high diversity |
| Species richness | Number of different species in an area | Does not account for relative abundance |
| Species evenness | Relative abundance of each species | Required for accurate diversity measurement |

## What's next

Measuring biodiversity is the foundation for all remaining topics in this unit on conservation and classification. Accurate measurement of biodiversity allows ecologists to assess the impact of human activity on ecosystems, monitor ecosystem recovery after disturbance, and prioritize areas for conservation protection. Mastering the sampling and calculation techniques covered here will prepare you to answer common data analysis questions in your final exam, which often require you to compare diversity between sites and interpret your results. Next, you will explore how species are classified, the threats that reduce global biodiversity, and the conservation strategies used to protect vulnerable ecosystems.

- [Classification](https://www.owlsprep.com/study/cie-9700-u19-classification/)
- [Conservation of endangered species](https://www.owlsprep.com/study/cie-9700-u19-conservation-of-endangered-species/)
- [Principles of gene technology](https://www.owlsprep.com/study/cie-9700-u19-principles-of-gene-technology/)

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