4.2.1 Biodiversity: the levels of biodiversity (habitat, species and genetic); how to sample plants and animals (random sampling, quadrats, transects and mark-release-recapture); the calculation and interpretation of Simpson's index of diversity; and the ecological, economic and aesthetic reasons for maintaining biodiversity.

What this dot point is asking

OCR wants you to define the levels of biodiversity, describe how to sample plants and animals (random sampling, quadrats, transects and mark-release-recapture), calculate and interpret Simpson's index of diversity, and explain the ecological, economic and aesthetic reasons for maintaining biodiversity.

The answer

Levels of biodiversity

Biodiversity is the variety of living organisms, measured at three levels:

• Habitat diversity: the range of different habitats in an area (for example woodland, meadow and pond on one site).
• Species diversity: the number of different species (species richness) and how evenly the individuals are spread between them (species evenness).
• Genetic diversity: the variety of alleles within the gene pool of a species.

High biodiversity generally indicates a stable ecosystem that can better resist environmental change.

Sampling methods

You cannot count every organism, so you sample and scale up. To avoid bias the sample must be random and large enough to be representative.

• Random sampling for non-motile organisms: lay out a grid with tape measures, choose coordinates with a random number generator, and place a quadrat at each.
• Quadrats: frame quadrats give density (count per area) or percentage cover (estimated by eye or with a point quadrat); take many and find a mean.
• Transects: a line (line transect) or series of quadrats (belt transect) used where the habitat changes across a gradient (for example up a shore), to study how distribution changes with an abiotic factor. This is systematic sampling, used deliberately where there is a gradient.
• Mark-release-recapture for motile animals: capture, mark and release a sample, then recapture later. Population $\approx \dfrac{(\text{first sample}) \times (\text{second sample})}{\text{number marked in the second sample}}$ (the Lincoln index). It assumes no births, deaths or migration, mixing of the marked individuals, and that the mark does not harm or make them more visible.

Simpson's index of diversity

OCR uses Simpson's index of diversity, $D$:

where $n$ is the number of individuals of each species and $N$ is the total number of all individuals. (An equivalent form is $D = 1 - \dfrac{\sum n(n-1)}{N(N-1)}$.) The value ranges from 0 to 1: a value closer to 1 means higher diversity (more species, more even), and a value closer to 0 means lower diversity (few species, or one dominant species). Higher diversity tends to mean a more stable ecosystem.

Why maintain biodiversity

• Ecological: diverse ecosystems are more stable and resilient; many species are interdependent (food webs, pollination), and keystone species support whole communities.
• Economic: wild species are sources of food, medicines, materials and crop genes; genetic diversity in crop wild relatives helps breed disease- and climate-resistant varieties; ecotourism brings income.
• Aesthetic and ethical: natural variety has intrinsic value and improves human wellbeing; many argue we have a duty to conserve it for future generations.

Examples in context

Example 1. Monoculture farming. A field of a single crop has very low species diversity (D near 0), so it is vulnerable to a pest or disease wiping out the whole crop, whereas a diverse hedgerow is far more stable.

Example 2. Crop wild relatives. Wild relatives of wheat and rice carry alleles for disease resistance and drought tolerance; conserving this genetic diversity lets breeders develop resilient varieties, a clear economic reason for maintaining biodiversity.

Try this

Q1. Explain why a large number of quadrats should be used when sampling. [2 marks]

• Cue. It reduces the effect of chance and makes the sample more representative, so the mean estimate is more reliable.

Q2. A community has Simpson's index of diversity of 0.15. Comment on its biodiversity. [2 marks]

• Cue. 0.15 is close to 0, indicating low diversity (few species or one dominant species), so the community is likely to be less stable and more vulnerable to environmental change.

Q3. State one economic reason for maintaining biodiversity. [1 mark]

• Cue. Wild species provide food, medicines, materials or useful genes for crop breeding (or income from ecotourism).