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Biodiversity encompasses the variety of life at all levels — from genetic diversity within species, to species diversity within habitats, to ecosystem diversity across landscapes. Classification provides a systematic framework for organising and understanding this diversity. This topic is central to A-Level Biology, connecting ecology, evolution, and conservation in a way that underpins much of the specification.
Key Definition: Biodiversity is the variety of living organisms in an area, encompassing genetic diversity, species diversity, and ecosystem diversity.
Biodiversity can be considered at three levels:
| Level | Description | Example |
|---|---|---|
| Genetic diversity | Variation in alleles within a species | Different alleles for coat colour in mice; different haemoglobin alleles in humans |
| Species diversity | Number and abundance of different species in a community | Tropical rainforests have high species diversity; a monoculture field has low species diversity |
| Ecosystem diversity | Range of different habitats and communities in an area | A region containing woodland, wetland, heathland, and grassland |
Genetic diversity is particularly important because it provides the raw material for natural selection. A population with high genetic diversity is more likely to contain individuals that can survive environmental changes, diseases, or new selection pressures. Genetic diversity can be assessed by looking at the number of different alleles present in a population, or by the proportion of loci that are heterozygous (heterozygosity index).
Exam Tip: In exam questions, be precise about which level of biodiversity is being discussed. "Biodiversity" does not simply mean "number of species" — that is species richness. Simpson's Index measures species diversity, which includes both richness and evenness.
A more informative measure that considers both richness and evenness:
D = 1 − Σ(n/N)²
Where:
Key Definition: Simpson's Index of Diversity (D) is a quantitative measure of biodiversity that takes into account species richness and the relative abundance (evenness) of each species. D = 1 − Σ(n/N)².
Worked Example 1 — Calculating Simpson's Index:
A student surveys plant species in a meadow and records the following data:
| Species | Count (n) | n/N | (n/N)² |
|---|---|---|---|
| Oak | 30 | 0.30 | 0.0900 |
| Beech | 25 | 0.25 | 0.0625 |
| Ash | 20 | 0.20 | 0.0400 |
| Birch | 15 | 0.15 | 0.0225 |
| Hazel | 10 | 0.10 | 0.0100 |
| Total | 100 | Σ = 0.2250 |
D = 1 − 0.2250 = 0.775 — moderately high diversity.
Now compare this with a second site dominated by one species:
| Species | Count (n) | n/N | (n/N)² |
|---|---|---|---|
| Oak | 80 | 0.80 | 0.6400 |
| Beech | 8 | 0.08 | 0.0064 |
| Ash | 5 | 0.05 | 0.0025 |
| Birch | 4 | 0.04 | 0.0016 |
| Hazel | 3 | 0.03 | 0.0009 |
| Total | 100 | Σ = 0.6514 |
D = 1 − 0.6514 = 0.349 — much lower diversity despite the same species richness.
Exam Tip: If the question asks you to "compare the biodiversity" of two habitats, calculate D for each and then state which has the higher value and therefore greater diversity. Always show your working — examiners award method marks even if the final answer is wrong.
Estimating biodiversity requires appropriate sampling techniques. The choice of method depends on the type of organism and the habitat.
Key Definition: A quadrat is a square frame of known area (commonly 0.25 m² or 1 m²) used to sample the abundance and distribution of organisms in a habitat.
Described diagram — Belt transect methodology. The diagram shows a bird's-eye view of a habitat with a clear environmental gradient — for example, a rocky shore running from the upper shore (left) down to the low-tide mark (right). A long tape measure is stretched in a straight line from one end of the gradient to the other, marked in metres. At regular intervals along the tape (e.g., every 2 metres), a quadrat (drawn as a square frame, labelled 0.5 m × 0.5 m) is placed on the ground immediately beside the tape. Inside each quadrat, small symbols represent the organisms recorded — different species are shown in different colours or shapes (e.g., green circles for algae, brown shapes for barnacles, star shapes for limpets). A data table beside the diagram lists the distance along the transect in the first column and the species recorded with their abundance (percentage cover or count) in the remaining columns. Arrows and labels indicate that the transect runs along the environmental gradient, and a caption reads: "Belt transects allow systematic sampling along an environmental gradient, revealing how species distribution and abundance change with distance."
For mobile animals that cannot be counted directly, the mark-release-recapture method is used:
Described diagram — Mark-release-recapture methodology. The diagram is arranged as four sequential panels connected by arrows, illustrating the steps of the technique. Panel 1 — Capture: A net or trap is shown collecting animals (e.g., beetles) from a habitat. A label states "First sample: capture n₁ individuals." The captured animals are drawn inside a container, and a count label reads "n₁ = 40." Panel 2 — Mark: The captured animals are shown being individually marked — a small dot of non-toxic paint is applied to each beetle's shell. A magnified inset shows a single beetle with a clearly visible paint mark on its back. A label emphasises: "Marks must not affect survival, behaviour, or predation risk." Panel 3 — Release and mixing: The marked beetles are released back into the habitat. Dashed arrows show them dispersing and mixing with unmarked individuals in the population. A clock icon and label indicate that sufficient time must pass (e.g., 24–48 hours) for the marked individuals to mix randomly with the rest of the population. Panel 4 — Recapture: A second sample is collected from the same habitat. The captured individuals are shown inside a container — some have paint marks and some do not. Labels indicate "Second sample: n₂ = 50 total captured" and "Marked recaptures: nₘ = 8." Below the four panels, the Lincoln Index formula is displayed: N = (n₁ × n₂) / nₘ, with the worked calculation shown.
The population size is estimated using the Lincoln Index:
N = (n₁ × n₂) / n_m
Where:
Worked Example 2 — Lincoln Index Calculation:
A biologist captures 40 woodlice in a garden, marks them with a small dot of paint, and releases them. Two days later, she captures a second sample of 50 woodlice. Of these, 8 have a paint mark.
N = (40 × 50) / 8 = 2000 / 8 = 250
The estimated population size is 250 woodlice.
Assumptions of the Lincoln Index:
Exam Tip: If a question asks you to evaluate the reliability of a mark-release-recapture estimate, consider whether these assumptions were met. For example, if some marks rubbed off, n_m would be smaller than it should be, giving an overestimate of population size.
Key Definition: An organism's ecological niche describes its role in the ecosystem, including its habitat, its feeding relationships, its interactions with other species, and the abiotic conditions it requires.
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