In simple terms
A friendly intro before the formal notes — no formulas yet.
Biodiversity
Cambridge 9700 Paper 4 — Biodiversity (18.2). This lesson covers the three levels of biodiversity, endemism, biodiversity hotspots, and the calculation of Simpson's Index of Diversity. It includes worked examples and flashcards to solidify understanding.
- 1
The value of D ranges from 0 to 1.
- 2
A value close to 1 indicates high biodiversity (high species richness and evenness).
- 3
A value close to 0 indicates low biodiversity (the habitat is dominated by one or a few species).
- 4
A higher D value signifies a more stable ecosystem, better able to withstand change.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 18.2.1
Define the terms ecosystem and niche
- 18.2.2
Explain that biodiversity can be assessed at different levels, including: • the number and range of different ecosystems and habitats • the number of species and their relative abundance • the genetic variation within each species
- 18.2.3
Explain the importance of random sampling in determining the biodiversity of an area
- 18.2.4
Describe and use suitable methods to assess the distribution and abundance of organisms in an area, limited to frame quadrats, line transects, belt transects and mark-release-recapture using the Lincoln index (the formula for the Lincoln index will be provided, as shown in the Mathematical requirements)
- 18.2.5
Use Spearman's rank correlation and Pearson's linear correlation to analyse the relationships between two variables, including how biotic and abiotic factors affect the distribution and abundance of species (the formulae for these correlations will be provided, as shown in the Mathematical requirements)
- 18.2.6
Use Simpson's index of diversity (D) to calculate the biodiversity of an area, and state the significance of different values of D (the formula for Simpson's index of diversity will be provided, as shown in the Mathematical requirements)
Explore the concept
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
What is Biodiversity?
At its core, biodiversity refers to the variety of life in all its forms, at all levels of organisation. It's not just about the number of different species, but a much richer, more nuanced concept. The Cambridge syllabus 9700 requires you to understand three distinct levels:
Measuring Biodiversity: Simpson's Index of Diversity
To compare the biodiversity of different habitats, we need a quantitative measure. Simpson's Index of Diversity (D) is a formula used to calculate a measure of diversity that takes into account both species richness and species evenness.
D = 1 - Σ(n/N)²
The value of D ranges from 0 to 1.
A value close to 1 indicates high biodiversity (high species richness and evenness).
A value close to 0 indicates low biodiversity (the habitat is dominated by one or a few species).
A higher D value signifies a more stable ecosystem, better able to withstand change.
Endemism: Uniquely Valuable
Endemism describes a species or group of species that is found exclusively in a particular geographical area and nowhere else in the world. Think of the lemurs of Madagascar or the Komodo dragon found only on a few Indonesian islands. Endemic species are particularly important in conservation because their restricted distribution makes them highly vulnerable to habitat loss, climate change, or other threats within their small range. If their specific habitat is destroyed, the species could go globally extinct.
Examiners love questions linking endemism to conservation. Emphasise that endemic species are disproportionately affected by habitat loss and thus require urgent, targeted conservation efforts to prevent global extinction.
Biodiversity Hotspots: Critical Areas
Biodiversity hotspots are regions of the Earth that are both biologically rich and deeply threatened. They are identified based on two strict criteria:
These hotspots cover less than 2.5% of the Earth's land surface but harbour a disproportionately high percentage of its terrestrial biodiversity. Identifying and protecting these areas is crucial for global conservation efforts, allowing limited resources to be focused where they can have the greatest impact.
Why Does Biodiversity Matter?
Maintaining high levels of biodiversity is fundamental for both human well-being and ecosystem stability:
For Human Well-being:
For Ecosystem Stability:
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
An ecologist sampled two different woodland areas, Woodland P and Woodland Q, to compare their biodiversity. The number of individuals for five tree species was recorded in the table below. Calculate Simpson's Index of Diversity (D) for both woodlands and determine which is more diverse.
| Species | Woodland P | Woodland Q |
|---|---|---|
| Oak | 50 | 15 |
| --- | --- | --- |
| Ash | 10 | 15 |
| Birch | 12 | 15 |
| Sycamore | 8 | 15 |
| Holly | 20 | 40 |
- 1
Step 1: Calculate N (total individuals) for each woodland.
- Woodland P: N = 50 + 10 + 12 + 8 + 20 = 100
- Woodland Q: N = 15 + 15 + 15 + 15 + 40 = 100
Outline three distinct levels of biodiversity. Explain why maintaining high levels of biodiversity is crucial for ecosystem stability.
- 1
Species diversity: The variety of different species within a particular area, encompassing both species richness (number of species) and species evenness (relative abundance of each species).
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What are the three levels of biodiversity?
- Species diversity: Variety of species.
- Genetic diversity: Variety of genes/alleles within a species.
- Ecosystem diversity: Variety of habitats and communities.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
The value of D ranges from 0 to 1.
- ✓
A value close to 1 indicates high biodiversity (high species richness and evenness).
- ✓
A value close to 0 indicates low biodiversity (the habitat is dominated by one or a few species).
- ✓
A higher D value signifies a more stable ecosystem, better able to withstand change.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9700/41 · Q9(b)(i)
A student planned a method to measure the relative abundance of Usnea and Xanthoria on trees along a transect from the town centre at 0 km to unpolluted countryside at 4 km. Suggest why measuring the relative abundance of the two types of lichen gives information that is useful for conservation.
Extra simulations & links
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Frequently asked
Checkpoint
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Before you move on: do 9700/41 · Q9(b)(i) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.