4.1.1Topic 4 · Biodiversity and Natural Resources
The Variety of Life — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
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4.1.1 The Variety of Life — Edexcel A Level Biology A (SNAB) 9BI0 notes: Define biodiversity at three levels, explain why it matters, and describe how it can be affected by human activity.Biodiversity is the variety of living organisms in an area.
- Habitat (ecosystem) diversity
- Species diversity
- Genetic diversity
- Ecological
- Economic and practical
- Ecosystem services
Section 1 of 9Defining biodiversity
Biodiversity is the variety of living organisms in an area. It is described at three levels:
- A population with high genetic diversity is better able to adapt to environmental change and resist disease.
A habitat is the place where an organism lives; a community is all the populations of different species living together in a habitat; an ecosystem is the community together with its non-living (abiotic) environment.
Section 2 of 9Why biodiversity is important
- Keystone species have a large effect on the community and their loss may affect many other species.
- and anti-cancer drugs from the rosy periwinkle), pollination and genetic resources for breeding crops.
- Biodiversity provides raw material for future discoveries.
Section 3 of 9Threats to biodiversity
| Threat | Effect on biodiversity |
|---|---|
| Habitat loss and fragmentation (agriculture, urbanisation, deforestation) | Reduced area and smaller populations; isolated patches limit gene flow |
| Monoculture farming | Replaces many species with one crop; fewer habitats, uses pesticides |
| Overexploitation (hunting, fishing, collection) | Populations fall faster than they can recover |
| Pollution (pesticides, fertiliser run-off, plastic) | Toxins accumulate; eutrophication reduces oxygen |
| Invasive species | Compete with, prey on or bring disease to native species |
| Climate change | Changes in temperature and rainfall shift ranges and break timing links such as flowering and insect emergence |
Extinction is the permanent loss of a species. The current rate of extinction is estimated to be many times the natural background rate.
Section 4 of 9Agriculture and biodiversity
Modern intensive agriculture can reduce biodiversity by removing hedgerows, draining wetland, applying herbicides and pesticides and planting a single crop variety over large areas.
- such as maintaining field margins and hedgerows, crop rotation, reduced chemical input and planting wildflower strips, can support pollinators and natural pest predators while still producing food.
- which is why seed banks keep wild relatives and traditional varieties as sources of useful alleles.
Section 5 of 9Describing diversity and linking forward
Biodiversity can be described by simple counts, but a better measure also takes into account the abundance of each species, as in an index of diversity (next lessons).
- but one dominated by a single species is less diverse than one in which all species have similar numbers.
- and a high number of different alleles suggests high diversity.
Section 6 of 9Exam focus and links
Command words matter in this topic. 'Define' needs a precise statement of what biodiversity includes. 'Explain' needs a reason, for example why a loss of habitat reduces species richness: smaller area supports smaller populations, which are more likely to go extinct by chance. 'Discuss' requires balanced arguments, for example the benefits and costs of protecting a habitat. When a data question gives numbers of species over time, calculate percentage changes and describe trends, quoting figures from the data.
Connect biodiversity to the concepts in the rest of this unit: it can be measured with an index, compared between habitats with statistical tests, explained by niches and adaptation, and protected through conservation measures both in the wild and outside it. Human population growth and food production are the underlying pressures, so questions on sustainability link them directly.
Section 7 of 9Exam-style questions
Distinguish between species diversity and genetic diversity. [2]2 marks
Species diversity is the number of species and their relative abundance in a community. [1]
Genetic diversity is the variety of alleles within the gene pool of one species. [1]
Explain how the removal of hedgerows to enlarge fields can reduce biodiversity. [3]3 marks
Exam-style questions1. Hedgerows are habitats and food sources, so removing them eliminates places for nesting, shelter and feeding. [1]2. Remaining populations are fragmented and smaller, and isolated, so gene flow is reduced and inbreeding may rise. [1]3. Fewer species survive, so species richness falls; food webs are simplified. [1]Explain why maintaining genetic diversity in crop species is important. [3]3 marks
Exam-style questions1. A wide range of alleles gives a chance that some individuals resist new diseases or pests. [1]2. It provides variation for selective breeding to produce new, improved varieties, for instance for drought tolerance. [1]3. Low genetic diversity can lead to the loss of an entire crop if a disease strikes. [1]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Biodiversity | The variety of living organisms in an area, at habitat, species and genetic levels. |
| Species richness | The number of different species in a community. |
| Gene pool | All the alleles of all the genes in a population. |
| Ecosystem | A community of organisms together with their physical environment. |
| Keystone species | A species with a disproportionately large effect on its community. |
| Habitat fragmentation | Division of a habitat into smaller isolated patches. |
| Extinction | The permanent loss of a species. |
Section 9 of 9Exam technique
Marked practiceQuizzes for this lesson
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