OCR A Level Biology A: Threats to biodiversity and conservation (H420) – Revision Notes

These revision notes condense outcomes 4.2.1(f) to 4.2.1(i) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. The study guide teaches each point with worked examples. When you can recall everything below, test yourself with the practice questions, tick off outcomes on the OCR Biology checklist, and see the hub for other topics. The free diagnostics help you pick what to revise next.

Key terms

Term	Meaning
Biodiversity	The variety and complexity of life
Monoculture	Growing one crop species over a large area, usually year after year
Keystone species	A species with an effect on its community far greater than its abundance suggests
Genetic resource	The alleles held in wild species, rare breeds and varieties, available for future use
In situ	Conservation in the natural habitat
Ex situ	Conservation away from the natural habitat
Captive breeding	Breeding endangered animals under managed conditions, usually in zoos
CITES	Convention on International Trade in Endangered Species
CBD	Rio Convention on Biological Diversity
CSS	Countryside Stewardship Scheme

4.2.1(f) Factors affecting biodiversity

Factor	Main effects on biodiversity
Human population growth	More land converted to farms, housing and roads; habitat loss and fragmentation; over-harvesting; pollution; more carbon dioxide released
Agriculture (monoculture)	Habitat replaced by one species; hedgerows and margins removed; pesticides kill non-target insects; fertiliser causes eutrophication; low genetic variation in the crop; soil mineral depletion
Climate change	Range shifts to poles or higher altitude; mismatched timing between species; coral bleaching; loss of ice and coastal habitats; local extinction where species cannot move

Method: describing and explaining data

1. Say what changes, with figures: start value, end value and direction.
2. Calculate a percentage change if you can: (new - old)/old x 100.
3. Give a rate if time is given: change divided by years.
4. Link to a biological cause, naming habitat, food supply or breeding.
5. Add a caution: correlation does not prove cause.

Worked reminder

A reef has 62% coral cover. Ten years later it has 41%. Percentage change = (41 - 62)/62 x 100 = -33.9%, which is a fall of 21 percentage points. Do not mix up percentage points with percent.

Another reminder: a region’s cropland rises from 2.4 to 3.0 million hectares over 20 years. Change = 0.6 million ha, which is +25.0%, or 0.03 million ha (30,000 ha) per year.

4.2.1(g) Reasons for maintaining biodiversity

Type	Points to make	Example
Ecological	Protect keystone species because organisms are interdependent; keep a genetic resource	Sea otters keep urchin numbers low so kelp survives; wild crop relatives carry disease-resistance alleles
Economic	Reduce soil depletion from continuous monoculture; ecosystem services; medicines; tourism	Rotating crops and keeping soil organisms cuts fertiliser costs; foxglove gave digoxin
Aesthetic	Protect landscapes people value for beauty and wellbeing	Meadows, heathland, coastlines

Must-know distinction

A keystone species is defined by its effect, not by its numbers or size. A species that is simply rare is not necessarily keystone.

4.2.1(h) In situ and ex situ methods

Method	Type	One advantage	One limitation
Marine conservation zone	In situ	Whole habitat and food web protected	Difficult to police
Wildlife reserve	In situ	Natural behaviour and interactions kept	Needs space; may be too small
Seed bank	Ex situ	Many species stored cheaply	Some seeds cannot be dried and frozen; seeds must be tested and regrown
Botanic garden	Ex situ	Research, education and propagation	Few individuals, so low genetic variation
Zoo	Ex situ	Safe breeding programmes	Small populations lose genetic diversity; animals may not cope when released

Method: evaluating a conservation method

1. Name the method and state in situ or ex situ.
2. Give two benefits linked to the species described.
3. Give two limitations, for example cost, space, genetic diversity, habitat not protected.
4. Finish with a judgement, such as using ex situ as a back-up for in situ work.

Worked reminder

A seed sample of 150 seeds gives 126 germinating seedlings. Viability = 126/150 x 100 = 84%. To obtain 100 seedlings: 100/0.84 = 119.05, so sow 120 seeds (round up).

4.2.1(i) Agreements

Agreement	Level	Key points
CITES	International	Regulates trade in listed species and products; the most threatened are listed so that commercial trade is prohibited; others need permits; countries must enforce it
CBD (Rio)	International	Conserve biodiversity; use its components sustainably; share benefits of genetic resources fairly; countries write their own plans
CSS	Local and national	Government pays farmers to manage land for wildlife, such as hedgerows and margins

Limits to remember: CITES covers trade only, not habitat loss; illegal trade continues where enforcement is weak; all three depend on funding and on people taking part.

Linking threats, reasons and responses

The specification says actions must be taken at local, national and global levels. A strong answer joins a threat to a reason and then to a response at a suitable level.

Threat	Why it matters	Response	Level
Hedgerow removal and monoculture	Loss of habitat and insect food; soil depleted	Countryside Stewardship Scheme payments	Local and national
Over-collection and trade in a rare species	Population falls; genetic resource lost	CITES listing and permits	International
Habitat destruction in a species-rich region	Keystone species and genetic resource lost	Wildlife reserve; CBD national plans	National and international
Overfishing	Food webs disrupted; fish stocks fall	Marine conservation zone	National
Plant species with a very small wild population	Alleles may be lost for ever	Seed bank and botanic garden as back-up	Ex situ
Rising temperature	Ranges shift; timing of food supply changes	Reduce carbon dioxide; protect corridors of habitat	Global and national

Mismatch and range-shift reminders

Two data patterns turn up often, so practise describing them.

• Timing mismatch. One species responds more to warming than another it depends on. Example: a food peak moves 6.3 days earlier per degree Celsius while the consumer moves 1.5 days earlier. The gap widens by about 4.8 days per degree, so the consumer misses the food peak.
• Range shift. A species is found 150 m higher after 40 years. That is 150/40 = 3.75 m per year. A species on a mountain top has nowhere higher to go, so its population shrinks or is lost locally.

Command words in this topic

Command word	In this topic
Describe	State what the data or method shows, using figures
Explain	Give the biological reason, linking cause to effect
Suggest	Apply knowledge to an unfamiliar case; give a sensible reason
Evaluate	Weigh benefits against limitations and reach a judgement
Calculate	Show the working, give units and round sensibly

Short answer frames

• “Explain why monoculture reduces biodiversity”: the habitat is a single plant species, so there is less food and shelter, fewer species survive, and pesticides and removed hedgerows reduce insects further.
• “Explain why genetic resource matters”: wild species hold alleles that crops lack, such as resistance to a pathogen, and once the wild species is lost the alleles cannot be recovered.
• “Evaluate a zoo breeding programme”: benefits are protection and managed breeding; limitations are small population size, loss of variation, cost and the difficulty of release.

Quick self-test

1. Give two ways monoculture reduces biodiversity.
2. A forest falls from 800 to 620 hectares. Calculate the percentage change.
3. Explain why a warmer spring can harm a bird that feeds its chicks on caterpillars.
4. What is a keystone species?
5. Give one economic reason for maintaining biodiversity.
6. Distinguish between in situ and ex situ conservation.
7. Give one advantage and one limitation of a seed bank.
8. A sample of 250 seeds has 190 germinating. Calculate the percentage viability.
9. Why can CITES not protect a species that is threatened only by land clearance?
10. Name the scheme that pays farmers in England to manage land for wildlife.

Answers

1. Habitat replaced by one species; fewer food sources; hedgerows removed; pesticides kill insects; fertiliser run-off (any two).
2. (620 - 800)/800 x 100 = -22.5%.
3. Caterpillars hatch earlier, so the food peak comes before the chicks need it; fewer chicks are fed and survive.
4. A species whose effect on its community is far greater than its abundance suggests, such that removing it changes many other species.
5. Continuous monoculture depletes soil, so varied crops reduce fertiliser cost; or pollination, medicines, tourism.
6. In situ is in the natural habitat; ex situ is outside it, for example in a zoo or seed bank.
7. Advantage: many species stored in little space. Limitation: some seeds cannot be dried and frozen, or the habitat is not protected.
8. 190/250 x 100 = 76%.
9. CITES regulates international trade only, so it does not address habitat destruction.
10. The Countryside Stewardship Scheme.

Where marks are usually lost

• Giving a percentage without the sign, or reporting a change in percentage points as a percentage.
• Saying “habitat destruction” without saying which habitat or what the species lose.
• Describing a factor but not linking it to biodiversity or to a species.
• Writing about zoos only as places to see animals, and omitting captive breeding and genetic diversity.
• Treating ex situ as better than in situ in every case.
• Mixing up CITES (trade) with the CBD (conservation and sustainable use).
• Claiming that correlation shows cause.
• Not rounding seed numbers up when calculating how many to sow.

Official syllabus

OCR A Level Biology A (H420), Version 4.1 (April 2026), for first assessment in 2025, published by OCR. These notes cover section 4.2.1(f) to 4.2.1(i).