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

This guide covers learning 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. It sits in Module 4, Biodiversity, evolution and disease, which the specification lists as content for component 02 (Biological diversity) and component 03 (Unified biology). The measuring side of biodiversity (sampling, richness, evenness, Simpson’s Index and genetic diversity) is outcomes (a) to (e) and is not repeated here.

Use the OCR A Level Biology hub to see every topic and the printable checklist to tick off outcomes. When you have read this guide, move to the revision notes and the practice questions. The free diagnostics show where your gaps are.

What the specification asks for

Outcome	You must be able to
4.2.1(f)	Explain the factors affecting biodiversity, to include human population growth, agriculture (monoculture) and climate change
4.2.1(g)	Give the ecological, economic and aesthetic reasons for maintaining biodiversity
4.2.1(h)	Describe in situ methods (marine conservation zones, wildlife reserves) and ex situ methods (seed banks, botanic gardens, zoos)
4.2.1(i)	Describe international and local agreements, including CITES, the Rio Convention on Biological Diversity (CBD) and the Countryside Stewardship Scheme (CSS)

The specification states that actions to maintain biodiversity must be taken at local, national and global levels. That sentence is a useful frame for the whole topic: each agreement and method works at one of those levels.

4.2.1(f) Factors affecting biodiversity

Biodiversity is the variety and complexity of life. Most of the threats you need are linked to human activity, and a good answer always connects the human cause to a change in habitat, resources or survival.

Human population growth

More people need more food, housing, water, fuel and materials. The effects on other species are indirect and add together.

• Land is converted from forest, wetland and grassland to farmland, roads and settlements. This destroys or fragments habitats, so populations become small and isolated.
• Extraction of resources (timber, fish, water) can exceed the rate at which populations recover.
• Waste and pollution, including fertiliser run-off and plastics, damage ecosystems.
• Higher demand for energy releases more carbon dioxide, which links to climate change.

Worked example 1. A country had 38 million people and 14.2 million hectares of forest. Forty years later it has 52 million people and 11.0 million hectares of forest. Describe the change in forest per person.

Population change = (52 - 38)/38 x 100 = +36.8 %
Forest change     = (11.0 - 14.2)/14.2 x 100 = -22.5 %
Forest per person, start = 14.2/38 = 0.374 ha
Forest per person, end   = 11.0/52 = 0.212 ha
Change per person = (0.2115 - 0.3737)/0.3737 x 100 = -43.4 %

Forest area fell by about a fifth while population rose by over a third, so forest per person fell by about 43%. The data show a link between more people and less habitat, but they do not on their own prove that population growth caused all the forest loss. Other factors, such as the type of farming, also matter.

Agriculture and monoculture

Monoculture means growing one crop species over a large area, often every year. For biodiversity it has several effects.

• Natural habitat is replaced by a single plant species, so few other species find food or shelter.
• Hedgerows, field margins and ponds are often removed to make fields larger.
• Pesticides kill non-target insects, and the loss of insects reduces food for birds and other animals.
• Fertiliser run-off causes eutrophication in nearby water, which can kill aquatic animals.
• The crop has little genetic variation, so one pest or disease can spread through all of it.
• The same crop takes the same minerals from the soil each year, so the soil is depleted unless fertiliser is added.

Worked example 2. A farming district had 4,820 km of hedgerow. Thirty years later 3,615 km remain. Calculate the percentage change and the mean loss per year.

Change = 3,615 - 4,820 = -1,205 km
Percentage change = -1,205/4,820 x 100 = -25.0 %
Mean loss per year = 1,205/30 = 40.2 km per year

Interpreting data like this is part of the specification’s guidance. A table of six farms shows the kind of pattern you may be asked to describe.

Farm	Field margin (% of area)	Bird species recorded
1	0	14
2	1	15
3	2	19
4	4	22
5	6	26
6	8	27

Farms 1 to 3 have a mean of 16 species and farms 4 to 6 have a mean of 25 species, which is 9 more, or 56% higher than 16. The positive correlation suggests that margins support more bird species. It does not prove cause: farms with more margin may also use less pesticide, so a second variable could be responsible.

Climate change

Rising mean temperatures and changing rainfall alter where species can live.

• Species may shift their range towards the poles or to higher altitude. Species that cannot move, or that run out of land (a mountain summit, an island), may become extinct locally.
• Seasonal events change. If one species responds to warmth more than another, a food supply may no longer match a consumer’s needs.
• Warmer, more acidic seas damage coral reefs through bleaching, which removes the habitat for many fish.
• Melting ice and sea-level rise remove habitats for polar and coastal species.

Worked example 3. A woodland bird feeds its chicks on caterpillars. Over three years the data are:

Year	Mean spring temperature (°C)	Day of peak caterpillars	Day of peak chick food demand
1	6.1	58	56
2	7.4	49	54
3	8.8	41	52

Gap (demand - caterpillar peak) = -2, +5, +11 days (positive = food peak earlier than demand)
Caterpillar peak moves (58 - 41)/(8.8 - 6.1) = 6.3 days earlier per °C
Chick demand moves (56 - 52)/(8.8 - 6.1) = 1.5 days earlier per °C

Caterpillars respond about four times as strongly to warmth as the birds do. By year 3 the peak of food falls well before the chicks need it, so fewer chicks are fed and the bird population may decline. This is a mismatch in timing, and it is a good example of why interdependent species are affected differently.

4.2.1(g) Reasons for maintaining biodiversity

The specification groups the reasons as ecological, economic and aesthetic.

Ecological.

• Keystone species have an effect on their community far greater than their abundance suggests. Sea otters eat sea urchins, which graze kelp. Where otters are lost, urchins increase and kelp forests collapse, along with the fish that shelter in them. This shows interdependence of organisms: removing one species changes many others.
• Genetic resource. Wild species and wild relatives of crops carry alleles that crops lack, such as resistance to a disease. If the wild species is lost, the alleles are lost for ever. Breeds and varieties with different alleles also give a population the variation it needs to adapt to change.

Economic.

• Continuous monoculture depletes soil minerals. Keeping varied crops, rotations and natural soil organisms reduces this loss and lowers fertiliser costs.
• Diverse ecosystems provide pollination, pest control and water purification. Wild species have also given medicines, such as digoxin from foxglove.
• Natural areas bring income through tourism.

Aesthetic. People value wildlife and landscapes for their beauty, for recreation and for wellbeing. Protecting landscapes such as heathland and meadows keeps these benefits for future generations. Aesthetic value is hard to give a money value, and that is why it needs its own argument.

4.2.1(h) In situ and ex situ conservation

In situ conservation protects species in their natural habitat. Ex situ conservation keeps them outside it.

Method	Type	How it works	Advantages	Limitations
Marine conservation zones	In situ	Areas of sea where damaging activity such as some fishing is restricted	Protect whole habitats and food webs; fish stocks can recover and spill into nearby waters	Hard to police a large sea area; need continued funding
Wildlife reserves	In situ	Land protected from development and managed for wildlife	Species keep natural behaviour and interactions; habitat protected	Needs space; poaching and invasive species still possible; reserve may be too small
Seed banks	Ex situ	Seeds dried and stored at low temperature	Many species in little space; protects genetic diversity cheaply	Some seeds do not survive drying; stored seeds must be tested and regrown; habitat not protected
Botanic gardens	Ex situ	Living plants grown, studied and propagated	Allow research and public education; plants can be reintroduced	Limited numbers, so low genetic variation; plants may adapt to garden conditions
Zoos	Ex situ	Captive breeding of endangered animals	Protected from predators and poachers; breeding managed to keep variation	Small populations lose genetic diversity; costly; animals may lose survival skills before release

Worked example 4. A seed bank tests 200 seeds from a wild plant. 172 germinate. How many seeds must it sow to get at least 50 seedlings?

Viability = 172/200 x 100 = 86 %
Seeds needed = 50/0.86 = 58.1, so sow at least 59

Always round up because you cannot sow part of a seed. Seed banks regularly repeat such tests: a fall in germination shows that the stored seeds are ageing and need regrowing to make fresh stock.

4.2.1(i) International and local agreements

Agreement	Level	What it does
CITES, the Convention on International Trade in Endangered Species	International	Governments agree to regulate or ban international trade in listed species and their products. The most threatened species are in a list where commercial trade is prohibited; others need permits.
Rio Convention on Biological Diversity (CBD), agreed at the 1992 Earth Summit	International	Sets goals for conserving biodiversity, using its components sustainably and sharing fairly the benefits of genetic resources. Countries draw up their own plans.
Countryside Stewardship Scheme (CSS)	Local and national	A government scheme in England that pays farmers and land managers to manage land to benefit wildlife, for example by keeping hedgerows and field margins.

Strengths and limits. Agreements work only if countries pass laws and enforce them. CITES covers trade, not habitat loss, so it cannot protect a species threatened mainly by land clearance. Illegal trade continues where enforcement is weak. The CBD depends on national action. The CSS works because it gives farmers a reason to protect wildlife, but it is voluntary and the payments must be high enough to compete with farming income.

Common errors

• Writing “monoculture kills plants”. Say that it removes habitats and food sources and increases the risk from pests.
• Writing that in situ means “in the wild” only. It means in the natural habitat, which includes a protected reserve.
• Treating seed banks as a replacement for habitat protection. They store genetic diversity but not ecosystems.
• Stating that a correlation proves cause.
• Forgetting to link a reason to a level: local (CSS), national (reserve law), global (CITES, CBD).

Official syllabus

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