OCR A Level Biology A: Populations, interactions and sustainable management (H420) – Revision Notes

These notes condense outcomes 6.3.2(a) to 6.3.2(e), Populations and sustainability, from the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. Read the full study guide for explanations and worked examples, then test yourself with the practice questions.

Other links: the OCR A Level Biology hub, the printable checklist, and the free diagnostics to find weak outcomes. For the biodiversity topics, see threats to biodiversity and conservation.

Key terms

Term	Meaning
Population	All individuals of one species in a habitat at one time
Limiting factor	A variable in short supply that keeps population size below its potential
Carrying capacity	The maximum population size an environment can sustain over a long period
Density-dependent factor	Effect gets stronger as population density rises (food, disease, predation, competition)
Density-independent factor	Effect does not depend on density (severe frost, flood, drought)
Intraspecific competition	Competition between members of the same species
Interspecific competition	Competition between different species
Conservation	Maintaining ecosystems and biodiversity through management, with sustainable human use
Preservation	Protecting an area or species as it is, with human interference minimised or excluded
Sustainable use	Harvesting no faster than the resource is replaced

6.3.2(a) Population size

Change in population = (births + immigration) - (deaths + emigration)

Percentage change = change / original size x 100

Growth phase	What is happening
Lag	Slow rise while individuals adjust
Exponential	Births exceed deaths, numbers multiply by a constant factor per equal time
Stationary	Limiting factors act, size fluctuates around the carrying capacity
Decline	Resources run out in a closed system, deaths exceed births

• Doublings = time / doubling time. Final number = start number x 2 raised to the number of doublings.
• Log10 of cell number against time gives a straight line during exponential growth.
• Final population size is set by the most limiting factor. Improve that factor and the carrying capacity rises until another factor limits.
• Density-dependent factors stabilise a population. Density-independent factors can cause a sudden fall.

Method: describe an S-shaped curve

1. Name the phases in order and say where the curve is steepest.
2. Quote values from the axes, with units.
3. Give the final level and call it the carrying capacity.
4. Explain each change in terms of births, deaths, resources and competition.

Reminder. A population of 600 with 90 births, 10 arrivals, 60 deaths and 25 departures changes by 100 - 85 = +15, which is 2.5%.

6.3.2(b) Interactions between populations

Predator-prey

• Prey peak first. Predator peak follows after a lag.
• More prey leads to more predator food, survival and breeding.
• More predators lead to more prey eaten, so prey fall. Less food then makes predators fall.
• Fewer predators lets prey recover, and the cycle repeats.
• Evidence is a correlation. Other factors (prey food supply, weather, alternative prey, refuges) also affect the cycle.

Method: explain a predator-prey graph

1. Describe both curves, with the lag in years or months.
2. Link each predator change to the prey change before it.
3. Say why the predator curve is delayed (time to find prey, breed and grow).
4. Add one limitation of the data.

Competition

	Intraspecific	Interspecific
Between	Same species	Different species
Needs	Identical	Overlapping
Effect	Helps hold the population near carrying capacity; density-dependent	One species may be reduced or excluded
Plant example	Mean mass per plant falls as sowing density rises	Two species sharing light and water

When density rises, total yield per unit area rises by less than the density does, because each individual gets a smaller share.

6.3.2(c) Conservation and preservation

	Conservation	Preservation
Aim	Maintain and manage	Keep unchanged
Human use	Sustainable use allowed	Restricted or excluded
Example	Coppicing a wood on rotation	A no-take marine zone

Type of reason	Points
Economic	Timber, food, fuels, medicines; genes in wild relatives of crops; ecosystem services; tourism income and jobs
Social	Health and recreation; education; cultural value; livelihoods of local communities
Ethical	Right of species to exist; duty to future generations; stewardship; extinction is irreversible

If a question asks for economic, social and ethical reasons, give one point for each type and link it to the situation in the question.

6.3.2(d) Sustainable management

Timber

• Coppicing: cut near ground level, regrowth from the stump, sections cut in turn.
• Selective felling keeps a canopy. Clear felling removes all trees.
• Replant after felling, ideally with native species.
• Thinning, pest control and leaving dead wood help biodiversity.
• Harvest must not exceed growth. Excess = harvest - growth.

Fishing

Measure	How it protects the stock
Larger net mesh	Young fish escape and breed later
Quota	Catch capped at a sustainable level
Closed season, no-take zone	Protects breeding and spawning
Limits on boats, licences, banned methods	Reduces fishing effort
Stock monitoring	Quotas follow the real stock
Fish farming	Reduces pressure on wild stocks but adds waste and disease issues

Quota = percentage x estimated stock. Shares for fleets are percentages of the quota.

Reminder. A wood grows 4.0 m3 per hectare per year over 90 ha, so growth is 360 m3 per year. A harvest of 400 m3 is 40 m3 too much, which is 11.1% above growth, so the stock will fall.

Method: evaluate a sustainable management plan

1. State the resource and the rate of harvest compared with the rate of replacement.
2. Describe each measure and how it protects breeding or regrowth.
3. Give a benefit and a cost for each group affected.
4. Judge the plan, mentioning the time scale and any data limits.

6.3.2(e) Conservation, preservation and human needs

Human activity	Effect on populations	Control
Habitat destruction	Smaller, isolated populations	Protected areas, planning law
Overharvesting or hunting	Adults removed faster than replaced	Quotas, licences, seasons
Pollution	Harms organisms	Regulation, reduced chemical use
Introduced species	Predation or competition on native species	Trapping, removal, biosecurity
Low numbers of a species	Risk of extinction	Captive breeding, reintroduction

• Zoned reserves combine a preserved core with a managed buffer. Local people gain from jobs, ecotourism and payment for conservation, so they have a reason to protect the area.
• Balance conflicts by naming each group, giving a benefit and cost, and judging over a short and a long time scale.

Must-know distinctions

• Limiting factor and carrying capacity: the factor sets the limit, the carrying capacity is the resulting maximum size.
• Density-dependent and density-independent.
• Conservation (managed, sustainable use) and preservation (left alone).
• Intraspecific and interspecific competition.
• Sustainable harvest and overharvest: compare the harvest with the growth rate.

Quick self-test

1. A population has 600 animals. In a year there are 90 births, 10 immigrants, 60 deaths and 25 emigrants. Calculate the percentage change.
2. A bacterial culture of 2.0 x 10^3 cells doubles every 30 minutes in its exponential phase. Calculate the number of cells after 3 hours.
3. Give two density-dependent factors that limit a population.
4. A predator population peaks after its prey population peaks. Explain the delay.
5. Define carrying capacity.
6. State one difference between intraspecific and interspecific competition.
7. State one difference between conservation and preservation.
8. A 64 ha wood is coppiced on an 8-year rotation, with equal areas cut each year. What area is cut each year?
9. A fish stock is estimated at 25 000 tonnes and the quota is 12% of the stock. Calculate the quota.
10. Explain why a larger net mesh helps maintain a fish stock.

Answers

1. Net change = (90 + 10) - (60 + 25) = +15. 15 / 600 x 100 = 2.5% increase.
2. 3 hours = 6 doublings. 2.0 x 10^3 x 2^6 = 2.0 x 10^3 x 64 = 1.28 x 10^5 cells.
3. Any two from competition for food or space, disease, predation.
4. Predators need time to find and catch the extra prey, breed and grow, so predator numbers rise after the prey peak.
5. The maximum population size that an environment can sustain over a long period, given its resources.
6. Intraspecific is between members of the same species; interspecific is between different species.
7. Conservation allows sustainable human use under management; preservation keeps an area as it is, with human interference minimised or excluded.
8. 64 / 8 = 8 ha each year.
9. 0.12 x 25 000 = 3000 tonnes.
10. Small immature fish pass through the mesh, so they survive to breed and replace the fish that are caught.

Where marks are usually lost

• Describing a graph without values or units from the axes.
• Reversing the order of the predator-prey peaks.
• Explaining a plateau as “no more growth” without naming the limiting factor and the births and deaths.
• Using “conservation” and “preservation” as if they meant the same.
• Giving two economic reasons when the question asks for economic, social and ethical.
• Saying a sustainable harvest is “small” instead of comparing it with the rate of replacement.
• Presenting predation as the only cause of a cycle when the data show a correlation.
• Evaluating a management plan without mentioning a group that gains or loses.

Official syllabus

OCR A Level in Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025, published by OCR. Section 6.3.2, Populations and sustainability.