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

This guide covers learning outcomes 6.3.2(a) to 6.3.2(e), Populations and sustainability, in the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. It sits in Module 6, Genetics, evolution and ecosystems. The specification says that ecosystems may need careful management for economic, social and ethical reasons, and that biological resources must be used sustainably.

Use the OCR A Level Biology hub and the printable checklist. After this guide, use the revision notes and the practice questions. The free diagnostics show your gaps. For biodiversity, see threats to biodiversity and conservation and measuring biodiversity.

What the specification asks for

Outcome	You must be able to
6.3.2(a)	Explain the factors that determine population size, including limiting factors, carrying capacity and the effect of these on final population size
6.3.2(b)	Describe interactions between populations: predator-prey relationships (effects on both populations), and interspecific and intraspecific competition
6.3.2(c)	Explain the reasons for, and differences between, conservation and preservation, including economic, social and ethical reasons for conserving biological resources
6.3.2(d)	Explain how managing an ecosystem can provide resources sustainably, limited to timber production and fishing
6.3.2(e)	Explain how ecosystems can be managed to balance conservation or preservation against human needs, and the effects of human activities on animal and plant populations and how these are controlled

The specification links (a) to its mathematical skills: estimating results, using powers and exponentials, logarithmic scales (for example a microorganism’s growth) and moving between graphs, numbers and equations.

6.3.2(a) What determines population size

A population is all the individuals of one species in a habitat at one time. Its size changes through four processes: births and immigration add individuals; deaths and emigration remove them.

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

Worked example 1. A reserve holds 840 deer. In one year there are 126 births, 18 animals arrive, 74 deaths and 40 animals leave.

Net change = (126 + 18) - (74 + 40) = +30
Percentage = 30 / 840 x 100 = 3.6 %      New size = 870
Birth rate = 126 / 840 x 100 = 15.0 %    Death rate = 74 / 840 x 100 = 8.8 %

Limiting factors and carrying capacity

A limiting factor is a variable that keeps a population below the size it could reach if the factor were not in short supply. The carrying capacity is the maximum population size that an environment can sustain over a long period, given its resources. It is set by whichever factor is most limiting.

Type	Examples	How it limits
Abiotic	Light, temperature, water, mineral ions, pH, oxygen, space	Sets the rate of growth, survival and reproduction
Biotic	Food supply, predation, disease and parasites, competition	Raises deaths or lowers births

Factors are also described by how their effect changes with population size. Density-dependent factors act more strongly as the population gets denser: competition for food, disease spread, predation. They stabilise a population around its carrying capacity. Density-independent factors affect a population whatever its density, for example a severe frost, drought or flood, and can cause a sudden fall below the carrying capacity.

Population growth curve

A population entering a new habitat usually shows these phases:

1. Lag phase. Numbers rise slowly while individuals adjust.
2. Exponential (log) phase. Resources are plentiful, births exceed deaths and numbers multiply by a constant factor in each equal interval.
3. Stationary phase. Limiting factors act, so births plus immigration equal deaths plus emigration. Size fluctuates around the carrying capacity.
4. Decline phase (in a closed culture). Resources run out, so deaths exceed births.

On a graph with a logarithmic y-axis, exponential growth is a straight line, because the logarithm turns repeated multiplication into repeated addition. This is why microbiology growth curves are often plotted as log10 of cell number.

Worked example 2. A yeast culture starts with 5.0 x 10^5 cells per cm3 and then doubles every 2 hours during its exponential phase. Find the number after 10 hours and the log10 values.

Doublings = 10 / 2 = 5
N = 5.0 x 10^5 x 2^5 = 1.6 x 10^7 cells per cm3
log10: 5.70 at the start, 7.20 after 10 h (0.30 per doubling)

Worked example 3. A small mammal was released on a predator-free island. Population counts each year were 20, 34, 58, 95, 140, 178, 196, 200, 201 (years 0 to 8).

Yearly increase: 14, 24, 37, 45, 38, 18, 4, 1
Greatest increase: 45 between year 3 (95) and year 4 (140)
Percentage growth then: 45 / 95 x 100 = 47.4 %
Last interval: 1 / 200 x 100 = 0.5 %

The curve is S-shaped. Growth is fastest at about half the final size and levels off near 200 animals, the carrying capacity. Explain the slowing with density-dependent limiting factors: food and nest sites become scarce, competition rises, births fall and deaths rise. Improving the most limiting factor (more food, say) raises the carrying capacity until another factor limits.

6.3.2(b) Interactions between populations

Predator-prey relationships

• Abundant prey feed more predators, which survive and breed, so predator numbers rise after the prey peak.
• A larger predator population eats more prey, and prey decline.
• Predators then go short of food and decline. Surviving prey breed again and the cycle restarts.

Worked example 4. Yearly estimates for a lemming population (per hectare) and its Arctic fox predators (per 100 square kilometres) are in the table.

Year	1	2	3	4	5	6	7	8	9
Lemmings	120	310	520	300	90	140	350	560	330
Arctic foxes	3	4	9	15	8	3	5	10	16

Prey peaks: years 3 and 8, so one cycle = 5 years
Predator peaks: years 4 and 9, a lag of 1 year
Prey fall, year 3 to 5: (90 - 520) / 520 x 100 = -82.7 %
Predator fall, year 4 to 6: (3 - 15) / 15 x 100 = -80.0 %

The predator curve follows the prey curve with a delay because predators need time to find prey, breed and grow. Describe the pattern as a lag plus a repeated rise and fall. Be cautious about cause: a correlation does not prove that predators alone control the prey. Prey also depend on their own food supply and weather, predators may switch to other prey, and prey may find refuges. Experiments that exclude predators or add food test the cause directly.

Competition

Intraspecific competition is between members of the same species. They have identical needs. It is density-dependent and helps hold a population near its carrying capacity.

Interspecific competition is between different species for the same limited resource. If one species is better adapted to the shared conditions, it can reduce the population of the other or exclude it from the habitat. The grey squirrel has largely replaced the red squirrel across much of England.

Worked example 5. Seedlings of one plant species were grown at three densities. Mean dry mass per plant was 8.0 g at 10 plants per m2, 5.2 g at 40 per m2 and 2.5 g at 160 per m2.

Total yield per m2: 10 x 8.0 = 80 g, 40 x 5.2 = 208 g, 160 x 2.5 = 400 g
Mass per plant, 10 to 160 plants: (2.5 - 8.0) / 8.0 x 100 = -68.8 %
Density rose 16 times but total yield rose 5 times (400 / 80)

Each plant gets less light, water and ions as density rises. That is intraspecific competition.

6.3.2(c) Conservation and preservation

Conservation is the maintenance of ecosystems, species and biodiversity through management, which allows sustainable human use and can involve active intervention. Examples are coppicing a woodland on a rotation, controlling an invasive species, or managing grazing on a meadow.

Preservation is the protection of an area or species to keep it as it is, with human interference minimised or excluded. Examples are a strict wilderness area with no extraction, or a no-take marine zone.

	Conservation	Preservation
Aim	Maintain and manage, with sustainable use	Keep unchanged
Human activity	Allowed within limits	Restricted or excluded

Reasons for conserving biological resources

• Economic. Wild species supply timber, food, fuels and medicines. Wild relatives hold genes useful for breeding crops. Ecosystems provide services such as pollination and flood control. Tourism creates jobs and income.
• Social. Green spaces support health, recreation and education, and many places and species have cultural value. Communities depend on resources, so conservation protects livelihoods.
• Ethical. Species may have a right to exist whatever their use to humans. People have a duty to future generations and to act as stewards. Extinction cannot be reversed.

6.3.2(d) Sustainable management: timber and fishing

Sustainable use means harvesting no faster than the resource is replaced.

Timber production

• Coppicing. Stems are cut near ground level and regrow from the stump. Cutting sections in turn gives a regular supply and habitats at different stages of growth.
• Selective felling removes some trees and keeps a canopy. Clear felling removes all trees from an area.
• Replanting after felling, ideally with native species.
• Thinning, pest control and leaving some dead wood for decomposers and insects.

Worked example 6. A 240 ha woodland is divided into 12 equal coupons and coppiced on a 12-year rotation. Wood grows at 6.5 m3 per hectare per year.

Cut each year = 240 / 12 = 20 ha
Annual growth = 6.5 x 240 = 1560 m3
Harvest of 2000 m3 per year: excess = 440 m3, which is 28.2 % above growth

The harvest exceeds growth, so the stock falls every year. A sustainable plan keeps the harvest at or below 1560 m3.

Fishing

Fish stocks renew only if enough adults survive to breed. Methods:

• Net mesh size. A larger mesh lets small, immature fish escape and breed.
• Quotas set a maximum catch of a species. Closed seasons and no-take zones protect breeding and spawning.
• Limits on fishing effort: licences, fewer vessels, banned methods.
• Monitoring stocks by sampling, so quotas follow the stock.
• Fish farming (aquaculture) reduces pressure on wild stocks but brings waste and disease problems.

Worked example 7. A stock is estimated at 48 000 tonnes. The quota is 15% of the stock, shared between three fleets in the ratio 50%, 30% and 20%. A small-mesh net sample had 84 of 200 fish below breeding size; a large-mesh net had 18 of 150.

Quota   = 0.15 x 48 000 = 7200 tonnes
Fleets  = 3600, 2160 and 1440 tonnes
Small mesh: 84 / 200 x 100 = 42 %    Large mesh: 18 / 150 x 100 = 12 %

6.3.2(e) Balancing conservation and human needs

People need food, fuel, housing and income. Conflict arises because those needs can reduce populations of wild species. Management tries to balance them.

Effects of human activities on populations

• Habitat destruction and fragmentation shrink populations and isolate them.
• Overharvesting and hunting remove animals, or cut and collected plants, faster than they are replaced.
• Pollution, such as fertiliser run-off or pesticides, harms organisms.
• Introduced species can prey on or out-compete native ones.

How these are controlled

• Laws and licences (hunting seasons, protected species).
• Zoned protected areas: a preserved core and a managed buffer where limited use is allowed. A 400 km2 reserve with 30% as no-take has 120 km2 fully protected.
• Captive breeding and reintroduction.
• Controlling invasive species, for example trapping.
• Involving local people, paying them for conservation, and ecotourism, so that they gain from protecting the area.

When evaluating a proposal, name the groups affected, give a benefit and a cost for each, use any data supplied, and reach a judgement that mentions time scale. Short-term loss of income can give long-term gain if stocks recover.

Common errors

• Treating carrying capacity as a fixed number. Numbers fluctuate around it.
• Giving the wrong peak order in a predator-prey cycle. The prey peak comes first.
• Treating conservation and preservation as the same. Say what human use each allows.

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.