OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(a) the factors that determine size of a population
Full specification wording
Populations and sustainability: (a) the factors that determine size of a population
Population size is the number of individuals of one species in a defined area.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 1 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Population size is the number of individuals of one species in a defined area. It is set by births and immigration adding individuals and by deaths and emigration removing them, so change in population = (births + immigration) − (deaths + emigration). These rates are themselves controlled by density-dependent factors, whose effect grows as numbers rise (competition for food, space or light; predation; disease; accumulation of toxic waste), and density-independent factors, whose effect does not depend on numbers (temperature extremes, drought, fire, pollution events). Carrying capacity is the maximum population an ecosystem can sustain; when numbers exceed it, mortality rises and the population falls back. Reproductive potential, generation time and the availability of resources also shape how quickly a population grows.
What earns the marks
- Identifies births and immigration as additions and deaths and emigration as losses to population size.
- Distinguishes density-dependent factors (competition, predation, disease) from density-independent factors (temperature, drought, fire).
- Explains carrying capacity as the maximum sustainable population set by available resources.
- Links resource availability and reproductive potential to the rate at which a population can grow.
- Relates population factors to sustainability, for example keeping harvesting within carrying capacity.
Common mistakes and how to put them right
Treating emigration as an addition to a population.
Put it right: Emigration removes individuals, immigration adds them.
Assuming all limiting factors depend on density.
Put it right: Some, such as drought or fire, act regardless of population size.
Confusing carrying capacity with unlimited growth.
Put it right: Carrying capacity is the ceiling set by resources, beyond which mortality increases.
Exam tips
- Write the population change equation explicitly before substituting any figures.
- Classify each named factor as density-dependent or density-independent and say why.
- Use the term carrying capacity precisely when explaining why a population levels off.
Try the lesson
The start of the Populations and sustainability: (a) the factors that determine size of a population lesson, made from checked teaching notes for this specification point.
Have a go first
Which of these is a density-independent factor that could reduce a population of insects?
Listen
Population size is the number of individuals of one species in a defined area. It is set by births and immigration adding individuals and by deaths and emigration removing them, so change in population = (births + immigration) − (deaths + emigration). These rates are themselves controlled by density-dependent factors, whose effect grows as numbers rise (competition for food, space or light; predation; disease; accumulation of toxic waste), and density-independent factors, whose effect does not depend on numbers (temperature extremes, drought, fire, pollution events). Carrying capacity is the maximum population an ecosystem can sustain; when numbers exceed it, mortality rises and the population falls back. Reproductive potential, generation time and the availability of resources also shape how quickly a population grows. A strong answer earns marks for these points. Identifies births and immigration as additions and deaths and emigration as losses to population size. Distinguishes density-dependent factors (competition, predation, disease) from density-independent factors (temperature, drought, fire). Explains carrying capacity as the maximum sustainable population set by available resources. Links resource availability and reproductive potential to the rate at which a population can grow. A common slip: Treating emigration as an addition to a population. Instead, emigration removes individuals, immigration adds them. Exam tip: Write the population change equation explicitly before substituting any figures.
See it
Key words
Say what each word means before you turn the card.
Births, deaths, immigration and emigration
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Carrying capacity and resource limits
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Reproductive potential and sustainability
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See every definition
- Births, deaths, immigration and emigration
- Births and immigration add individuals to a population, while deaths and emigration remove them, so change in population = (births + immigration) − (deaths + emigration).
- Carrying capacity and resource limits
- The maximum population an ecosystem can sustain; when numbers exceed it, mortality rises and the population falls back.
- Reproductive potential and sustainability
- Reproductive potential, generation time and the availability of resources shape how quickly a population grows, and population factors relate to sustainability, for example keeping harvesting within carrying capacity.
Label the diagram
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1Box 1
2Box 2
3Box 3
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