Populations — Cambridge IGCSE Biology
Populations, communities and ecosystems, the factors that affect population growth, and the phases of the sigmoid growth curve.
Revision notes
2 short notes, in the order of the specification. Each one in short:
A population is a group of organisms of one species, living in the same area, at the same time. A community is all the populations of different species in an ecosystem, and an ecosystem is the community and its environment interacting together. The rate of population growth depends on food supply, competition, predation and disease.
A sigmoid curve is the S-shaped growth curve of a population in an environment with limited resources. It has four phases: a lag phase of slow growth, an exponential phase of rapid growth, a stationary phase where limiting factors make birth rate equal death rate, and a death phase where the death rate is higher and the population falls.
Exam questions
6 exam-style questions (21 marks), each with its mark scheme.
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10 cards: flip them, mark what you knew, and practise the rest.
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The whole of organisms and their environment on one page, so you can see where this subtopic fits.
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Quick check questions
Define a population.
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A group of organisms of one species, living in the same area, at the same time.
Define a community.
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All of the populations of different species in an ecosystem.
State the four factors that affect the rate of population growth.
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Food supply, competition, predation and disease.
Name the four phases of the sigmoid growth curve in order.
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Lag, exponential (log), stationary and death.
Supplement Why does a population level off in the stationary phase?
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Limiting factors mean the birth rate equals the death rate.
Frequently asked questions
What is the difference between a population and a community?
A population is a group of organisms of one species, living in the same area, at the same time, such as all the frogs in a pond. A community is all of the populations of different species in an ecosystem, such as all the plants, animals and microorganisms in that pond.
What are the phases of a population growth curve?
A sigmoid population growth curve has four phases. In the lag phase, growth is slow with few individuals. In the exponential, or log, phase, growth is rapid. In the stationary phase, the population stays roughly constant as the curve levels off. In the death phase, the population falls.
Why does a population stop growing in the stationary phase?
Supplement Population growth stops in the stationary phase because limiting factors, such as food supply, space and the build-up of waste, restrict growth. The birth rate equals the death rate, so the population stays constant. Organisms are still being born and still dying, but the numbers balance each other out.
What factors affect the rate of population growth?
The rate of population growth depends on food supply, competition, predation and disease. More food means more organisms survive and reproduce, so the population grows faster. Competition for limited resources, such as food, water, light and space, slows growth. More predators slow prey population growth, and disease can kill or weaken organisms.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026