Populations, communities and ecosystems

Organisms and their environment · Populations · note 1 of 2

Spec 19.4.1, 19.4.2, 19.4.3, 19.4.4Download all Populations notes (PDF)Download all notes (PDF)5 pages

Populations, communities and ecosystemsSpec 19.4.1, 19.4.2, 19.4.3, 19.4.4

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.

Population: A group of organisms of one species, living in the same area, at the same time.
Community: All of the populations of different species in an ecosystem.
Ecosystem: A unit containing the community of organisms and their environment, interacting together.

Levels of organisation, using a pond as the example
Level	Meaning	Pond example
Organism	A single living thing	One frog
Population	One species, in the same area, at the same time	All the frogs in the pond
Community	All the populations of different species in the ecosystem	All the plants, animals and microorganisms in the pond
Ecosystem	The community and its environment, interacting together	The pond community plus the water, light and temperature

Nested circles showing an organism (one frog) inside a population (all the frogs) inside a community (all the species in the pond) inside an ecosystem, with arrows for light, water and temperature acting on the ecosystem from outside. — Each level contains the one before it. The ecosystem is the community plus its environment.

Factors affecting the rate of population growth

The rate at which a population grows depends on four factors: food supply, competition, predation and disease.

Factors affecting population growth
Factor	Effect on population growth
Food supply	More food means more organisms survive and reproduce, so the population grows faster. Less food slows growth or makes the population fall.
Competition	Organisms compete for limited resources such as food, water, light and space. The more competition, the slower the growth.
Predation	Predators eat prey, so more predators slow the growth of the prey population.
Disease	Pathogens can kill or weaken organisms, so disease slows growth or reduces numbers.

Common mistake:

A population is one species only. If an answer includes different species living together, it is describing a community. Always include 'at the same time' in the definition of a population.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

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.

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