Population, community, habitat and ecosystem; biotic and abiotic factorsSpec 5.11, 5.12
In short
A population is all the organisms of one species in a habitat at one time. A community is all the populations of different species living together in a habitat. A habitat is where organisms live, and an ecosystem is the community plus its abiotic environment. Biotic and abiotic factors control the numbers and distribution of organisms.
- Population
- All the organisms of one species living in a habitat at the same time.
- Community
- All the populations of different species living and interacting in a habitat.
- Habitat
- The place where an organism, population or community lives, with its own set of environmental conditions.
- Ecosystem
- A community of organisms together with the non-living (abiotic) components of their environment, and the interactions between them.
For example, in a pond the population of frogs, the population of pondweed and the populations of all the other species make up the community. The pond water, sediment, light, dissolved oxygen and temperature are abiotic parts of the ecosystem.
Biotic and abiotic factors
The numbers and distribution of organisms in a habitat are controlled by both kinds of factor. Distribution describes where organisms are found, and abundance how many there are.
| Biotic factors | Abiotic factors | |
|---|---|---|
| Meaning | Factors caused by living organisms | Non-living physical and chemical factors |
| Examples | Competition (for food, light, space, mates), predation, parasitism and disease, grazing, availability of food | Temperature, light intensity, water availability, pH, salinity, oxygen concentration, mineral ions, wind speed, humidity |
| Effects | Intraspecific competition limits population size; interspecific competition and predation can exclude a species from part of a habitat | Set the range of conditions in which a species can survive and reproduce, for example plants that are not tolerant to salt are limited to areas of low salinity |
Biotic factors are often density-dependent: their effect increases as the population gets larger, for example disease and competition. Abiotic factors such as a severe frost affect a population regardless of its size.
In a data question, say whether the factor is biotic or abiotic, and then link it to the effect on numbers or distribution, for example 'an increase in predators lowers prey numbers' rather than just 'predation'.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026