Living organisms may form populations of single species, communities of…

The idea

Living organisms do not live alone. A population is all the organisms of one species in a habitat. A community is all the populations of different species living together in a habitat. An ecosystem is the community plus the non-living conditions of its environment, such as light, temperature, water and soil. Organisms interact with each other in many ways, including competition for resources, predation and mutualism. They also interact with their environment, for example plants take in carbon dioxide and water for photosynthesis, while animals depend on plants for food and oxygen. Humans interact with ecosystems by farming, fishing, polluting, conserving habitats and introducing species. These interactions can affect population sizes and the balance of a community, so changes to one part of an ecosystem can have knock-on effects on other parts.

What earns the marks

• A population is all the organisms of one species living in a particular habitat.
• A community is all the populations of different species living together in a habitat.
• An ecosystem includes the community of organisms and the non-living conditions of their environment.
• Organisms interact with each other through competition, predation and mutualism, which can affect population sizes.
• Organisms interact with their environment, for example plants exchange gases with the air and take in mineral ions from the soil.
• Humans interact with ecosystems in many ways, including farming, pollution, conservation and the introduction of new species.
• Changes in one part of an ecosystem can affect other organisms because the community and environment are linked.

Common mistakes and how to put them right

• Using population and community as if they mean the same thing.

  Put it right by stating that a population is one species, while a community includes all species in the habitat.

• Defining an ecosystem as only the living organisms.

  Put it right by including the non-living conditions, such as temperature, light and soil.

• Assuming that all interactions between organisms are harmful.

  Put it right by giving examples of mutualism, where both species benefit, as well as competition and predation.

Exam tips

• Define each ecological term precisely before using it in an explanation, especially population, community and ecosystem.
• When describing an interaction, name both organisms and state the effect on each, for example predators reduce prey numbers while gaining food.
• Use a named local example, such as a pond or woodland, to show how organisms, the environment and humans interact.