AQA A Level Biology复习笔记7.4.1 Communities

The Definition of a Community


  • Species do not exist by themselves in their own isolated environment, they interact with other species forming communities
  • A community can be defined as:

Multiple populations (of different species) living and interacting in the same area

  • For example, a garden pond community is made up of populations of frogs, newts, pond snails, damselflies and dragonflies and their larvea, water beetles, water boatmen, pond skaters and many other species all living and interacting within the pond habitat


  • Communities interact with the non-living components of the environment they live in, forming ecosystems
  • An ecosystem can be defined as:

The interaction between a community (the living, biotic part) and the non-living (abiotic) parts of the environment

  • An ecosystem is a relatively self-contained community of interacting organisms and the environment they live in, and interact with
  • There is a flow of energy within an ecosystem and the nutrients within it are recycled (e.g. the carbon, nitrogen and phosphorus cycles)
  • There are both living (biotic) components and non-living (abiotic) components within an ecosystem
  • Ecosystems vary greatly in size and scale
    • Both a small pond in a back garden and the open ocean could be described as ecosystems
    • A human being could also be described as an ecosystem (there are thousands of species of bacteria living on and in every person)


  • Ecosystems vary in complexity:
    • A desert is a relatively simple ecosystem
    • A tropical rainforest is a very complex ecosystem


  • No ecosystem is completely self-contained as organisms from one ecosystem are often linked to organisms from another
    • For example, many birds species are able to migrate long distances to find food sources or breeding locations from various ecosystems


Example of an ecosystem

  • A forest is a perfect example of a complex ecosystem
  • There is a large community of organisms including trees, birds, small and large mammals, insects and fungi
  • The non-living components of the ecosystem include the soil, dead leaves, water from the rain and streams, the rocks and any other physical or chemical factors
  • The non-living components of the ecosystem influence the community of organisms




Levels of organisation in an ecosystem