Communities (AQA GCSE Biology): Revision Note

Levels of organisation in an ecosystem

• There are four levels of organisation within an ecosystem:

  • Individual organism: a single member of a species

  • Population: a number of individuals of the same species living in the same area at the same time

  • Community: all the populations of all the different species (all of the organisms) living and interacting in the same habitat

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

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Competition & adaptation

• If organisms in a community need the same limited resource in order to survive and reproduce, they will compete with each other

• Examples of resources that may be limited include:

  • food

  • mates

  • light (plants)

• Competition can be between:

  • members of the same species

  • members of different species

• When different species compete for the same resource, the better-adapted species may outcompete the less well-adapted species

  • In this situation the outcompeted species may go locally extinct, or be forced to move to another habitat

• Adaptations are features that aid survival and reproduction in a particular environment; a species may be better adapted if it is, e.g.:

  • more effectively camouflaged

  • more aggressive / larger and stronger when fighting for food or space

  • resistant to the effects of a disease

  • able to absorb more water from food in a dry environment

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Ecosystems

• An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment

• Ecosystems can vary greatly in size and scale

  • Some ecosystems are small – a single tree can be seen as an ecosystem.

  • Some ecosystems are very large – the whole of Antarctica can be seen as one ecosystem.

• Some other examples of ecosystems include:

  • a garden pond

  • a woodland

  • a coral reef

  • a desert

Interactions within an ecosystem

• In order to survive and reproduce (have offspring), organisms need certain resources from their surroundings (from the ecosystem they are living in)

• This means that members of a species will often interact with members of its own species or other species

• Some examples of these interactions include:

  • Predators (carnivores) eating prey

  • Herbivores eating plants

  • Plant species being pollinated by bees

Competition in plants & animals

• Plants and animals in a community or habitat may compete with each other for limited resources

Competition in plants

• Plants may compete for:

  • Light: plants require light for photosynthesis in order to produce glucose, which provides them with energy for growth

  • Space: plants require space above soil (so leaves can absorb maximum sunlight) and below soil (so roots can absorb water and mineral ions)

  • Water (from soil): water is essential for photosynthesis and therefore plant growth

  • Mineral ions (from soil): plants require various mineral ions such as nitrate ions (used to make proteins) and magnesium ions (used to make chlorophyll)

Competition in animals

• Animals may compete for:

  • Food: food provides animals with the energy they require for growth and reproduction

  • Mates: animals require mates in order to reproduce (produce offspring) and pass on their genes

  • Territory: some animals compete for territory. This is an area of habitat that provides an individual with resources such as water, food, shelter and mates

Interdependence

• Within a community, each species depends on other species for food, shelter, pollination, seed dispersal etc

• If one species is removed it can affect the whole community

• This is called interdependence

• A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant

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• For example, in the food web above, if the population of earthworms decreased:

  • The population of grass plants would increase as there are now fewer species feeding off them

  • The populations of frogs and mice would decrease significantly as earthworms are their only food source

  • The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)