Communities (AQA GCSE Biology: Combined Science)

There are 4 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: multiple populations (of different species) living and interacting in the same area
• Ecosystem: the interaction between a community (the living, biotic part) and the non-living (abiotic) parts of the environment

Levels of organisation in an ecosystem

You should understand the following 2 terms:
1. Competition:

• If a group of organisms all need the same resource in order to survive and reproduce (have offspring) but there is a limited amount of the resource available, they are said to compete for the resource
• Competition can be between members of the same species (intraspecific competition) or between members of different species (interspecific competition)

Intraspecific competition between two grey squirrels (same species) for a limited resource

Interspecific competition between a grey squirrel and a red squirrel (different species) for a limited resource

2. Adaptation:

• If an organism has certain features, behaviours, or other characteristics that help it to survive and reproduce in its habitat, it is said to be adapted to its habitat

• 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

• 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

• Plants in a community or habitat may compete with each other for certain limited resources:

Resources competed for by plants table

• Animals in a community or habitat may compete with each other for certain limited resources:

Resources competed for by animals table

• 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
• 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

A food web shows the interdependence of organisms

• 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)