Interdependence and competition

All organisms in an ecosystem depend upon each other. If the populations of one organism rises or falls, then this can affect the rest of the ecosystem.

Within a community, each species depends on other species, for instance for:

  • food
  • shelter
  • successful pollination
  • successful seed dispersal

The most common interdependence is for food. For example, in the simple food chain:

grass → rabbit → fox

If the foxes in this food chain were killed, the population of rabbits would increase because they are no longer prey to the foxes. As a result, the amount of grass would decrease because the increased population of rabbits would be eating it.

Very small changes to ecosystems have large consequences, which can be difficult to predict. This means that all the organisms in an ecosystem are dependent upon each other. We call this interdependence.

A stable community is one in which the size of the populations of all species remain relatively constant over time. In the example above, the amount of grass, and the numbers of rabbits and foxes would all remain relatively constant. The different populations are living in a healthy balance within their environment.


Often, the organisms within an ecosystem compete for resources.

Competition can be interspecific or intraspecific depending on whether organisms from different species or the same species are competing for these resources.

Competition in plants

Plants compete for:

  • light, which is necessary for photosynthesis
  • space, avoiding shading so the plant can get maximum light for photosynthesis
  • water from the soil, which is necessary as a solvent for chemical reactions and for photosynthesis
  • minerals from the soil, so that the plant can form other products from the glucose produced by photosynthesis

Competition in animals

Animals might compete for:

  • food
  • a mate, so they can pass on their genes to offspring
  • territory - an animal may defend the territory where it lives
Diagram showing the things that animals and plants compete for in an ecosystem

Competition creates a 'struggle for existence'. Individuals within a species that cannot compete effectively are unlikely to reproduce, and may die. This can be referred to as 'survival of the fittest' and is a driver for evolution. Species that are unable to effectively compete may become extinct.