Interdependence within a community


Organisms in an ecosystem rely on each other for their survival. This relationship is called interdependence. When organisms interact with one another it affects their survival. This becomes obvious when studying predator-prey cycles, mutualistic relationships and parasitism.

Predator-prey cycles

The graph below shows characteristic repeating patterns called predator-prey cycles.

Predator-prey cycles:

  1. There are always more prey than predators.
  2. The number of predators increases because there are more prey, so there is more food for them to eat.
  3. The number of prey reduces because there are more predators, so more get eaten.
  4. The number of predators reduces because there is less prey so less food.

There is interdependence between the predator and the prey. Any change in numbers of prey affects the numbers of predators and vice versa. In a healthy, balanced ecosystem the numbers of predators and prey remain fairly constant.


Some organisms rely on the presence of organisms of a different species. For example, oxpecker birds eat ticks and larvae infesting the skin of buffalo and other large animals. For this reason oxpeckers are called a cleaner species. This is an example of mutualism (new definition) - both species benefit from the arrangement.

Other examples of mutualism:

  • Lichens - formed by algae and fungi living together. Algae can photosynthesise and make food, which is shared by the fungus. The fungus in turn shelters the algae from a harsh climate.
  • Leguminous plants - peas, beans and clover have colonies of nitrogen-fixing bacteria in nodules attached to their roots. The plants gain nitrogen containing compounds from the bacteria, and the bacteria gain sugars from the plants.
Plant growing in soil with roots magnified to show the root nodules containing nitrogen-fixing bacteriaLegumes such as beans and clovers have root nodules containing nitrogen-fixing bacteria


Parasites are organisms that live on or in a host organism. The parasite benefits from this arrangement, but the host suffers as a result. Parasites do not usually kill the host because this would cut off their food supply.


  • Fleas live on the skin of other animals and suck their blood, this feeds the flea but weakens the host.
A tapeworm
A tapeworm
  • Tapeworms live inside other animals, attaching themselves to the host’s gut and absorbing its food. The host loses nutrition, and may develop weight loss, diarrhoea and vomiting.
  • Headlice bite other animals in order to feed off their blood.
  • Mistletoe roots grow into the veins of the host tree to absorb nutrients and minerals.
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