Communities: This is called interdependence.

The idea

Interdependence means that organisms in a community depend on each other for survival. Within a community, each species relies on others for resources such as food, shelter, pollination and seed dispersal. A change in one population can therefore affect many others. For example, if a plant species declines, herbivores that feed on it may decline, and predators that eat those herbivores may also be affected. Interdependence also includes relationships such as mutualism, where both species benefit, and parasitism, where one benefits at the expense of the other. Understanding interdependence helps explain why removing or adding a species can destabilise a community and alter population sizes.

What earns the marks

• Defines interdependence as the dependence of organisms within a community on one another for survival.
• Identifies ways organisms depend on each other, such as food, shelter, pollination and seed dispersal.
• Explains that a change in one population can affect other populations in the community.
• Uses a named example, such as a plant and its herbivore or a predator and its prey, to show interdependence.
• Distinguishes interdependence from simple competition by emphasising mutual reliance within a community.

Common mistakes and how to put them right

• Treating interdependence as only competition for resources.

  Put it right by explaining that organisms also rely on each other, for example for pollination or food.

• Assuming a change affects only one species.

  Put it right by tracing the effect through a food web to other populations.

• Confusing interdependence with a single organism's needs.

  Put it right by describing relationships between different species in a community.

Exam tips

• Use the word community when explaining interdependence, because it refers to all the populations living together in a habitat.
• Give a specific example and state the direction of the effect, such as fewer plants leading to fewer herbivores.
• Link interdependence to food webs or pollination when answering questions about changes in population size.