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Biology · Communities in Ecosystems

Interdependence and Competition

No species lives alone. This lesson explains how living things depend on each other, through predation, mutualism and parasitism, how they compete for resources, and why interdependence matters for the stability of a community.

  • 8 key terms
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Learning Objectives

  1. 1Describe the importance of interdependence in a community, relating it to predation, mutualism and parasitism.
  2. 2Describe what living things compete for, and the importance of competition in a community.
  3. 3Explain how a change in one population can affect others in a community.
  4. 4Interpret graphs of predator and prey populations.

Retrieval practice

  1. 1

    What is a community?

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    All the populations of different species living in an area.

  2. 2

    What is a food chain?

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    A diagram that shows what eats what in a community.

  3. 3

    What do plants compete for?

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    Light, water, mineral ions and space.

  4. 4

    What is a parasite?

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    An organism that lives on or in another organism and feeds on it.

Living things need each other

A bee gets nectar from a flower and the flower gets pollinated. A fox eats a rabbit, and the rabbit eats grass. A tick feeds on the blood of a deer. In every ecosystem, species depend on each other for food, shelter, pollination and seed dispersal. This is interdependence, and it means that a change in one species can affect many others.

Interdependence

Species in a community are interdependent, which means each one depends on others.

  • Food

    Animals depend on plants or other animals for food, and plants depend on animals for pollination and seed dispersal.

  • Shelter

    Many animals use plants for shelter, such as birds nesting in trees.

  • Stable community

    When all the species are present, the populations stay fairly steady. This is a stable community.

  • Removing a species

    If one species is removed, the species that depend on it are affected, which can change the whole community.

Predation, mutualism and parasitism

There are three main ways in which two species can depend on each other.

  • Predation

    A predator hunts and eats its prey. The predator benefits and the prey is harmed. Predators also help to keep the number of prey in balance.

  • Mutualism

    Both organisms benefit. Nitrogen-fixing bacteria live in the root nodules of clover, peas and beans. The plant gives them sugars, and they give the plant nitrogen compounds. Another example is a cleaner fish that eats parasites from a larger fish.

  • Parasitism

    A parasite lives on or in a host and feeds on it. The parasite benefits and the host is harmed. Examples are a tapeworm in a gut, a tick or a flea on an animal, and a mistletoe on a tree.

  • Pollination

    Insects get food from flowers, and the flowers get pollinated, which is another example of mutualism.

Competition

Competition happens when two or more organisms need the same limited resource.

  • Plants compete for

    Light, water, mineral ions and space.

  • Animals compete for

    Food, water, mates, shelter and territory.

  • Within a species

    Individuals of the same species compete with each other, because they need exactly the same things.

  • Between species

    Different species compete if they need the same resource, such as red and grey squirrels.

  • The result

    The organisms that are best adapted get more of the resource, so they survive and reproduce more. Competition limits the size of a population.

Interactions between two species

Predation and parasitism

  • One species benefits
  • The other is harmed
  • A predator kills its prey
  • A parasite usually keeps its host alive

Mutualism and competition

  • Mutualism: both benefit
  • Competition: both are harmed
  • Mutualism helps both species to survive
  • Competition limits the numbers of both

Predator and prey numbers

The populations of a predator and its prey often rise and fall together.

  • Prey increases

    There is more food for the predators.

  • Predators increase

    They eat more prey, so the prey population falls.

  • Prey decreases

    Now there is less food, so the predators starve and their population falls.

  • The cycle repeats

    With fewer predators, the prey can increase again. The predator peak comes after the prey peak.

Case study

Losing the wolves

In some places wolves were removed from a national park. The deer, which were no longer hunted, increased in number, and they ate so many young trees and shrubs along the rivers that other species lost their food and shelter. When the wolves were brought back, the number of deer fell and the plants recovered. The case shows how species depend on each other, and how removing one species can change the whole community.

Interdependence Wolves, deer and plants depend on each other
Effect Removing the predator changes the whole community

The big idea

Species in a community are interdependent through predation, mutualism and parasitism, and they compete for limited resources, so a change in the population of one species can change the others.

Interdependence and competition

Interdependence and competition

  • Predation

    • predator benefits
    • prey harmed

  • Mutualism

    • both organisms benefit

  • Parasitism

    • parasite benefits
    • host harmed

  • Competition

    • both harmed
    • for food, light, water, space

  • Stable community

    • all the species keep the populations steady

Summary and exam focus

  • Species depend on each other, which is called interdependence.
  • In predation the predator benefits and the prey is harmed.
  • In mutualism both species benefit, and in parasitism the parasite benefits and the host is harmed.
  • Organisms compete for light, water, minerals and space, or food, mates and territory.
  • Predator and prey populations rise and fall in cycles.

Exam focus

Describe what happens to the population of prey, and the population of predators, when a disease kills many of the predators. (3 marks) (3 marks)

Start with the immediate effect and then follow it through. Fewer predators means fewer prey are eaten, so the prey population increases. Then the extra food allows the predators to recover.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Interdependence
The way in which species in a community depend on each other.
Predation
The hunting and eating of one animal by another.
Mutualism
An interaction in which both organisms benefit.
Parasitism
An interaction in which a parasite lives on or in a host and harms it.
Competition
The struggle between organisms for the same limited resource.
Predator
An animal that hunts and eats other animals.
Prey
An animal that is eaten by a predator.
Stable community
A community in which the populations stay fairly constant.

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