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Competition between animals is usually for:

• Food
• Water
• Territory
• Mates

Competition between plants is usually for:

• Light
• Water
• Minerals
• Space

Competition for these resources can affect population growth. Organisms have adapted to their individual environments and to any competition for resources.

Humans can also influence the normal balance of an ecosystem.

Competition affecting organism populations

Red and grey squirrels

The grey squirrel is not native to the UK - it was introduced from North America.

There is great competition between the red and grey squirrel with the population of the grey outweighing that of the red in some areas.

This has happened for a number of reasons:

• The grey squirrel eats a wider range of food and can survive in areas of mixed woodland whilst the red squirrel cannot.
• The grey squirrel carries a disease that is fatal to red squirrels while the grey remains unaffected.

However, grey squirrels are much larger than red squirrels and need more food to survive.

In areas where seeds are small (e.g. conifer forests) the grey squirrel struggles to obtain enough food and the red squirrel dominates.