all the people living in a particular country, area, or place
population
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(an) illness of people, animals, plants, etc., caused by infection or a failure of health rather than by an accident
disease
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to gradually become less, worse, or lower
decline
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marking something as clearly different from others
distinctive
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an ability or willingness to change in order to suit different conditions
adaptability
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the protection of plants and animals, natural areas, and interesting and important structures and buildings, especially from the damaging effects of human activity
conservation
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The grey / red squirrel was introduced to Britain and has become very successful since then.
grey
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Why do you think grey squirrels are regarded as a pest?
Because they damage trees, they eat humans’ waste food and they carry a virus that kills red squirrels.
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The main reason why the red squirrel is less successful is that the grey squirrel is fatter / thinner so it is less affected by cold weather.
fatter
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How many red squirrels are left in the UK?
Fewer than 140,000
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Another reason is that grey squirrels are unable / able to live in cities.