They first arrived on British ships almost 200 years ago, with the aim of cutting back a rapid growth of rat (鼠) population．Then the cats themselves became the problem．After dealing with the rats', they turned to birds-and now hundreds of them have been killed in a move to protect rare (稀有) birds．Even cat lovers have been persuaded to back the move, after being told that there was no other choice.
When British sailors took control of Ascension Island in 1815, they found nobody living on it but that it was overrun with rats which had escaped from sinking ships．They introduced cats to kill the rats-and were soon overrun by cats．By 1820 they were sending back to England for dogs to kill the cats, but that didn' t work．Ever since the wild cat population has been attacking the birds.
Man introduced these cats, and it has disturbed the balance．Ascension Island is a most important seabird gathering place．We have to undo man' s mistake in bringing cats here．Action has been taken to kill off wild cats for the protection of birds．Only cats raised as pets are allowed．Since they are micro-chipped (加电子芯片的), if they get caught they can be sent back to their owner．The action can be considered rather successful．The cat population is down to the last few-they're the most difficult ones to catch.
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