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It's time to go again to the A Moment of Science mailbag. A listener writes:
又到了《科學一刻》聽眾來信時間了。一位聽眾寫道：
Dear A Moment of Science,
親愛的《科學一刻》欄目組：
I know that cats kill a lot of wild birds and other animals.
據我所知貓會殺死許多野生鳥類和其它動物。
But as a cat owner and cat lover I can't help but be a little defensive.
但是作為一個養貓和愛貓之人，我忍不住想為其辯護。
And I wonder: don't other pets kills wild animals, too?
我想知道，難道其它寵物就不會殺害動物嗎？
Yes, they do. In fact, after cats, one of the main culprits when it comes to harming wildlife is none other than man's best friend: dogs.
是的，其它的寵物也會。事實上，除了貓，殺死其它野生動物的主犯正是人類最好的朋友：狗。
Cats in the United States kill up to 3.7 billion birds every year and as many as 20 billion small mammals.
在美國，貓每年殺害37億只禽類和200億只小型哺乳動物。
And many cats roam free while dogs are either kept on leashes or in fenced yards.
許多貓行動自由，而狗卻被拴著或是關在圍墻院子中。
So how can dogs possibly do as much damage as cats?
這樣，狗為何能與貓造成同樣大的破壞大呢？
Well, dogs don't kill as many animals as cats, especially not in the United States.
狗殺死的動物沒有貓殺的多，尤其是在美國。
But consider that in many other countries dogs are not kept fenced in.
但在其他許多國家，狗沒有被圈養。
Experts estimate that around 75% of dogs around the world are free roamers.
專家們估計全球大約75%的狗是自由的。
And when given free reign, dogs rack up their share of kills.
一旦行動自由，狗殺死動物的比例會大幅上升。
For example, in the 1980s researchers found that a single stray dog in New Zealand killed as many as 500 flightless birds called Kiwis.
比如，在80年代，研究人員發現在新西蘭一只流浪狗殺害了多達500只名叫幾維的無翼鳥。
In 2006, a small pack of owner-less dogs on Viwa Island in the South Pacific came close to wiping out a species of ground frog.
2006年，在南太平洋維瓦島上，一小群流浪狗幾乎使一種陸地蛙瀕臨滅絕。
The point is that cats are not the only culprits. Any ecosystem is vulnerable when a foreign species without natural predators invades.
問題的關鍵在于貓并不是唯一的兇手。當一種無天敵的外來物種入侵時，任何生態系統都是脆弱的。
