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Killer¹ Cats: Cats Kill Up To 3.7 Billion Birds Each Year
杀手猫：猫每年杀死37亿只禽类

It’s time to go again to the A Moment of Science mailbag. A listener writes:
又到了《科学一刻》听众来信时间了。一位听众写道：

Dear A Moment of Science,
亲爱的《科学一刻》栏目组：

I know that cats kill a lot of wild birds and other animals. But as a cat owner and cat lover I can’t help but be a little defensive². And I wonder: don’t other pets kills wild animals, too?
据我所知猫会杀死许多野生鸟类和其它动物。但是作为一个养猫和爱猫之人，我忍不住想为其辩护。我想知道，难道其它宠物就不会杀害动物吗？

Killer Cats
杀手猫

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，其它的宠物也会。事实上，除了猫，杀死其它野生动物的主犯正是人类最好的朋友：狗。

Cats in the United States kill up to 3.7 billion birds every year and as many as 20 billion small mammals. And many cats roam free while dogs are either kept on leashes³ or in fenced yards. So how can dogs possibly do as much damage as cats?
在美国，猫每年杀害37亿只禽类和200亿只小型哺乳动物。许多猫行动自由，而狗却被拴着或是关在围墙院子中。这样，狗为何能与猫造成同样大的破坏大呢？

Dogs vs Cats
狗与猫的较量

Well, dogs don’t kill as many animals as cats, especially not in the United States. But consider that in many other countries dogs are not kept fenced in. Experts estimate that around 75% of dogs around the world are free roamers.
狗杀死的动物没有猫杀的多，尤其是在美国。但在其他许多国家，狗没有被圈养。专家们估计全球大约75%的狗是自由的。

And when given free reign⁴, dogs rack up their share of kills. For example, in the 1980s researchers found that a single stray dog in New Zealand killed as many as 500 flightless birds called Kiwis. 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.
一旦行动自由，狗杀死动物的比例会大幅上升。比如，在80年代，研究人员发现在新西兰一只流浪狗杀害了多达500只名叫几维的无翼鸟。2006年，在南太平洋维瓦岛上，一小群流浪狗几乎使一种陆地蛙濒临灭绝。

The point is that cats are not the only culprits. Any ecosystem⁵ is vulnerable when a foreign species without natural predators⁶ invades.
 问题的关键在于猫并不是唯一的凶手。当一种无天敌的外来物种入侵时，任何生态系统都是脆弱的。