英語(yǔ)聽力文摘689 殺手貓：貓每年殺死37億只禽類
It’s time to go again to the A Moment of Science mailbag. A listener writes:
Dear A Moment of Science,
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 defensive. And I wonder: don’t other pets kills wild animals, too?
Killer Cats
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.
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 leashes or in fenced yards. So how can dogs possibly do as much damage as cats?
Dogs vs Cats
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.
And when given free reign, 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.
The point is that cats are not the only culprits. Any ecosystem is vulnerable when a foreign species without natural predators invades.
