Feral cats are thought to be responsible for the extinction of no less than 20 native Australian mammal species, reported Weston Williams in the Australia edition of the Christian Science Monitor. The population density is smaller than the density in North America and Europe, but their impact on the wildlife Down Under is of grave concern.

Australia is not alone. A 2013 study found that cats kill as many as 3.7 billion birds and 20.7 billion small mammals in the U.S. every year.

"All outdoor cats can pose risks to wildlife," said Niamh Quinn, UC Cooperative Extension...