The World’s Worst Invasive Mammals

Animals as common as goats, deer, rabbits or mice can have a devastating effect on other wildlife

Long-tailed Macaque (© Arco Images GmbH / Alamy)

Feral Cats

Feral Cats
(© Duncan Usher / Alamy)
There are an estimated 60 million feral cats (Felis catus) in the United States alone. Together, they kill around 480 million birds every year. Cats were domesticated (or perhaps domesticated themselves, according to some scientists) in the Fertile Crescent region of the Mediterranean in the early days of human civilization; the cats eradicated mice and rats before they could get to grain reserves. The cats did their job and proliferated throughout the world, thanks in large part to their popularity as human pets.

When house cats are allowed free range outdoors by their owners, however, or simply don’t have owners, they not only wreak havoc as opportunistic hunters, they can also spread disease. In addition to carrying rabies, 62 to 82 percent of cats in a recent study tested positive for toxoplasmosis, a parasite that has been shown to cause neurological damage to sea otters and other marine mammals that are exposed when heavy rainfall washes infected cat feces into the water. Cats have also hurt populations of birds, reptiles and other creatures. The black stilt of New Zealand (a seabird), the Okinawa woodpecker and the Cayman Island ground iguana are just a few of the dozens of endangered species at risk due to the proliferation of feral cats.

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