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Invaders among us

Non-native plants and animals
crowd out indigenous species, cost money

The brown tree snake is considered on invader species

CNN's Natalie Pawelski reports on some unnatural parts of nature
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ATLANTA (CNN) -- All nature is not natural -- at least not to the place where it thrives.

Invader species are plants and animals thriving where they don't belong. And a new study says they're causing $123 billion in damage every year.

Once introduced, nuisance species are usually there to stay.

"We've exterminated extremely few either animals or plants once they have become established in the U.S.," said David Pimentel, the Cornell University ecologist who conducted the study.

Some invaders arrived by accident. The zebra mussel, which clogs water pipes around the Great Lakes, hitched a ride in ships' ballast water.

The brown tree snake, native to eastern Indonesia, the Solomon Islands, New Guinea, and parts of Australia, has found its way to Guam, where it has virtually wiped out the island's native forest birds. The snake also has a propensity for crawling onto power lines, causing power outages and damaging lines.

Feral pigs can be a nuisance  

Others, like feral pigs in Hawaii and other states, were brought to the United States on purpose. Once they were domesticated, but now they uproot trees and trample habitats.

Even pets take their toll. Pimentel's study says America's 90 million cats -- two-thirds of them domestic -- kill 200 million birds each year. And dog bites send an estimated 800,000 people to emergency rooms each year, costing about $30 million.

The undisputed king of invaders is the rat. The United States has a billion of them. Rats thrive despite centuries of eradication efforts, causing $19 billion damage to crops, buildings and human health each year.

Emerging threats include long-horned beetles, which kill shade trees, and Formosan termites, which are reducing quaint New Orleans homes to powder.

Even plants can be troublesome. The colorful purple loosestrife replaces native water plants in the nation's wetlands and along streams, taking away natural cover for other species. And cheatgrass speeds the spread and frequency of wildfires on rangelands.

With more trains, planes and ships circling the globe, non-native species have more chances than ever to hitch a ride, take hold and take over.

Correspondent Natalie Pawelski contributed to this report.  
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