The threat of invasive species

Giant African snails, invasive species
CBS News

An earlier version of this story was broadcast on Oct. 30, 2011.

Monsters aren't just figments of our imagination. THEY'RE HERE! Just ask Illinois Governor Pat Quinn who in June called for the total separation of the Mississippi River Basin from the Great Lakes hoping against hope to stop the advance of one very aggressive fish. A fish that's just one part of a far bigger problem. Our Cover Story is reported by Mark Strassmann:

Meet the king of the Illinois River, and a big showoff.

Fast, easy to startle, and voracious eaters, Asian silver carp are a jarring sight here, and a hit on YouTube. but fisheries research scientist Greg Sass...says that in the long history of this waterway, what's also remarkable is when this fish got here...just 20 years ago.

"Very muscular," said Greg Sass. "This thing'll hurt you. They can jump out of the water at high speeds. They can jump a great distance out of the water, and so they're quite dangerous when you're out on the river."

But fisheries research scientist Greg Sass...says that in the long history of this waterway, what's also remarkable is when this fish got here...just 20 years ago.

"By the mid-2000s, it was becoming a crisis," Sass told Strassmann. "Not only with fish jumping everywhere, but just what was showing up in our catches, the amount of Asian carp versus the native fishes."

With no natural predators in this stretch of the river in Central Illinois, up to nine out of ten fish here are now Asian carp.

"We've seen explosive population growth, a population that has almost doubled every year," Sass said. "We're fairly confident we have the highest wild densities of Asian carp anywhere in the world."

Which is quite a feat considering this fish is from China.

The Illinois river has been invaded.

"As a scientist or even a recreational fisherman or a boater, you go, 'Wow, this is a huge, huge problem,'" he said. "These fish are behaving in a way that we're not used to. They're doing things to the ecosystem that we're not completely sure what kind of effect there's going to be yet. And there's just an incredible abundance of them."

It's an invasion from coast to coast, from Africanized killer honey bees in the Southwest to South American nutria in Louisiana, to the spread of the Burmese python in the Florida Everglades ... all part of a scary trend.

"Everywhere we look, we see species that are spreading and damaging our natural ecosystems," said University of Notre Dame biologist David Lodge. "And when scientists look into the future, they see the potential for many more damaging species."

"You make it sound like we're under attack everywhere," said Strassmann.

"We ARE under attack everywhere."

Lodge says our agricultural system depends on plant and animal species imported to America, like wheat and cows and pigs - but that doesn't make them invaders.

"An invasive species is a species that's been transported from one part of the planet to the other by people, and a species that has a harmful impact," Lodge said.

"Typically transported by design, by accident, or little bit of both?" asked Strassmann.

"Both. Lots of invasive species have been transported by accident in the ballast water of ships or as a hidden pest on plants that have been imported. But many have also been imported intentionally."

Like the Shakespeare fan who, back in the 1890s, set out to bring to America every bird mentioned in the Bard's plays ...