The recent evidence (if that link doesn't work, try this) that the virulence of the West Nile virus is due to hybridization of mosquitos is very interesting. The specter of "alien species" brings to mind the decimation of island ecosystems by rats, cats and snakes, or the spread of the zebra muscle in the Great Lakes or the threat of the Africanized Honey Bee.

You can look to Reason for a contrarian take on "alien invaders." One thing is certain though, the transplantation of many life forms across continents have altered ecosystems and "natural" history irreversibly.

If a day comes that a super-plague eliminates humanity, there would still be:

• Red Deer in New Zealand.
• Camels in Australia.
• Horses in North America.
• Nutria and Racoons in Europe and Asia.

...to give a few "sexy" examples, and neglecting the much more common export of plants.