I was reading a story in today's Sunday Mail about new worries that Scotland's iconic red squirrel may be extinct within a decade.  The culprit, it seems, is a deadly "squirrelpox virus," introduced to the U.K. by imported North American gray squirrels, who infect the reds through drey (nest) sharing.

The story, which is sad enough, reminded me that for all the attention that global warming and habitat destruction gets when it comes to wildlife conservation, people often forget that invasive species (nonnative animals, plants, viruses and bacteria) introduced, either accidentally or intentionally into new environments is one of the leading causes of extinction in the word.  And the United States is no exception.

It was an invasive species (Asian bark fungus) that virtually wiped out the American Chestnut, once the dominant canopy tree in the Northeastern United States.  From feral pigs in Hawaii to Asian carp in the Midwest, the spread of invasive species is another major conservation hurdle that both international and U.S. law and policy have yet to effectively address.