Across northern England and Scotland, a do-or-die battle is being waged against an aggressive invader: the American gray squirrel.

Researchers have discovered that the gray squirrel is threatening Britain's red squirrel by destroying its habitat and spreading disease. Miles Barne, chairman of the London-based European Squirrel Initiative—yes, there is such a thing—says Victorians first brought the American gray squirrel to England in 1876, thinking they would be a nice addition to their forests. They were wrong! The gray squirrel, which is almost double the size of the native red squirrel, turned out to be lethal. In addition to hoarding food, the gray squirrel also carries a strain of pox that somehow doesn't hurt the gray squirrel, but is deadly for the red squirrel.

Who would have thought something so small and seemingly innocent could cause so much harm?