What's the story on North American earthworms?
Dear Straight Dope:
What's the true history of North American earthworms? Through various haphazard library and online research, I have come across three theories. Please confirm or debunk the following:
1) North America lost all earthworms in the last few ice ages and the ones we see today are immigrants from the human colonists from Europe (hmm . . . which also had many ice ages).
2) North America has many species of its own earthworms, but they are being almost totally ousted by European imports.
3) The worms we see in our backyards are and always have been native to North America.
Please sort out this all out.
I'll vote for (4), none of the above.
First off, all of North America was not scraped down to bare rock during the ice ages, and that's about what it would take to wipe out all the earthworms. It might be fair to say that much of Canada was stripped of its native earthworms last time around, but relatively little of the U.S., and none of Mexico, was affected.
Second, the native worms aren't being wiped out. There are currently 17 native species and 13 European species in the eastern United States, Lumbricus terrestris being the most common import, and in many areas the dominant species in the topsoil. The native species may be less abundant than they once were, but if they were going to be "ousted," they'd be gone by now. I'm not aware of any ongoing flux in the system, and think it's pretty much stabilized over the last hundred years.
Finally, it's unlikely that all the worms you see in your backyard are native to North America, but not impossible, especially if you live in a spot like Arizona or SoCal or New Mexico, etc., where the imports don't do as well (European species don't adapt well to deserts). If you live in the East, you're just going to have to learn to recognize 30 virtually identical species if you want to know for sure. Welcome to the world of invertebrate taxonomy!