Blimey! Who'd have thought it? The meek and mild common earthworm, beloved of Charles Darwin, is crawling amok in the good old U.S. of A.
According to an article in this week's New Scientist [full article available to subscribers only]:
… Earthworms have a reputation as environmental good-guys, churning and enriching the earth as they munch their way through soil and leaf litter. The trouble is that Minnesota shouldn't have any worms. Nor should anywhere else in the US and Canada north of a line that runs roughly west from Boston—at least not since the end of the last ice age 10,000 years ago. These worms are invaders who hitched a ride with goods and settlers from Europe three centuries ago, and are now brought in as fishing bait…
In Europe, where the species is native, L[umbricus] terrestris and other earthworms play an important role in incorporating nutrients from fallen leaves into the earth. Their burrows aerate the soil and create handy channels for growing roots, water infiltration and gas exchange. Their casts form hotspots for nitrifying bacteria, which fix nitrogen into a form that plants can absorb. In Minnesota, though, the soil is aerated by soil insects, such as beetles, centipedes and millipedes, and other duff [i.e. leaf litter] inhabitants like salamanders or small mammals that move through the duff and keep it loose. "A more efficient system of aerating the soil is displaced when the earthworm invades," says [Lee] Frelich [University of Minnesota]. As a result it becomes denser, native plants and tree seedlings can't take root in the packed forest floor, and those that do are promptly polished off by burgeoning populations of deer.
Sweet revenge for what the Yanks' nasty grey squirrels did to our reds, you might be tempted to think, but invasive species are becoming a bigger and bigger problem throughout the world. Whether they be cane toads, cats and rabbits in Australia; crayfish, Himalayan balsam, ragwort and rhododendrons in the UK; rats and goats on the Galápagos Islands; or dear old Mrs Tiggywinkle on the Western Isles—introduced species, once they establish themselves, are almost always disastrous news for the local flora and fauna.
In many places (either literally or metaphorically) the cat is out of the bag. There is very little we can do to put right the mess that we have created (for it is generally us who have introduced the invasive species, either deliberately or unintentionally). In other places, such as the Galápagos, we are doing our best to make amends.
You can't blame the invasive species for the damage they're doing. They're just doing what they evolved to do: fighting the good Darwinian fight, tooth and claw; trying to fill their new-found ecological niches.
You can't blame them, but you don't have to like it.