North American earthworms may be a mixed blessing

Well, you learn something new every day. Apparently, all types of earthworms in North America were wiped out during the last ice age, some twelve thousand years ago. The fact that we now have earthworms at all is due to invasive species brought over by invasive European settlers.

From the 1600s onwards, waves of settlers arrived here from Britain, Spain, France and the Netherlands, bringing with them familiar plants and trees to plant. They also brought worms in the soil and roots of these plants, and even in the ballast of their ships, worms that gleefully prospered in the worm-less soils of the the New World, at least in the northern parts of the USA and Canada. Many different species of earthworms, including the common and familiar red earthworm, Lumbricus Rubellus, flourished, and almost all of the worms we see in the northern parts of North America (and increasingly even in more southern parts) are invasive species.

Now, earthworms are usually considered a "good thing" by gardeners and agriculturalists, for stirring up and aerating he soil and making nutrients more available to plants. But that may not be case, or at least not the whole story. After the Ice Age, North American forests evolved other ways to get nutrients from the ground, and the invasive European earthworms  may actually encourage more invasive plants to take hold.

More recently, a German study looking at earthworms in Canada(!) has shown that these invasive European earthworms are increasingly outcompeting local organisms like protists, nematodes, bacteria, fungi, and arthropods, all of which play there own roles in the ecosystem. For example, areas higher in earthworms are impoverished in other detritivores, particularly arthropods (although, for reasons not fully understood, the numbers of carnivorous arthropods were actually higher). The knock-on effect of all this further up the food chain in not well understood either, but an effect there must be.

So, next time you are gardening, you might look at those worms in a slightly different way. At best, they seem to be a mixed blessing.

Of course, this all raises another question that no-one seems to have addressed: why were North American earthworms wiped out by the Ice Age and not European ones?