May — Earthworms

One evening this spring I took my dog out for his late-night constitutional when I heard some rustling in the leaves. Thinking it was the paparazzi yet again trying to obtain some illicit photos of this reporter’s private life, I turned and made a beeline for the door. I soon discovered that the dog was not only not keeping up, but was stationary and otherwise occupied. When the leash grew taut and he was disrupted from his repose, I surprisingly came face to face with the source of the rustling: earthworms. As the worms hightailed it or, more accurately, low-tailed it back into their holes, and the dog delivered a not very subtle “what-is-your-deal?!?” look, I began to ponder worm-life as we know it.

First of all, their numbers are simply astounding. There are about 1,500 species of segmented worms or annelids on the planet. They live throughout the world, except in the Arctic and Antarctic. You should also know that all earthworms found in Minnesota are non-native. Yes, sports fans, there are about 15 species of earthworms in our state, all introduced from Europe and Asia.

Next point to ponder: why do earthworms exit their burrows? During heavy rains, worms will exit their burrows to escape drowning. Makes sense. At other times, worms will move to the level of soil that meets their needs. If the upper reaches dry out, they will move down to moister areas. Moisture is important to terrestrial worms because they obtain oxygen through their skin. Blood in the skin distributes the oxygen to other parts of its body.

Earthworms also have interesting reproduction methods, as they are both hermaphroditic and homosexual. Each can function as both female and male, but only as one sex at a time. During mating, both partners are male and they exchange sperm. Later, each becomes female and develops mature eggs. There isn’t space here to discuss the exact process; suffice it to say that the individual worm uses sperm obtained during its encounter with the other male to fertilize its eggs. After a few weeks, these hatch as small but fully formed earthworms, except for a lack of the sex structures, which develop two to three months later.

Image courtesy of the Minnesota Department of Natural Resources.

While it may be hard to believe, Minnesota’s natural communities developed over the millennia without earthworms, i.e. earthworms are not native. In fact, earthworms can have a significant negative impact on our forests. As the Minnesota Department of Natural Resources’ website explains: “Without worms, fallen leaves decompose slowly, creating a spongy layer of organic ‘duff’. This duff layer is the natural growing environment for native woodland wildflowers. It also provides habitat for ground-dwelling animals and helps prevent soil erosion.” Now, worms eat these fallen leaves, eliminating this duff layer. Tree seedlings and herbaceous plants cannot survive in these conditions.

Please do not release your unused worms — be it from fishing or your home composting bin — into the woods or anywhere outside. Dispose of them in the garbage.

Learn more about the earthworm problem from the Minnesota Department of Natural Resources.

— Tom Lewanski, Conservation Director