Are worms good for the soil?

Reading interpretive signs on some of our local nature trails, I learned that the last ice age killed off Minnesota’s earthworms. European settlers brought them back unintentionally, but there is still an advancing line where the worms have not yet returned.

Without worms, forests develop a thick, spongy layer that is very slowly consumed by fungi and bacteria. In the last 10,000 years, many native plants have adapted well to life in this soil.

When earthworms return, they quickly consume the spongy layer, often eating it down to bare soil by summertime. These new soil conditions then cause a rapid decline in understory plants.

I always thought earthworms were good for the soil, but, as usual, it’s more complicated than that. Every environment is unique. Some environments are defined as much by what isn’t
there as by what is. When you reintroduce the “missing” piece, everything changes.