We’re all familiar with this sweet scene of a robin feeding earthworms to its young, but did you know this worm is non-native and invasive?
It’s true. 10,000 years ago the glaciers killed North America’s native earthworms. Though there are still some natives in the south they work deeper underground than the European and Asian worms that arrived with immigrants in potted plants, root balls and dry ballast (soil).
Until quite recently I thought earthworms were native. All my life I’ve watched robins yank them out of the soil and seen them on the sidewalk after heavy rain. Gardeners and composters are happy with them, too, but…
What’s good for the garden is lousy for North America’s forests. Earthworms churn the soil column and devour leaf litter, invertebrates and fungi that our northern forests rely on. The result is a lack of ground cover and poor regeneration of the trees.
The problems are especially acute at the edge of the earthworm advance around the 45th parallel, Minnesota for example. Studies have shown this lowly garden friend is responsible for the decline of ovenbirds in northern Midwest forests and the decline of forest orchids. Oh my!
Like the emerald ash borer, we humans have accidentally introduced a species that’s bad for the forest. The only way to stop it is for us to stop moving worms and soil. Composters and gardeners take note! If you’re on the edge of the earthworm advance — in Minnesota or Maine, for instance — don’t buy worms. (Pittsburgh isn’t on that edge; earthworms have been here a very long time.)
Meanwhile, thank heaven that robins eat them!
Want to learn more? Watch this 10 minute video from the Smithsonian Environmental Research Center.
(robin photo by William Majoros via Wikimedia Commons. Earthworm photo by James Lindsey at Ecology of Commanster, Belgium via Wikimedia Commons. Click on the images to see the originals)