Earthworms were the superheroes of my elementary-school science class. They aerated the soil, added organic matter, created structures that held water, turned garbage into gold. Seeing them and their lovely, rich castings in my garden and in the compost pile, all these many years later, makes me feel proud and virtuous.
But there’s more to the story. Here’s what they didn’t tell us in school: In the parts of North America that were glaciated (including the northeastern and midwestern regions of the U.S.), the glaciers wiped out all the terrestrial earthworms. Over the next ten thousand years, as the glaciers receded, the ecosystems adapted to this wormless state. Leaf litter piled up in the forests, and trees and understory plants evolved to gather nutrients directly from the litter, with help from fungi and bacteria. They also evolved to plant their seeds directly into the leaf litter. These forests had little or no topsoil, just mineral soil covered by a “duff layer” – leaf litter, twigs, bark, and so forth.
Fast forward to the 1600s, when the Europeans who settled the area unknowingly brought worm eggs in their cargo. They cleared forests and planted (mostly European) crops; the worm eggs hatched; fields and pastures filled up with earthworms and their castings. Today, there are some 15 species of earthworms in the area, all from Europe or Asia.
Earthworms damage the forests – we think
The worms are great for lawns and gardens (I think they’ve been poisoned out of most industrial-type farms), but – according to today’s accepted wisdom – they are making a mess of forests. They eat the duff layer, making it hard for trees and other forest plants to gather nutrients and for seeds to sprout. They eat tree roots, which weakens the trees. They greatly reduce the amount of carbon that trees can sequester. Forest scientists think earthworms cause as much damage as deer overpopulation.
I read all this recently, while I was trying to research the deer problem (see Regenerating trees), and I was horrified. Our compost pile is at the edge of the woods, and it’s full of worms. (My guess is that the worms come from the hay that we use as food and bedding for the rabbits – much of which ends up in the compost pile – but who knows.) Not only the compost pile but also the area around it is rich with worm castings. Suddenly, that stopped being a source of pride and became a source of anxiety. Are we setting off a worm apocalypse in our woods? Will all our efforts to re-create native woodlands be for naught?
There’s no open area suitable for the compost pile, but we decided that, once the weather gets warm, we’ll move the pile up to the chicken coop. There, we can count on the chickens to eat the worms as fast as the worms can multiply. It will make the chickens very happy, too.
But the more I think about it, the more confused I become. The Northeast was largely reforested in the last century or two after farmers abandoned their fields and pastures. How did native forests arise successfully on these earthworm-infested lands? Do the trees or the fungi fight back with earthworm-inhibiting chemicals? Or are they evolving back to their pre-Ice Age, earthworm-friendly genetics? Or are the post-agricultural forests all inherently weak and unhealthy?
And then another question arises: How are agroforestry and permaculture possible in the once-glaciated regions of the United States? People are planting rows of canopy trees (oak, chestnut, hickory), understory trees (hazelnut, native and non-native fruits), berry bushes and vines, with alleys of grass and even annual crops such as squashes between them. (One person doing this type of agriculture is Mark Shepard, who describes his experiences in his wonderful book Restoration Agriculture. He’s in Wisconsin, and I was wondering how he managed this problem till I realized that he’s in the Driftless Region, a high plateau that escaped glaciation. So he probably still has native earthworms. However, people are using similar practices in once-glaciated areas.) Do these “artificial edge” communities have earthworms, or not? If the pasture/garden alleys have earthworms, what prevents the worms from getting into the wooded areas?
More research to be done….
And any insights welcome.