Darwin Loved Worms and I Hate Them
Charles Darwin loved earthworms. When he wasn’t messing around trying to figure out why animals were the way they were, he was pondering earthworms. In the 19th century most folks thought worms were a pest. But Darwin was convinced otherwise.
Darwin believed they provided an important yet unnoticed service; worms slowly turned over the soil. So he conjured up an experiment. He spread small pieces of coal across a field behind his house outside of London and left them. Decades later, he dug a trench and looked in the walls of the trench to see how far down the pieces of coal had sunk through the action of the worms turning over the soil. The soil increased in depth by 0.2 of an inch per year. After 10 years a chunk of coal on the soil surface sinks two inches. Indeed, as Darwin believed, earthworms do add a layer of digested matter on the surface bit by bit.
In Darwin’s field, that was fine and dandy. The worms were native. Here in the glaciated Northeast, they are one ugly brute. There were no earthworms as the continental glacier left a clean slate some 12,000 years ago. Any native worm species are found far to the south and move northward very slowly.
Enter the European invasion. Worms from the old world hitchhiked and found an empty new world to colonize. Today, we continue to transport them to new areas, intentionally and unintentionally, through dumping of unused fishing bait, transport of compost and mulch, and moving topsoil around.
So what’s the big deal?
Earthworms literally eat tons of leaf litter when they are introduced to the northern hardwood forest. In worm-free forests, this is done very slowly by fungi, bacteria and other native detrivores resulting in a rich, thick layer of rotting material called “duff”. Many understory plants are dependent on duff. It protects seeds from predation, extreme cold and drought and adds lots of nutrients. Add earthworms to a thick duff layer, and as Darwin found out, soon it is eaten and turned into the soil. The forest floor completely changes.
Biologists are just now beginning to understand the ramifications of this. But already one thing is clear. These little wigglers are a big problem. A lot of native understory plants appear to be very sensitive to these changes, including economic powerhouses like sugar maple and red oak seedlings. Experimental plots with and without earthworms look drastically different. Those without worms have a thick, lush growth of understory plants and those with worms are comparatively barren.
And with introduced earthworms come introduced plants. Recently, scientists have found that earthworm invasions are facilitating non-native plant invasions, probably through alteration of soil nutrients and disruption of fungal mycorrhizae that are needed by plants to help absorb nutrients.
Change the duff layer and the understory plant community and of course you change the animal community too. Recently, studies in Pennsylvania and New York found that Red-backed Salamanders, usually the most abundant vertebrate in hardwood forests, are profoundly affected by the presence of earthworms. Red-backs rely on a thick duff layer with a healthy population of tiny invertebrates to eat, both of which decline with increasing worm densities.
In their native habitat the work of earthworms might have pleased Darwin, but in a northern hardwood forest it is becoming clear that these little wigglers are a big problem.