The Unexpected Threat to Northern Forests
Owners of dogs and cats are well aware of the problems caused when their pets have worms. Luckily, there are remedies for those mammalian issues.
But now there's another serious invasive forest pest, one that we have welcomed, and thought was completely beneficial; one that has insidious behavior, yet seems so benign and desirable to have around. It's advancing into our northern forests, altering their composition, even destroying them. And there's no remedy, no pill to stop it. What hideous critter is this?
Scientists call it "Lumbricus terrestris". Most of us know it better as ... the common nightcrawler ! Actually, it's not just the nightcrawler that's a problem, but a number of earthworm species, none of which belong here. In the northern reaches of the U.S., where glaciers occurred, there are no native earthworms (states south of the glacier's limit do have native earthworms, which are not a problem in those forests).
The Forest FloorFirst, the background story. Ten thousand and more years ago, huge glaciers covered the northern portion of our continent, as far south as New Jersey in the east, west through the Great Lakes region to the west coast. No native earthworm species, if they were here prior to the glaciers, survived under the ice sheets, which were thousands of feet thick. In the post-glacial period, our forests grew back, following the melting glaciers northward. But native earthworms in the unglaciated regions to the south only advance in the soil a couple dozen feet or so per year, so they couldn't move northward as fast as the forest did when glaciers retreated; thus, our northern forests have existed for millennia without them, and are adapted to the worm-free habitat.
Each autumn, as leaves fell, they accumulated on the forest floor; they were slowly decomposed by fungi, microbes, and other organisms.
|A natural forest floor duff layer
Understory plants, including wildflowers, shrubs, and tree seedlings, take root in that layer, which lies above the mineral soil. They depend upon that carpet of duff for nutrients, and as a rooting medium. In a good site
|Intact herbaceous plants
In an undisturbed forest, that floor layer becomes a giant web of mycorrhizae, which are root cells modified by the presence of certain fungi (eg, amanitas, boletes, chanterelles, etc).
|Chanterelles, a mycorrhizal mushroom
The largest percentage of plant and tree species (80% or more) have mycorrhizal relationships, which are highly important to plants, particularly in poor soils. Mycorrhizal fungi excel at gathering phosphorous from the soil, and trees in nutrient-poor soil do far better when they can form mycorrhizae. That's how a proper forest should be working.
Today's Forest Floor Problem
In our northern states where the non-native earthworms have invaded the hardwood forests, they rapidly eat newly fallen leaves and small twigs, consuming the duff layer, and mixing it into the lower soil layers via their movements into the ground. The duff layer is not replenished as quickly as it is eaten, and disappears. Earthworm activity dramatically decreases the mycorrhizal relationships, and thereby harms the plants that rely on them and the duff layer. The makeup of the soil layers is changed, to the detriment of the forest community that has occupied the land for millennia. Worms break up the organic layer to the point where important elements such as nitrogen, phosphorous, and carbon are leached deeper into the soil, where young plant roots simply can't reach them. Herbaceous plants can virtually disappear, leaving the ground barren and brown. The soil becomes drier due to aeration by the worms. Compare this photo of stark, worm-damaged woods to those of the undamaged old growth forests above:
|Worm damaged, impoverished forest floor
Notice how there's no duff under the thin layer of leaves (not to mention the obvious lack of green plant life on the ground). The photo was taken at the height of the growing season, in a suburban forest.
There are some plant species that don't participate in mycorrhizal relationships, and may survive the onslaught. So you may see a diminished complex of plants, comprising a sparse variety of species, with things still looking green. For example, Pennsylvania sedge may take over the area. You may find Jack-in-the-pulpit, or False Solomon's Seal.
Some of the trees that appear to be particularly sensitive to the disappearance of the forest floor layer are Sugar Maple, Red Maple, Red Oak, Basswood,
Serviceberry (Shadbush), and Hophornbeam. Their seedlings don't do well without the duff's benefits. Ash seedlings, however, can do well (but they're under severe attack now by the Emerald Ash Borer). Nightcrawlers prefer to eat the most nutritious leaves, so the fallen basswood and maple leaves tend to go first; when they're gone, oak leaves will be eaten. The population of worms will be greater in the forests with the greatest quantities of the leaves they prefer, so basswood/maple forests tend to be affected more than oak/beech woods.
The forest is a complex ecosystem, so animal species are also affected. With the presence of earthworms and resultant loss of the floor layer, tiny invertebrates and microscopic organisms decline, which can affect the entire food web, from amphibians and reptiles to birds, small mammals, and large mammals.
Salamanders, for instance, go into decline because their young can't find enough of the duff layer-resident food they depend on. And with the drying of the uppermost ground layer, their habitat is degraded.
Where do these earthworms come from?
It's believed that the non-native ones that are found here were first brought in ships from Europe. Soil was used as ballast in the holds of those ships, and it no doubt contained earthworms. Potted plants were also brought here over time. So that's what started it all. Now, we also have some from Asia, including the latest invader, the "Jumping Snake Worm" (aka "Asian Jumping Worm", "Crazy Snake Worm"), Amynthas agrestis. The movement of the invasion front radiates outward from wherever the worms are introduced; it happens much more quickly as a result of human activities than from the natural advancement of the worms.
Now that the worms have become established, we spread them by various means. We move topsoil from one location to another; fishermen may discard unused bait worms; gardeners encourage them in compost piles; well-intended community composting sites provide soil containing worms to anyone who wants it; landscaping practices move plants with soil and worms long distances. Worms deposit tiny eggs and cocoons in the soil, and these can easily be undetected and transported into forest interiors (eg, in soil caught in the tread of tires, or even boots).
Heavy deer populations compound the problem when they browse herbaceous plants and tree seedlings. Add logging to that, and you really have a recipe for declining forest cover.