Spring has arrived, and with it have come the robins. These harbingers of warm weather are fun to watch as they scurry and hop on the wet grass in front of the Hunnewell Building and in the Bradley Rosaceous Garden, prying hapless earthworms from the soil. However, the sad earthworms in question are all non-native interlopers, and although they may lose their battles with robins, they are collectively winning the long war on New England’s native ecological communities.
One of the earthworms that these early birds snack on is Lumbricus terrestris, better known as the common earthworm. At the Arboretum, you can find thousands of these earthworms during the early spring and fall without even picking up a shovel. Channeling your inner robin, look down at patches of bare soil or between clumps of grass. Almost anywhere you go, you will see hundreds upon hundreds of earthen pyramids, called middens, spread over the landscape. Middens are actually just piles of worm poop that mark the entrances to vertical burrows (up to six feet deep) within which rests a single earthworm.
Every night, especially after a good spring rain, Lumbricus terrestris comes to the surface to search for food. This habit is responsible for their second common name, “nightcrawler.” Reaching out from their burrows each worm uses a lip like appendage called the prostomium to grab leaf litter and other organic material laying near its midden. The worm pulls this material back inside, where it decomposes and is consumed. However, since the worms do not eat the hard stems, these remain protruding from the tops of their middens like flags on a fort.
During the heat of the day, as well as through the hot mid-summer months, L. terrestris retreat down their burrows to avoid desiccation. If a keen eyed robin manages to grab a resting nightcrawler in its burrow, however, the worm uses tiny hairs on its body, called setae, to anchor itself to its burrow walls. That is why you may see a robin stretching a worm like a rubber band, only for the worm to pop free and disappear back into the soil.
However, unlike its avian nemesis, the “common earthworm” is not even native to New England. In fact, before the arrival of Europeans, New England had not had native earthworms for at least 12,000 years (the end of the last ice age), if ever at all. These days, the region is home to over two dozen non-native species. The introduction of Lumbricus terrestris and Amynthas agrestis, among others, has changed the very soil beneath our feet, with profound implications for native plants.
Although earthworms provide benefits to agricultural soil, such as incorporating organic material and increasing air and water infiltration, in New England’s hardwood forests they also consume the thick leaf layer, mix soil horizons, decrease fungal presence, and increase nitrogen leeching. Changing the soil affects the above ground plant community as well. Trees including Acer rubrum (red maple), Acer saccharum (sugar maple), Quercus rubra (red oak), and Tilia americana (american linden) are just a few of the many plants that struggle to recruit seedlings in forests invaded by these exotic earthworms.
In turn, changing plant communities effect the animals that depend on them, from birds that depend on young trees for nesting, to amphibians that breed in the leaf layer, to mammals that forage on hardwood understory plants. For this reason, earthworms such as Lumbricus terrestris are rightly regarded as a keystone organisms, since they have an inordinate amount of influence on the other species in the ecosystems in which they live. So, the next time you see a spring robin triumphantly pull a sad looking worm from the ground, think about it this way: although the early bird may get the worm, the worms may have the last laugh.