The Invasive Species Wriggling Beneath Your Feet

In this guest post, student Jason Chen explains how some of the most invasive species can be wriggling out of sight, right beneath our feet.

Earthworms are the last animal most people would expect to be invasive in North America. They definitely don't look like they're very good at being nefarious: they sort of ooze along burrows in the soil, eating leaf litter, and only make their existence known to the outside world when they crawl out of their burrows to escape being drowned (after which many of them dry into a disgusting crust on the sidewalk when the sun comes back out). Earthworms don't have the gluttonous reputation of the brown tree snakes of Guam, which have eliminated multiple endemic species of birds from the island. Nor do they have the noxiousness of the brown marmorated stink bug, now infamous in eastern North America for its population booms and pungent defensive odor, or the destructive beauty of the purple loosestrife, which blankets wetlands in tall spikes of attractive magenta flowers. If anything, they've been praised as the farmer's friend since Charles Darwin, who wrote an entire book extolling the humble worm for speeding up the decomposition of organic matter and making tunnels that aerate the soil.

However, that same ability to alter soil architecture that is so valuable to gardeners and farmers is also dangerous for habitats in which worms should not be present. Even worse, people don't seem to realize that the earthworms tunneling through their flower beds and lawns are most likely introductions that arrived as early as the early 17th century. Today, roughly 45 European and Asian species, including the well-known nightcrawler (Lumbricus terrestris), of baitshop fame, and the redworm (Eisenia fetida), frequently used in vermicomposting, have joined the hundred-odd native worm species that survived the last ice age, along with exotic species from less well-known groups such as Amynthas, Octolasion, and Aporrectodea. They have embedded themselves so deeply in their new home that many of us have accepted that these earthworms have always been here, busily tilling and enriching the soil for our benefit. Without a second thought, people continue to distribute and release invasive earthworm species in community composting heaps, potted plants, and bait shops, just as early English colonists dumped soil ballast crawling with worms from their ships hundreds of years ago.

What have these introduced earthworms accomplished during the several centuries after they set up house and established themselves on the North American continent? Earthworms are canonical ecosystem engineers: organisms that play such an important role in creating and maintaining their habitats that their activity can decide which other species benefit or lose out in that environment. Just as beavers create ponds and meadows by cutting trees and building dams, worms of different species can influence natural processes in soils. Native earthworms probably shape different kinds of soils differently from each other and from non-native worms. In the Great Lakes region, which has no native earthworms, invasive earthworms have been found to affect nitrogen levels in stream water by directly excreting ammonium waste into the soil, possibly contributing to nitrogen pollution in the dead zone of the Gulf of Mexico. Introduced earthworms also compete for resources with native species, including soil microarthropods, native earthworms, and millipedes.

Even worse, they can act as agents for the establishment of a host other non-native plants, which can in turn aid the establishment of other species: taking ‘ecosystem engineering' to its ultimate conclusion by building a new community of species within the old, in a process aptly termed ‘invasional meltdown'. Nightcrawlers have been found to facilitate the survival of seedlings of at least one non-native shrub, the European buckthorn, by eliminating competing plants on forest floors. The interaction also works the other way around: Buckthorn leaf litter is more nutritious than the leaves of native trees and shrubs, such that removing buckthorn bushes in an area can actually decrease nightcrawler numbers in an area. Essentially, nightcrawlers and buckthorns are engaged in a mutualism that aids the spread of both partners into native habitats. Other invasive plants, including garlic mustard (Alliaria petiolata), Japanese barberry (Berberis thunbergii), and Japanese stiltgrass (Microstegium vimineum) also seem to benefit by associating with earthworms.

Unfortunately, it is probably too late to do anything about the unfolding earthworm invasion. All biologists can do now is to monitor the spread of the worms across the continent and document its effects on native ecosystems. If there's anything we should take away from all this, it's that we humans urgently need to address our vast ignorance of the far-reaching consequences of events that happen beneath our feet, even when most of us don't even realize how complex . Several attempts to educate the public about this quiet conquest have indeed been established, and in the course of writing this I ran into excellent information websites about invasive earthworms by the University of Minnesota, the University of Alberta, and various state agencies. Whether these programs work is another question: The University of Alberta researchers evaluating the effectiveness of their public outreach, which included appearances on radio programs, magazine articles, and television clips, found that they reached less than 5% of Alberta anglers, their target audience. The program's impact was dismal: The percentage of anglers they surveyed that knew that earthworms were non-native in Alberta barely changed before and after implementing the program, and even worse, nearly half said they would not change how they disposed of extra bait worms, "with many commenting that they did not believe earthworms could be harmful."

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Jason is an undergraduate at the University of Pittsburgh, studying ecology & evolution and classics. He likes to do things all normal undergraduates do these days: eating, doing homework, watching TV, and raising giant caterpillars in his room.

Image credits:

• The European nightcrawler, Lumbricus terrestris. Image by Michael Linnenbach, made available under the Creative Commons Attribution-Share Alike 3.0 Unported license.
• Japanese stiltgrass, Microstegium vimineum. Image by Siddharth Patil, made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication