Earthworms: An Invasive Helper
Earthworms are often considered a gardener's best friend. They’re used around the world for composting and improving soil quality - but did you know that they aren’t always a boon to your backyard? Beneath the helpful position in our microenvironments, there’s a complex ecological history behind the humble earthworm - and it isn’t always good!
Earthworms in their Natural Habitat
The earthworm is all over - but, it’s actually native to Europe and Asia! They can be found all over the globe, and in every continent - with the exception of Antarctica. In their native ecosystems, earthworms play a vital role in soil health and nutrient cycling. Through their burrowing activities, earthworms actually aerate the soil, which in turn allows for better water infiltration and more root growth. Earthworms also consume organic matter, and break it down into nutrient-rich castings. The castings enrich the soil and provide essential nutrients for growth. All in all, their interactions with microorganisms enhance soil fertility and promote healthy plant growth. As if the earthworm didn’t do enough to promote plant growth, they’re also vital food source for numerous native species, including birds, mammals, and invertebrates.
The earthworm is very involved in the intricate webs of life in their ecosystems. Remember, though, they’re invasive - and no matter how useful an invasive species may be, they can still cause issues in non-native environments. To that end, our gardener’s best friend actually poses a major problem in North America.
Earthworm Classification
To understand why the earthworm was such an issue in the section ahead, it is important to know how earthworms are classified. Earthworm species are usually categorized environmentally as being epigeic, endogeic, or anecic.
Epigeic species live in the organic layer of litter near the soil surface and have short life cycles. Epigeic species, like the earthworm, are often used commercially for composting.
Endogeic species live in the mineral soil and seldom rise to the surface, so they are not frequently encountered or utilized by humans!
Anecic species are those that form permanent burrows in the mineral layer, but come to the surface to feed at night. Funnily enough, you probably are familiar with one of these species - nightcrawlers are anecic!
North America had plenty of endogenic and anecic worms - but, we didn’t have any native epigeic worms until the European settlers reintroduced them to our area. Those earthworms are epigeic - so they caused a big problem in our forests!
Earthworms Invade!
Historically, North America hasn't had a native earthworm species since the glaciers from the Pleistocene ice age wiped out our native population roughly 10,000 years ago.
While many areas remained ice-free (like the southeastern US, Central America, and the Pacific Northwest) and could maintain an earthworm population, our northeast - stretching as far down as Pennsylvania - wasn’t so lucky. The entire northeast has no native earthworm species!
Without our earthworm friends, thick blankets of leaf litter would fall in old-growth forests and remain for several years. These leaf blankets would break down slowly, providing nutrients to young saplings while covering and protecting them. This continued for a while, promoting dense and healthy forest growth, until European settlers re-introduced earthworms to North America. It’s thought they may have brought the worms as fishing bait, but ecology was still young, and they had no idea the consequences of their simple bait roaming free. To that end, suddenly these dense floor covers were colonized quickly by the new worms, and the old-growth forest of the northeast suddenly lost its protective layer of leaf mulch. The young saplings now needed to fend for themselves.
As you can probably guess, a sapling doesn’t do very well against a herbivore.
Reshaping the Commonwealth with Earthworms
While the northeast region still had detritivores (animals that feed on dead organic material) and other insects like ants, crickets, and millipedes, they were no match for an invasive species. The native insects and detritivores that ate leaf litter and aerated the soil were outpaced rather quickly. The earthworms spread rapidly, outcompeting the rest of the native species.
As they spread, the earthworms pulled nutrients away from the leaf layers as they tunneled towards the dense root growth of old trees. The young saplings now had no protection, and no nutrients. Naturally, fewer young trees were able to replace the older trees that were cut for lumber or dead/dying. Due to the lack of new tree growth, grassland species were able to move in and quickly took land that originally was uninhabitable pre-earthworms. This changed the ecosystem of the area quite heavily, and likely contributed to an ecosystem collapse.
Balance: Essential Conservation
While earthworms indirectly caused the fall of an ecosystem in the northeast, they are still extremely beneficial in many areas. For the southeast US, earthworms play an important role in regulating our soil for grasslands and open-canopy forests. They do so while providing an important food source for many animals. At this point, earthworms are ingrained in our ecosystems and contribute in even our daily lives. Any attempt to remove or control them would now cause significant harm with no real positive return.
Understanding the history of these worms and the unforeseen damage they caused ultimately helps us understand the delicate balance of our ecosystems. By learning and studying such things, we can do a better job of protecting our native environments in the future - with a watchful eye for invasive species, of course.
