Myth: Earthworms Are Native to the U.S.

Spring is upon us. With the snow off the ground (for now at least), and little new growth to obscure our view of the soil, it is easy to see the early stirrings of earthworm activity. Earthworm castings, neat piles of worm poop deposited on the surface as the worms busy themselves with their soil-work, are already accumulating on the lawns and parkways. Although nothing seems as natural or as necessary as this silent toiling, earthworms are, in fact, not native to the Chicago area.

As crazy as it may sound, nearly every earthworm you’ve seen in Chicago is an import! Native or not, their presence in gardens is often welcome as they increase soil fertility, making for bigger plants. However, in natural areas, where plant and animal communities have developed over thousands of years on worm-less soils, earthworm presence can be disruptive to these communities. Worms may be excellent companions in our vegetable plots, but are not so welcome in our natural areas.

Impact of Non-Native Worms

Earthworms are themselves a pretty species rich group. Several thousand species are known. As is true of many other biological groups though, increasingly worm species are being moved from one part of the world to another. Many European and Asian species have become very prevalent in the United States. Though this may be good news for farmers and gardeners, the presence of non-native worms is not such good news for people who want to restore or protect native plants.

This is because alien worms can invade areas where there are no native earthworms, like in much of the Midwest. Concerns about earthworms have been increasing in recent years, due the greater scientific understanding of the damage these alien species can cause, including being linked to the decline of an endangered plant species.

The fact that every earthworm you have seen in the Chicago region has been a non-native species originally from Europe or Asia is so surprising to the non-specialist (yes, there are several ecologists who specialize in the study of earthworm, a testament to the importance of these “ecosystem engineers”) that the observation is sometimes met with disbelief. It is however fairly easy to understand the reasons why there are no native earthworms in this region. Much of the Midwest was covered in glaciers until a little over 10,000 years ago!

Alien earthworms probably first arrived in plants and soil brought to the Midwest from Europe on ships during the late 1800s and early 1900s. Soil and rocks were often used as ballast to adjust the weight of European ships and then dumped here; European plants desired by settlers were imported for landscaping purposes and their resident earthworms along with them. More recently bait has become an issue because all common fishing bait worms are non-native species.

No less a naturalist than Charles Darwin took a lively interest in the humble earthworm.In fact, he devoted an entire book to them. That book entitled The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits (1881) culminated an almost lifelong interest in worms. Nothing escaped his attention: their density in the soil, their taste preferences, and even their unusual sexual habits (their “passion” he said, “is strong enough to overcome for a time their dread of light.”) In particular, he meticulously quantified the rate at which worms convert leaves into soil, thereby increasing the fertility of the soil.

Darwin’s influential early work has been followed by a sustained interest in earthworms by soil ecologists.  In recent years, ecologists have adopted the term “ecosystem engineers” to refer to animals such as earthworms that can directly or indirectly alter the availability of resources to other species. That is, earthworms engaged in relatively simple activities such as burrowing through the soil, consuming leaves, and even depositing layers of mucous in their burrows, change the physical nature of the soil and its fertility in such a way that it can greatly benefit other organisms.

Generally, earthworm activity results in greater soil fertility and more plant growth. However, oftentimes only one or two plant species benefit (for instance, our crops, or, less desirable, fast-growing weeds). Where earthworms abound in the Chicago region, there typically will be fewer native plant species that you would find in areas without earthworms.

And When They Got Together…

Soils without native earthworms track the extent of the last glaciations. Areas extending north through Canada that were covered by glaciers have no native earthworms and the native earthworms which can be found in the southern U.S. are very slow to re-colonize northwards, moving approximately 5-10 meters per year. Alien earthworms move just as slowly. So, unless humans move alien worms into local woodlands and other native areas, it is very difficult for the worms to get to these areas on their own!

Although there can be few complaints about the mighty feats performed by worms in our gardens, there are concerns in the Chicago region that earthworms make the protection of native plants and animals in nature preserve even more difficult. For instance, the voraciousness of earthworms for leaf material noted by Darwin is responsible for reducing the buildup of leaf litter layer in our woodlands. The forest relies on the natural organic compost that occurs when leaf litter decays slowly. However non-native earthworms eat the leaves in this litter and cause it to disappear completely, resulting in the loss of the natural fertilizer produced by leaf decay and in fewer plants and wildflowers growing on the forest floor.

Leaf litter also is home to an exceptionally rich community of small creatures including soil organisms, insects, spiders, small mammals, salamanders and toads (see EcoMyth article January 2010). As a result, there may be a cataclysmic loss of litter critters—almost a local mini mass extinction—in these natural areas. In addition, we now know that the greatest abundance of worms is found where the invasive shrub European buckthorn is at its peak density. It seems as if the success of this earthworm invader is assisting in the spread of the plant invader (ecologists have coined the term “invasional meltdown” to describe this phenomenon). Currently, researchers are investigating the plausible idea that ecological restoration which improves degraded habitat also reduces earthworm populations. Preliminary results of this work suggest that areas with higher plant diversity that have been under restoration or management for longer periods of time have lower earthworm populations, but that earthworm-invaded areas may stay invaded for several years after restoration has begun.  That is, supporting ecological management of our natural areas may reduce problems associated with alien earthworms.

What You Can Do

  • Support and/or participate in ecological restoration in our natural areas!
  • Keep non-native compost worms out of the woodlands: Non-native earthworms are commonly sold around the country for home compost piles. Freeze the compost for at least one week to kill worms in the compost before you use homemade compost near a wooded area.
  • Don’t dump fishing bait or other worms in the woods; toss any unused bait in the garbage.

Written by Liam Heneghan and Lauren Umek, Institute of Nature and Culture, DePaul University