Blog Archive:

Beneath Our Feet

GSBI 2018-05-02 GSBI 2018-05-02

Earthworms in the National Parks

By: Mac Callaham, USDA Forest Service Southern Research Station, Georgia, USA

Photo from George Washington Memorial Parkway.

I plop down into my seat as I board the flight. I give a brief nod of greeting to my neighbor, and notice that he is (like me) of an age which might indicate that he could expect to pass the flight time in conversation, rather than totally absorbed in a liquid crystal display.

I start with a standard question, “Are you headed out, or headed home?” And we’re off!

When it’s my turn, I tell him that I’m headed out to a conference where I’ll give a presentation on my research on earthworms. I know this is a topic that will keep the conversation rolling all the way from Atlanta to New York… fortunately, we have refreshments!

It’s generally true that people are interested in earthworms, and even more so when I tell them that I study invasive earthworms. People are also quite interested – and protective – of their National Parks, so I find that I usually have an attentive audience when I talk about my work on earthworms in the Parks.

“Invasive earthworms?!?” they exclaim. “There’s more than one kind?” they gasp. Then in rapid succession: “Where do they come from? What do they do? How do you know they’re invasive?” and more. I do my best to answer them, and eventually get around to describing the work that my colleagues and I have been doing at the Great Smoky Mountains National Park (GSMNP) in Tennessee, and the George Washington Memorial Parkway (GWMP) in Virginia and Washington, DC.

In the GSMNP it’s an introduced Asian worm that’s moving into the Park from the edges. This worm is one that’s sold as bait, and goes by the trade name of “Alabama Jumper.” It’s aptly named, because (although not technically from Alabama) they definitely jump. When molested these worms can flip and twist and throw themselves up off the ground, sometimes catching several inches of air in the process. This species is a cause for concern because where it becomes established, it can reach very high densities, and it consumes much of the fallen leaves on the forest floor. These leaves are habitat for lots of other invertebrates, as well as some larger animals like salamanders. It’s no surprise that the GSMNP is considered a hotspot of biodiversity for forest floor creatures like millipedes and salamanders considering the rugged, inaccessible terrain, the lush vegetation, and the warm and wet conditions that predominate. On the other hand, it is a bit surprising, and alarming that one introduced earthworm species can outcompete these other forest floor dwellers, and impact their numbers and diversity, but this is exactly what our team has documented in recent years.

At the George Washington Memorial Parkway (GWMP) in Washington DC, we sampled earthworms and other soil invertebrates to help catalog the existing biodiversity of the Parkway and its constituent Parks (including Great Falls NP). We were also interested in whether the past history of human uses of the land could be detected in the soil animal community. There’s evidence that major disturbances can contribute to the establishment of non-native earthworm species, and the GWMP provided a great opportunity to examine this relationship with well documented history of soil disturbances ranging from the canal-building activities at Great Falls Park in the 1780s, through the Civil War entrenchments, and on into contemporary disturbances. We’ve uncovered good news and bad news with this work. On the one hand we’ve found what we believe to be an undescribed native species of millipede at one of the GWMP network parks (Turkey Run Park), but on the other hand, we found several individuals of a European earthworm species that has never been reported in North America before our sampling.

Alabama Jumper.Photo © Susan Day / UW Madison Arboretum — Alabama Jumper.
Photo © Susan Day / UW Madison Arboretum

All this brings up a couple of important points. First, we still don’t seem to have a good handle on the species diversity that we have native to the soils of North America, and second, there are new species introductions happening all the time, and we don’t have a good handle on what these introductions will mean for the native species. Will this latest introduced species become invasive? Will it choke out some native species, or otherwise decrease our native biodiversity? Is there any way to control these invasive species?

Out the window, our flight from Atlanta to New York has skirted the eastern slope of the Smoky Mountains, and passed within sight of the National Mall in DC. My new friend leans over and looks down on these landscapes. He is reflective.

“Wow, there’s a lot going on down there that I never thought about before…”

I nod knowingly. I tell him that few people ever really think about what’s going on right under their feet, but I tell him that now he’s one of the lucky ones.

GSBI 2017-07-17 GSBI 2017-07-17

Earthworm invasions in northern forests

By Erin Cameron, Postdoctoral Researcher, Helsinki University, Finland

Lumbricus terrestris. Image by E. Cameron — *Lumbricus terrestris.* Image by E. Cameron

Finishing my paper route always took longer on rainy days when I was a kid – I had to walk slowly to avoid stepping on earthworms and occasionally I stopped to move them off the sidewalk. I would never have believed that earthworms were invasive in much of Canada and the northern United States! In those previously glaciated areas, there are no native earthworms, but instead only European earthworms that were introduced to North America with the arrival of European settlers.

Earthworm populations can only expand about 5 to 15 meters per year on their own, and consequently people play a key role in their spread. For my master’s thesis with Dr. Erin Bayne at the University of Alberta, I tried to determine how earthworms were spreading in Alberta’s boreal forest. I was sold on the research question once I realized that I’d need to canoe or kayak across lakes to test whether earthworms were most common near boat launches where anglers might dump their earthworm bait. After a couple near misses but only one capsizing incident, we found that earthworms were present at approximately 70% of the boat launches and roads sampled, but only 35% of far shores and less than 15% of forest interiors. They were also more likely to occur at older roads than more recently built roads, suggesting that earthworms were introduced by vehicle traffic (their eggs can become stuck in tire treads) not during construction of the roads.

After examining how earthworms were being introduced, we started to investigate their effects in the boreal forest. Surprisingly to most people, earthworms do not always improve soil health or benefit other organisms. When exotic earthworms invade forests where there are no native earthworms, they consume leaf litter layers, mix organic and mineral soil horizons, and affect nutrient cycling. These impacts on soil structure and ecosystem functioning can then lead to cascading effects on other organisms. In northern Alberta, we found that earthworms decreased the thickness of the leaf litter layer, reduced the abundance and diversity of microarthropods, and decreased plant biomass, depending on the species. Not all species were negatively affected though – one of the key predators of earthworms, the American robin, was more likely to occur in areas where earthworms were present.

Sampling for earthworms in the boreal forest. Image by R. Rocha

Earthworm invasions are at an earlier stage in northern boreal forests than temperate hardwood forests, where the deep burrowing and mineral soil dwelling species that cause the largest changes are more widespread. At our study sites, the most common species is a litter dwelling species called Dendrobaena octaedra. But because most people are not aware that earthworms are invasive, they continue to introduce earthworms by dumping their bait, moving soil, or not cleaning their tires when travelling to remote areas. We started a citizen science project to collect data on earthworm distributions across Alberta, which at the same time serves to increase public awareness about earthworm invasions: http://worms.educ.ualberta.ca

However, earthworm invasions are occurring globally, rather than only in North America. We also lack data on distributions of native and exotic species of earthworms at broad scales, making it difficult to determine the key factors driving their distributions. To address this issue, we started a working group (sWORM; https://www.idiv.de/?id=429) at the German Centre for Integrative Biodiversity Research (iDiv) to synthesize data on earthworm distributions. Let us know if you have earthworm data and want to participate!