Andy Holdsworth is a PhD candidate in conservation biology at the University of Minnesota. He studies the ecological effects and conservation implications of nonnative earthworms in Minnesota and Wisconsin.
Tuesday, 2 Sep 2003
There is a blurry line between work and play in my life. My wife, Hillary, learned this lesson for the Nth time this Labor Day weekend. She agreed to help me with one day of field research in exchange for a day of unscheduled canoeing and a day visiting a friend at a cabin in northern Wisconsin. As agreed, after six hours (less than a day!) of counting bunches of sedges (grass-like plants) in my research plots, we gleefully headed off to play in the Porcupine Lake Wilderness Area of the Chequamegon National Forest in northern Wisconsin. We portaged Moxie (our canoe) a half mile and slipped onto the lake for a leisurely afternoon paddle. With Hillary napping in the bow and me gazing at the wheeling and courting dragonflies, leisure seemed to have set in very nicely. Then I suggested we paddle to the shore and go for a little hike along a tributary stream …
We sauntered along admiring the pretty forest. Within five minutes, however, I couldn’t resist the urge to start scraping at the dead leaves on the ground. I had seen a distinctive lumpiness in the layer of leaves. I grabbed one of these lumps and below it I found what I expected: the burrow of Lumbricus terrestris, the earthworm that we all know as the “nightcrawler.” A foot away I found another and another and they continued for half a mile along the stream. Chatting with Hillary about her dreams of traveling to Cuba, I thought my earthworm curiosity was satisfied until I walked about 200 yards away from the stream and found “the leading edge.” “Hillary, we can go to Cuba if we find the rest of this leading edge.”
On the stream side of the edge, the layer of leaves was very thin and the ground was very hard underfoot, like pavement. Once we crossed over the leading edge the ground was like walking on a cushion with a thick covering of dead leaves. What made the difference? Earthworms. Apparently, fishermen dumped their leftover bait worms beside the stream and they spread about 200 yards into the forest. Across the leading edge there were no earthworms. I dispatched Hillary to confirm there were no earthworms further in the forest while I whipped out my GPS unit to map this leading edge. Wow, cool isn’t it? Okay, what other response would you expect from a guy whose friends call him “worm boy” or “earthworm Andy”? What is the big deal?
The leading edge we found was made by several species of nonnative earthworms. In fact, all of these earthworms are from Europe. Until European settlement of Minnesota and Wisconsin (and most of the northern edge of the United States), there were no earthworms working the soil. Yup, the last glaciation that ended about 11,000 years ago wiped out any earthworms that were in this region. For thousands of years, the forests and grasslands of this region developed without the influence of earthworms. When European settlers brought their fruit trees and other plants to the New Country, they inadvertently brought their Old Country earthworms. The use of earthworms as fishing bait has further scattered nonnative earthworms across the landscape. Until about 30 years ago, nobody thought anything of this.
Another graduate student in my lab has started studying these leading edges of earthworm invasion. She has found that the invasion of nonnative earthworms reduces the number of understory plant species by half and significantly reduces the cover of these plants, leaving mostly sedges and Jack-in-the-pulpit (check out photos and more info). This change results in the complete loss of the duff layer, or the partially decomposed leaf fragments (see photos of this, too). Without earthworms to incorporate this thick leaf mulch into the topsoil, forest herbs and tree seedlings rooted themselves in the duff layer. When earthworms invaded, the plants were essentially uprooted. (To learn more about these and many other changes caused by nonnative earthworms, visit the Minnesota Worm Watch website.)
But how will forests adjust to the invasion of “nature’s plow”? Which plant populations will recover and why? How will nonnative earthworms affect the productivity of northern hardwood forests? What are the implications of nonnative earthworms for forest conservation efforts? Many forest managers are asking these questions. I hope my PhD research provides useful answers.
Now it is back to the nitty-gritty of research work. It’s time to pick up my field equipment and field assistant, and head out to harvest samples from a field experiment. The work and play in the Porcupine Lake Wilderness Area seems so far away.