An earthworm on my driveway (student guest post)

It was a damp afternoon; it had been raining for most of the day and humidity was still in the air when I returned from my run and took some time to recover by stretching my legs. Seating myself on the driveway as I stretched, I noticed a six-inch-long earthworm winding its way slowly across the asphalt. I have often seen dead, desiccated earthworms on asphalt and concrete surfaces –my understanding of the phenomenon is that earthworms burrow up out of the ground during heavy rain to avoid being drowned as the soil becomes saturated. Usually, a few individuals seem to have the misfortune of ending up on impermeable asphalt or concrete surfaces and dying before they are able to find their way back to moist soil after the rain ends. I watched this particular earthworm as it progressed slowly across the porous surface of the asphalt, sticking its anterior end into each chink and crevice it came upon, apparently trying to burrow back down into the ground. Of course, each attempt at burrowing was thwarted by the impermeability of the asphalt. It was sad to watch the futility of the earthworm, guided by instinct, trying to burrow into this strange environment.

A quick Wikipedia search of earthworm physiology and behavior revealed the mechanism by which earthworms move through soil. These mechanisms appeared useless on asphalt—an environment the earthworm is not evolutionarily equipped to deal with. The earthworm’s slow pace brought to mind the issue of scale. The short distance of a driveway, traversable quickly in human terms, is for the tiny earthworm a much more formidable expanse. Watching the confused route the earthworm was taking, I couldn’t help but think it would probably live out the rest of its short life on that driveway, like so many of the poor, desiccated carcasses I had seen before. We often think of the effects that roadways and other manmade barriers have on larger animals like deer, but habitat destruction and fragmentation clearly operate at all scales, including the very small. By placing artificial barriers like roads or driveways in the middle of an earthworm habitat, could humans change the environmental conditions governing the evolution of earthworm populations?  Islands of earthworm populations subject to the sorts of processes that take place on oceanic islands?  Insular populations separated by concrete barriers?

A few days after my earthworm observations, we discussed the ecological role earthworms and other “soil biota” play in helping regulate soil health in my Science of Environmental Issues course. The soil is a habitat like any other, home to a diverse and extensive ecosystem of organisms – plants, fungi, arthropods, and microbes – that are linked to the aboveground ecosystem through the food web.  By recycling nutrients like nitrogen and phosphorus and making them available to plants, soil biota help maintain soil quality and productivity. According to many sources, earthworms in particular help to increase water infiltration and water retention by soil through their tunneling and burrowing activities; they also mix and aerate the soil. These functions are thought to help to make soil more arable so that plant species to take root and develop more easily.  But is this true?  Apparently not all scientists agree.

Listen to a story on invasive earthworms featured on NPR’s Talk of the Nation

Listening to the above NPR Science Friday podcast suggests a more nuanced perspective on the ecological significance of earthworms. It turns out that earthworms, like the one I saw exploring the topography of my driveway, may not be as innocuous or even as beneficial as I originally thought. Apparently, most earthworm species in the forests of the northeastern United States are non-native transplants from Europe and Asia and may be deteriorating soil quality in these ecosystems. It seems hard to believe, but the emblematic nightcrawler species, whose uses as bait and perceived importance in gardening make it such a visible part of American cultural life, is actually an invasive European species.  Native North American earthworm species are not found in higher-latitude states because they were killed around 15,000 years ago when the land was glaciated. Considering that I live north of the glacial boundary, it is very likely that the earthworm I saw on my driveway was one of the interloper species.

Students in general ecology (EES-152) studying the density of earthworms on South Mountain in 2009. (Photo: RK Booth)

Conventional wisdom among farmers and gardeners has long held that earthworms improve soil quality—exactly as I was led to believe from my previous class discussions.  However, some scientists refute this claim, arguing that while the movement of earthworms has been observed to aerate compacted soil, leading to increased productivity, soil in most agricultural and forest systems is not compacted and receives adequate aeration without earthworm activity. Also, many studies that have attempted to show that earthworms enhance soil productivity in agricultural systems have been anecdotal. Although earthworm activity likely does help to accelerate the movement of water through soil, this may not always be a good thing for soil productivity, as too much earthworm activity can result in water leeching from the soil too quickly.

Effect of invasive earthworms on forest understory and soil profiles. From Wisconsin DNR.

The main problem with non-native earthworm species in the forest ecosystems of the Northeast is that they consume the rich layer of organic matter that coats the forest floor. This organic “carpet” is essential for soil productivity and the health of the forest ecosystem, as it protects soil from erosion; provides proper environmental conditions for certain plant species, like spring beauty, trillium and trout lily, to take root; provides a habitat for certain animal species, like salamanders and beetles; and stores carbon and nitrogen, preventing the release of these elements into the atmosphere or nearby water systems. However, earthworm populations are extremely hard to remove once they’ve been established. Human efforts should probably be aimed at halting their spread rather than trying to eliminate existing populations. Education and outreach to people who use earthworms in different aspects of their daily lives, he said, is the most effective way to control the problem. Accidental transfer by humans is, of course, the way these organisms likely arrived in the non-native environment of North America to begin with–as is the case with so many invasive species.

All from watching a little earthworm trying to make its way through our world….

-kag-

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Posted on March 27, 2012, in Conservation & Biodiversity (EES-28), Student Guest Posts and tagged , , , , , , , . Bookmark the permalink. Leave a comment.

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