Around this time last year, I started my first garden. I grew up digging in the loamy earth of an old riverbed in Connecticut, picking out earthworms to feed my pet duck. As such, I was completely unprepared for the obstinate brick of Central Valley clay waiting for me in my backyard. It was not only devoid of life, it was a place where earthworms went to die.
By the time it was all said and done, an entire Saturday (and the first two layers of skin on my hands) was gone. But I felt a quiet rush of happiness when I saw the first worm castings appear on the soil surface a few days later. I built it and they came!
Worms are so-called ecosystem engineers, changing the physical structure of the soil as they move through it and digest it. In doing so, they change the availability of resources for other organisms and even change the flow of energy through an ecosystem.
Although North American soils are teeming with earthworms, it may surprise you to learn that the majority of these worms are non-native species. The reason for the mass extinction of native earthworms? According to one leading hypothesis, as the glaciers receded ten thousand years ago, they scraped off the top layer of soil and drove all of the native earthworms in the Northern US and much of Canada extinct.
So, over the last 10,000 years, forests in these regions have evolved without earthworms. Without worms to help decompose organic matter such as leaf litter, other detritivores (i.e. fungi and bacteria) became wholly responsible for recycling nutrients. Because these organisms are a bit slower than earthworms at decomposing organic matter, gradually a forest floor accumulated. This layer of dead plant matter served as habitat for all manner of creatures, including insects, spiders, fungi, and small vertebrates.
Flash forward to the 1700s. European colonists have arrived in North America, carrying an array of goods in the hulls of their ships. In the hustle and bustle of colonization, Europeans also unknowingly transported a few stowaways, including viral diseases and earthworms.
As these non-native earthworms colonized forests in the Northern US, they also ate. In fact, they ate so much that the layer of leaf litter on the forest floor all but disappeared. In the Great Lakes region, populations of ground-nesting songbirds that depended on the litter for shelter declined. The litter had also provided protection for seedlings from predators and extreme temperatures. Slowly, the thick carpet of vegetation on the forest floor disappeared.
To better understand the distribution of these invasive earthworms, researchers at the University of Minnesota-Duluth created Worm Watch, a citizen science initiative. So far, Worm Watch has received over 25,000 earthworm specimens collected by residents in the Great Lakes region as well as Alaska and Canada.
“We’re are not only using data collected by citizen scientists to document the distribution and density of worms across the landscape, but we’re also identifying earthworm-free areas for preservation,” says Ryan Hueffmeier, a researcher with Worm Watch.
In California, earthworm awareness is on the rise, though not necessarily due to their detrimental effects on forest ecology. Although the increasing popularity of worm-composting (a.k.a. vermiculture) operations has led to concerns of non-native species introductions, Hueffmeier points out that the most commonly used species, the Red wiggler (Eisenia fetida) cannot survive cold Northern winters.
Proponents of vermiculture defend it fiercely, arguing that it binds nutrients to aggregated soil particles and increases microbial activity, leading to a slow release of nutrients.
“The earthworms are actually putting the beneficial microorganisms back into the soil that need to be there for the plants to take up nutrients naturally,” says Larry Royal, owner of Earthworm Soil Factory. “It’s very similar to the forest—nobody fertilizes the forest, yet it continues to grow.”
Whether you’re an earthworm aficionado or an ecologist tracking the movement of invasive species across the landscape, few can disagree that these invertebrates are quiet revolutionaries. The humblest of creatures, the earthworm, is changing the face of an entire forest ecosystem while we unknowingly go about our lives aboveground.