Friend or foe? Invasive earthworms can benefit agriculture but harm forests
Featured Photo Credit: T. Brown via Flickr
Reference: Frelich, L.E., B. Blossey, E.K. Cameron, A. Davalos, N. Eisenhauer, T. Fahey, O. Ferlian, P.M. Groffman, E. Larson, S.R. Loss, J.C. Maerz, V. Nuzzo, K. Yoo, and P.B. Reich. 2019. “Side-swiped: ecological cascades emanating from earthworm invasions” Frontiers in Ecology and the Environment. DOI: 10.1002/fee.2099
The earth under our feet is teeming with diverse lifeforms, sustained in a nearly boundless network of cryptic but intricate ecological connections. Pause to consider the life of the soil. If there is one creature that comes to mind, a sort of mascot of the below-ground realm, it is the earthworm. We have befriended earthworms, welcoming them as heroic engineers and caretakers of the soil—churning, aerating, fertilizing, invisibly toiling on behalf of the food ecosystems that sustain us.
And so I regret to tell you: there’s some bad news about earthworms. In nature there are no true heroes and villains. So much depends on context. That’s why it’s possible for these heroes of the farm to be villains of the forest. The same creatures that are beloved benefactors to garden humus wreak ecological havoc when they plow and chew through the leaf litter on the forest floor.
As a child I dug into my great-uncle’s garden to snatch earthworms from the crumbling earth. I stored a tangle of them in a cup, then one-by-one threaded them onto a hook to catch pan fish in the neighbor’s pond. This rite of passage seemed as homegrown as any American pastime I could imagine. So I felt a gut punch when I learned these companions of my childhood were in fact nonnative, invasive species. And a double whammy: the popularity of worms as fishing bait has fueled their spread through a vast swath of North American soil, from north to south, east to west, from prairie to farm to forest.
In agricultural contexts invasive earthworms have been welcomed, but it’s a different story in the forest. Why? Forest soil has a different structure and—what is obvious to any casual observer—supports an entirely different community of plants and animals. Importantly, forest soil is topped by a layer of leaf litter (biological “detritus”), which is slowly decomposed and incorporated into the soil by microbes and small animals including earthworms.
The trouble with earthworms in the forest, the authors of a recent paper explain, is that they are “in essence, stepping on the gas pedal for processing detritus.” They do their work too fast, shrinking the thick top layer of leaf litter that would otherwise protect underlying soils against erosion and temperature swings, not to mention providing essential habitat for plants, animals and microbes.
Without the insulating effects of leaf litter, soil can become warmer and drier in midsummer. Nutrient leaching increases. Complex changes in soil chemistry unfold over time. The physical structure of the soil changes, becoming inhospitable to many organisms while nurturing a few into dominance. The nature and mechanisms of these changes are complex and interrelated, but the combined effect of earthworm activities is to decrease fertility and biodiversity in forest soils.
The list of potential ecological outcomes in earthworm-invaded forests is daunting. Earthworms can incite chains of events that affect carbon storage, interact with other disturbances (such as fire) to increase tree mortality, promote erosion and decrease water quality, increase drought susceptibility (especially alarming in the context of climate change), facilitate non-native plants, and change resources available to the wider community of forest wildlife, including vertebrates.
Some of these impacts have clear potential to affect human health, although specific outcomes are difficult to predict. For example, there is evidence that earthworms cause a cascade of effects that promotes ragweed, a common human allergen. Earthworms can also indirectly affect habitat for the tick that transmits Lyme disease.
Earthworm impacts are extensive and also feed into a massive complex of other ecological relationships and alterations, with potential to affect not only the environment, but also the economy and human health. Amid so many changes tracing a clear path from cause to effect requires dedicated scientific bushwhacking. The authors stress that an interdisciplinary approach is crucial for finding a way forward and understanding how invasive earthworms are changing the North American landscape.
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