The worms in the bottom of a compost cone (Image: Brendan McGarry)

Amidst the pungent, half decomposed pile at my feet writhed a mass of life. Countless gnats whirled about, a Devil’s Coachman Beetle scurried to cover, and small, indeterminate grubs wriggled through coffee grounds and slimy banana peels.

And there were so many worms.

Staring at them, I realized a few things simultaneously. First, I have no idea what species I was observing. And two that I didn’t know if we have many native worms on the Hill or elsewhere in the Pacific Northwest.

I’ve been composting since childhood, in charge of emptying our family food waste into a worm bin and later into a lazy pile in our regular compost after my parents decided they couldn’t bother with worm bin upkeep. Memories of rodents scurrying about my feet during nocturnal visits to deposit kitchen scraps encouraged me to start using a plastic cone with a porous basket buried underground for our household waste. It restricts rodent access while allowing other creepy crawlies access to do their job.

The vast majority of us have positive attitudes towards worms. We’ve been told they do good work as detritivores, that they build soil fertility, and that our gardens need them. And while in some instances these things can be true (the market for worm castings doesn’t lie), it’s not the complete picture.

One of the many things we can blame on early European settlers is the introduction of non-native earthworms to our continent. By disgorging their boat ballasts of soil from the homeland onto North American shores, pilgrims and the like, unwittingly introduced a huge problem for the ecosystems that stretch from New Jersey across the midwest into Western Canada. This part of the world was essentially earthworm free after the last ice age, when glaciers covered a large swath of the continent and froze them out. Introduced European worms found ecosystems without any competitors and got to work swallowing the natural, slowly decomposing leaf litter of these healthy forests, along with the seeds of native plants, slow growing seedlings, and much. This type of slow burn degradation mingled with a suite of poor land use practices that is the hallmark of colonialism.

To be fair, we can’t just blame early immigrants for bringing earthworms over, because alone these species don’t spread quickly (afterall native earthworms hadn’t inched their way North after the last glacial maximum around 10,000 years ago). But people who fish for fun, raise and sell plants, and farm crops all collaborated in spreading soil contaminated with non-native worms both on purpose and unknowingly. Today the worms we see on the Hill may not be humble soil patriots, but interlopers who aren’t as overtly beneficial as we might think.

In our part of the world, it’s worth noting that we do have native worms. The glacial maximums in our region didn’t touch every corner of the Pacific Northwest and native earthworms persisted in various corners of the region. The Southeastern US is noted for having a decent diversity of native earthworms along with the less desiccated corners of Mexico and Central America. However, for the 180 or so native earthworms across the continent, there are at least 60 introduced species, most of whose impacts and distribution we don’t fully understand.

Taking another step back, let’s consider “earthworms” broadly. The first sentence of the Wikipedia entry offers this summary: An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. However, this phylum, the segmented worms, contains over 22,000 extant species that live in the deep ocean, in the intertidal zones, in freshwater, and of course, in the soil. Earthworm isn’t a very exact term but it generally refers to a group of related species in the suborder Lumbricinia. All of them are noted for being detritivores, eating dead and decaying matter, along with a smattering of still alive material.

Charles Darwin’s final scientific book before his death summarized his research and passion for earthworms. Despite being ridiculed by his peers, the book The Formation of Vegetable Mould Through the Action of Worms, with Observations on their Habits was a hit in garden-obsessed England (you can read a pdf of the book here). Darwin’s observations demonstrated that earthworms were doing important things for soil in England and truly did move earth through their diligent munching and casting. His devotion to these simple creatures was a runway towards much scientific attention and today we know even more about their genetics, anatomy, and behavior (fascinating stuff and beyond the scope of our focus today).

The species you have seen poking out from the soil in your garden or laid out on the sidewalk after a heavy rain are almost undoubtedly the large European Earthworm, Lumbricus terrestris. Other common introduced species are the European Nightcrawlers, Dendrobaena veneta and the star of many a worm bin, Red Wrigglers, Eisenia Fetida. All these species have the same basic anatomy, a head and a tail (a mouth and a butt), a long tube like gut, and the raised portion of their length, called a clitellum (which is part of their reproductive anatomy). Less obvious are their setae, the tiny hairs that line their length and help them move about and the pores that both release and collect sperm (all these species are hermaphrodites).

European Earthworm, Lumbricus terrestris (Image: Dunnock_D via Flickr)

There are three general categories of earthworm natural histories, which result in different impacts on soil composition and nutrient cycling. Closest to the surface are epigeic species which live in the upper few inches of the soil, often living in the duff and leaf litter. Just below them in the upper soil horizons are endogeic species that feed off the decaying matter close to the surface but rarely come up themselves (Red Wrigglers are a good example). And finally there are the anecic species that create deep vertical burrows that they crawl in and out of to eat and deposit castings (European Earthworms fall into this category).

Undoubtedly you are curious if any of the earthworms we see on the Hill are native. I was too when I found myself staring at the worms enjoying my kitchen scraps because I definitely didn’t put those creatures there. There are good accounts of two Pacific Northwest species – the Giant Palouse earthworm, Driloleirus americanus, and the Oregon giant earthworm, Driloleirus macelfreshi who inhabit the natural prairies of southeastern Washington and the Willamette Valley respectively and are both rare because they live only in untilled soils and hard to find because they are anecic species. But information about other native species is largely lacking except in passing reference and most of the research I dug up was focused on the impacts of worms in grain growing in the Columbia Basin (where most species are introduced). My effort to hail a particular invertebrate expert was unanswered. So I can’t say if we have a single native worm on the Hill or if we ever did at some point in the past (this area was also under considerable glacial ice during the last maximum).

If I had to take an educated guess, I’d say that there’s a strong chance we don’t. Native species are tied to specific soil types, available foods, and ecosystems and live in a tight balance. That balance doesn’t exist in our urban, novel context. Non-native earthworms while eaten by many other species do not exist with the same relationships and as with many introduced species, can run rampant without checks and balances. We may not have the same issues as the forests of New England due to earthworms, at least as far as we can tell, yet.

It may sound ridiculous to be worried about earthworms, but ecologists across our region are on the lookout for newcomers that are quickly spreading here. Three species of earthworms commonly called jumping worms are becoming more and more common in the Pacific Northwest, spreading up and down the I-5 corridor as we unwittingly move them about with topsoils, potted plants, and refuse. Because all three of these species live in the upper six inches on the soil (they are endogeic), they can have an outsized impact on the rate of leaf litter decay and speed things up, changing nutrient cycling that our local ecosystems have developed with and eventually excluding native plants and the animals that rely on them. (In the NE this has results in problems for salamanders and ground nesting birds who have a harder time finding places to live in a forest floor denuded of leaf litter.)

Jumping worms are noted for having a large mouth. If you see something say something by www.invasivespecies.wa.gov/report-a-sighting, even if you aren’t quite sure of the ID. (Image: Linda Tucker Serniak and Joanne Schneidermann Dole)

Despite concerns, it’s also completely true that earthworms are important contributors to soil health in the places they naturally exist and can even be a boon to our gardens by helping process decaying material into nutrients more readily available to plants. They have also been doing hard work in worm bins, creating rich soil from what we’d otherwise consider to be waste. What I now think are Eisenia Fetida, Red Wrigglers in my compost cone, are clearly doing good work because below the layer of recently deposited egg shells and lemon peels was black gold for next year’s vegetables. I might be harboring a little bit of ecological dread for the yet untold impacts of introduced earthworms, but I am also glad to have detritivores picking away at my kitchen scraps.

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