Falling Leaves and Jumping Worms
After four years living with the muted Autumn colors of the Chesapeake watershed, I’ve been very happy these last few years to be back up north, enjoying the dazzling splendor of Fall in New England. From the excitable red maples to the patient hemlocks, the bone-shaking breezes, the footfalls of plummeting acorns, and the scolding cries of sun-soaked blue jays — Fall is my favorite season for a walk in the woods and the one which makes me feel most at home outside or in.
In addition to its personal and cultural significance for New Englanders like me, the Fall season and its namesake, slow-motion evacuation of leaves from canopy to forest floor is of crucial ecological importance. At its most abstract, the “system” part of an ecosystem consists of a complex web of matter and energy moving back and forth between living creatures and the physical environment. When the deciduous trees in a forest dump their leaves all at once onto the ground, it represents a significant transfer of matter from the canopy to the forest floor, from the living bodies of the trees to the “dead” substrate of the soil. This seasonal loss of foliage, along with the more everyday loss of spent or dried out leaves during the Spring and Summer, is what creates the thick layer of leaf litter that defines temperate forests not only here in New England, but across the upper latitudes.
At first glance, there may not seem to be much going on in the leaf litter, but this is just the understandably biased perspective of an upright walking ape; in truth, the moment a Fall leaf hits the ground is far from the end of its story. On the forest floor, dead leaves and the various tiny organisms living on them are fed upon by a rich community of invertebrates, including nematodes, springtails, pill bugs, millipedes, and slugs. These critters shred up the leaves, breaking them down into small pieces that are more manageable for bacteria and fungi, the real workhorses of decomposition. Over time, this mostly microscopic community of soil organisms slowly transforms the litter into a thin covering of organic matter — called the O-horizon — which makes up the top layer of the soil. This layer is very important for plant growth because it contains much of the nutrients extracted by decomposers that plants need to survive. The litter layer and soil together act as a kind of recycling plant, mediating the transfer of materials from dead plant matter to living plants and then to the whole ecosystem worth of creatures that feed upon them.
The litter layer also acts as a habitat in its own right for thousands of invertebrate animals and millions of microbes, as wells as various amphibians, reptiles, small mammals, and even ground nesting birds! Depending on factors such as its depth and composition, the litter layer shapes the conditions in which these creatures live via its impacts on soil moisture, pH, and erosion. The plant and animal community living on a carpet of acidic pine needles, for example, will be very different from that which is found in a layer of oak and maples leaves. Here in New England, where low winter temperatures produce a much slower, less efficient decomposition process compared to forests growing further south, the plants and soil organisms are specifically adapted to exploit the nutrient cycles and microclimates associated with a thick layer of leaf litter.
Fallen Leaves Under Threat:
Today, these conditions and the creatures that depend on them are threatened by a very cryptic and perhaps somewhat unexpected group of invasive organisms— the earthworms.* If you are surprised to learn that earthworms can be invasive, don’t worry, you are not alone. Most people (including myself) grew up being taught that earthworms are an indispensable environmental asset, and this is not entirely untrue. Burrowing earthworms such as the common night crawler (Lumbricus terrestris) can be very helpful in gardens and agricultural lands, where their tunnels make it easier for water and air to flow underground. Other species, such as the red wiggler (Eisenia fetida), play an important role in compost piles, where they help drive the processes that transform old food into a useful soil additive.
The problem is that not all earthworms are the same and not all ecosystems benefit from the activates of different earthworm species in the way that are our gardens do. This is especially true in places like New England, which actually lost all of their native earthworms during the last Ice Age. As such, when European settlers colonized the region in the 1600s, they found forests that had been without earthworms for over 10,000 years, resulting in a very specific set of environmental conditions that native plants and animals had evolved to exploit. While most of the nonnative earthworm species that colonists brought with them have been fairly harmless, a handful, including the common nightcrawler and the Asian jumping or “crazy” worm (Amyhtas agrestis), have become invasive, wreaking havoc on the forest floor in ways that ripple out through whole ecosystems.
By far the most visible impact that invasive earthworms have on a forest is the depletion or all out destruction of the litter layer and the O-horizon (Hale et al. 2005; Heneghan et al. 2007; Snyder et al. 2011). Without much in the way of predators or competition from other decomposers, invasive earthworms can multiply rapidly, bulldozing through the organic matter that carpets the forest floor and, in the case of the Asian jumping warm, converting it into a loose, dark layer of pellets known as casts (basically, worm poop). As the litter disappears, it drives increases in soil temperature and decreases in soil moisture content, changing the forest floor microclimate in ways that can be harmful to native species. Shallow rooted plants such as maple seedlings, for example, seem to rely on the litter layer to help them retain moisture and so can have a hard time becoming established in worm-infested forests (Corio et al. 2009; Dobson et al. 2017). Litter also protects young plants and seeds from sunlight, winter weather, and predators such as insects and small mammals, so less litter could cause increased stress or mortality from these sources (Frelich et al. 2006).
Earthworms can also harm native plants more directly as they go about their business on and beneath the forest floor. Many native plants in New England rely on a symbiotic relationship with certain kinds of fungi that help provide them with nutrients and water in return for sugars produced by the plant. These fungi, called mycorrhiza, are often negatively affected by invasive earthworms, which damage fungal strands with their burrowing activities; feed directly on the fungi and their spores; and perhaps disrupt the relationship between fungi and plants by providing the latter with patches of extra nutrients in the short term (Lawrence et al. 2003; Paudel et al. 2016). The resulting damage to mycorrhiza populations, along with stress from leaf litter depletion and possibly even worms feeding directly on roots, can eventually lead to declines in native plant diversity (Cortez and Bouché 1992; Gilbert et al. 2014; Craven et al. 2017; Dobson et al. 2017).
When native understory plants decline due to earthworm activity, they leave space open for other species that can survive in a changed forest. The plant community that emerges in the aftermath of earthworm invasions often includes certain native species of grass and sedge (Hale et al. 2006; Holdsworth et al. 2007), but also lots of nonnative, invasive plants (Craven et al. 2017). At least here in New England, most invasive plants are from the same parts of Europe and Asia that the invasive earthworms are from, and so are likely to be better prepared evolutionarily to withstand their shenanigans. As such, earthworm infested forests will often have understories dominated by invasive species such as buckthorn, garlic mustard, and Japanese barberry (Frelich et al. 2006; Heneghan et al. 2007; Nuzzo et al. 2009). Interestingly, there is at least one documented example of invasive plant removal resulting in declining worm populations, suggesting that the two might be mutually reinforcing each other's invasive activates in some cases (Madritch and Lindroth 2009). Both invasions by nonnative plants and worms and the resulting declines in native plant species are also probably exacerbated by over browsing from hyper abundant white-tailed deer (Fisichelli et al. 2013; Dobson and Blossey 2015; Dávalos 2015).
As invasive earthworms change the soil, the litter layer, and the understory, they change foundational aspects of the forest ecosystem that all sorts of other animals depend upon and cause complicated ripple effects throughout the community. Unsurprisingly, the depletion of the litter layer and direct competition with invasive earthworms has been found to result in the decline of various native decomposers, although this may be less of a problem with certain burrowing worms that create tunnel habitats rich in food (Eisenhauer 2010; Snyder et al. 2011; McCay and Scull 2019). Worm infested forests may also have fewer insects and other arthropods more generally, including those that do not inhabit the litter layer (Jochum et al. 2022).
Higher up the food chain, many species of salamanders feed on earthworms and some also use the old tunnels of larger burrowing species for shelter (Ransom 2012; Ransom 2017). However, these positive impacts may be stymied somewhat by depletion of the litter layer and competition for cover objects (logs, rocks, etc.) with surface dwelling species, like the Asian jumping worm (Ziemba et al. 2015; Ziemba et al. 2016). Finally, even bird populations can be impacted by worms, with the depletion of the litter layer possibly resulting in less successful reproduction for ground nesting species that use the litter to hide their young (Loss and Blair 2011).
Doing the Worm:
Given all these complex, often negative impacts that invasive earthworms are having on forest ecosystems, you are probably wondering what we might be able to do to control them. Unfortunately, this is not an easy task. Removing plants by pulling them up and applying herbicides is quite difficult; removing a much smaller, cryptic soil organism without undue harm to the rest of the ecosystem is even harder.
One possible method of controlling certain earthworm species is prescribed fire. That may sound like “undue harm,” but limited, controlled fires can actually have lots of positive impacts for certain ecosystems, even beyond helping to get rid of invasives. One study from 2015 found that controlled burns significantly reduced the number of viable cocoons for the Asian jumping worm (Ikeda et al. 2015). This is a pretty exciting result, because jumping worm are an annual species with adults that only live for one season. This means that destroying the cocoons in which young worms spend the winter could significantly reduce the overall population, even though adults can generally escape fires by digging down deeper into the soil. For now, it isn’t clear how well fire will work as a control method for other invasive worm species, including those that have multi-year life cycles, but it's certainly worth looking into.
While scientists are working on methods for controlling existing populations of invasive worms, the best thing that you can do is avoid starting any new ones. If you like to fish using live bait, avoid buying worms labeled as “jumping worms,” “jumpers,” “crazy worms” or any variation on those names, as they are especially invasive. No matter what kind of worms you are using and even if you collect them yourself, be sure to dispose of any unused bait properly by freezing it and then throwing it in the trash. Do not throw worms into the water or release them at you fishing site. If you are using worms for making compost, buy red wigglers, as this species has a hard time surviving in the wild and so does not become invasive. Finally, if you have to dispose of soil or litter for any reason, bring it to your local dump — do not dump in the forest! It is very likely that a few worms will be hitching a ride.
In addition to these more direct steps, it is also worthwhile to reflect a little bit on the story of invasive earthworms and the lessons it can teach us as aspiring good citizens of our ecosystems. First, invasive earthworms are a really good example of just how complicated ecological systems can be — I mean, just look at the citation list for this article! Earthworms touch so many different aspects of our ecosystems that it is very hard (read, impossible) to capture everything they are doing in just a few paragraphs or a handful of scientific studies. I’ve tried to capture that complexity by diving a little more into the details than other popular articles have, but I still had to leave out a lot to keep down the length and keep things reasonably accessible. All that complexity is part of the reason why it is so hard to predict when a species will become invasive and why it's so dangerous when they do, but it’s also what makes ecology such an interesting field of science!
The other important lesson that earthworms teach us is that in ecology, specifics matter. Just because a species behaves a certain way or has a certain impact in one geographic location or habitat type does not mean that it will be the same in another. This rule even applies within the category of “invasive earthworm.” I kept things fairly general in this article, but the truth is that different worms feed in different ways and so have different impacts on the ecosystem — all the changes I described above are caused at least in part by invasive earthworms, but not all invasive earthworms contribute the same to any given change. Likewise, different starting conditions in different forests are associated with different impacts from earthworm invasions, resulting in some variation in the results that different studies get (Craven et al. 2017). If you talk to an ecologist, you may hear them say that the moto of their field is “it depends,” and that’s because when you ask one of us a question, it is very likely that our answer will begin with that phrase.
So next time you go for a walk on a beautiful fall day, I hope that you’ll take a moment to look around and think a little about just how absurdly, amazingly complicated the nature all around you is. I know the goal of these sorts of walks is generally to relax and calm your mind a bit, but I also think that they are great opportunities to experience wonder in the everyday. If a little earthworm can have so many complex impacts on a whole forest, just imagine the amazing stories that are playing out in every square foot of your local park — and what kind of an impact your own activates can have in the ecosystem you call home.
*Throughout this article, I use the terms “earthworm” and “worm” interchangeably, which can be a little bit misleading. Take a look at the Animal Kingdom family tree and you’ll see that we call a lot of things “worms” that really aren’t all that closely related to each other. When I use the term here, I am only talking about animals in the phylum Annelida (the earthworms). You don’t really need to know what that means, but just keep in mind that what I am saying here does not necessarily apply to everything that you might think of as a “worm.” Also, if I use the general terms “earthworm” or “worm” I will be talking about invasive members of that group unless overwise specified.
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Photo Credits:
(1) "Leaf Litter - Guelph, Ontario" - Ryan Hodnett (CC By-SA 4.0)
(2) “Invasive earthworm Amyhtas agrestis [tentative ID]” — by Tom Potterfield (CC BY-NC-SA 2.0)
(3) “Seiurus aurocapilla nest Maine 2” — by Fredlyfish4 (CC BY-SA 4.0)
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