May 9, 2011

Do Composting Worms Pose a Threat as Invasive Species?

Concern over the potential for non-native earthworms to become invasive, and have a negative impact on ecosystems in various parts of North America has gained considerable attention in recent years. The full scope of the problem has yet to be determined, but there is substantial academic research evidence to suggest that various species of earthworms have been significantly altering native ecosystems in regions where they’ve been introduced – particular in areas where no earthworms have been previously found. (Callaham et al. 2006).

My own introduction to this topic came in the form of an online article I happened upon several years ago entitled “Wisconsin cracking down on ‘infected bait’” – and I actually made mention of it here on the blog (see “Nightcrawlers Ruining Northern Forests?” – unfortunately it looks as though the original article is no longer available). In all honesty, while the topic certainly caught my attention and I took the claims seriously – the full extent of the issue (and the potential implication of composting worms as threats) did not fully register until fairly recently. On that note, I’d like to take the opportunity to say thanks to my good friends Mark (“from Kansas”) and Allison Jack for helping to get this topic back on my radar screen. Both of them have been in contact with researchers in this field, and they were able to get me pointed in the right direction so I could learn a lot more myself.

I’ve been a pretty vocal proponent of the use of Red Worms (Eisenia fetida/andrei) in outdoor systems over the past couple of years, and the upcoming release of my “Better Backyard Composting” series (which will certainly offer outdoor vermicomposting as a beneficial strategy) makes this all the more timely – and my writing about it all the more important! I should point out that my intent with this first article is not to provide complete coverage of this topic – I simply want to get the ball rolling, introduce the topic to those who might not be familiar, and generally provide some commentary based on what I’ve gathered thus far!

Some things to consider…

  • Much of the concern seems focused on northern deciduous and boreal forest ecosystems – particularly those with no native species of earthworms. It is believed that glaciation – which affected much of northern North America – wiped out all native earthworms, and thus all species found in this part of the continent are thought to be introduced (mainly from Europe). Some of the areas where research has been conducted include the northern states (particularly Minnesota), Alberta Canada, New York State and the Great Lakes Region (Larson et al. 2010).
    **This is NOT to suggest that there is no potential for concerns in other areas, however**

  • One of the main issues is the resulting loss of the “O Horizon” – the uppermost layer in the soil profile (consisting primarily of fallen leaves and other organic matter in forest environments) and mixing of organic matter into the “A Horizon” (next layer down) once certain invasive species become established. When no earthworms are present, this organic matter breakdown generally takes place over the course of years – whereas the process is often greatly accelerated when earthworms are present.

  • This rapid breakdown of O Horizon can greatly impact nutrient cycling – some evidence to suggest that certain key nutrients such as nitrogen (N) and phosphorus (P) become less available, or end up lost via leaching as a result (Hale et al. 2005; Frelich et al. 2006). It has been found to reduce understory vegetation and result in the loss of certain plant species altogether (namely those dependent on the habitat, and other benefits provided by the O Horizon).

  • Full grasp of the potential long-term impact this could have has not yet been determined. As per usual in the world of scientific research – much more work is needed.

  • There seems to be some debate among researchers regarding the importance of the natural spread of established earth worm populations VS human-assisted introduction/spread (Hale 2008b) – the latter certainly being the one that gets the most attention.

  • Different invasive earthworms species have different habitat/feeding preferences (belong to different “functional groups”) and thus different impacts on the ecosystems into which they spread or are introduced. Different combinations of species can result in different impacts as well (Hale et al. 2008).
    Bottom-line, it’s important not to make generalizations.

  • Threat posed by more recently introduced species, such as Amynthas sp (“Jumpers”) has not yet been adequately assessed – more work also needed in this area (Callaham, 2006).

Here are a couple of BIG questions on my mind (will then addresses each of these based on what I know so far):

1) Is there ANY research evidence to suggest that Red Wigglers (Eisenia fetida/andrei) are negatively disrupting native ecosystems in any part of North America (or the world for that matter)? Is there even evidence to suggest that they would if given the opportunity?

2) If YES (for #1), what is the evidence (what sort of studies have been conducted, results etc)? If NO – are there other widely-used composting worms that pose real (or at least potential) threat?

Red Wigglers | Eisenia fetida/andrei

Based on information I’ve come across thus far, feedback from the earthworm professionals I’ve been in touch with, and my own extensive experience with these worms, there doesn’t seem to be strong evidence suggesting that they pose a threat to natural ecosystems (but this is certainly still something I am looking into). Firstly, I should explain (for those not already aware) that E. fetida and E. andrei, while determined to be two distinct species (Dominguez et al. 2005), tend to occur in mixed populations in the same habitats – my understanding is that they can only conclusively be distinguished from one another via molecular analysis. As such, it makes sense just to consider them together.

For one thing, these worms don’t tend to occur in “natural” habitats at all – but rather, tend to be found in concentrations of very rich organic matter such as manure and compost heaps. It’s not so much that they won’t survive in decaying leaf litter (they likely would) – it’s more that the chances of them actually thriving in this environment seems quite unlikely. Some experts cite the inability of Red Worms to withstand cold winter temperatures as one of the main reasons for there being less of a concern with them – I’ve actually found them to be quite cold-tolerant, so I’m somewhat less inclined to build an argument around that, but I guess it would be location dependent (they seem to do ok with minimal protection here in Southern Ontario – but in more northerly regions, and/or in more of a “natural” environment, perhaps they don’t overwinter nearly as easily).

As luck would have it, I happen to live about 15 minutes from a world-leading earthworm researcher and worm ID specialist, Dr. John Reynolds. That’s not to say that we’ve met (haha) – but I have been having email exchanges with him for quite some time. As soon as the invasives issue ended up on my radar screen I decided to get back in touch with Dr. Reynolds to chat about potentially sending in some worms for identification (more on that in a minute). While I was at it, I ended up asking for his professional opinion on the E. fetida/andrei (as invasive species) situation. My question was basically, “in your professional opinion, are these worms a potential threat?” – to which he responded:

No. Eisenia foetida [note: alternate spelling] because of its high optimum developmental temperature requirements it is not now found widely in nature. Since the decline of horses in agriculture, most populations are in articifial situations. If you looked at three of my recent monographs covering eastern North America you would see the limited distribution of Eisenia foetida.

[I’ve included a listing of the monographs mentioned in the References section]

My own “wild” encounters with this worm seem to be very much in-line with Dr. Reynolds’ findings. Quite literally, the ONLY place I’ve ever encountered these worms, other than in composting systems, is in aged, outdoor manure heaps – usually those containing mostly bedded horse manure.

So what about OTHER composting worms?

Lumbricus rubellus

This is the worm I am now very keenly focused on. It is certainly mentioned frequently in the literature as an invasive species (Hale et al. 2005; Larson et al. 2010; Frelich et al. 2006 – among numerous others), and over the years it has been repeatedly referred to as a “composting worm”. What’s intriguing is the fact that I’ve come across multiple mentions of key “surveys” examining the presence (or lack thereof) of L. rubellus in samples obtained from worm farms in Australia, Europe and North America. One such mention can be found in Edwards and Bohlen (1996) – p.251:

“In surveys of commercial earthworm farms in the US and Europe by Edwards, and Australia by Buckerfield and Baker, the earthworms sold under the name L. rubellus were all E. fetida or E. andrei.”

I just HAD to get to the bottom of this, so I contacted Dr. Edwards himself. Unfortunately, his response only served to muddy the situation even further (for me anyway). Here is some of what he said:

The picture with L. rubellus is somewhat confused, the report in the Vermiculture Technology book on the survey on worm farms dates back to the 1980s and is no longer valid. This review was never published except mentioned in various papers.

The current situation is that L. rubellus is being used in vermicomposting. For instance, a earthworm farm in Illinois run by Bill Kreitzer. L. rubellus is interesting because it is one of the few vermicomposting earthworms in soil improvement. Hence, there is quite a lot of interest in this species. The same situation exists with Dendrobaena veneta.


Bottom-line, this is definitely something I want to look into a LOT more. My own sneaking suspicion is that L. rubellus is not employed in typical vermicomposting situations quite as much as some people might think. That’s not to say it won’t sometimes be found in vermicomposting systems (especially those in outdoor locations).

According to Dr. Reynolds, separating L. rubellus from E. fetida is “not difficult” based on “a couple of characters which can be seen with a magnifing glass, even if they are not mature.” (including common features like coloration, and morphology of tail region). As such, I think there needs to be a lot more focus on educating people about the differences between these species.

I’m pretty sure that L. rubellus is quite common in my region (and I think I’ve found some already), so I’m hoping to track down a bunch of them and A) get a positive ID on them (by sending in to Dr. Reynolds) and B) test them out as “composting worms”.

European Nightcrawlers (Eisenia hortensis / Dendrobaena veneta)

This is variety of worms (or several closely related variety of worms – yet another issue I’m interested in getting to the bottom of) that MAY pose a potential threat (hard to say for sure). While they are fairly similar to Red Worms in terms of habitat/food and general environmental requirements, I get the feeling they are able to adapt to semi-soil environments somewhat more easily than Reds. I have seen no mention of these worms in any of the literature I’ve come across thus far, so they at least don’t seem to be a major known threat – but my suspicion is that their frequent use as a fishing worm, and their potential ability to live in more of a “natural” environment, may leave their status in question.

African Nightcrawlers (Eudrilus eugeniae) & Blue Worms (Perionyx excavatus)

These species MAY have the potential for causing issues in sensitive ecosystems located in warmer regions, but they certainly won’t pose any threat in northern regions of North America. Both of them die once temps drop below 10 C / 50 F, so there is very little chance of them over-wintering in most areas, let alone thriving.

I’ve had Blue Worms in outdoor systems previously, and while they certainly did well during the summer, once cooler temps of fall arrived their activity declined rapidly and they started to disappear. I was unable to locate any the following growing season.


OK – well, I think I’ve provided enough of an “eye-full” for one blog post! This is definitely a topic I plan to revisit on an ongoing basis – so no need to get it all out today! haha

Obviously, given my entrenched (no pun intended) position in the world of vermicomposting – not to mention my financial interests in, and reliance on, this field – it’s no-brainer to assume that my views on all this are going to be at least a little bit biased. I think what’s helped me remain reasonably objective here is the fact that I am a life-long naturalist with a strong background in ecology and conservation biology. Bottom-line, I wouldn’t have brought this up at all if it wasn’t something that I felt the vermicomposting community needed to be aware of, and learn more about.

My intent here is not to leave people with any sort of final consensus or conclusions regarding the issue of invasive earthworms – the scientific research in this field has not reached that point, so I’m certainly not in a position to do so. Conversely, I don’t think it’s something that should be swept under the carpet either.

My hope is that we’ll be able to open up a respectful dialogue (hopefully with some of those involved in the research as well) so as to develop a better understanding of the current situation.

Supporting Literature (Including References Cited)

Callaham, M.A.Jr., Gonzalez, G., Hale, C.M., Lachnicht, S.L., Zou, X. 2006. Policy and management responses to earthworm invasions in North America. Biological Invasions 8: 1317-1329.

Dominguez, J., Velando, A., Ferreiro, A. 2005. Are Eisenia fetida (Savigny, 1826) and Eisenia andrei Bouche´ (1972) (Oligochaeta, Lumbricidae) different biological species? Pedobiologia 49: 81-87.

Edwards, C.A. and P.J. Bohlen. 1996. The biology and ecology of earthworms (3rd Edition). Chapman & Hall, London, 426pp.

Frelich, L.E., Hale, C.M., Sheu, S., Holdsworth, A.R., Heneghan, L. Bohlen, P.J., Reich, P.B. 2006. Earthworm invasion into previously earthworm-free temperate and boreal forests. Biological Invasions 8: 1235-1245.

Hale, C.M. 2008. Evidence for human-mediated dispersal of exotic earthworms: support for exploring strategies to limit further spread. Molecular Ecology 17: 1165-1169.

Hale, C.M., Frelich, L.E., Reich, P.B., Pastor, J. 2008. Exotic earthworm effects on hardwood forest floor, nutrient availability and native plants: a mesocosm study. Oecologia 155: 509-518.

269. Reynolds, J.W. 2010. Earthworms (Oligochaeta: Lumbricidae, Sparganophilidae) of the Atlantic Maritime Ecozone. Chapter 13, Pp. 225-281. In: Assessment of Species Diversity in the Atlantic Maritime Ecozone, D.F. McAlpine and I. Smith
(eds.). Ottawa: NRC Press, xii + 785 p. [paper in English; abstracts English and French].

273. Reynolds, J.W. 2010. The earthworms (Oligochaeta: Acanthodrilidae, Lumbricidae, Megascolecidae and Sparganophilidae) of northeastern United States, revisited. Megadrilogica 14(7): 101-157. [paper in English; abstracts in English, French, Spanish, Italian]

275. Reynolds, J.W. 2011. The earthworms (Oligochaeta: Acanthodrilidae, Eudrilidae, Glossoscolecidae, Lumbricidae,
Lutodrilidae, Ocnerodrilidae, Octochaetidae, Megascolecidae and Sparganophilidae) of southeastern United States.
Megadrilogica 14(9-12): 175- 318. [paper in English; abstracts in English, French, Hungarian, Spanish]

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