Since we talked about earthworms last week…how about a little research?

When I was a kid I was taught that earthworms were good. Lots of worms was a sign of a healthy soil.

As I got older, I discovered that isn’t entirely true–some midwest soils didn’t have earthworms until Europeans showed up.  Some soils had a whole fauna of American worms that were displaced by the invasive, introduced earthworms.  There are at least 45 different species of non-native earthworms in the US right now.

It leaves me rather conflicted about earthworms, as a gardener. :(

Earthworms cause basic changes in the structure, biology, and chemistry of soil.  In gardens and (many) agricultural lands, worms are a good thing. They increase soil drainage and mix organic matter down into the soil.

But…hardwood forests in the Midwest are not used to having worms.  The last glaciation killed the native worms off.  Several different researchers have documented that as biomass of earthworms increases, the amount of forest understory growth decreases, and fewer trees had seedlings. That means the forest is less likely to regenerate itself as trees age and die. Not good.

Recently, a team of researchers wanted to see if they could find out what earthworms are doing to the chemistry of carbon cycling in forests. On clearing the floor:

“The earthworms that the team studies were brought to North America by early European colonists, probably in the ships’ ballasts or in plant soil….In some areas of the forest, more than 350 worms can be found in one square meter.  “The impact of that many worms is huge for the forest ecosystem as from spring to fall they actively consume litter and mix it into the soil, leaving only a bare surface by year’s end.” Filley said.

In contrast, sites that have no earthworms have many years of accumulated litter and organic matter above the soil. This has implications for plant seed germination, water holding capacity and infiltration of the forest floor, among other things.

Decomposition of leaves and twigs by bacteria and fungi is normally the primary source of nutrients in the forest.  Cycling of nutrients from leaves and other materials that fall to the forest floor is critical to maintaining the health of the forest.

This study concluded that part of the problem is that earth worms poop out lots of lignin–a very difficult compound for soil bacteria to break apart. Interestingly, the net effect of this could be to latch onto more carbon, (“carbon sequestration”), rather than to release it for use to plants or in the atmosphere.  So while worms are not so good for the forest, they could potentially be good for keeping carbon out of the atmosphere.

Which to choose? In some places, there is no choice. The worms are here to stay.

If you live in the Midwest, please check out the Great Lakes Worm Watch before you order worms for composting, or let that fishing worm go!


Filley, T., McCormick, M., Crow, S., Szlavecz, K., Whigham, D., Johnston, C., & van den Heuvel, R. (2008). Comparison of the chemical alteration trajectory of leaf litter among forests with different earthworm abundance
Journal of Geophysical Research, 113 (G1) DOI: 10.1029/2007JG000542

Additional references:
Hale, C., Frelich, L., & Reich, P. (2006). CHANGES IN HARDWOOD FOREST UNDERSTORY PLANT COMMUNITIES IN RESPONSE TO EUROPEAN EARTHWORM INVASIONS Ecology, 87 (7), 1637-1649 DOI: 10.1890/0012-9658(2006)87[1637:CIHFUP]2.0.CO;2

Posted by Gwen Pearson

Writer. Nerd. Insect Evangelist. Have you heard the good news? BUGS!


  1. if they were wiped out only in the last glaciation, that must mean that the current dynamic in the forest is a fairly recent development… couldn’t this be an example of Pleistocene rewilding?

  2. I hate the damn things. Midwestern forests have a number of really amazing ant species that are litter inhabitants- cryptic predators in the genus Pyramica, for instance- whose habitat is being consumed by the worms.

  3. Let’s be clear about one thing: Earthworms are evil and must be destroyed, because they have a devastating impact on oribatid mite communities, as several published studies have shown. Outside the Rockies, any lumbricid in Alberta in likely to be an invasive alien and I earn my living at the moment from oribatid mites. Therefore, I have good reason to be concerned.

    Every time I dig up one of those evil monsters that are turning my clay hard pan into something a plant can grow in I glare at the halves. On the other hand, when I find a worm drying up on the sidewalk, I invariably put it back in the mulch. I used to blame Walt Disney for this schizophrenia, but now I blame EC Pielou. I recently reread her “After the Ice Ages” and was extremely disconcerted to realize that 10,000 years ago nothing was ‘native’ to Alberta except a kilometre or two of ice. Actually, with this time perspective, one could say that everything in Canada is an invasive species. Urgh, argh, what to do?

    Wazza may be on to something. I think I’m going to stop worrying about worms. After all, the literature I use to guide my work on oribatids is almost entirely European and worms and mites seem to have worked out a solution there long ago.

  4. That’s the problem with a used planet–how far back do you want to try to turn the clock?
    And is the energy and time you’re going to spend to do that worth it?

    I go back and forth on this–sometimes the enormity of what our ecological imperialism has done just overwhelms me.

  5. Well, here we have further proof that the problem is not earthworms but man. If we didn’t bring so many invasive species from one place to another, everything would work fine. I once told my friend, humans shouldn’t have evolved. Their evolution meant the inevitable destruction of this fine planet that works great on its own.

    And I still adore earthworms. =)

  6. Moving species willy-nilly from one place to another is an annoying and potentially destructive aspect of human behaviour (just ask all those animals that used to depend on the American Chestnut), but there never was a Garden of Eden that we destroyed, except in the Bible. Nature seems to have lots of destructive powers (ice ages, volcanic eruptions, asteroid impacts) that make us look like pikers and at least we have the potential to use our brains to do better. Well, maybe. Some days I feel all optimistic about the ‘sapiens’ tag. This planet will inevitably be destroyed. I’m betting on the sun as the culprit, but I’m not putting much money on it.

  7. Catherine Sherman May 21, 2009 at 1:51 pm

    This is very interesting. I had no idea that the earthworms we know and love in the Midwest are aliens from Europe. I’m still happy to see them in my soil!

  8. First, I think it’s still true that Darwin was the first to really take a hard, scientific look at what earthworms do. His paper on worms and “leaf mould” is available at several sites, and still a good, fun read, I think. Sort of a Renaissance man.

    Second, once I read a paper on plotting the extent of the glaciers from the last ice age by the species of worm in the area. The evidence the paper cited basically had native worms from the south up to the limit of the glaciers, and European invasives north of the line. Interesting way to map things out. Worm geography.

    Finally, if one wanted to get rid of worms in a plot of land — say, to save a lake in Minnesota and the forest around it — how would one do that? (Don’t say “DDT.”)

  9. There is some hope that parasites might be used to remove the earthworms, but mostly, I think it’s a lost cause.

    The best thing to do is to stop moving worms from place to place as bait or in soil, so they don’t infest new areas.

  10. Hi Ed – Can you remember where you saw that paper on worm distribution? It sounds interesting.

    I think Bug Girl is right about it being a lost cause, and perhaps it isn’t as important as it sometimes seems (things do move around, assemblages of species change, and there is no Garden of Eden other than in our minds). This particular problem is important to me because I have a quarter section of bush land in a knob and kettle (formed by stagnant hunks of retreating glaciers producing pothole lakes surrounded by small ridges) region just east of Elk Island National Park. The land is “agricultural”, but the soil is too poor to grow crops (e.g. I planted a kilogram of seed potatoes and harvested 270 g) and it hasn’t even been grazed for 40 years. There’s no road access (surrounded by small lakes and beaver ponds), so there are few weeds and no earthworms as far as I’ve seen – but as soon as the County decides to extend the road network (or as soon as I do something as dumb as plant an Evans Cherry), the earthworms will get introduced on the treads of the bulldozers or in a potted plant. The worms will come someday and I wonder if that is really bad or if having earthworms won’t actually restore the system to something closer to what is was before the Ice Ages when there undoubtedly were earthworms?

  11. If only I could remember where I saw that paper . . .

  12. There’s a discussion of earthworms at Garden Web, covering some of the distribution issues — a heated discussion! — but they aren’t good about citations:

  13. Hi Ed – Thanks for the link to the earthworm ‘discussion’ at GardenWeb. I’ve been struggling with the native vs alien dichotomy myself and was embarrassed to find out that sometime during the last 20 years I had lost my scientific perspective and had half-converted to the invasive species religion without even realizing it. If I hadn’t read about the native plant movement in Nazi Germany, I wonder if I would have ever questioned my beliefs? It is really kind of creepy how politics/religion is infiltrating every aspect of environmental science. Anyway, I think I’ll appreciate the alien worms that have turned the clay hardpan that once was my yard into something that plants can grow in, but avoid moving them to the country. If they can get their on their own, though, I’m not going to waste any time killing them

  14. Hi Ed et al. – the paper with the earthworm distributions is possibly:

    Reynolds, J.W. and M.J. Wetzel. 2008. Terrestrial Oligiochaeta (Annelida: Clitellata) in North America, including Mexico, Puerto Rico, Hawaii, and Bermuda. Megadrilogica 12(12): 155-208. (253 species with maps for each)

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