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as the worms turn

My field site is a typical urban forest, loaded with invasive species. The primary woody invaders are buckthorns (Rhamnus catartica and frangula).  I’ve wondered why the mono-patches of buckthorn usually had no litter layer beneath them.  It turns out this is likely due to a synergistic relationship between buckthorn and exotic earthworms.

Wiscglac First, a little background on the worms. Michigan has no native earthworms.  Neither does most of the upper Midwest, much of New England, or Canada.  The Wisconsin glaciation exterminated whatever species may have occurred in these areas.  Click on the map; the blue area is the extent of this last glaciation.  If you live in these areas, all the worms you find when you turn a spade of earth likely have European or Eurasian origins. Some arrived in soil and plant material brought by colonial settlers.  More recently, there has been a much larger influx from worms imported and raised as fish bait and for vermiculture. North America’s non-native wormifauna is comprised of nearly four dozen species, including the common species we call nightcrawlers (Lumbricus terrestris) and red wigglers (L. rubellus). Non-native earthworms are common in urban, suburban, and rural areas.  Wilderness areas are invaded by the dumping of unused fish bait, and the transport of soil in logging operations, for example.

Northern forests evolved to function with a thick layer of organic matter which acts as a mulch.  Soil properties, including delicate fungal relationships and low soil pH, are maintained when this litter is broken down slowly by native soil decomposers. Earthworms are very efficient decomposers.  They quickly break down leaf litter, and reduce or eliminate the organic soil horizon and change soil nutrient composition and availability. These actions have a cascading effect in the ecosystem, altering the soil microbial community and the composition of plant life. In one instance, a rare fern was extirpated because the loss and alteration of the litter layer disrupted the fungi relationship it needed to survive. Declines in some salamanders may be linked to exotic earthworms invasions, which reduce litter-dependent prey available to young salamanders.

I ran across a paper in the Chicago Wilderness Journal on the synergy of introduced earthworms and introduced buckthorn.  It notes that buckthorn leaves are high in nitrogen.  This makes them attractive to earthworms and other decomposers, so they break down rapidly. Mixed-species leaf litter, if it also contains buckthorn, also decomposes faster than it would if buckthorn were not present. Thus, buckthorn accelerates the reduction of forest floor litter, especially where non-native earthworms are present, exacerbating the problems associated with the lack of litter and changes in associated processes.  Where buckthorn and worms are present together, ecosystems receive a double-whammy, since buckthorn-dominated patches have elevated nitrogen, higher pH, and higher water content, all factors that can make restoration of habitats difficult even after the removal of the buckthorn.

I’m intrigued by the impacts of non-native species on ecosystems.  It seems we are just now beginning to scratch the surface of non-native/non-native and native/non-native interactions.


Filed in Natural history, Science

Comments on this entry are closed.

  • John August 25, 2005, 2:57 pm

    It's amazing the havoc one small creature can wreak on an ecosystem.

  • Aydin August 25, 2005, 9:58 pm

    But, who would be eating all those dead leaves if there were no introduced worms?

  • Nuthatch August 26, 2005, 6:31 am

    All the other little detritivores — millipedes and beetles and fungi, etc. They do it much more slowly, and the flora of the forests evolved to flourish in the slowly decomposing, moisture-retaining mulch and soils that were rich with nutrients that come from this slow decomposition.

  • TroutGrrrl August 27, 2005, 12:51 pm

    Great post. I've read about the 'no-worm zone' before but sort of forgot about it. I am also interested in the litter decomposition/soil carbon topic, but in an agricultural context. I've got a lot of learning to do – I think I need to include more forestry science reading….

  • doulicia August 30, 2005, 10:18 am

    Catching up on my reading here…I had no idea our garden variety (pun intended) worms were non-native. Thanks for the post. 100% new information to me!

  • Ronald September 2, 2005, 2:56 am

    A no worm zone? I've never heard of such a thing. Very interesting! Mind you, you can't beat Australia for earthworms. Six feet long, some of them…

  • Nuthatch September 2, 2005, 6:20 am

    Pity, they only get about six inches here. Eh-hem.

  • Amy Stewart October 4, 2005, 1:58 pm

    Great post. I am loving the Circus of the Spineless!

  • George G. Brown March 24, 2006, 10:23 am

    Hello earthworm friends:

    Just a point of clarification. Michigan DOES have native earthworms, at least three species of Bimastos (probably in their northernmost range of occurrence), one species of Diplocardia and Sparganophilus.
    Please see the paper of Murchie on the topic:
    Murchie, W.R. (1956) Survey of the Michigan earthworm fauna. Papers of the Michigan Academy of Science, Arts and Letters 41: 53-72.
    His PhD dissertation also has very interesting distribution maps of the species.
    For further information on the topic, you can also contact Sam James, who is the earthworm taxonomy specialist of the USA (sjames@ku.edu).

  • Rob May 3, 2008, 4:16 pm

    Interesting information for worm composters. I was going to release some of the red wigglers from my worm bin into my garden and thought I'd investigate their origin first. I really had to search to find out they are non-native to Canada due to the many worm composting websites and fans who think worms can do no wrong. Thank you. They are probably already there but I don't want to add to the problem of non-native species introduction. I can dig non-native worms in my backyard, go to a local lake or stream and use them to catch non-native rainbow trout. And I'm Scottish…

  • Larry May 15, 2008, 9:10 pm

    I have a small wood lot surrounding my home that I am trying to restore to its native state. I eradicated all the buckthorn six years ago and I have introduced many native species of plants. I have a plethora of earthworms that eat the leaf litter causing the soil to dry out and get compacted which is not natural. If the earthworms are removed, the leaves will remain longer on the ground, keeping the soil moist and hopefully build up a good humus layer of soil over time. How do I eliminate the earthworms without harming the flora and fauna?


  • Nuthatch May 15, 2008, 9:35 pm

    Larry, I'm not sure this is possible. I know it's not even easy to get rid of them in a small plot to do research studies. Perhaps your best bet is to continue to supplement the organic matter with compost.