more on non-native earthworms

Another very interesting paper [1] has come out on the impacts of non-native earthworms on forests, which I wrote about a year ago. As you recall, much of northern North America has no native earthworms, and microbes are primarily responsible for recycling leaf litter and thus controlling nutrient availability in northern forests. The rate of decomposition of leaf litter and nutrient cycling is crucial, because the forest floor is the physical foundation for all the native plants and trees of the forest community.

These communities evolved without earthworms.  European earthworms have been introduced into northern North America in large part through their use as fishing bait. Earthworms are detritivores, which means they eat and process this leaf litter, and have the ability to completely alter the physical, chemical, and biotic characteristics of the forest floor and upper soil horizons, notes the paper.

This study took place in Minnesota over four years, comparing vegetation in plots that had no earthworms to that in plots which had a suite of non-native earthworms.  Findings include:

• As total earthworm biomass increased, density and abundance of herbaceous plants in half the study sites decreased, and the density and abundance of tree seedlings decreased in 75% of the study sites.
• Regardless of biomass, sites with the most species of earthworms had the lowest plant diversity.
• This could be due to a synergistic relationship between certain worms. Worms of the genus Aporrectodea did not appear to consume leaf litter until it was partly processed by other species.  Then they could quickly go about removing forest floor.
• The species with the most impact was Lumbricus rubellus (often called redworms or red wigglers, and used not only as bait, but in vermicomposting). Where the biomass of L. rubellus was high, the herbaceous plant community was either absent or dominated by a common sedge and jack-in-the-pulpit.

The forest floor gets literally eaten out from under native plant communities.  Those with small seeds that can germinate on thin forest substrate (like Garlic Mustard) will have an advantage over native species with complex seed dormancy needs, and the root zones of plants that have chemical compounds that deter herbivory (such as jack-in-the-pulpit) are also sometimes avoided by worms.

In the conclusion, the authors stated:

“Although local control of invasions may be possible in some situations, the magnitude and regional scale of earthworm invasions seem to suggest that in the next few decades a majority of hardwood forest will be impacted to some degree by earthworms.”

It was mentioned that because of their disproportionate impact, introductions of L. rubellus should be prevented even in areas already infested by other species.  This is a species sold to people who use them to compost food and yard waste. These worms would eventually end up in the garden, so maybe it’s a good idea, if you live in a northern state or province, to be careful about what type of worms you have.  There is a cool key to Canadian worms at WormWatch (or you can print this one out).

A fascinating — and well-written — paper.

[1] Hale, C.M., L. E. Frelich, and P.B. Reich.  2006.  Changes in hardwood forest understory plant communities in response to European earthworm invasions.  Ecology 87:1637-1649.