Refuge Notebook: Earthworms seen as invasives on the Kenai

Posted: Friday, August 06, 2010

Until recently, I have given little thought to the small earthworms that are common around dwellings here on the Kenai. I viewed earthworms as beneficial because they aerate the soil, mix organic material into the soil profile, and process coarse organic debris into a form that can increase nutrient availability. All these things are good for garden productivity.

Photo By Matt Bowser
Photo By Matt Bowser
Earthworm (Octagonal-tailed)

Unfortunately, the very fact that worms can alter soil properties means that when they successfully invade regions naturally devoid of earthworms, something is likely to change.

There are no earthworms known to be native to Alaska. If any earthworms had existed in Alaska, they would have been wiped out by the extensive glaciations over the past 100,000 years. The earthworms that we commonly see around dwellings here are recent introductions.

I have been finding that the octagonal-tailed worm (Dendrobaena octaedra) is already well-established around my house in Kasilof and at many places near roads on the Kenai National Wildlife Refuge (see photo). Native to Northern Europe, this species has now colonized much of northern North America.

An especially cold-hardy worm, the octagonal-tailed worm can survive sub-freezing soil temperatures down to about 7 F by allowing most of its body fluids to freeze. It also thrives in acidic, conifer-dominated forests where most other earthworm species cannot survive.

In aspen forests recently invaded by octagonal-tailed worms in Alberta, they reached densities of up to 243 worms per square foot, increasing nutrient turnover rates and boosting the growth of grasses.

Fortunately for the Kenai, octagonal-tailed worms generally cause only minor changes in soil nutrient cycling and soil structure where they become established, leaving the character and composition of forests mainly intact. There are other earthworm species, however, which have the potential to cause much more damage.

Introduction of two European species, red marsh worms (Lumbricus rubellus) and nightcrawlers (Lumbricus terrestris), has rapidly brought about dramatic changes to formerly worm-free Minnesota hardwood forests. Historically, leaf litter in these forests was broken down slowly, leaving a thick layer of litter and duff on the forest floor. The introduced earthworms quickly consumed the organic material from the forest floor, leaving bare earth. This happened so quickly in some places (up to 3 inches of duff consumed per year) that plants were left with their roots exposed above bare soil. Some native plant species that are dependent on a thick organic layer have been extirpated from portions of their range where these worms have invaded. Local declines of ovenbirds, red-backed voles, shrews, and salamanders have been attributed to loss forest floor thickness in forests of the Upper Midwest.

Perhaps the worst consequence of earthworm invasion is "invasional meltdown," where exotic species interact positively. In this case, earthworms encourage the growth of invasive plants. Having come from Europe where soils are thoroughly mixed by the indigenous earthworm fauna, most of our exotic weeds are well-adapted to soils that have been modified by earthworms. By favoring exotic weeds, the activities of earthworms will make our native habitats more susceptible to successful invasion by additional exotic weed species.

Last Tuesday I noticed a dead earthworm on the pavement near the boat launch at Hidden Lake campground, an animal too large to be an octagonal-tailed worm. A quick search in the leaf litter nearby yielded a large red marsh worm, the species found to cause the most rapid damage to forests in Minnesota. Its presence at the boat launch was almost certainly the consequence of someone discarding worms used as fishing bait.

Earthworms do not get around very quickly on their own, but they may be dispersed rapidly by people. Eggs and cocoons can be easily trapped in mud in vehicle tires and spread on road systems, logging roads, and ATV trails. This is probably how the octagonal-tailed worm has become so widely distributed on the Kenai.

Worms are also spread by transport of potted plants, garden soil, and compost, as well as through intentional introductions of worms into gardens and worm farms, and by abandonment of live fishing bait. According to Alaska fishing regulations, no live animals may be used as bait in fresh waters, yet exotic European nightcrawlers are sold at local tackle shops for this purpose.

Octagonal-tailed worms are already so well-distributed on the Kenai that their eventual colonization of most our forests on the Kenai seems inevitable. Fortunately, the species with the potential to cause the most damage are not yet well-established. To prevent the spread of red marsh worms, nightcrawlers, and other exotic worms, I would recommend that live worms should not be used as bait and should not be dumped out on the ground.

Further information on the impact of introduced earthworms on forests can be found at: http://www.nrri.umn.edu/worms/forest/.

Matt Bowser has served the Kenai National Wildlife Refuge as an entomologist since 2009.

Previous Refuge Notebook articles can be viewed on our website http://kenai.fws.gov/. You can check on local birds or report your bird sighting on the Kenai National Wildlife Refuge Birding Hotline (907) 262-2300.



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