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On the Trails: Who cares about earthworms?

Posted: January 17, 2014 - 1:01am
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Earthworms, like the one pictured, often pull dead leaves into their burrows. They eat the dead leaves, as well as small particles of mineral soil, mixing up all that material and commonly depositing their digestive excreta up on the surface.   Photo by Bob Armstrong
Photo by Bob Armstrong
Earthworms, like the one pictured, often pull dead leaves into their burrows. They eat the dead leaves, as well as small particles of mineral soil, mixing up all that material and commonly depositing their digestive excreta up on the surface.

Worms? Who cares?! Down in the dirt, out of sight and out of mind …

Well, some fisher-persons care. In some parts of the continent, bait shops have bins of worms for sale. And kids have been known to sell little buckets of worms at strategic locations during the fishing season.

Gardeners certainly care. They cosset their worm populations in compost piles, as their protégés diligently turn organic debris into soil. Out in natural environments, earthworms often pull dead leaves into their burrows. They eat the dead leaves, as well as small particles of mineral soil, mixing up all that material and commonly depositing their digestive excreta up on the surface. Long ago, Darwin’s experiments and observations, described in his final book, showed that as a result of worm activity, soil depth can increase by as much as several inches per decade. This gradually buries objects such as bones and stones that lie on the surface. The burrows help aerate the soil and facilitate the penetration of plant roots. Good gardeners and agriculturalists love worms.

There’s more to the story, of course. What Darwin apparently did not measure was the effect of earthworms on the leaf litter itself. If worms are pulling leaves into their burrows, that must deplete the layer of dead leaves on the surface. So what?

A natural experiment on a grand scale provided the conditions for beginning to answer that question. Pleistocene glaciations sent the ice sheets grinding down over northern lands; glaciers of the Little Ice Age did it again in our part of North America much more recently. In addition to all the grinding actions, the crushing ice sat on the earth for hundreds up to thousands of years. It’s not likely that any native earthworms could survive that kind of treatment.

Therefore, across much of northern North America (and Eurasia) earthworms were probably eliminated. Native worms still inhabited lands to the south of the glaciers and probably survived in Little Ice Age refugia on the west coast, at least in British Columbia. As the glaciers retreat, worms slowly recolonize the ice-free land. But, on their own, they move very slowly indeed — only a few meters a year. So even long after the ice was gone, vast areas of the north still lack native earthworms. As far as I can tell, there is only one possibly native earthworm recorded from Alaska (in Kodiak), and it is not clear if the term means native to North America or to Alaska per se. In any case, it probably arrived by means of human transport.

Where earthworms are absent, decomposition of leaf litter is controlled mainly by bacteria and fungi, and the litter decomposes more slowly. So that litter gradually accumulates, building up a layer of mulch or “duff.” The duff layer is important for the germination of seeds and establishment of tree seedlings and understory plants. Duff holds moisture and provides insulation from temperature extremes. It slows down the drying-out of logs that then are more usable by insects and fungi and mosses. The native plant communities of deglaciated terrain have developed mostly in an earthworm-free environment and use the duff layer intensively.

However, over a dozen species of non-native earthworms have invaded many areas of the north, transported by fisher-persons who dumped excess bait or in dirt stuck in the tires of vehicles. Exotic worms have also escaped from the custody of gardeners and moved into northern forests. A growing body of evidence indicates that the invaders are having a negative effect on forest communities. The precise effects depend on many factors, including exactly which exotic species are the first to arrive, as well as litter palatability, soil acidity, seasonal soil temperatures and other environmental conditions.

In deciduous forests of the Great Lakes region and the northeastern portion of North America, invasive earthworms have reduced the leaf litter layer in deciduous forests, with concomitant reductions of abundance and diversity of native tree seedlings and understory plants. In addition, the exotic worms sometimes impair colonization by mycorrhizal (symbiotic) fungi that help sustain so many native plants. Reduced abundance of plants exacerbates the problem of a thinner layer of leaf litter because fewer leaves are added to the layer each year. A thin litter layer supports a lower abundance of insects and spiders that birds and some small mammals eat. Research on two species of ground-nesting songbirds revealed that bird density and nest survival tended to be lower in areas with the exotic earthworms, perhaps because of the reduced food supply or because of poorer nest concealment.

In Canada, exotic earthworms have invaded both aspen and pine forests east of the Rockies; they are present in some boreal-zone forests too. The invasive worms are known from many parts of Alaska. They have also invaded in the coastal hemlock rain forests in British Columbia. Although their effects in the Canadian forests have not been as well studied as in the eastern deciduous forests, preliminary studies show that the consequences of exotic earthworm invasion are similar. As elsewhere, earthworms change the chemical and physical properties of the soil, sometimes replacing native earthworms, and change the distribution of small arthropods and the understory vegetation.

I have not found any research papers that address the consequences of invasive earthworms on our local forests. But the growing body of evidence from other forests, including those of coastal rainforest just south of us, suggests that Southeast Alaska is likely to be subject to the same negative effects as found elsewhere. The worms are certainly here: hikers often see them lying in the paths. But how widespread they are, and their ecological impact, remains to be measured.

• Mary F. Willson is a retired professor of ecology.

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