Night of the Invasive Earthworms: A Horror Story for Northern Forests

We all learned from an early age that earthworms are God’s good gift to gardeners.   They rework the soil and help to degrade leaf litter thus helping to make nutrients and water available to plant roots.   Our appreciation of earthworms can be traced back to Charles Darwin who spent more than 40 years studying these creatures.  In 1881 shortly before his death he published one of his best selling books. It was titled: The Formation of Vegetable Mould through the Action of Worms, with Observations of their Habits.  The “mould” in the title would refer to the organic rich topsoil in a soil profile.  In this book Darwin reveals the biology and ecology of worms that has become the basis of how we understand worms today.  The concept of bioturbation (the reworking of the earth’s soils and even how soils are formed) are thus one of the many ways in which we, without controversy, are  all Darwinian in our thinking.  But are earthworms all a benevolent as we have come to believe?

Image left:  A forest without earthworms has a rich understory of herbaceous plants, tree seedlings, and shrubs, and a thick, spongy leaf litter layer. (Photo by Scott Loss)

A forest without earthworms has a rich understory of herbaceous plants, tree seedlings, and shrubs, and a thick, spongy leaf litter layer. (Photo by Scott Loss) Click for source.

I was prompted to think about our wriggly friends by a seminar speaker this week who is researching the possible effects of both deer and earthworms on the reduction in floral diversity in northern forests.   Yes, earthworms are being blamed, at least partially, for the degradation of forests here in Ohio and points north.    It seems that there is a growing body of evidence that suggests that over-eager earthworms are removing too much leaf litter and causing too much disturbance in the soil both of which lead to leaching of nutrients from the soil.

The problem isn’t that worms are bad in general. The problem is that we have the wrong worms.  Most of our worms are invasive exotics from Europe and Asia.  Just 100 miles north of where I live there are no native worms at all!  North American worms are practically unknown in Canada. The worms there are all invasive forms and while they may be good for gardens on balance they are likely causing less than desirable changes to the forests.

Image right: A forest experiencing heavy earthworm invasion often has few remaining herbaceous plants and seedlings, no intact litter layer, and extensive patches of bare soil. (Photo by Scott Loss)

A forest experiencing heavy earthworm invasion often has few remaining herbaceous plants and seedlings, no intact litter layer, and extensive patches of bare soil. (Photo by Scott Loss).  Click for source.

Why aren’t there native worms in northern North America?

You might think that exotic night-crawlers have  simply driven out our native worms. While exotics are a problem in southern latitudes this isn’t why Canada lacks native worms.  The explanation really quite simple but probably surprising to many.  The ice age did it!  The last ice age covered much of North America with a large ice sheet. Worms would have not been able to survive under this very thick layer of ice and thus at the height of the last ice age worms would have only been able to live as far north as central Indiana and Ohio.   When the ice sheet melted and pulled back animals and plants moved back to repopulate the land but worms are not highly mobile creatures.  It was suggested at the seminar I attended that a typical worm population could migrate up to 50 meters per year.  As a result, worms simply have not had enough time since the Ice Age to repopulate the northern portion of North America.

The extent of the glaciers during the most recent ice age.

The extent of the glaciers during the most recent ice age.

I did a quick back-of-the-envelope calculation of worm migration based on this 50 meter estimate.   At that rate it would take a worm population 33 years to migrate a mile.  So it would take about 7000  to 10000 years for native worms to have migrated from the glacier line at the height of the ice age to their present populations about 200 to 300 miles north of that line.   North of that they simply have not had enough time to get there.  These times fit will with the estimated time of the last ice age based on many other methods of aging.  So northern forests have been living without worms for some 10,000 years and getting along just fine.   Then, in just the last 200 years worms from Europe and Asia began arriving in North America likely as stowaways in pots of bulbs brought to plant in gardens.  These worms have escaped into the wild and invaded our northern forests.

Worms gone wild:

These worms interact differently with our North American plants than they do with plants in Europe which have lived with them for a long time and like so many exotic plants they thrive without the presence of as many of their natural predators and diseases.  They are larger and work the soil at a much faster pace than most native North American worms would.   Today there are 182 recognized species of worms in North America of which 60 are invasive species.  These invasive species are also causing problems in southern latitudes where they are slowly replacing some native species and that will cause problems in the future for those forests but the northern forests are faced with dealing with all invasive species.

Spring wildflowers are disappearing quickly from our woods in past decades. This has usually been blamed on over populations of deer that browse down all vegetation.  Deer are clearly a huge problem as they are found in higher abundance in our forests today than probably at any time in history given they have few natural predators.   They eat everything green reducing the ground vegetation including even the saplings of new trees making it hard for forests to replace their own members.   But now it appears that just reducing deer populations might not solve the forest understory problem in northern forests.  Exotic worms are removing all the leaf litter than normally would decay and release nutrients slowly over time. With reduced leaf litter the soil is left bare and nutrients are washed away reducing their availability to trees and to spring wildflowers and exposing the roots of trees.  The plants reduced health makes them more susceptible to drought and other harsh conditions.

A group of invasive earthworms. Well, invasive if they are on a continent where they are not natively found.

A group of invasive earthworms. Well, invasive if they are on a continent where they are not natively found.

But the effect of deer and earthworms goes much further, along with the loss of native plants and thick ground litter on the forest floor the habitat for ground nesting birds is also being reduced.   With the loss of native plants invasive European and Asian plants have been moving to cover large portions of forest floors with monocultures of other plants that don’t provide the habitat or food resources that our native animals would prefer and so often can’t use.

It is amazing that something as simple as an earthworm can have such profound effects.  Earthworms play an important role in our environment just as Darwin theorized.  They really do impact the health of plants around them.   In most places earthworms and plants have struck something of a balance when the two have lived with each other for long periods of time.   But like almost all living things, when they are put into a new environment, all members of that community have to adjust to the particular behaviors of that new member.   Time will sort things out but for those of us that are used to a rich diversity of wildflowers on our spring forest floors we may have to recognize that the new balance that is struck may include the loss of many of those plants and their replacement with plants that can handle these more aggressive earthworms.

Other resources:

Earthworms to blame for decline in Ovenbirds in northern Midwest forests, study reveals.  

U. Kutschera and J. M. Elliott, “Charles Darwin’s Observations on the Behaviour of Earthworms and the Evolutionary History of a Giant Endemic Species from Germany, Lumbricus badensis (Oligochaeta: Lumbricidae),” Applied and Environmental Soil Science, vol. 2010, Article ID 823047, 11 pages, 2010. doi:10.1155/2010/823047

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