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Quibbling with the terminology of invasion biology

November 15, 2011

We believe the key word in Invasion Biology—invasion—is overused and that the adjective—invasive—is used indiscriminately to describe non-native plants, whether they are invasive or not.  We have already told our readers about photographic evidence of open space in the Bay Area taken over decades which proves the eucalypts and Monterey pines are not expanding their range.  Yet, these are two of the nearly 200 species of non-native plants on the California Invasive Plant Council’s list of “invasive species.”

Now we will use another non-native species as an example of how invasiveness is exaggerated by invasion biology.  Apparently many earthworms in the United States are non-native.  That’s because much of the population of native earthworms was wiped out by the glaciers that receded about 10,000 years ago. 

When Europeans began to colonize the country 500 years ago, earthworms were one of many species of plants and animals that they brought with them.  Like many non-native species, earthworms were transported unwittingly, first in the ballast of ships and then in the soil that accompanied plants that were purposefully transported.  In modern times, most earthworms are said to be introduced into new areas primarily by fishermen who use them as bait and then dispose of their surplus bait by throwing them on the ground.

Some 500 years later, invasion biologists are concerned about the impact that non-native earthworms are having on the ecology that evolved without earthworms.  Since glaciers did not extend as far south and west as California and the Bay Area, we don’t know if there are native earthworms here.  So, as you read about the impact that earthworms are said to have on the northern hardwood forests of the Great Lakes Region (Minnesota, Wisconsin, Michigan) and the northeast, you can’t assume that these observations apply locally.

The last period of glaciation in the United States

Since the hardwood forests of the northern and eastern states of the country evolved without earthworms for about 10,000 years, the duff (organic matter) on the forest floor is essential to the sustainability and health of the understory as well as the regeneration of the forest.  Earthworms digest the duff, incorporating it into the soil, and thereby deprive the native plants and trees of the nutrients that they depend upon for germination and survival. 

Although we accept that non-native earthworms may be having a negative impact on the native forests in these areas, we don’t think it is appropriate to call the earthworms invasive because they aren’t moving.  They exist only where man has introduced them.  Genetic studies have determined that isolated populations of non-native earthworms are unrelated to one another.  Their location in urban areas and around lakes and streams where fishermen visit implies that the earthworms are dispersed by man.  And they move very slowly, one-half to 1 kilometer (5/8th of a mile) every 100 years!*

Yet, those who study the earthworms and the managers of public lands who are trying to prevent the further introductions of earthworms consistently describe the worms as “invasive.”  Granted, it is a quibble, but we think this is a misleading description because it implies that the earthworm is to blame for whatever damage is being done to the native ecology when the blame clearly belongs to man.  The earthworms aren’t going anywhere.  Wherever they are found, they were put there by man.  The earthworm is another scapegoat for the harmful activities of man. 

On the bright side, invasion biologists and land managers aren’t trying to eradicate the non-native earthworm.  They are merely trying to educate fishermen to dispose of their surplus worms in the trash, rather than leaving them on the ground.  They concede that it’s impossible to get rid of earthworms where they now exist.  So, at least the soil isn’t being poisoned with pesticides in a futile attempt to return the land to its wormless history.

Once again, we urge invasion biologists to be more discriminating with their designation of “invasive species.”  We believe this term should only be applied to species that are expanding their ranges rapidly and that are actually causing damage in the environment.  Given the impact of climate change, native species are as likely to become invasive as non-native species.  If the term is used more accurately and less often, fewer efforts to eradicate hundreds of plant and animal species would be required.  Therefore those eradication efforts that are truly necessary would be less destructive and more successful.  The field of invasion biology would be more credible and enjoy more public support.

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* Hale, Cindy, “Evidence for human-mediated dispersal of exotic earthworms:  support for exploring strategies to limit further spread,” Molecular Ecology, (2008) 17, 1165-1169

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