Number of identified North American species:* ~160,000
Number of “invasive species” in North America: ~6,500
* Not counting nematodes, microorganisms, or flatworms.
The gardener loves him/her*; the northern forestry manager fears him/her. A worried observation from the Minnesota Department of Natural Resources:
“All of the terrestrial earthworms in Minnesota are non-native, invasive species from Europe and Asia. … [A]t least seven species are invading our hardwood forests and causing the loss of tree seedlings, wildflowers, and ferns.”
A hundred native North American worm varieties happily navigate the red and black earth of the American south, and a few more cling on in the Pacific Northwest. In Canada, New England, Pennsylvania, and the upper Midwest, though, the last Ice Age froze off the local worms above a latitude roughly matching that of the Mason-Dixon Line.** The 15 or so worm species common in these parts now are all descendants of European worms brought in after the Mayflower landing. As such, they are “invasive species,” spreading north and west at a pace of about 50 feet per year, and forestry science views them as dangerous pests. Eating off the forest “duff layer” of dead leaves and organic matter as they go, the Euro-worms trigger a long and depressing chain of events: soil compaction, loss of small forest plants and tree seedlings, decline of native orchids and wildflowers, reduction of habitat for small animals, slower forest regeneration, increased vulnerability to pests, ultimately “a grave threat to the biodiversity and long-term stability of hardwood forest ecosystems.”
Dollying up and back for a robin’s-eye view of the matter, the 15 worms are one group of many invaders. Excluding microbes, flatworms and nematodes — not because they don’t matter, but they’re hard to count — North America is home to about 165,000 species of animals, fungi, and plants. Ignoring the traditional kingdom/phylum/order courtesies, they are arranged as follows:
Ignoring the traditional kingdom/phylum/order courtesies, they are arranged as follows: 90,000 insects; 45,000 fungi; 17,000 plants; 8,000 arachnids; 4,261 vertebrates (including 2,400 fish, 900 birds,442 mammals, 382 reptiles, and 261 amphibians); 1,600 crustaceans; 1,300 coelenterates (e.g. jellyfish, corals, and sea anemones), 755 mollusks, and 115 worms.
About 6,500 of these species by the U.S. Geological Survey’s estimate are newcomers, termed “invasive,” as they arrived after 1492. Celebrated examples include the Japanese kudzu vine in the Southeast, the Black Sea’s zebra mussel in the Great Lakes, the Southeast Asian snakehead fish in the Potomac and Chesapeake, the British starling everywhere but Canada, the Burmese pythons multiplying in the Everglades, the Argentine ants crawling up the California coast, and the very recent arrivals of “murder hornets” and Joro spider from Japan and Korea. Together they are said to cause roughly $20 billion in annual damage to the U.S. economy; the United States Geological Survey (USGS) believes that the “current annual environmental, economic, and health-related costs of invasive species exceed those of all other natural disasters combined.”
What, if anything, can be done about this? “Prevention” options are ideal, including laws banning deliberate introduction of alien species; inspection, sterilization, and quarantine rules at seaports and air terminals; and regulations requiring ships arriving from overseas to dump ballast water in mid-ocean and exchange it for sterile water as to prevent further introductions of shallow-water mollusks, worms, and crustaceans to new habitats. Attempts to extirpate the invaders after arrival, though, are at best partial defenses and sometimes hopeless; the U.S. Geological Survey sadly says that “the odds of eradicating an introduced population of reptiles once it has spread across a large area are very low,” and the worms are another example, as they advance, a foot or two every month, further into the northern woods.
* Per the National Wildlife Federation, “Earthworms are hermaphrodites, meaning an individual worm has both male and female reproductive organs.”
** Drawn by two 18th-century surveyors, Charles Mason and Jerry Dixon, to mark Pennsylvania’s border with Maryland and West Virginia.
The U.S. Government invasive species gateway.
In Hawaii, 282 of 1,100 native species are threatened or endangered. Hawaii’s Invasive Species Council cam be found here.
A global invasive species database, including a list of the top 100 invasive-species threats worldwide.
For an international comparison, New Zealand’s quarantine system.
At sea: The International Maritime Bureau explains the International Ballast Convention, an agreement meant to prevent transcontinental movements of shallow-water clams, shrimp, mussels, fish, and other animals via ballast-water, which entered into force in 2017.
Damages: A French survey tries to estimate the cost of invasive species worldwide, reviewing 1,900 local estimates to arrive at a guess at $1.3 trillion worldwide since 1970; annual costs have steadily escalated to about $167 billion per year as of 2017. The most costly invasions are those of malarial mosquitoes, rats, cats, fire ants, and termites.
And some invaders, with estimated dates of entry
Spiders (2013): A University of Georgia release on the Joro spider, full of gleefully mock-reassuring innuendo (they are “relatively harmless to people,” and “their fangs are often not large enough to break human skin”).
Worms (~1600): Detail from the University of Minnesota on worms and the northern forest:
The non-native worms consume the duff (leaf litter) layer of forest floors as they eat their way across the continent’s forests from thousands of points of introduction, initially by European settlers and more recently by their use as live fishing bait. The worms alter the physical and chemical properties of soils, changing the pH, nutrient and water cycles, and disrupting symbiotic relationships between soil fungi and roots (mycorrhizas). The earthworms also amplify the negative effects of droughts, warming climate, and deer grazing on native plants, [UMinn Research Associate Lee] Frelich said. “Many native plant species, such as trillium and native orchids, cannot thrive under these changed circumstances.” Conversely, the worms literally prepare the soil for non-native plants from Europe, which are co-evolved with the earthworms on their home continent, including buckthorn, garlic mustard, Japanese barberry [ed. – obviously not a ”European” plant, but still an invader], tatarian honeysuckle, and hedge nettle.
The U. of Minn. reports on earthworm risk and damage to northern forests.
Birds (1890): The starling was introduced by Gilded Age New Yorker Eugene Schieffelin, a pharmaceutical magnate and Bronx Zoo donor, who brought a flock of about 120 birds from England in 1890 as part of a scheme to introduce all of the 64 bird species found in the plays of William Shakespeare to Central Park. Most of his skylarks, thrushes, and so forth died off. Starlings thrived, to the detriment of local bluebirds and woodpeckers; Schieffelin’s original cageful has grown to 200 million across North America. Smithsonian Magazine explains here.
Snakes (1942 for the brown tree snake in Guam, ~1980 for the Burmese python in the Everglades): The brown tree snake, native to the Solomon Islands, was accidentally introduced to Guam during World War II, apparently in the wheel wells of military aircraft. Within fifty years it had wiped out 10 of Guam’s 13 native birds, 2 of its 3 mammals, and 6 of its 12 lizards. USGS comment:
The impacts of these introduced species, and particularly the brown Treesnake, are so severe that they have been compared to and found to have more lasting effects on the ecological diversity of an island ecosystem than did the naval bombardment and leveling of forests that occurred on Guam during World War II.
Burmese pythons are a more recent migrant, first found in the Everglades in the mid-1980s, and now number “in the tens of thousands.” The U.S. Geological Survey notes that Florida is home to 53 invasive reptiles, and says “the odds of eradicating an introduced population of reptiles once it has spread across a large area are very low.”
The USGS on invasive reptiles.
A more optimistic take from National Geographic, as the pythons find a foe in the egg-eating native bobcat.
Ed Gresser is Vice President and Director for Trade and Global Markets at PPI.
Ed returns to PPI after working for the think tank from 2001-2011. He most recently served as the Assistant U.S. Trade Representative for Trade Policy and Economics at the Office of the United States Trade Representative (USTR). In this position, he led USTR’s economic research unit from 2015-2021, and chaired the 21-agency Trade Policy Staff Committee.
Ed began his career on Capitol Hill before serving USTR as Policy Advisor to USTR Charlene Barshefsky from 1998 to 2001. He then led PPI’s Trade and Global Markets Project from 2001 to 2011. After PPI, he co-founded and directed the independent think tank ProgressiveEconomy until rejoining USTR in 2015. In 2013, the Washington International Trade Association presented him with its Lighthouse Award, awarded annually to an individual or group for significant contributions to trade policy.
Ed is the author of Freedom from Want: American Liberalism and the Global Economy (2007). He has published in a variety of journals and newspapers, and his research has been cited by leading academics and international organizations including the WTO, World Bank, and International Monetary Fund. He is a graduate of Stanford University and holds a Master’s Degree in International Affairs from Columbia Universities and a certificate from the Averell Harriman Institute for Advanced Study of the Soviet Union.
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