USA Forest-invading earthworms hit the news

Geoff Read at
Tue Aug 15 17:31:10 EST 2000

Hold the front page for:

Exhibit A: the Ecol. Soc. America paper abstract

Impacts of invading European earthworms on understory plant communities in 
previously worm-free hardwood forests of Minnesota. HALE, C.M., 

Abstract: During the last few decades, European earthworm species have 
been invading hardwood forest ecosystems in the northern tier of states in 
the U.S. These hardwood forests have developed since the last glaciation in 
the absence of native earthworms, and many stands historically had thick 
forest floor layers, that served as rooting medium for many species of 
forest herbs and tree seedlings. Leading edges of earthworm invasion have 
been identified in the Chippewa National Forest of northern Minnesota and 
a pilot study was conducted. Our objectives were to elucidate the rates and 
patterns of change in earthworm populations, forest floor and upper soil 
horizons, herbaceous understory vegetation and seedling demography 
related to the visible leading edge of earthworm invasion in previously worm-
free hardwood forests. A 30 by 150m grid of nested sample plots (45 
sample points) was established perpendicular to the visible leading edge in 
4 northern hardwood sites. Forest floor thickness, herbaceous plant 
diversity and tree seedling density decreased dramatically with increasing 
earthworm biomass. Successive appearance of 5 different earthworm 
species and replacement of the E horizon by an A horizon were also 
associated with the visible leading edge. Heavily impacted stands have 
been observed with only one species of native herb and virtually no tree 
seedlings remaining. Therefore, concerns have been raised about the 
widespread loss of native forest plant species and the stability of hardwood-
forest ecosystems.  

Exhibit B: The news story

“SNOWBIRD, UTAH -- Exotic earthworm species are wreaking ecological 
havoc in the northern Great Lakes region, researchers warned here on 7 
August at the annual meeting of the Ecological Society of America. In the 
most heavily affected areas, the invading worms have devastated plants 
and turned a forest floor once carpeted with leaf litter into bare soil.         

“Some 18,000 years ago, thick ice sheets killed all the native earthworms in 
the northern half of North America. Recolonization by native worms has 
been slow, and there are still no native species in the northern part of the 
United States and Canada. When European colonists arrived in the East 
toting plants and seeds, Old World earthworms came along, too--as many 
as 14 species. Now ensconced in soils around cities and farms, the worms 
are on the march. And once again, humans--by transporting worms for fish 
bait and dumping out the extras, for example--are helping them spread.       

“To see what vanishes when worms crawl in, Cindy Hale, a forest ecologist 
at the University of Minnesota, Twin Cities, and her colleagues studied the 
advancing worm front in the Chippewa National Forest in Minnesota. 
During the last 2 years, the frontier of the worm invasion moved about 10 
meters into the forest. The forest floor, in turn, lost about 75% of its plant 
cover. The invaded areas had only one or two species of forest floor plants, 
while 150 meters away, the worm- free zones had more than 10. And tree 
seedlings, usually found in densities of 100 per square meter in the worm-
free zone, were almost entirely absent. It's not entirely clear yet how the 
worms disrupt the ecosystem, but the long-term results could be dramatic, 
says Hale: "A worst-case scenario is the total extirpation of a whole suite of 
understory plants, many of which are our most beloved spring flowers, 
including trilliums and spring beauties."         

“The findings surprised some scientists. "I've never seen somebody link a 
decrease in biodiversity to invasion by earthworms," says Mark Hunter, an 
ecologist from the University of Georgia, Athens. Nor have many other 
ecologists, says Hale. When she asks forest ecologists if there are worms 
in the soil they study, she says, "nobody knows!"        --MARI N. JENSEN  

  Geoff Read < at>

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