Lee Frelich has done much work on the effects of earthworms on the soil. Basically the earthworms change the structure of the forest floor. In areas where they are not present there is a thick layer of duff- bits of leaves in various states of decay. Without worms, fallen leaves decompose slowly, creating a spongy layer of organic "duff." This duff layer is the natural growing environment for native woodland wildflowers. It also provides habitat for ground-dwelling animals and helps prevent soil erosion.
Most of the nutrient cycling takes place in the duff layer and organic rich soil and decayed leaf mixture of the upper soil zone. This nutrient cycling is mediated by fungal dominated processes. The earthworms eat this duff. I areas where they are present the soil is not overlain by the duff and becomes more compacted. The decay process are no longer taking place in the duff layer and upper soil horizons but within the mineral layers of the deeper soil. Here they are more likely to become mineralized and leached away instead of being held by the organic layer and available for plants. "According to Peter Groffman, a microbial ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., northern hardwood forests have relied on thick layers of leaf litter that serve as a rooting medium. The earthworms, Groffman reports, “come into an area with a thick organic mat, and two to five years later that layer is gone.” As a result, some northern hardwood forests that once had a lush understory now have but a single species of native herb and virtually no tree seedlings. Evidently, earthworms change the forest soils from a fungal to a bacterial-dominated system, which speeds up the conversion of leaf detritus to mineral compounds and thereby potentially robs plants of organic nutrients."
In addition with the absence of the duff layer the soil tends to dry out and erode rather than retain water. The soils tend to dry out quickly and shallow rooted plants become stressed, others starting from seed die before being able to root deep enough to find adequate water. The absence of the duff layer dramatically affects the rate of water infiltration into the soil. Water being nutrients quickly seeps into the deeper subsurface or to the water table rather than being retained in the shallow soil zone and being made available to shallow rooted plants and seedlings.
Many native species root in this duff layer and upper soil zone and are eliminated from the ecosystem. Big trees survive, but many young seedlings perish, along with many ferns and wildflowers. Some species return after the initial invasion, but others disappear. In areas heavily infested by earthworms, soil erosion and leaching of nutrients may reduce the productivity of forests and ultimately degrade fish habitat. The presence of earthworms also facilitates the invasion of non-native plants into the environment through the elimination of many native species unable to reproduce adequately when the duff layer is gone.
You can see the sharp boundary in the forest between areas heavily infested by non-native earthworms and those areas which are not. Perhaps in Europe they are not a problem, but there are not many natural predators that limit the populations of the invasive earthworms in our soils and they are overwhelming the natural soil forming and nutrient cycling processes. There is material on the web talking about some of this. Much of it is in English, and not really very detailed and overly simplified. One site to look at is found here: http://www.nrri.umn.edu/worms/forest/soil.html
another blog that is OK is found here: http://www.wildlifegardeners.org/forum/ ... rests.html
If Lee responds he can direct you to some better articles in refereed scientific journals.
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky