Melvyn, Earthworms are not a native species here in North America.
I'm on Melvin's side on this one. There are native earthworms in North America, although worms not native to northern forests are causing some ecological difficulties there. Here's some material gleaned quickly from the web:
From WikipediaA total of approximately 182 earthworm taxa in 12 families are reported from America north of Mexico, i.e., USA & Canada, of which 60 (ca. 33%) are exotic/introduced. Only two genera of Lumbricid earthworms are indigenous to North America while introduced genera have spread to areas where earthworms did not formerly exist, especially in the north where forest development relies on a large amount of undecayed leaf matter. When worms decompose that leaf layer, the ecology may shift making the habitat unsurvivable for certain species of trees, ferns and wildflowers. Another possible ecologic impact of greater earthworm numbers: larger earthworms (e.g. the night crawler, Lumbricus terrestris, and the Alabama jumper, Amynthas agrestis) can be eaten by adult salamanders, and when the salamanders do consume the earthworms they are more successful at reproduction. However, those earthworms are too large for juvenile salamanders to consume, which leads to a net loss in salamander population.
Currently there is no economically feasible method for controlling invasive earthworms in forests. Earthworms normally spread slowly, but can be quickly introduced by human activities such as construction earthmoving, or by fishermen releasing bait, or by plantings.
From US Department of AgricultureWHAT DO EARTHWORMS DO?
Earthworms dramatically alter soil structure, water movement, nutrient dynamics, and plant growth. They are not essential to all healthy soil systems, but their presence is usually an indicator of a healthy system. Earthworms perform several beneficial functions.
Stimulate microbial activity. Although earthworms derive their nutrition from microorganisms, many more microorganisms are present in their feces or casts than in the organic matter that they consume. As organic matter passes through their intestines, it is fragmented and inoculated with microorganisms. Increased microbial activity facilitates the cycling of nutrients from organic matter and their conversion into forms readily taken up by plants.
Mix and aggregate soil. As they consume organic matter and mineral particles, earthworms excrete wastes in the form of casts, a type of soil aggregate. Charles Darwin calculated that earthworms can move large amounts of soil from the lower strata to the surface and also carry organic matter down into deeper soil layers. A large proportion of soil passes through the guts of earthworms, and they can turn over the top six inches (15 cm) of soil in ten to twenty years.
Increase infiltration. Earthworms enhance porosity as they move through the soil. Some species make permanent burrows deep into the soil. These burrows can persist long after the inhabitant has died, and can be a major conduit for soil drainage, particularly under heavy rainfall. At the same time, the burrows minimize surface water erosion. The horizontal burrowing of other species in the top several inches of soil increases overall porosity and drainage.
Improve water-holding capacity. By fragmenting organic matter, and increasing soil porosity and aggregation, earthworms can significantly increase the water-holding capacity of soils.
Provide channels for root growth. The channels made by deep-burrowing earthworms are lined with readily available nutrients and make it easier for roots to penetrate deep into the soil.
Bury and shred plant residue. Plant and crop residue are gradually buried by cast material deposited on the surface and as earthworms pull surface residue into their burrows.