There around 6,000 species of earthworm, only about 120 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.
Australia has 650 known species of native earthworm that survive in both rich and in nutrient-poor conditions where they may be sensitive to changes in the environment. Introduced species are commonly found in agricultural environments along with persistent natives. Most of the 75 or so exotics have been accidentally introduced into Australia. The total species numbers are predicted to exceed 2,000.
A total of approximately 182 earthworm taxa in 12 families are reported from America north of Mexico, USA & Canada, of which 60 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. 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. These earthworms are too large for juvenile salamanders to consume, which leads to a net loss in salamander population. Earthworms normally spread slowly, but can be quickly introduced by human activities such as construction earthmoving, or by fishermen releasing bait, or by plantings.
A recent threat to earthworm populations in the UK is the New Zealand Flatworm (Artiposthia triangulata), which feeds upon the earthworm, but in the UK, has no natural predator itself. At present sightings of the New Zealand flatworm have been mainly localised, but this is no reason for complacency as it has spread extensively.
The families, with distribution of the main ones:
Acanthodrilidae: Africa, midland and southeastern North America, Central and South America, Australia and Oceania.
Almidae Africa, South America.
Eudrilidae: Tropical Africa.
Exxidae: Central America/Caribbean.
Glossoscolecidae: Central and northern South America.
Lumbricidae: Temperate Northern Hemisphere from Vancouver Island, Canada to Japan, mostly Eurasia.
Megascolecidae: South East Asia, Australasia and Oceania, northwestern North America.
Ocnerodrilidae: Central and South America, Africa.
Octochaetidae: Central/South America, western Africa, India, New Zealand, Australia.
Sparganophilidae: North America.