Not all earthworms are good and earthworms are not native to many parts of the northern United States. Places like Minnesota and Pennsylvania are having problems with these non-native, invasive species from Europe and Asia killing the forests.
Parts of the northern US that had glacieral activity, do not have native earthworms. When the Europeans arrived they had soil on the ships and this is how some of the first European species got here.
What causes the problem in the north is earthworms feed voraciously on the leaf litter, breaking it down too fast and flooding the soil with nutrients, especially nitrogen. The second factor is the worms actually change the pH of the soil with their body secretions. This is changing the native soil chemistry and pH so native plants are having a hard time growing.
Since Colonial times about 45 exotic earthworm species have been introduced to North America. Now in the south, there are over 100 species of native North American earthworms. The glaciers never made it to the south.
So keep in mind that on the north earthworms are damaging the ecology, but in the south the invasives do not seem to be causing a problem.
there are many website out there on this subject if you want to learn more.
Minnesota Worm Watch Invasive Earthworms—A Threat to North American Forests
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