I hate invasive species, and I have read all about foreign earthworms. However, I am not askeered of European Nightcrawlers because they seem not to be able to handle certain sorts of winters, and they will not travel far.
So every couple of years I order 2500 of these rapid breeders. If you have decent soil, to a decent depth, regularly replenished with good mulch and fallen leaves, these guys will do a good job for your borders and other gardens while feeding the worm-eaters.
All of my shrub, lawn, and vegetable gardens have been carefully prepared. No pesticides, etc, with good deep loam fortified with maure, etc. I let grass clippings lie, and fallen leaves too (until autumn). Feed the worms!
How can you tell whether your soil is lousy? If you dig up a shovel full of earth and do not find 5 or 6 wigglers, it's either not very good or it's been a rough winter for them.