Do you know if your area has problems with flatworm? They come in on foreign plants (usually from New Zealand, but not always) and decimate the worm population.
http://www.gardenersworld.com/how-to/pr ... flatworms/
Gardener's World don't seem to think they are a real threat - they certainly are up here in Scotland, where much of Southern Scotland and the Central Belt are suffering. I found one in my garden last year - only one, which I killed (but I doubt they work alone, and they are bound to lay eggs in abundance), but have since been advised not to invest in an outdoor wormery, as the flatworms will eat the worms. They like being under things like slabs, so it's good to have some in your garden to trap the buggers. I've also been told that I'm better off with container gardening, as there is less chance of the blighters getting into containers, as long as they are rested on sharp gravel (as the flatworms don't like it), or plantings are surrounded by a trench of sharp gravel treated with an organic solution that deters them.
Since destroying my flatworm, I have seen a slight increase in earthworms in my garden, but nowhere near the level they were at a few years ago. It might not be the dreaded worm, but it's worth being on the lookout because it is spreading, by all accounts. Introducing ground beetles can help keep them under control, it seems.
Good luck, let us know how you get on.