There are 27 species of earthworm in the UK and over 2000 worldwide (some of which are 2 metres long!) The important thing to note is that whilst bees get all the news coverage worms are just as essential to life as bees. All soil needs earthworms, they keep it aerated, full of life and nutrient rich. Lots of other birds, mammals and amphibians rely on worms as an important part of their diet, from song thrush to blackbird, to hedgehog and fox. Worms are an important part of our food chain and of course they actually play a serious part on making the very soil that we depend on to grow our food.
In our garden look out for the deep burrowing lob worm that makes lots of wiggly vertical burrows throughout the soil. On damp summer nights they emerge to seek food and to mate. They are partial to a lawn clipping, a leave and any other plant leftovers!
Charles Darwin spent around 40 years studying earthworms and ended up writing a riveting book (!) The Formation of Vegetable Mould through the action of Worms with Observations on their Habits - Not a very snappy title but his conclusion was - It may be doubted whether there are many other animals in the world which have played so important a part in the history of the world than the earthworm.
If you are lacking in worms you can add a boost http://www.wigglywigglers.co.uk/garden-worms.html They're easy to add and help with drainage and soil building.