The decline of the honey bee is increasingly well documented. But there are many other bee species that are essential in the pollination of plants and the production of food crops, and they are also under threat. There are a number of causes. The Varroa mite and certain agricultural pesticides like Bayer's Clothianidin, a neonicotinoid, are cited as primary suspects, together with loss of habitat due to modern farming practices..
We used to have 27 species of Bumble Bee in the UK. 2 have become extinct in the last 70 years and several more are on the critical list. Much needs doing beyond the garden gate, but there is plenty we gardeners can do. Wherever you live in the UK, you should be able to attract at least 6 bumblebee species to your garden, and perhaps as many at 10.
Think about your choice of plants. Native wildflowers and cottage garden style plants are the best (see full list below), while doubles, exotics and annuals such as Busy Lizzies and Begonia are the worst. Try and provide bee friendly habitats and nesting sites and join http://bumblebeeconservation.org/about-bees/.