There’s an interesting article here http://tinyurl.com/6ay4c3o on experiments in America with flowering hedgerows, which are designed to encourage pollinating insects.
In this case the hedgerows are areas left to grow a little wild, with flowers for the insects – rather different to a typical British hedgerow, which is made up of trees and woody shrubs. However British hedges play a similar role – insects are encouraged both by the trees and the plant species and flowers that grow in the base of the hedge and along the hedge margins.
To encourage populations of bees in particular and insects in general it is a good idea for farmers to leave a fairly wide hedge margin rather than ploughing right up to the hedge. This also encourages birds and other small mammals as it gives them a habitat in which to feed or live. And finally predators such as hawks and bats are also thus encouraged, so the positive effect of hedges goes all the way back up the food chain.