This amazing infographic shows how a multitude of factors impact on the numbers of dogs in rescue in the UK. It was produced as a result of a scoping study done for the RSPCA by The OR Society's pro bono section. Four volunteers from DECC did the actual research and you can download the report here.
The main "take-home" message is that the solution to so many unwanted dogs is not simply "neuter your dog" (although that obviously helps) because so many other factors are involved.
(You may need to zoom in to see the graphic properly).