Undoubtedly the majority.
there is no reason that, with training, “pitbull” types can not excel in those functions.
There are distinct reasons why certain breeds are most prevalent in search and rescue, tracking, bird hunting, rabbit hunting, field trials, herding, pulling, guide/service dogs, etc. They've been bred for and have the natural characteristics to do certain tasks more effectively and successfully than other breeds. Not all purebreds are great at what they were bred and trained for. Occasionally even a Heinz 57 will excel at something one wouldn't normally expect. However, the majority of the time the beagle will be a far superior rabbit hunter to a St. Bernard. A Newfie will be far superior to an Akita at water rescue, a border collie will be far superior to a golden retriever at herding, etc., etc., etc.
Doncha just love it when things fall into place, accidentally?
http://lsspbc.com/blog/adba-info/weight-pulling/
:)