Bullying may be "natural," but it ain't polite.
I'm not a proponent of the let-the-dogs-work-it out theory of canine intra-species social relations. Sure, the dogs will work it out, but what they work out may be the ostracizing of one of the dogs from the pack or, worst-case scenario, the injury of death of one of the dogs. Our pets do not live in a fictional survival-of-the-fittest world. They live in our homes and in our neighborhoods.
Maybe the reason that otherwise responsible dog owners allow their dogs to gang up on other dogs is because they do not know what bullying looks like. I've seen it here and here. Yes, my little forty-pound standard poodle can bully with the best of them. I don't tolerate it, and I certainly don't encourage it.
Think about it. How many responsible parents do you know that subscribe to a theory of child-rearing that says "let the kids work it out." Bullying is anti-social (if not a-social). We don't tolerate bullying in our our playgrounds; we should not tolerate it in our dog parks.
Too often, what happens on the playground is that the parent of the child that is being bullied is forced to leave the park with her child. This does nothing to teach the lesson to our children that bullying is wrong. Likewise, any self-respecting owner of a dog that is being bullied will leave the dog park. This is a profoundly negative experience for the bullied dog: "Other dogs are mean, and other dogs mean I don't get to have any fun." What should happen, both on the playground and in the dog park, is that the adult who is responsible for the bully should put a stop to the undesirable behavior. A parent may be able to achieve the desired affect by telling Billy "Play nice or we're going home." A dog owner must say the same thing, but in that special language by which humans and canines communicate: training.
An effective training tool for dealing with bullies is "negative punishment." Don't be misled by the name: In this case, two negatives do make a positive. Negative punishment is a part of the positive training technique known as "operant conditioning." Most dog owners are familiar with operant conditioning in the popular training method known as "clicker training." Negative punishment is the humane way to train your dog to abandon undesirable behaviors, just as positive reinforcement—"click and treat"—is the humane way to train your dog to adopt desirable behaviors (Sit!).
Bullying is a natural behavior, but it is not a desirable behavior. Dog owners who advocate off-leash recreation as necessary for proper socialization of our dogs and then allow their dogs to bully other dogs are paying lip service to a politically correct buzz-word: They are not practicing responsible dog ownership.