Toy/water bowl aggression with other dogs
My German shepherd recently turned 1, and his toy aggression with other dogs has gotten worse.
I love to take him to the dog park all the time, and he gets along with other dogs really well. But if there's a tennis ball involved, he will NOT play with them. He will sit down and naw on the ball and if any other dogs come near him he will growl and now recently try to bite them. I can't even get the ball away from him, he wont listen to me at all and just run away. And just recently if there's a water bowl on the ground he'll guard it and try to bite any dog that'll come near.
I have no problems at home, I can stick my hand in his food bowl/water dish, take away his toys no problem, tell him no he can't have something and he'll wait until I say it's okay.
I've looked everywhere online, and all I've found was aggression towards people and how to "fix" that. But It's just towards other dogs. How do I correct this behavior?