Toy Possessiveness:
I have two Australian shepherds. One Male that is a year old and a two year old female. They have never attacked anyone or anything over toys or food but I am afraid that they are beginning to be possessive over their toys.

At the dog park my one year( the same one who is a little reactive on the leash, but is getting better) old will find toys and play keep away with all the dogs. Then he will take it and lay down with it. When another dog comes by him he will get really still...almost like he is telling the dogs to stay away. Sometimes he will show his teeth, sometimes he will slam his paw down on the toy and then look at the dog challenging it.

He also goes to visit a friend every now and again. When he visits the other dog and him with play and mouth each other, which is pretty normal from what I understand. But my dog will hoard all of the toys, he will collect them all and put them around himself, and then protect them.

Both of the dogs are possessive with the bones that we have at home. We have at least one bone around all the time so that they have things to keep themselves occupied with. If one of them is chewing on it and then leaves to go do something else, if the other even walks by with no intention to take the bone, the one that was previously chewing on the bone will sprint to the bone and put it in their mouth and look at the other dog with a still stare like saying " it is mine stay away."

Like I said before, neither of them have attacked each other or another dog. I just think that they are developing bad habits that could be worse if I don't stop it now. Neither of them will growl at me if I claim or take their food or toys, so they are respectful of me and other humans but other dogs is what I am afraid of.

I have a feeling the reason why my one year old is worried about his toys is because the two year old when he was little and even sometimes now will take toys away from him. I correct it most of the time, but I find it hard to determine if they are playing or she is trying to take it from him.

Any tips to prevent my two dogs from escalating their possessive tendencies? What should I let the dogs do and not do at home?

Dog park question:
My one year old likes to grab my two year old's collar whenever the two year old gets excited. Then he will start playing with her, play growl and kind of mouthing. They do this at home and it is just playing but they can get a little vocal. When we are at the dog park when he does this and they start playing it gets the attention of other dogs and people. The dogs get excited and it gets everyone is a tizzy. Even though they are playing I feel like everyone thinks that they are being vicious. I try my hardest to keep him form biting her collar but he wants to do it every time she gets excited ( I throw the ball or frisbee). Any goo tips on getting them to stop? Should I let them play like that at home?