It's normal to protect what is yours. However, it's not appropriate for dogs living in a human society. They may not find it fair, but when dogs live with humans, nothing is theirs to protect. Don't feel too bad for them. They're doing just fine. When you don't own anything, you also have no reponsibility! If your dog didn't get that memo, it's time to give him the message.
Did my dog read the memo?
Territorial aggression is when a dog protects an area, such as a home, yard, or even a vehicle. The goal in this case is not to harm the intruder but to make him go away. The dog will growl, bark, chase, and do anything threatening to make the intruder leave. Territoral dogs may chase cars, bicyclists, skateboarders, other dogs, other animals, and people. The difference in territorial chasing and predatory chasing lies in the goal. Since the goal of predatory chasing is to catch the prey, the dog will be silent and stealthy. The territorial dog does not want to catch the intruder but drive them away, so he will be noisy and obvious.
If the intruder does NOT leave, then the dog may bite to get his point across. This can be a real problem when the dog decides he doesn't want someone who LIVES in the house to be IN his house. In the wild, a dog who doesn't want another dog in his territory would simply chase him away. Only rarely does the intruding dog fight to stay. In a domestic environment, dogs don't get a choice in who shares their space. This can result in fights between dogs in the same household over who gets to rule the roost. Dogs can also decide they don't want YOU in their territory, even if their territory is just a small area inside the home - the couch, for example.
How do I give my dog the message?
The first thing to work on is your leadership role. Your dog needs to understand that it's your territory, and you will protect it as you see fit. It's not for him to worry about. Use the leadership exercises outlined on the Training & Behavior page. This will go more smoothly if your dog is not protecting territory from you. If you are on the receiving end of the aggression, tread lightly. As always, aggression problems are best handled by professionals. But, if you insist on doing it yourself, please be safe! If there is concern that your dog may bite you, muzzle him while you work on your leadership exercises. For example, it will be necessary to physically place your dog into position several times when you first begin to work on Long Down. If your dog doesn't think you have a right to physically manipulate him, he may bite! A muzzle will protect you while you teach him that you do indeed have the right to physically manipulate him (gently, of course!).
If the territory your dog is protecting is relatively small, such as the corner where his bed sits or the recliner in the living room, restrict his access to those areas until you complete leadership training. That way, he will not have a chance to practice the behavior. Then, you can gradually give him limited access to those areas while conditioning him not to guard them. If he is guarding those areas from you and you're not allowed to sit in your own recliner, use treats at first to lure him while you teach the Off command. As he becomes more pliant, gradually phase out the treats and use a leash to guide him instead. Work on this until your dog will get off the recliner on your voice command alone. Until this is achieved, do not allow him on the recliner unless you invite him. Put a laundry basket or a pile of books on the recliner if you have to so that he can't get up there unless you say it's ok. Once he is 100% compliant with your Off command, you can give unrestricted access again. Just keep an eye out for any signs that he is reverting!
If your dog is guarding the entire property, it won't be possible to restrict access (unless your neighbors are willing to dog sit!). That means you have a bit more of a challenge on your hands because he will have opportunities to practice the behavior, especially when you're not home. To reduce those opportunities, crate him when you're not home with no view to outside. Don't worry, it's not forever. Better that he miss out on a few weeks of scenery than be euthanized because he bit the meter-reader! What do we do when we want to get better at something? We practice! Take away his opportunities to practice so he can't hone his territorial aggression skill.
When you are at home, interupt any territorial behaviors. If he's running along the fence barking at passersby, tell him 'Wrong!' and bring him inside. Put him in a Down/Stay for a few minutes to calm him down. The more consistent you are with this, the faster the training will go. If he does it with guests who come into the home, crate him where he can't see them. As he learns not to get snippy with your company, he will earn more access.
Don't get discouraged!
It can seem like it's taking forever, and he's never going to get it. Just be patient and CONSISTENT. There may be little change in the beginning, then suddenly it seems like he "got it" overnight.
An excellent Territorial Aggression treatment program
Dog-Dog Territorial Aggression treatment program
Territorial and Dominance Aggression are related