Tom:
The question of liability is generally based on the status of the person injured. That can vary from "trespasser" to whom no duty of care is owed to "business invitee," e.g., customer, to whom a high duty of care is owed.
Each state has its own case law and in some instances statutory law which may govern the situation. You can always consult a lawyer in your own jurisdiction.
Ward's suggestion is good-you should have insurance whether it be a homeowner's policy or a business policy. Which policy best fits your particular needs is best left to you and your insurance agent?
As a final thought, I also have problems with groundhogs and my peach trees. A quarter stick of dynamite down one of the holes can usually take care of the den.
Chris Patterson