I have known a number of people who take over care of animals if there is a tragedy. Usually they are family members, sometimes friends. I have a couple of friends who have no family, and I have offered to adopt or foster their pets. In the case of natural disasters, for example, hurricane Katrina, groups like state sponsored SPCAs, shelter groups et al, comprised mainly of volunteers, worked tirelessly to either reunite or rehome. In my experience, friends, neighbors, and/or family are likely to care for them.