Dolly tried to kill Buck because she got the disease rabies and she was turning into a mad dog. Perrault and Francois have to kill her otherwise she will bite other dogs and they will also get rabies like her. That was the reason she did not survive.
Spitz took Buckś spot while he was not there and they got in a fight when Buck came back and saw him. Buck made him strenghtless and hungry huskies ate him. That was the reason Spitz did not survive.
Mercedes, Hal, Charles, and other dogs except of Buck did not survive because they did not listen to Thornton. The advice he gave them was to not go out because the ice was melting but they did not listen.
Buck survived because he understood the law of club and fang. Also he knew who the leader was and he would do whatever the leader said. In addition, he was the first dog from California that survived because he was really careful around other dogs and people.
Thornton almost drowned in a river but then Buck jumped in the river too then Hans and Pete helped Thornton get out because Buck could not do it alone.
Hans and Pete were Thornton's friends and they did not survive because when Buck was with the moose, the Native Americans came and killed Hans, Pete, and some other dogs from the team.