Какие природные факторы, определяющие численность видов живых организмов на той или иной территории, являются основными?

I think is happens becouse sometimes young people don't have a clue about their future and their interest and hobbies. They don't know sometimes where to go and what to do,so they join youth organisations to spend their free time among the same people as they are. They think it will help them to find friends, ortheir destination in future life, or just to have fun with another young people.