I need evidence proving that nature is an active character in "Lord of the Flies".
Actually, nature is not an active character as such but takes on the characteristics of a person. The word you are looking for is personification. The beach where half the boys stay reflects their need for social perimenters and security; the jungle beckons the more courageous of the boys such as Jack to discover the "wildness" of its interior. This is an implict metaphor, too, of a psychological journey withîn the depths of the human heart (again a universal metaphor here difficult to avoid).
There are other examples of personification: The pig's decapitated head attracting flies and the corpse of the parachutist represent the instint of savagery that somehow has its appeal. Simon's epiliptic seizures represent the daze and mental confusion the boys are in, being lost on the island and without the usual reference points (and constraints) of civilization. On the other hand, the conch "calls" the boys to order.
It is intersting to note that the fire takes on opposing roles: it is a symbol of loss of complete control ("Fire on the Mountain") but at the same time it is the "lighthouse" that gets the attention of the ship at sea - which finally comes to shore and rescues the boys.
For further insight into Coral Island as a microcosm of society, check out the reference below.