Where in the Lord of the Flies is there a conflict of man vs nature?

Man vs. nature is one of the three major external conflicts that can be found in many narratives. A man vs. nature conflict occurs when a character must struggle against the natural world in some way. Stories that have characters dealing with survival in the wilderness or an epic natural disaster have man vs. nature as their main conflict. Some examples of man vs. nature films are Into the Wild, San Andreas, or (most relevant to Lord of the Flies) Castaway.

In Lord of the Flies, the man vs. nature conflict is the first to emerge, as the school children find themselves stranded on the island. They need to find food and fresh water, they need to create shelter from the hot sun, they need to navigate the vine creepers and dangerous pink granite rock. All of this is compounded by the man vs. society conflicts and man vs man conflicts that crop up among the boys: Ralph and Piggy struggle to get the others to form a productive society that can work to get them rescued, Jack and Ralph fight over who will be leader, etc. These other conflicts make the boys' mastery over the natural world of the island that much harder. 

As the novel progresses, the man vs. nature conflict becomes two-fold. The boys are still trying to survive in a wild and sometimes dangerous setting, but they are also each fighting what Golding positions as the natural instincts of wildness and destruction that lie dormant in humankind. Examples of this litter the latter part of the novel.

One example is the hunters' struggle to hunt and kill the island's pigs for meat. While this is a classic man vs. nature conflict, it is deepened when the hunters do a blood lust dance, eventually getting so caught up in pretending to hunt Robert (who is acting as the pig) that they actually hurt him. Later, when the hunters worship the head of the pig as the "Lord of the Flies," they demonstrate the evil nature that lurks in the hearts of each of them, bringing a new, more chilling look at the man vs. nature conflict.

Another example of this is the signal fire. At first, the fire represents the boys' mastery over the natural world (they have learned to control it and to use green branches to produce smoke that is thicker and easier to see for potential rescuers). As the novel progresses, however, the fire becomes a way for the hunters to hunt and kill Ralph. It is thus another tool to perpetuate the natural evil in mankind. 

Even when the boys are rescued by the amazed and horrified naval officer and can leave the natural world of the island behind, they are heading for a warship, evidence of mankind's continued natural evil and desired for destruction.