The boys find themselves surrounded by natural beauty on the island:
This was filled with a blue flower ... and hung down the vent and spilled lavishly among the canopy of the forest. The air was thick with butterflies, lifting, fluttering, settling ... clambering among the pink rocks, with the sea on either side, and the crystal heights of air, they had known by some instinct that the sea lay on every side.
This world exists as a paradise until the boys' arrival: and with them comes destruction. They deplete the natural resources, set fire to the beauty of the island, make brutal sport out of murdering the animals, and then resort to murdering each other.
Near this same description of the island in chapter 1, Ralph turns to the boys and proclaims: "This belongs to us." With this, he must claim the "scar" they create even from the beginning:
Beyond falls and cliffs there was a gash visible in the trees; there were the splintered trunks and then the drag, leaving only a fringe of palm between the scar and the sea.
The scar is an area which has been devastated by the plane's crash—the wreckage ripping through trees and natural beauty to destroy the paradise in that spot.
The scar then symbolizes the power of mankind to devastate the delicate and beautiful balances in nature. Sometimes, this is done through technological progress, as seen in the plane. But sometimes this is seen through mankind's quest to exert power over nature itself, as seen in the murder of the pig. Either way, nature suffers because of mankind's presence.
While Ralph's initial claims to the island project mankind's quest to rule over nature, he is also claiming the scar, and therefore the devastation, that mankind creates.