How does Golding increase tension in the final chapter?

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From the text:

Ralph fumbled to hold his spear so that it was point foremost; and now he saw that the stick was sharpened at both ends. The savage stopped fifteen yards away and uttered his cry. Perhaps he can hear my heart over the noises of the fire. Don’t scream. Get ready.

A herd of pigs came squealing out of the greenery behind the savage and rushed away into the forest. Birds were screaming, mice shrieking, and a little hopping thing came under the mat and cowered.

All at once the lights flickering ahead of him merged together, the roar of the forest rose to thunder and a tall bush directly in his path burst into a great fan-shaped flame.

The ululation rose behind him and spread along, a series of short sharp cries, the sighting call. A brown figure showed up at his right and fell away. They were all running, all crying out madly. He could hear them crashing in the undergrowth and on the left was the hot, bright thunder of the fire. He forgot his wounds, his hunger and thirst, and became fear; hopeless fear on flying feet, rushing through the forest toward the open beach.