Midworld

Born was a child of the rain forest that covered Midworld, part of the primitive society that the peaceful jungle planet had sustained for hundreds of years. He was wise in the ways of his world, and he knew well the precarious natural balance that governed all things.

Then one day the aliens came. Giants. They knew nothing of the Upper or Lower Hell -- and they cared less. Born had risked his life to save them, to guide them through the myriad tangled boughs, past unseen, unsuspected dangers lurking in the underbrush. But worse than their ignorance of how to survive, the aliens had plans for Midworld, plans that could utterly destroy the globe-spanning forest that his people called home.

As the days passed, Born realized his mistake. And as he had once hunted only to live, he knew now that he would be forced to live only to kill...
(Goodreads.com)