I had an idea.
Spread over the healthy parts of the island live frightening beasts that protect the spaces they've claimed for their own, and they take the forms of immensely powerful Awakened animals, vermin, and plants, and they control divine power over nature itself. Their habitats represent nature in its rawest glory, where plants grow to great size, animals are wilder and stronger, and the boundaries between reality, Faerie, and the planes of nature are extremely thin. Fey creatures protect and serve the great beasts, and many of the islanders worship them as powers unto themselves.
The great beasts oppose the destruction of their portions of the island, and thus repel incursions. However, they are NOT friendly to the islanders, and though they will accept help in destroying those who defile nature, they will not hesitate to devour those who intrude on their domains, even if they allied with them before. Think the swamp monster from
this, only it's a great beast actually intending to devour you.