Hard corals built up layers of coralline limestone to create a protecting barrier reef around the shallow lagoon where the central island used to be, and the action of wind, waves, and tides build small islands of coral sand.

The endangered Sesbania tomentosa or ‘ohai is native to Nihoa, Necker, and the larger Hawaiian Islands.

[3] The ecoregion is home to a distinct fauna, including the endangered Hawaiian monk seal (Neomonachus schauinslandi), three endemic species of birds – the Laysan duck (Anas laysanensis), Laysan finch (Telespiza cantans), and Nihoa finch (Telespiza ultima) – and the Nihoa millerbird (Acrocephalus familiaris kingi), an endemic subspecies.

[4] The Northwestern Hawaiian Islands are an important feeding, nesting, and nursery habitat for seabirds, sea turtles, and cetaceans.

Papahānaumokuākea Marine National Monument was created in 2006, and protects the islands and the surrounding oceans.