Howland Island National Wildlife Refuge, 50 miles north of the equator and 1,600 miles southwest of Honolulu, is a low, flat, sandy island with a narrow fringing reef. The refuge is over 32,000 acres, including 400-acre Howland Island. The majority of the refuge is marine habitat, including extensive coral reefs and other inshore tropical ocean habitats. Uninhabited and vegetated only by grasses, prostrate vines, and low-growing shrubs, due to scant rainfall and intense sun, the refuge is managed primarily as nesting and roosting habitat for 20 species of seabirds and shorebirds. Threatened green sea turtles and endangered hawksbill turtles forage in the shallow waters on, and seaward of the reef along with hundreds of species of fishes, corals, and other invertebrates. Howland was exploited for commercial guano harvesting during the 19th century. "Guano" is essentially bird droppings, which were used as fertilizer. Its guano deposits are now exhausted.