Contact with Americans and Europeans in the early 19th century brought more settlers to the island, and had a lasting negative ecological effect. On lower slopes, vast tracts of koa–ōhia forest were converted to farmland. Higher up, feral animals that escaped from ranches found refuge in, and damaged extensively, Mauna Kea's native māmane–naio forest. Non-native plants are the other serious threat; there are over 4,600 introduced species on the island, whereas the number of native species is estimated at just 1,000.
