In many scattered places in the Philippines are rock units known as ophiolites that weather into soil that is known as ultrabasic or ultramafic. These soils are rich in certain minerals such as nickel, magnesium, and iron, but very poor in others, such as silica, potassium, calcium, and phosphorus. Because the latter minerals are essential nutrients for many plants, places with ultrabasic soils tend to have sparse, slow-growing plants, and many of the plant species are adapted to this habitat and are rare or absent elsewhere. Any trees and shrubs tend to have thick, leathery leaves, and the plants are often stunted, sometimes only one or two meters tall. The ground vegetation often includes many species of ferns and some carnivorous plants such as sundews and pitcher-plants. Little is known about the vertebrates that live in such places, but it appears that their density tends to be low, and at least some species of mammals appear to be restricted to this habitat, but too little is know to be certain. Because these areas have concentrations (at low density) of unusual minerals, they are often targeted for intensive and extensive mining.