The Loess Hills have abundant oak-hickory hardwood forests and some of the last remaining stands of prairie grass in the region. The invasion of prairie and oak savanna areas by woodland species such as red cedar (not native to the Hills) is threatening the stability of the fragile soils, as well as diminishing the native ecosystems found there. The areas of native prairie comprise big bluestem and little bluestem, Indian grass, sideoats grama, and forbs such as yucca, pasque flower and lead plant (false indigo). Many of the prairie species found in the Loess Hills are outside of their normal range of distribution, with plants like spiny-leafed yucca and prickly pear cactus being more common further west, in the Sandhills of central Nebraska. In 1984, a previously undiscovered fern, the Prairie Moonwort, was discovered in the Hills and is thought to be endemic to the Hills.