Before Euro-American settlers moved into the Loess Hills, the land was a healthy system of native grasses, flowers, and scattered trees. Today, human-facilitated introduction of non-native, invasive species pose a huge threat to native biodiversity in the Loess Hills. Leafy spurge, smooth brome, and sweet clover compete with native grasses and flowers and crowd them out of natural areas. Garlic mustard has become a big problem in woodlands and is displacing native woodland wildflowers and herbaceous plants. In all of these cases, without the native plants, the native animals and insects are declining.