Exotic species (also known as invasive or non-native species) are often unintentionally introduced by people traveling from outside the forest by sticking to vehicle tires, shoes, or cattle and are usually found near roadways, campgrounds, and other areas used by people. The Forest Service has an invasive species control effort that identifies and attempts to contain the further spread of non-native plants. Invasive plants of particular concern in the forest include spotted knapweed, yellow starthistle, rush skeletonweed, leafy spurge, and cheatgrass.