Weeds typically become established in highly disturbed sites such as river and stream banks, trailheads, roadsides, building sites, trails, wildlife bedding grounds, overgrazed areas, and campgrounds. Consequently, well-managed land is the best defense against the spread of invasive weeds. However, even well managed land in good condition is susceptible to invasion when natural disturbances (such as wind, water, fire, and a wide variety of wildlife, including birds and mammals) open niches in the plant community and distribute plant parts and seeds. Once established, weeds can be spread by many vectors, including vehicles, wind, recreationists, waterways, animals, and weed-contaminated hay.