Invasive species are great hitchhikers. They float in the ballast of ships, lurk in luggage, stick to unwashed sports gear, and cling to the soles of hiking boots. Scientists focus on stopping them from spreading because, once a new species gets rooted, it is expensive to manage and nearly impossible to remove.Shipping and industry are the major pathways for invasive species, but studies have also shown that tourists can spread them into protected wilderness.