Do tourists disperse weed seed? A global review of unintentional human-mediated terrestrial seed dispersal on clothing, vehicles and horses

Human-mediated seed dispersal is recognised as an important, but under-researched, issue. To assess the potential for tourists to act as unintentional seed dispersal agents, we reviewed published and unpublished data on seed dispersal via clothing, vehicles (cars) and in/on horses and donkeys, all of which can be used by tourists. Seeds from 754 species of terrestrial plants have been collected from these vectors, 15% of which are internationally recognised environmental weeds. Seeds were collected from personal clothing and equipment (228 species), the fur of donkeys and horses (42 species), horse dung (216 species) and vehicles (505 species). Most were herbs (429 species) or graminoids (237 species) and native to Europe. Annual Poa, White Clover, Kentucky Bluegrass and Yorkshire Fog were the most frequent species. There have been eight studies specifically on tourists, which identified 12 species on clothing, 26 on vehicles and 133 from horse dung. Methods that minimise the risk of tourists as human-mediated dispersal agents may therefore be appropriate for some tourism activities/destinations: suggestions are made. Further sampling using standardised experimental techniques is required to assess the relative risk associated with specific tourist activities and locations and determine which, and how much, seed is transported.