the most amount of physical impact, what some may see to be damage, to the land occurs with the least amount of use. impact can happen fairly rapidly, but once it occurs each extra visitor does relatively little damage. that being said, the most harm mountain bikes do to an area is the trail itself, assuming that people do not ride off trail. Someone already pointed out that riding a bike on a trail has an impact just about equal to a hiker, and that equestrains have a larger impact than the two. That being said, the amount of impact also depends a lot on the vegetation, climate, and types of use in recreation areas ie cross country riding vs downhill where skidding and heavier breaking are more common handling methods. ladders, bridges, carved burms, and other man made features are all human impacts and maybe viewed by some to be destructive to the natural environment. Even though many of these features may be created using resources from the area and seem to be natural enough, they are products of man. People who do not bike, hikers bird wathers equestrians and other nature appreciators, may find these features to distract from the natural setting and the intrinsic beauty of the area. It is a very grey area and almost deals entirely with user groups interests. What one person believes to be an improvement to an area may infact completely ruin another users view of the area as being wildlands. Afterall, wildlands are defined as a place where the works of nature are the main attraction rather than the works of man. With all that being said, those are the main reasons bicycles are not allowed into wilderness zones. this is an awesome thread. Stay cool and happy trails!