I am researching this for school, and I found this article that bugged me in a way. Here is part of it.
Some of it makes sence, but I think he has the wrong image of a true mountain biker, the all mountain rider."Some things are obvious: mountain bikes do more damage to the land than hikers. To think otherwise ignores the story told by the ground. Although I have never ridden a mountain bike, I am very familiar with their impacts. For the last seven years I have regularly run three to six miles several times a week on a network of trails in the Sandia Mountain foothills two blocks from my home. These trails receive use from walkers, runners, and mountain bikers; they are closed to motorized vehicles. Because I'm clumsy, I keep my eyes on the trail in front of me. I run or walk in all seasons, in all kinds of weather. I have watched the growing erosion on these trails from mountain bike use. The basic difference between feet and tires is that tire tracks are continuous and foot tracks are discontinuous. Water finds that narrow, continuous tire tracks are a rill in which to flow. Also, because many mountain bikers are after thrills and speed, their tires cut into the ground. Slamming on the brakes after zooming downhill, sliding around sharp corners, and digging in to go uphill: I see the results of this behavior weekly. I regularly see mountain bikers cutting off cross-country, even on steep slopes, for more of a challenge. They seem blind and deaf to the damage they cause. Admittedly, backpackers and horsepackers can cause damage to wilderness trails. But this is a poor argument to suggest that we add another source of damage to those trails." Dave Foreman