I know, because I met another hiker there who saw them skid down the front of the hill, where there is a big gash easily visible from the road. One of them had gone over his handlebars and crashed. I guess they live for illicit thrills like that. The mountain bikers are obviously able-bodied. Most of them are young and healthy-looking, and many are extremely muscular.
Any trail is half uphill and half downhill.
And it is well known that on level ground, the bicycle uses less energy per mile than any other form of transportation, including walking. Obviously, there is no better way to enjoy nature, than to walk. I only saw one ranger, Larry Perkins, who was in the ranger station. We cannot afford to subject every square inch of our scanty remaining wildlife habitat to these hooligans. I did not visit any other area of the park, but I suspect that they are the same or worse.