FWIW, I've posted a link to the original article on the mountain-biking forum I frequent, and made a plea for people to think about where they are riding, and what may be under their tyres.

While many mountain-bikers (and walkers) do have a conscience as far as environmental / heritage issues are concerned, there are also those for whom such matters never cross their minds. Hopefully, the provision of information / education will get the message through to some, but sadly there are always going to be those who simply don't give a damn about anything or anybody other than themselves. :-(