In all honesty, one trail, with how much forest there is won't mean anything for the environment. A car will do more damage than a bike ever will. IMO, stopping mtb just because you think its bad for the environment is well.. a very extreme thing to do. If you feel that way good for you. Whatever makes you happy. Plus, is it really worth quitting mtb just to save some bugs and moss? Seems like you have other reasons to stop mtb'ing besides the environment.
Is this what you mean by the riders going off course?

yes, yes, I know, I'm bad. But it was FUN. It is kinda bad how much I rooster up with my 2.8in tires though. Takes me an hour to clean it all off my bike.
here is what some local boys made. There is a reason these guys are some of the best dh racers around. They train. DH training includes big freeride.
http://www.pinkbike.com/video/14844/
ahh look at all that nice timber. I bet a few bugs died in the process of it being made

(just teasing)
Don't worry, I'm all for the environment... environment use that is. Might as well enjoy life while we are here. (can't you tell I'm a repub by now...

)
it's good though you stopped. The more people that go to road biking the less crowded the trails will be meaning we can just go faster!!!
and btw, I would never "burn a tire on your lawn" per say, wish harm on you, or bash on you. We each have our own opinions. I respect yours, and your choice. In the end, each person can do their own thing, and their own part. Whatever makes each of us happy in our own way. If you find peace by doing that, more power to you. I definitely can't with myself quitting mtb for any reason other than being severly injured. In the end, it's the dirt bikers you really need to worry about. They are the ones that kill trails. Bikes somewhat, but dirtbikes absolutely slaughter them. Snowmobiling is one thing, and I love it, but mx, that's a whole diff thing. They destroy everything they ride on. Just think, by riding mtb, your one less guy on a dirtbike, so in return your doing your part by riding a pedal bike. All perception really.
(and btw, that was just grass in a city park in the pic... not the forest. It would be much worse of a rooster if it was in real back country...)
Also to add... I figured I would mention that the sport of DH mtb is pretty much all backwoods riding in most cases. It's growing while bmx is slowing down. People get older than they progress up to bigger things. this is an example of a real big legit dh race...
http://www.pinkbike.com/video/77804/