In reply to Franco cookson: Just for a minute, I'll take you seriously. Environmental damage is mostly caused by selfishness. Be that on a large scale - Co2 emissions, deforestation, or a smaller scale - dropping litter, unneccessary erosion in the uplands. The biggest threat to any environment isn't any one of these things by itself, it's the attitude of "It doesn't matter what I do". These things are related.
Lets take another upland example. Great damage was done to the uplands in the past through airborne industrial pollution and bad agricultural management. Your BIG issues, if you like. The biggest present threats to restoring this damage are from walkers, climbers, mountain bikes and the like - erosion, car transport, dropped ciggies, whatever. These are the little issues that you don't think are important. Individual actions might be trivial, but if you add them together...
To go back to your screes, the lichen and higher plant communities they are important for, are being stressed through airborne pollution and climate change. Additional erosion through careless scree running could perhaps be the tipping point. Little factors and big factors, working together. Ask your teacher on Monday.
To the OP: I'm most definately not having a go at you - I hope the link was of use / interest.