In reply to 08JepsonH:
My two penneth (and I think they will all have proper published material associated)
ilegal disturbance (very rare, but not unheard of) - destruction of nests, deliberate (rather than inadvertent) gardening of protected species
Seasonal disturbance - bird bans, flowering plants, also stuff like differening erosion rates in different weather, traffic pollutants behaving differently etc
physical erosion/ damage - boots on the ground, polish, crampon scratches etc
proximity (want of a better word) damage/ disturbance - disturbance of birds like ring ouzel/ golden plover/ some raptors which are sensitive to people in their territory, damage/ compaction to drainage uphill effecting erosion downhill, etc
physical pollution from people and their stuff - I give you the wasdale portaloos! but also vehicles, camping etc
As to postive practice - there's loads of stuff from the NT, RSPB, wildlife Trusts in the UK context and also some useable European examples. I remember from looking at this a few years ago, there isn't that much research actually quantifying damage/disturbance specifically from climbers - that might have changed though.
Also a lot of examples of climbers helping with roped access stuff for ringing/ nest counts/ habitat improvement
Sure there's loads I've missed. There's potential for a really good piece of work if you do the research!