Building the shelter of course disturbs local wildlife, but so does being there at all. Dismantling it after use disturbs the wildlife a second time. I'm thinking of the insects that will have settled in it overnight. As for natural shelter vs bivi, when we're thinking ecologically we can't ignore the industry and infrastructure that goes into a mass-produced bivi. It may be less of a disturbance than raking up leaves but only very locally. More of a disturbance globally. Camp fires, well I'm sure the wildlife would prefer we didn't. But there are lots of things wildlife does that we'd prefer they didn't. I like to leave shelters intact for others to see, especially kids. What reason is there to care about the wild if we don't feel a part of it? To me 'leave no trace' (meaning no trace visible to us, and us alone) only serves to reinforce our separateness from the wild, which does nothing to protect it in the long run, only dooms it to further mistreatment.