Actually, wherever humans have moved, they have decimated the local species. To be honest, there is really no such thing as a sustainable lifestyle. Trees drop leaves. If the leaves accumulated, and weren t broken down by other members of the ecosystem, the trees would eventually bury themselves, be unable to reproduce, and die. The lifestyle of the tree can only approach sustainability by being part of an ecosystem that processes its wastes.
Trees also produce oxygen.
Humans, however lightly they live on the Earth, need to kill to eat. Every time you kill an organism, you risk destroying unique biodiversity. That is harm. We would not be here, without them. So the individual you kill may be the one with a new gene that would allow its species to survive global warming.