If we have moral obligations to other humans (future humans as well) then we have moral obligations to preserving the earth for them considering deforesting, mountaintop removal, pollution, etc all threaten the continuity of the human species(and all other life on earth and as far as we know the only life in the known universe). In this regard I think it is only fitting that we extend our definition of ethics to all organic life.

Yes slave owners excused their behavior by viewing black people as animals and not fully humans. If there is no moral standard by which to judge an action then there is no good or bad just an opinion held by that particular culture. This leads to the view that there are many truths that apply only to particular cultures.

Here is a great video by Richard Dawkins touching upon the subject, he basically says everything I am trying to articulate.
YouTube - Richard Dawkins and Postmodernism

Of course he isn't addressing postmodernism in general but I suppose plenty of the postmodernist have been accused of relativism