You should check out Hard Green by David Roberts, posted at Gristmill:
What, then, is environmentalism? We want to save the set of ecosystems and species that happen to exist at this point in the earth’s history. Why this set? What makes it more special than the six other sets that have come and gone?
Here’s an obvious answer: This set includes us.
I don’t quite agree with the implicit logic that since we don’t feel very bad about the Permian-Triassic extinction event, we don’t grant animals any intrinsic value, but I like his central point.