We might agree that one human is worth more than one spotted owl; but not that the ten-billionth human is worth more than the last spotted owl.

How does that work? Do you choose the ten-billionth human by lottery and then tell her, sorry you lost, now it's you being weighed against the last spotted owl?

Added: also, 'last' isn't very different from 'first'. How about this scenario: in this universe where spotted owls never existed, I have bio-crafted the First Spotted Owl, a work of art! Unfortunately, it can't breed without a Second Spotted Owl, and I don't have the resources to make that. This makes my First Owl the Last Owl as well, and so worth more than the ten-billionth human. I thus get the moral right to kill said human and transmute it into the Second Owl!

(I have given Spotted Owls onwards the ability of hunting humans for breakfast, so I don't need to worry about the chicks.)