Retribution doesn’t work as a reason because if you’ve just made the human race extinct, there’s no-one to come back to get revenge - and no-one to get revenge upon.
This seems like a pretty good one, even if the “moral crime” in question then becomes the killing of potential, rather than the extinction itself.
But also the more life that is consumed by those who are not us. Ecosystems balance, eventually.
I think the difference between killing a couple of individuals of a common species, vs. causing the extinction of a species, is in the loss of uniqueness. Species have developed over many thousands or even millions of years, and we lose something irrecoverable when they vanish. Even if they don’t produce anything directly useful to us such a medicine, each species provides us with knowledge of how evolution occurs, so if we lose nothing else, we lose important knowledge. (Even if cloning makes it possible to restore lost species, it may be impossible to restore all the behavioral characteristics of the species, or for it to recover its place in its original ecosystem.)
For a contrasting view, I like the following quote from Charley Douglas, an early explorer of New Zealand and naturalist:
Fortunately, Charlie’s meal did not turn out to cause extinction of the species.
I’m not understanding how it’s a moral outrage to kill the last two of a species. Aside from human arrogance. If a species is already virtually extinct the ecosystem has already begun to adjust. I could understand if this would cause some butterfly effect but the fact is the planet adjusts. How many species have already been driven to extinction yet we are still here. Eventually, we’ll be gone also and something will still be here. Other than creating hardship for our own species, I can’t see a true tragedy here.
I wasn’t considering the entire human race being extinct - because, after all, you’d be gone then too, so you wouldn’t be able to worry about retribution at all! I mean that it’s good to avoid killing the last of a species because that also carries on to avoiding killng the last of a family or race. Sorry for the Godwin’s law thing here, but it’s part of the reason that Hitler’s government attempting to kill all Jews and Gypsies is seen as worse than a government killing the same number of people of different races.
We don’t need to worry much about the last two of a species consuming more stuff - odds are, we’ll eat them in turn or find some other use for them. Ecosystems don’t always balance out, either; at least, not to the benefit of humans, which is all that really matters to us.