Frankly, I think only sentient beings can have ultimate moral value. Other things can have it only instrumentally. However, the matter is a little more complicated than that. I will argue my view here.
“Sentient” means having inner experience, being able to feel something. As far as I can see, there is good reason to think that animals with a sufficiently complicated nervous system — and it doesn’t have to be very complicated — all have sentience, and other things don’t. It’s not really a question of whether the thing is an animal specifically or whether it has just what we call a nervous system, but you need something like that, and the only thing we know of at the moment that seems to fit are animals.
Sentience apparently implies the capacity to feel something like pleasure and pain, to experience things as positive or negative. As I see it, that is the core of moral value. Sentient things have a claim not to be hurt. Things like greater consciousness and intelligence in a more complicated being give it even more moral value, but this kind of sentience is the minimum.
If we agree that sentient beings can have moral value, what about other things? It’s sometimes been suggested (and I confess I don’t even know these suggestions well, so don’t take me to say the last word on them) that other things like parts of unliving or non-animal nature or species besides their individuals should also be seen as having value. Not just the value they have for sentient beings, which may of course be considerable, but value just in themselves.
Here’s why I don’t think so.
Sentient beings provide their own value. Whatever you think about them, there is always their own experience as well. Their subjective experience exists objectively, and I hope you don’t think that sounds contradictory because it’s barely even a paradox. Now, suppose you just have some “moral” idea that something is wrong even though it does no harm; say, that homosexuality just is wrong. That doesn’t make it wrong, and it certainly doesn’t give you the right to restrict what others can do — and therefore do harm against their objectively existing subjectivity (the sentience thing). You can throw around some abstract principles detached from concrete reality, and you can use different principles and get different results, but what harms a sentient being (or doesn’t) is always an objective fact.
So if you can’t just make something like that morally true based on some principle or just because you feel so, can you make something morally valuable the same way? Nope. And in fact it’s the same thing, because the traditionalist opposing something for no reason other than tradition is making the tradition inherently valuable. I like things like nature or species more than restrictive tradition, but they’re the same in the relevant sense here. If you say something non-sentient is valuable, the only thing making it valuable is your sentience, so this gives it only instrumental value. The outside world does not objectively give the thing any value. Sentience is the way the universe starts caring about itself, and only that gives value to a universe that’s just mechanical to begin with. Besides, if you assign something non-sentient value, then presumably you may in some circumstances value it higher than you value something sentient — and then you may do objective harm. Like in the homosexuality example. Again, it doesn’t matter if you’re defending something nicer like nature.
Of course, you may have the right to want to protect something non-sentient if it’s valuable enough for sentient beings. There’s nothing wrong with that. But to ascribe it inherent value takes the discussion off course and imposes your subjectivity outside itself.
That all said, it may still be better to take an attitude towards some non-sentient things that practically assigns them value directly, not just instrumentally. To respect nature, for example. Things like the ecosystem are very important for the sentient beings within them. This is instrumental value, and theoretically one might think it’s enough to think of the ecosystem (or whatever) as a useful thing to be used by sentient beings. However, this may not be enough. We can’t simply think completely analytically about what we need to do to be rational in all situations. Having the wrong kind of attitude towards the most precious instrumental values may lead to not taking them seriously enough. I may think that the value of rest of nature comes from its value to the sentient parts, but that does not mean I think it’s a good idea to treat it by always thinking only of that.