But the commenter I was replying to clearly seemed to be arguing that biodiversity has instrumental value.

I reference here only the difference between Utilitarianism and Consequentialism (with the former being often referenced but largely naive).

Come to think of it if 'providing happiness or pleasure as summed among all sentient beings' is actually the measure of instrumental value then you really only need a dozen species of plant and you've got all the 'happiness and pleasure' humans are likely to need.