I don't know what it means for something to be good for an environment in the absence of people living in that environment.

Why ever not? It means the same thing as it does when humans are there. Good for the environment isn't conventionally defined purely by what is seen by the local humans.

It isn't??? An environment is something that surrounds something else. Many environmentalists are so precisely for the benefits that a good environment gives to humans, although others also care about other animals (and even plants, although I don't really know what it means for something to be good for plants in themselves other than just helping them to grow). I'm not trying to be cute here, I really don't understand what you mean!

Perhaps you mean that wild animals (say mammals) would be better off.

No. I mean the environment.

That links helps a bit; it suggests that you mean that nature would be better off without humans, which is along the lines of what I was thinking. (I focussed on mammals simply because it's most clear to me what it means for something to be good for them in themselves.) However, it doesn't really explain how we know what's good for the natural environment. Please tell me what you mean!

Possibly you mean these items in bullet points. In my opinion, these things are good only because they are good for humans or (at least some) other animals. Obviously, your values may differ. If you mean, for example, that high biodiversity is good in its own right (or at least good for some reason not dependent on humans), then that's fine; please confirm or say instead what you do mean.