You're making the assumption that life only has meaning if our species survives indefinitely, or at least long enough to become interplanetary or interstellar.

I'm not advocating a nihilist approach to life, merely that I don't think the relative meaning (or lack of meaning) of life isn't derived by the life lead by our successors. What meaning we give it now in these moments we have, no matter how absurd, is all that is required to live a life more in harmony with other living things. The great barrier reef and the other natural wonders are worth saving because now, because they're worth experiencing now.