99% of species are extinct

[Piers]

[ QUOTE ]
what role could it play in various arguments, such as the necessity of protecting habitat, or biodiversity

[/ QUOTE ]

Not much. Species becoming extinct is part of how the world works. Species can become extinct because they all die out, or because they evolve into another species. There are often times where species die out or change much faster than others. I don’t think it can justifiably be used the way see to want to use it.

[ QUOTE ]
99% of species are extinct

[/ QUOTE ]

How far do you need to go back in time before this ceases to be true?

I guess a few billion years, although a big confusion here is the difficulty in defining species in bacteria and archaea.