Maybe they're "extinct here", because they just left.
BTW. this is actually a big theme in David Brin's Uplift saga[0]. Basically, you have all these alien civilizations, more and less friendly to each other, but bound by some common rules, some of which regulate sustainable colonization. This includes an exclusive right to settle and use a habitable planet with a biosphere for $whatever number of millenia, and after that time, the civilization there is supposed to pack up and leave, fix environmental damage, and erase any trace of their existence, commonly achieved by dumping everything into the planet's subduction zones so it gets naturally recycled; after that, the planet is to lie fallow for ${some other number} of millenia, to give nature a chance at creating more biodiversity and stuff.
Quite ingenious setting, if you ask me.
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