The very notion of protecting "endangered" species goes against nature. If they can not adapt and evolve to the world they live in then they no longer are fit for survival. Look at Pandas, they absolutely refuse to -blam!- and repopulate. Why should we bother trying to help a species that doesn't care about themselves?
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To maintain a certain grade of biodiversity. Yes, species die out eventually, but the current rate of animal extinction is relatively high thanks to human encroachment and agricultural monocultures that have replaced regional varieties. Do you want to live in a world that only has humans, cats, dogs, cows and pigeons?