More than 20 animals and one plant were declared extinct by the Fish and Wildlife Service last week — one of the longer extinction announcements in the history of the agency.

Declaring extinction is not as straightforward as you might think. Many of the species on this list likely disappeared decades ago but knowing for certain can be difficult.

And climate change is only exacerbating the problem. A report released by the United Nations last year estimates that one million plant and animal species are at risk of being gone for good.

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