The extinction crisis we’re experiencing today is hard to get our arms around. It can be tough even to just know when a species really has become extinct, and not just hiding from people. But scientists also want to know how species become extinct. Once we disturb a place, how long do we have to wait before the species there start to disappear? If we can understand the path towards extinction, we may be better able to stop the stampede. For this week’s “Matter” column in the New York Times, I  look at a rare opportunity to test out ideas of “extinction debt,” created by a dam in Thailand. It turns out that species can vanish from fragmented forests with