Although humanity has hunted a number of species to extinction, most of the problems we've created are inadvertent, in that they stem from habitat loss. By clearing land for agriculture or housing, we've eliminated or badly fragmented the native habitats of many species, driving many to the brink of extinction or beyond. Estimates of the species lost to habitat destruction have caused some to propose that humanity was in the process of creating the sixth mass extinction. We may very well be doing so, but a paper in today's Nature indicates that the models we use for species persistence are almost certainly exaggerating the rate of species loss.
Scientists were already aware that there were problems with estimates of species loss. Some early calculations had suggested we'd have lost nearly half the species on Earth by now, and data suggests that this is nowhere close to being accurate. One proposed solution for the discrepancy is the concept of extinction debt. Once habitat destruction reaches a certain point, the remaining populations of species consist of isolated individuals or lack the genetic diversity to be sustainable in the long-term. The species may still be around technically, but it's destined to go extinct within a few generations (which, depending on the species' longevity, could take dozens of years). In this case, the estimates are close to right, but the impact of the extinctions haven't been felt yet.
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