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Research Digest Synopses of Research Articles On the Brink: How Biology and Humans Affect Extinction Risk Close to a quarter of the world’s mammals are at high risk of extinction. Save for the periodic “great extinctions,” mammalian extinction has been a relatively rare event in geological terms, with one species disappearing from the fossil record every 1,000 years or so. Over the past 400 years, species have been disappearing 50 times faster than this “background” rate, with one vanishing every sixteen years. Human population growth and all its consequences—habitat destruction, propagation of invasive species, poaching—are largely to blame. Top predators often suffer heavily from encounters with humans, especially when those predators are perceived as economic threats. Thirty-four Mexican gray wolves have been reintroduced in Arizona since 1998, and fi ve have been shot, reportedly by ranchers. Species in the most densely populated areas are expected to face the greatest risk, yet some survive while others perish, suggesting biological factors play a role in their fate. If, for The IUCN lists lions as vulnerable. Photo, with permission, by example, the same external force drastically reduces populations Nicky Jenner, Institute of Zoology, London. of species with different biological profi les, then a species with While it’s possible that the direct effects of human population a relatively short gestation period may stand a better chance of density are past—that is, species most sensitive to human recovering than a long-gestating species. incursions are already gone—human population density likely Effective conservation strategies depend on understanding modulates biology. That might explain why gestation length which factors are likely to increase extinction risk, but it’s unclear didn’t predict risk