The worldwide decline in frogs and other amphibians has been well documented, though the reasons for it are less clear. Scientists have suggested many possible factors, including climate change, disease and increased exposure to ultraviolet radiation and pesticides. But for some species, a major reason could simply be that they are being eaten — by predators introduced into their habitat. A researcher at the University of California at Berkeley has shown this to be the case for a frog in the Sierra Nevadas. The researcher, Dr. Vance T. Vredenburg of the Museum of Vertebrate Zoology at the university, demonstrated...