After more than a decade of puzzling over what is killing off frog species around the world, scientists are now postulating that there is no one factor to blame but rather a combination of factors. Contributing problems include pollution, habitat destruction, the introduction of non-native species, crop fertilizers that cause high concentrations of toxic nitrates, and rising levels of ultraviolet-B radiation in sunlight, say amphibian scientists convening this week at the annual meeting of the American Association for the Advancement of Science. Andrew Blaustein, a zoologist at Oregon State University, said the frogs’ plight seems to indicate that a combination of environmental changes can lead to worldwide trouble, even if no single factor is enough to cause a problem. “Some of the things that are killing frogs almost certainly have implications for other animal species, including humans,” Blaustein said.