Introduced trout indirectly impact native amphibians via facilitation of a shared predator

Direct negative effects of introduced fish on amphibians have been studied extensively, but potential additional indirect effects are poorly understood. Based on our analyses of the diet, distribution and density of the aquatic garter snake (Thamnophis atratus) relative to the sympatric common garter snake (T. sirtalis), we found strong evidence that the introduction of trout has facilitated populations of T. atratus in the Klamath Mountains by providing an alternative common prey supply. Diet of T. atratus included trout and amphibians whereas T. sirtalis preyed solely upon amphibians. The distribution and density of T. atratus followed the distribution and density of introduced trout instead of native amphibian prey. When T. atratus was present in basins, numbers of native Cascades Frogs (Rana cascadae) were lower than in basins without T. atratus. The term “hyperpredation” well describes this case where a non-indigenous prey indirectly affects native prey via facilitation of a shared predator.