The role of introduced trout in the decline of the spotted tree frog (Litoria Spenceri) in south-eastern Australia Original Research Article

The spotted tree frog (Litoria spenceri) is one of numerous amphibian species in Australia that have suffered population declines in recent years, and is currently listed nationally as critically endangered. It occurs in upland streams in south-eastern Australia, along with two other lotic frog species: Lesueurʹs frog (Litoria lesueuri) and the leaf-green tree frog (Litoria phyllochroa). L. phyllochroa is uncommon along upland streams and may also have declined. In contrast L. lesueuri remains common in upland streams and has not declined. Brown trout (Salmo trutta) and rainbow trout (Onchorhynchus mykiss) have been introduced throughout the range of L. spenceri. The geographic pattern of decline of L. spenceri suggests that trout may have contributed to population declines. To assess this hypothesis, predation of tadpoles of L. spenceri and sympatric frog species by trout was experimentally evaluated. Experiments were conducted to compare the relative susceptibility of tadpoles of L. spenceri, L. phyllochroa and L. lesueuri to predation by two native fish species, mountain galaxias (Galaxias olidus) and two-spined blackfish (Gadopsis bispinosus), and S. trutta. Trout predation upon tadpoles was also investigated within a natural stream enclosure. Native fish species ate none or only few tadpoles of the riverine species, despite preying heavily upon tadpoles of a non-riverine species. In contrast, trout preyed upon all species to varying degrees. Fingerling trout preyed heavily upon hatchling tadpoles of L. spenceri and L. phyllochroa, but predation levels on tadpoles of L. lesueuri were much lower. L. lesueuri is the species that remains common in upland streams in the region. Trout were able to significantly reduce survivorship of tadpoles of L. spenceri and L. phyllochroa over short time periods within a stream pool enclosure where alternative prey and tadpole refugia were available. These findings indicate that introduced trout may have played a major role in the decline of L. spenceri.