Unpalatable prey species often have toxins that protect from direct consumption. However, negative effects of prey manipulation by predators are rarely considered and may understate impacts to unpalatable species. When both prey and predator are species of concern, stronger understanding of the predator–prey relationship is warranted. We explored direct and indirect effects of greenback cutthroat trout (Oncorhynchus clarkii stomias) on boreal toad (Anaxyrus boreas boreas) tadpole survival, growth, and development, as well as postmetamorphosis survival and growth. We found exposure to O. clarkii stomias delayed metamorphosis by 1–2.5 days and reduced tadpole survival by up to 25%, even though only a single tadpole was consumed. Tadpoles from captive-bred eggs had lower survival, growth, and greater loss in body condition after metamorphosis compared with tadpoles from wild eggs. We found that O. clarkii stomias do not innately avoid A. boreas boreas but instead strike tadpoles to determine prey palatability and repeated gustation likely reduced survival. Our results shed light on unresolved issues with A. boreas boreas captive breeding and demonstrate the nonconsumptive negative impacts of predators.
Introduced salmonids can impact aquatic ecosystems through direct predation and indirect effects. We explored the effects of introduced Cutthroat Trout (Onchoryncus clarki) on Boreal Toad (Anaxyrus boreas boreas) survival and habitat use during two aquatic life stages, the embryo and tadpole, at Boreal Toad breeding sites with and without Cutthroat Trout. We found no difference in embryo survival and higher tadpole survival at the site with Cutthroat Trout. Cutthroat Trout are unlikely to use the shallow areas where Boreal Toad eggs are deposited; however, during the tadpole stage, Cutthroat Trout and tadpoles overlap broadly in near-shore aquatic habitats. Frequency of tadpole habitat use is lower in Cutthroat Trout-used areas, but we observed no behavioral or temporal avoidance of Cutthroat Trout by tadpoles. Our results suggest that Cutthroat Trout do not have a negative effect on Boreal Toad embryo or tadpole survival in wild settings and that Cutthroat Trout presence does not preclude tadpoles from using habitats.
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