Unpalatable, yet unprotected: trout reduce survival and development rate of rare toad tadpoles despite chemical defense

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.