Resource instability undermines predictable plasticity‐mediated morphological responses to diet in a postglacial fish

Phenotypic plasticity has been presented as a potential rapid‐response mechanism with which organisms may confront swift environmental change and increasing instability. Among the many difficulties potentially facing freshwater fishes in recently glaciated ecosystems is that of invertebrate prey communities becoming significantly altered in species composition and relative abundance. To test how the rapidity of diet resource change may affect phenotypic responses during development, we subjected juvenile brown trout to pelagic‐type or littoral‐type diets that alternated either daily, sub‐seasonally, or not at all over a single growth season. The proportional intake of each diet was traced with stable isotopes of carbon and nitrogen and modelled with morphometric data on head and jaw shape. While those trout exposed to a single diet type developed predictable morphologies associated with pelagic or littoral foragers, those raised on alternating diets expressed more unpredictable morphologies. With extreme (daily) or even sub‐seasonal (monthly) resource instability, the association of diet type with the phenotype was overwhelmed, calling into question the efficacy of plasticity as a means of adaptation to environments with rapidly fluctuating prey resources. Phenotypic plasticity has been presented as a potential rapid‐response mechanism with which organisms may confront swift environmental change and increasing instability. We subjected juvenile brown trout to diet regimes, traced by stable isotopes of carbon and nitrogen, alternating at various temporal scales. We found that with extreme or even sub‐seasonal resource instability, the association of diet type with the phenotype was overwhelmed, calling into question the efficacy of plasticity as a means of adaptation to environments with rapidly fluctuating prey resources.