Consequences of dam‐altered thermal regimes for a riverine herbivore's digestive efficiency, growth and vulnerability to predation

Abstract

Abstract Rivers around the world are undergoing shifts in thermal regime due to climate change and human appropriation of water resources. The local impacts of thermal regime change are challenging to predict because water temperature can influence aquatic organisms and communities at multiple levels simultaneously. For example, thermal change can influence the phenology of periphyton blooms, primary consumer physiology and behaviour, and interspecific interactions with predators. Using tadpoles of the threatened river‐breeding foothill yellow‐legged frog (Rana boylii, Ranidae), their algal foods and their invertebrate predators, we mimicked dam‐induced changes in temperature and observed the outcome of consumer–resource interactions. In stream enclosures, we reared tadpoles across a gradient of cold to warm conditions, quantified the ash content of diet, digestive efficiency and growth rate, and assessed their vulnerability to hemipteran and odonate predators. Tadpoles reared in a cool stream (15.5°C daily mean) digested epilithic periphyton poorly (6.6 ± 0.4% assimilation efficiency). In contrast, average assimilation efficiency of tadpoles reared at 19°C was 11.6 ± 0.1%. Access to nutritious diatoms (i.e. dinitrogen‐fixing Epithemia spp., Rhopalodiaceae) increased assimilation efficiency to 10.1 ± 0.1% and 13.8 ± 0.1% in the cool and warm treatments, respectively. Assimilation efficiency correlated positively with tadpole growth rate (R = 0.66, p < .001). The effect of temperature on mortality due to predation was mediated by low tadpole growth rates at cool temperatures and not by the temperature of predator exposure. Non‐lethal effects of predators on tadpole growth and tail injury, however, depended on both rearing temperature and exposure temperature. Contrary to the expectation that the cost of predator avoidance behaviours may be greater at warmer exposure temperatures because basal metabolic rates are higher, our results indicated that the energetic cost of foraging less was amplified at cool temperatures. Our results show that when thermal conditions impair digestion in a species’ early life stages, recruitment bottlenecks are likely to arise due to the combined negative effects of cool temperatures on assimilation efficiency and growth, and increased predation by invertebrates.