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

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.

Effects of coexistence and predator experience on antipredatory responses of montane amphibian larvae towards native and introduced salmonids

“…Although growth may be not directly compromised and individuals may increase their short-term survival probabilities by shifting from the aquatic to the terrestrial phase, they might incur costs in later stages. Also, because the physiology of growth and development strongly varies with temperature (Smith-Gill and Berven 1979;Atkinson, 1994Atkinson, , 1996Álvarez and Nicieza 2002), the potential for amphibian larvae to co-exist with predators depends on both behavioral responses and environmental conditions (Catenazzi and Kupferberg 2018). Nonetheless, the adaptive phenotypic plasticity we observed might increase population resilience of amphibians in montane habitats suffering from fish introductions.…”

Identifying drivers of population dynamics for a stream breeding amphibian using time series of egg mass counts

“…Our analysis of 36 stream reaches with repeated census data for R. boylii revealed how population dynamics were related to stream temperature and streamflow. Colder stream temperatures were associated with lower intrinsic growth rates, which is biologically plausible given the faster growth, development, and lower risk of predation for larval R. boylii in warmer temperatures (Catenazzi & Kupferberg, 2017, 2018). Regulated stream reaches had much lower density of egg masses on average.…”

“…Model‐predicted temperatures in the NorWeST dataset do not capture the localized water temperature at breeding and tadpole‐rearing sites within streams (Catenazzi & Kupferberg, 2017) because of the spatial and temporal heterogeneity in water temperature that occurs both longitudinally along a stream segment and laterally across the width of a channel. The optimal temperature for tadpole food assimilation, growth, and survival in R. boylii likely peaks around 18–22°C (Catenazzi & Kupferberg, 2018) and declines at higher temperatures (Catenazzi & Kupferberg, 2013, 2017). Temperatures above the optimal range for tadpole development have several potential negative effects.…”

Macroinvertebrate and diatom community responses to thermal alterations below water supply reservoirs

“…The presence of a reservoir can severely modify the thermal regime of the river downstream, including increasing winter water temperatures (Dickson, Carrivick, & Brown, 2012; Raddum, 1985), reducing summer temperatures (Soja & Wiejaczka, 2014; Vinson, 2001), increasing diel constancy (Casado, Hannah, Peiry, & Campo, 2013; Horne, Rutherford, & Wehrly, 2004), and changing the timing of thermal events (Cowx, Young, & Booth, 1987; Preece & Jones, 2002). Thermal alterations within regulated rivers have been shown to prompt a range of ecological responses, including behavioural changes (Catenazzi & Kupferberg, 2018), demographic shifts (e.g., birth rate, recruitment—Rolls et al, 2013), the composition and diversity of biotic communities (Benítez‐Mora & Camargo, 2014; Lessard & Hayes, 2003), ecosystem functionality (Bruno et al, 2019; White et al, 2017), and food‐web complexity (Cross et al, 2013).…”