Future winters present a complex energetic landscape of decreased costs and reduced risk for a freeze‐tolerant amphibian, the Wood Frog (Lithobates sylvaticus)

Abstract

Winter climate warming is rapidly leading to changes in snow depth and soil temperatures across mid-and high-latitude ecosystems, with important implications for survival and distribution of species that overwinter beneath the snow. Amphibians are a particularly vulnerable group to winter climate change because of the tight coupling between their body temperature and metabolic rate. Here, we used a mechanistic microclimate model coupled to an animal biophysics model to predict the spatially explicit effects of future climate change on the wintering energetics of a freeze-tolerant amphibian, the Wood Frog (Lithobates sylvaticus), across its distributional range in the eastern United States. Our below-the-snow microclimate simulations were driven by dynamically downscaled climate projections from a regional climate model coupled to a one-dimensional model of the Laurentian Great Lakes. We found that warming soil temperatures and decreasing winter length have opposing effects on Wood Frog winter energy requirements, leading to geographically heterogeneous implications for Wood Frogs. While energy expenditures and peak body ice content were predicted to decline in Wood Frogs across most of our study region, we identified an area of heightened energetic risk in the northwestern part of the Great Lakes region where energy requirements were predicted to increase. Because Wood Frogs rely on body stores acquired in fall to fuel winter survival and spring breeding, increased winter energy requirements have the potential to impact local survival and reproduction. Given the geographically variable and intertwined drivers of future under-snow conditions (e.g., declining snow depths, rising air temperatures, shortening winters), spatially explicit assessments of species energetics and risk will be important to understanding the vulnerability of subnivium-adapted species.

Snow modulates winter energy use and cold exposure across an elevation gradient in a montane ectotherm

“…As would be predicted from declining air temperatures, we found that both energy use and energy stress decline with increasing elevation, despite increasing winter length at high elevations. This is in contrast to conclusions from a mechanistic niche model using the wood frog Lithobates sylvatica, which suggested that the opposing gradients in winter length and soil temperatures would completely cancel each other out across a latitudinal gradient in North America (Fitzpatrick et al, 2019(Fitzpatrick et al, , 2020. One key difference between these approaches is that mechanistic niche models initiate and terminate "winter" according to the microclimate conditions rather than calendar date (Fitzpatrick et al, 2019(Fitzpatrick et al, , 2020, whereas we used fixed thresholds based on natural history observations, effectively assuming that phenology is independent of site-to-site microclimate variation.…”

“…Climate change-induced reductions in snow cover will therefore have very different fitness implications for organisms living at different elevations, and these fitness impacts cannot be predicted based on air temperatures alone. This highlights the importance of refining our understanding of how snow interacts with air and soil temperatures to determine organismal performance and fitness, in order to predict the biological impacts of climate change (Fitzpatrick et al, 2019(Fitzpatrick et al, , 2020Kearney, 2020;Kelsey et al, 2021).…”

“…Toward higher latitudes and elevations, snow cover increases while air temperatures decrease (Ford et al, 2013;Rice et al, 2011). Across both latitudinal and elevational gradients in the United States, snow reduces or even eliminates gradients in soil compared to air temperature (Fitzpatrick et al, 2019(Fitzpatrick et al, , 2020Maurer & Bowling, 2014). Deep and persistent snowpack at high latitudes and elevations buffers the soil from cold air temperatures, and loss of snow cover will expose soil to more frequent cold snaps (Campbell et al, 2005;Fitzpatrick et al, 2019;Kearney, 2020;Slatyer et al, 2017;Zhu et al, 2019).…”

Earlier Snowmelt Advances Breeding Phenology of the Common Frog (Rana temporaria) but Increases the Risk of Frost Exposure and Wetland Drying

“…Depending on the targeted amphibian species and study site, the effects of earlier onset of breeding phenology on survival and population abundance can be positive or negativefor example a longer snow-free season can be favorable for growth, reproduction and resource acquisition in energy-limited mountain environments (Carey and Alexander, 2003;McCaffery and Maxell, 2010). It can also be negative (Reading, 2007;Tomaševic et al, 2007;Blaustein et al, 2010;Todd et al, 2010;Garner et al, 2011;Wassens et al, 2013;Carter et al, 2018;Fitzpatrick et al, 2020), for instance if earlier breeding phenology increases the risk of exposure of embryos to cold air temperatures or drought (Loman, 2009;Benard, 2015). Hence, with the documented earlier onset of snowmelt occurring in response to warmer winter and spring temperatures in documented in the European Alps (Beniston, 2012;Hall et al, 2015;Klein et al, 2016), surface pond embryos may be exposed to a greater risk of frost, which in turn would influence embryonic survival (Beattie, 1987;Frisbie et al, 2000;Muir et al, 2014) and population dynamics.…”

Three questions about the eco‐physiology of overwintering underground

“…Ectotherms living in cold climates may be dormant underground or in other retreats for months. Some individuals may remain in retreats until they emerge in spring, and conditions in their retreats will affect risk of freezing or cold injury as well as energy reserves in spring (Fitzpatrick et al ., 2020). Others emerge, bask, and move about on sufficiently sunny days; and their mid‐winter activity may enable feeding or physiological adjustments (see below); but it may also expose them to predation by endotherms.…”