Freezing in a warming climate: Marked declines of a subnivean hibernator after a snow drought

Abstract

Recent snow droughts associated with unusually warm winters are predicted to increase in frequency and affect species dependent upon snowpack for winter survival. Changes in populations of some cold‐adapted species have been attributed to heat stress or indirect effects on habitat from unusually warm summers, but little is known about the importance of winter weather to population dynamics and how responses to snow drought vary among sympatric species. We evaluated changes in abundance of hoary marmots (Marmota caligata) over a period that included a year of record‐low snowpack to identify mechanisms associated with weather and snowpack. To consider interspecies comparisons, our analysis used the same a priori model set as a concurrent study that evaluated responses of American pikas (Ochotona princeps) to weather and snowpack in the same study area of North Cascades National Park, Washington, USA. We hypothesized that marmot abundance reflected mechanisms related to heat stress, cold stress, cold exposure without an insulating snowpack, snowpack duration, atmospheric moisture, growing‐season precipitation, or select combinations of these mechanisms. Changes in marmot abundances included a 74% decline from 2007 to 2016 and were best explained by an interaction of chronic dryness with exposure to acute cold without snowpack in winter. Physiological stress during hibernation from exposure to cold, dry air appeared to be the most likely mechanism of change in marmot abundance. Alternative mechanisms associated with changes to winter weather, including early emergence from hibernation or altered vegetation dynamics, had less support. A post hoc assessment of vegetative phenology and productivity did not support vegetation dynamics as a primary driver of marmot abundance across years. Although marmot and pika abundances were explained by strikingly similar models over periods of many years, details of the mechanisms involved likely differ between species because pika abundances increased in areas where marmots declined. Such differences may lead to diverging geographic distributions of these species as global change continues.

The shifting importance of abiotic and biotic factors across the life cycles of wild pollinators

“…Ozgul et al, 2010). For example, in other snow-dominated mountain ecosystems, population abundance of small mammals was most strongly affected by winter conditions compared with other alternative mechanisms, including resource availability and phenology (Johnston et al, 2021).…”

“…For example, abiotic factors, such as short winters or hot summers, might exert stronger effects on certain parts of an organism's life cycle compared with other stages, where biotic factors, such as food resources, may prevail instead (e.g. Johnston et al, 2021; Radchuk et al, 2013). Nevertheless, the effects of abiotic and biotic factors on different parts of an organism's life cycle are often not accounted for, which limits our understanding of the effects of environmental variation on populations (e.g.…”

Dynamic winter weather moderates movement and resource selection of wild turkeys at high‐latitude range limits

“…It should be noted that this sequential approach ignores uncertainty in behavioral classifications, whereas a joint approach which directly propagates uncertainty between models would be better suited to deal with such issues. While a major focus of conservation efforts is providing breeding and foraging habitat, areas of respite from inclement winter weather can be equally important for individual fitness (Johnston et al, 2021;Shipley et al, 2020). For resident individuals in temperate climates, conservation of winter shelter should be considered in conservation assessments (Greiser et al, 2020;Keppel et al, 2012Keppel et al, , 2015.…”

Factors influencing distributional shifts and abundance at the range core of a climate‐sensitive mammal

“…Furthermore, Johnston et al (2019) found drastic declines in pika abundances the year following a snow drought, which led to a functionally colder winter (i.e., less snowpack reduced thermal buffering from fluctuating above‐snow temperatures). This relationship between warmer winter conditions and cold stress may seem paradoxical; however, recent investigations of other species that occupy similar habitats as pikas, yellow‐bellied marmots ( Marmota flaviventris ) and hoary marmots ( Marmota caligata ), have documented declines in population size and over‐winter survival in response to lower snowpack since both species require sufficient snow for thermal insulation while hibernating (Cordes et al, 2020; Johnston et al, 2021). Such investigations exemplify how cold‐adapted species like pikas are excellent indicator species for both quantifying the direct and indirect effects of climate change and illustrating sensitivity to short‐term weather conditions (but see Smith & Millar, 2018, who used site occupancy as a proxy for over‐winter survival at n = 37 patches).…”