I love when nostalgia for a project, place, or species intersects with a current interest, as happened this week for me with a paper by Cordes et al. 2020, about the contrasting effects of climate change on the seasonal survival of yellow-bellied marmots in the Colorado Rocky Mountains.
In this paper, the authors use 40 years of data on survival across age groups of marmots as well as environmental variables like mean minimum and maximum seasonal temperatures, total snowfall, and drought severity to examine the effects of climate change on seasonal mortality risk. Organisms living in temperate, arctic, and alpine environments experience ‘seasons’ or distinct cyclic variation in both biotic and abiotic factors that drive both ecosystem structure as well as the behavior and life history of many species. Generally this means that animals experience short, mild summers with peak food availability in which they give birth and also prepare for the coming winter, which usually is long and harsh with little food availability and during which many animals rely on fat reserves or even go into hibernation. Because climate change is impacting each season differently and also altering the frequency, severity, and duration of extreme events like droughts, the effects of climate change on seasonal mortality might differ by season. For example, warmer temperatures might reduce mortality in the winter and extend the foraging season in the summer, but a drier summer may reduce the amount of forage available at any given time, increasing mortality the winter after a dry summer. Conversely, warmer temperatures also decrease snow and ice pack, which negatively influences survival of animals that depend on, for example, sea ice for giving birth or hunting. Marmots and other high-elevation and high-latitude species are some of the first species to be significantly affected by climate change.
Marmots, one of the largest members of the squirrel family, are found all over North America and Eurasia; however, the population of yellow-bellied marmots studied here is located in western North American at Rocky Mountain Biological Lab in the Colorado Rocky Mountains. Across the year, Marmots here can lose nearly 40% of their body weight from peak weight at the end of the summer to minimal weight after 8 months of hibernation. The marmots at RMBL have been studied consistently for the last 40 years (I know because I was part of this work as an undergrad!) and since 1972, billy barr has been recording environmental data at RMBL, which has been an incredible resource for scientists across many fields (to read more about billy, check out these articles 1 and 2 – he’s an amazing person)! billy has documented the effects of climate change at RMBL, which, in the past 40 years, has seen a significant 3.5 m decline in predicted snowfall, a rise in temperatures of nearly 2°C, an increase in the growing season of nearly 50 days, and an increase in the severity of droughts. Thus this site and this population are a great resource to ask these kinds of questions.
The authors found that across pups, yearlings, and adults, seasonal mortality generally has decreased in the summer and increased in the winter, but that these trends were much more pronounced for pups and yearlings than for adults (Figure 1). Over the past 40 years, pup survival increased by 9% in summer but decrease by 8% in the winter, resulting in a negative change in survival of 3%. In contrast, yearling summer survival increased by 20% while winter survival remained relatively constant, resulting in an overall 8% increase in survival. Although the trends appear in the adult graph, the changes in survival were not significant, resulting in no significant overall change in survival for adults.
The authors then used mechanistic modeling to determine which environmental measures seemed to predict or drive changes in survival by age-class. Pup summer survival increased with decreased total snowfall whereas yearling summer survival was positively influenced by mean maximum summer temperatures and length of growing season. Pup winter survival was driven by conditions the previous summer, specifically pups were less likely to survive winter hibernation after longer, drier summers. There was no clear relationship with yearling or adult winter survival and any of the environmental variables, but adult summer survival was negatively associated with age.
In general, younger marmots are more affected by the interaction of climate change and seasonality, whereas adults seem to be more able to stand changes in seasonal conditions. However, these trends are driven by different environmental covariates depending on if the individual is a pup or a yearling. In fact, for some variables, like length of growing season, the age classes respond differently. In other words, there is no ‘one size fits all’ effect of climate change on survival for this large rodent and we may need to include seasonality and age class in other explorations like this one.
Line S. Cordes, Daniel T. Blumstein, Kenneth B. Armitage, Paul J. CaraDonna, Dylan Z. Childs, Brian D. Gerber, Julien G. A. Martin, Madan K. Oli, Arpat Ozgul. 2020. Contrasting effects of climate change on seasonal survival of a hibernating mammal Proceedings of the National Academy of Sciences 117 (30) 18119-18126; DOI: 10.1073/pnas.1918584117