Delayed Snowmelt Reduces Rocky Mountain Ground Squirrel Populations

Columbian ground squirrels (Urocitellus columbianus) are small (<1 kg) rodents that live in the high alpine and subalpine areas of the North American Rocky Mountains. Due to the short growing season of the plants they consume, these ground squirrels spend 8-9 months hibernating. During the active summer season, female ground squirrels spend the first 1.5-2.0 months rearing young and the last 1.5-2.0 months fattening up for the winter hibernation. A previous study (Ozgul et al., 2010) found that earlier snowmelt was associated with an earlier-than-usual emergence from hibernation and an increase in survival in yellow-bellied marmots (Marmota flaviventris) - another subalpine rodent - which led to an increase in population size. However, what would happen when snowmelt was delayed?

For twenty years (1991-2011), Lane et al. (2012) followed 150 female ground squirrels from a well-studied population where all animals were individually tagged for easy recognition. The population lived at 1,500 m above sea level on a mountain-top in Alberta, Canada (50°N, 114°W). Date of snowmelt and spring temperatures were recorded at a nearby weather station for each year of the study.

Over the course of the 20-year study, the researchers found that the date of emergence from hibernation in spring of adult female ground squirrels was delayed 0.47 days per year. The date of snowmelt was delayed by about 2.8 days (± 0.9 days) per year but there was not a significant trend in spring temperature. During the first half of the study, there was only one year when a substantial snowfall (>5 cm) fell after the middle of April and only one year (out of ten) when the population declined. However, during the second half of the study, seven years had late spring snowstorms and four out of nine years experienced a population decline. The researchers concluded that the delay in emergence from 1991-2011 due to delayed snowmelt resulted in a significant decline in individual fitness for female ground squirrels and subsequent declines in the overall population.

Lane et al. sum up the implications of their study this way: "the overwhelming majority of research to date on the effects of climate change in natural systems has focused on increasing temperatures (for example, a previous study summarized over 25,000 climate-change-induced phenological shifts and all of these were in response to temperature). Spring temperature is certainly the most ecologically relevant cue for many species, but over many areas of the globe its influence will be moderated by the date of snowmelt. Historically, these two cues should have been correlated as warmer springs led to earlier melting of the snow pack. However, over the past 20 years, this correlation has been weak at our study site [(statistics given)] as the date of snowmelt is now heavily influenced by an increasing prevalence of late-season snowstorms. The results presented here highlight the extent to which aspects of climate change other than increasing temperature may affect natural populations."

In other words, late spring snowstorms and delayed spring snowmelt were found to have detrimental effects on alpine/subalpine ground squirrels, while earlier snowmelt was found to have beneficial effects on subalpine yellow-bellied marmots (Ozgul et al., 2010). Of the two, it would appear that earlier snowmelt was more beneficial to ground-dwelling alpine/subalpine rodents but surprisingly, spring temperature did not appear to be a significant factor for either species.

Additional Reference
Ozgul, A., Childs, D.Z., Oli, M.K., Armitage, K.B., Blumstein, D.T., Olson, L.E., Tuljapurkar, S. and Coulson, T. 2010. Coupled dynamics of body mass and population growth in response to environmental change. Nature 466: 482-485.