Noel DiPaola Title page inserted -TK Winter Ecology – Spring 2009 Mountain Research Station – University of Colorado, Boulder

Climate change responsible for declining lemming population in Norway Current Event: Climate change responsible for declining lemming population in Norway ✪Lemmings: -Are subnivean creatures -Typically experience cyclical vs. stable patterns N/A. (2008). “Climate change pushing lemmings to cliff”. Retrieved on Feb. 26, 2009 from http://news.bbc.co.uk/2/hi/science/nature/7711709.stm The BBC news reported earlier this winter in November that climate change may be the root cause of declining populations of lemmings. Lemmings are small rodent mammals that survive through the winter not by migration or hibernation, but by simply sustaining under the snow in the subnivean space. The main argument of the article is that there is a trend towards stabilization in population density patterns. Typically lemming population growth proceeds in a cyclical pattern where about every 3 to 5 years the population will explode. Recently though, due to “wetter” winters the typical oscillation pattern has not been occurring. 2

In what way is climate change affecting the small rodent community? Change in population dynamics Leading to an overall drop in population explosions. We will be looking at what physical environmental variables (in response to climate change) are causing changes with lemming population growth. Lima et al. (2006). Feedback structures of northern small rodent populations. OIKOS, 112, 555-564. Climate change is causing rodent population dynamics to go from cyclical to stable. Stabilization means that the average of one population increase every 3 to 5 years is not longer happening. Therefore, growth rates are not as high and may even be declining for various lemming species.

More on Lemmings in general The subnivean space One study (Kausrud et al., 2008) specifically looked at this causal relationship of climate change and cyclical to stable population growth. Since the lemmings have such a “numerical dominance” over most all other rodent species it makes them a good indicator of what the subnivean conditions for other small mammals are as well. The subnivean space becomes the home of lemmings and many other small creatures during the winter because it provides insulation from the cold, access to food and protection from predators (foxes, owls, corvids, and raptors) (Kausrud et al., 2008). The subnivean space alsp serves as a time/place when the lemmings can start their reproduction process. Since the lemmings are dependent on the subnivean space, the duration and strength of the habitat will affect the “performance of the rodent community” (Kausrud et al., 2008).

What exactly is causing the stabilization? Less protective snow cover Increased hardness of snowpack Note: Duration of the snow cover is not as important There are two possible things that could be causing the stabilization pattern in populations growth experienced by lemmings: Less protective snow cover and increased hardness of the snowpack. These things could be working independently of one another or be working together to create more stable growth patterns for subnivean species. One might initially think that the duration of the snow cover is a factor in the phenomenon as well. However, scientific findings point to the conclusion that duration of snow cover has an insignificant impact on the rate of density change (Kausrud et al., 2008).

Reasons for stabilization Less Protective Snow Cover ★Increases predation rates (May account for 40-70% of variation in cycles) Net population growth of small mammals, such as lemmings, are dependent on the previous population density and the number of predator interactions (Lima et al., 2006). For the first reason of stabilization - the argument is that the less protective snow cover there is the more predator-rodent interactions there will be, causing a stabilization in population growth. Increased temperatures as a result of global climate change is reducing the amount of snow cover once it falls to the ground by the principle of melt metamorphism. Lima et al. (2006) found that as predation rates increased they did so with consistency across location and species. This study found predation to be responsible for about 40-70% of the variation in cycles, as opposed to the other possible reasons for stabilization only accounting for 10-30% (Lima et al., 2006).

Predation Rates and Snow Cover How would less snow affect predation rates? -Less protection for small animals? How would more snow affect predations rates? -Will increase sowpack density -Overall decrease?  Doesn’t explain recent stabilization patterns It’s really not 100% clear as to whether climate change, followed by increased air temperature and humidity, will generate more or less precipitation. What I am finding is that it really depends on what article you look at. I know that Derek supports the idea that there will be increased precipitation at higher elevations and less snow in the lower areas. If there were to be less precipitation due to more melting in any given area how would this affect predation rates? Less snow cover means less protection for the small subnivean species so predation rates may increase. The picture becomes a little more complex though because warmer temperatures will change both small mammal species’ and predator species’ growth cycles. On the other hand, If more precipitation occurs what will then happen to predation rates? We know that more snow will cause the snowpack to have a greater density (which will be talked more about in just a moment). This may mean that predators have a harder time getting into the subnivean space where their prey reside and/or it may mean that subnivean creatures themselves will decline in population due to a less conducive environment. Both these scenarios would probably cause an overall decrease in predation rates.

Reasons for Stabilization Increased Hardness of the Snowpack Optimal subnivean space = TG snow conditions Hardness develops because of the development of EQ snow, lose of the dynamic temperature gradient, and melting and freezing. The subnivean space is possible due to the formation of “snow crystals with weak cohesion near the ground” (Kausrud et al., 2008). This crystal structure is indicative of the temperature gradient type of metamorphism we learned about in class. “The end result is the formation, at the base of the snow pack, of very brittle, loosely arranged crystals referred to as ‘depth hoar’” (Marchand, 1996). This weakened snow pack allows small mammals to burrow and make a winter community for themselves. Kausrud et al. (2008) propose that “as global temperatures are expected to rise, that temperature is a highly significant predictor of hardness.” In terms of temperature gradient snow, the surface of the snow will be warmer than it once was originally causing more of the snow to become that warmer temperature more quickly throughout. Melting of the snow will occur in the snow due to the higher temperatures which will then cause water to trickle down through the snow crystals (Halfpenny & Ozanne, 1989). “Later freezing sinters (bonds) the crystals, increasing the strength and density of the snowpack” (p.45). This melting and freezing scenario will have the same effect but is not the same thing as equitemperature snow or destructive metamorphism. Equitemperature snow is the transformation of a snowflake into a “roughly spherical particle with a net reduction in surface area” (Marchand, 1996), which occurs from excessive sublimation. If the snow is too hard for the animals to burrow under then it not longer can be considered a subnivean space. Contraction of available subnivean space decreases winter survival of small mammals, making the chance of population explosion less likely.

Works Cited Halfpenny, J. C. & Ozanne, R. D. (1989). Winter: An ecological handbook. Boulder: Johnson Publishing Company. Kausrud, K.L., Mysterud, A., Steen, H., Vik, J. O., Ostbye, E., Cazelle, B., Framstad, E., Eikeset, A.M., Mysterud, L., Solhoy, T., & Stenseth, N.C. (2008). Linking climate change to lemming cycles. Nature, 256(10), 1692-1703. Lima, M., Berryman, A. A., & Stenseth, N. (2006). Feedback structures of northern small rodent populations. OIKOS, 112, 555-564. Marchand, P. J. (1996). Life in the cold. Hanover: University Press of New England.

Summary Stabilization is one effect that climate change has on subnivean species. It occurs because of loss of snow coverage (which in turn increases predation rates) and because of the increased hardness of the snowpack disallowing small mammals to burrow; ruining their winter habitat so that there’s less of over-winter survival among species.