Population biology

Case of the absent lemmings

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Changing weather patterns, producing the wrong kind of snow, have transformed the population dynamics of lemmings in northern Scandinavia. The knock-on effects have been felt throughout the ecosystem.

Credit: E. LESLIE

A colleague from Oslo once told me that when the Bible was translated into Norwegian, mention of plagues of locusts was replaced with plagues of lemmings. The logic behind this change was that most Norwegians knew nothing of locusts, but were all too familiar with periodic explosions in lemming numbers. The story is apocryphal, with references to lemmings only scrawled by the translator in the margin. Yet these scribbles suggest that lemming outbreaks have been a feature of northern ecosystems for the past millennium. But now the outbreaks, at least in some areas, have stopped. On page 93 of this issue, Kausrud et al.1 explore the underlying reasons.

Norway lemmings (Lemmus lemmus) are remarkable animals. These rodents can live for three or four years, spending their winters beneath the snow and feeding mostly on moss. A female can produce up to three litters a year, with as many as 12 young per litter. Lemmings occasionally become super-abundant when large numbers of young survive2. In northern Norway in 1970, lemmings were so common that snowploughs were used to clear the vast numbers of squashed animals from roads. Outbreaks don't last long: food becomes scarce, and lemmings will then often disperse en masse in search of greener pastures. On occasion, desperate to find food, they jump into water and start swimming. This behaviour led to the myth that lemmings commit suicide.

In northern Scandinavia, lemming outbreaks typically occur once every three to five years. Or they used to. In the past 15 years, localized outbreaks have either stopped or occur less frequently3. The cause of this change is the subject of debate, partly because the reason that rodent populations often show periodic outbreaks is itself controversial4,5,6,7,8. Fluctuating predation, food availability or quality, and climate variability have all been proposed as plausible mechanisms generating these population cycles. Whatever the cause, it is clear that in parts of northern Europe something is now preventing these rodents from periodically producing large numbers of surviving young3.

Kausrud et al.1 analyse a 27-year time series of lemming numbers from one site in Norway. They first demonstrate statistically that climate change means that Norway now gets a lot of the wrong sort of snow. Lemmings do well when warmth from the ground melts a small layer of snow above it, leaving a gap between ground and snow. This subnivean space provides warmth and allows lemmings to feed in relative safety from many of the animals that eat them. Climate change now means that the subnivean space does not exist for as much of each year as it used to. Worse still, the space itself is less likely to form: warmer temperatures mean that snow melts and refreezes, producing a sheet of ice that prevents lemmings from feeding on the moss.

The wrong sort of snow therefore means that food is hard to come by, keeping warm is challenging, and being eaten is more likely. Kausrud et al. use their statistical associations to construct a predictive model of lemming dynamics. This model, fitted to data from before the outbreaks stopped, predicts the observed cessation, providing compelling evidence that changing snow conditions are a major factor in the change in lemming population dynamics.

The researchers then go on to show that the reduction in the frequency of lemming outbreaks has knock-on consequences for the wider ecosystem. They argue that the scarcity of lemmings means that predators such as foxes turn their attention to other species, including willow grouse and ptarmigan, adversely affecting their populations. Evidence for changes in the numbers of species other than lemmings in these northern ecosystems is convincing. But although the mechanism that Kausrud et al.1 propose — a shift in predation patterns — is plausible, it is speculative.

The critical reader will complain that the story is based on correlations. Although this is true, it is often the only way to study populations and the consequences of changing climate for ecosystems9. The collection of detailed long-term data on the dynamics of free-living populations of animals and plants rarely attracts the same excitement as genomics or particle physics, yet such data are vital in characterizing the consequences of climate change for the natural world on which we depend. Describing and predicting such effects of climate change will help us prepare for, and possibly minimize, adverse affects. Kausrud et al.1 elegantly show the value of detailed long-term ecological data, and demonstrate the benefits of maintaining existing studies and instigating new ones.

By the time the Norwegian translator of the Bible got to the book of Revelations, he had stopped making references to lemmings, so we do not know whether the cessation of outbreaks foretells the imminent arrival of the four horsemen of the apocalypse. However, we do now understand that climate change has made lemming outbreaks much less common, which has in turn affected the fragile ecology of northern ecosystems. This research1 provides a striking example of how climate change can modify the workings of the natural world — raising the question of what consequences such change might have for us.

References

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    Kausrud, K. L. et al. Nature 456, 93–97 (2008).

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    Lambin, X. & Krebs, C. J. Oikos 61, 126–132 (1991).

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    Angerbjorn, A., Tannerfeldt, M. & Erlinge, S. J. Anim. Ecol. 68, 34–49 (1999).

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    Coulson, T. et al. Science 292, 1528–1531 (2001).

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Coulson, T., Malo, A. Case of the absent lemmings. Nature 456, 43–44 (2008) doi:10.1038/456043a

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