Lemmings (Lemmus and Dicrostonyx spp.)
D.G. Reid1, R.A. Ims2, N.M. Schmidt3, G. Gauthier4, D. Ehrich2
1Wildlife Conservation Society, Whitehorse, Canada
2Department of Arctic and Marine Biology, University of Tromsø, Norway
3Department of Biosciences - Arctic Environment, Aarhus University, Denmark
4Department of Biology, Laval University, Canada
November 7, 2012
- Changes in lemming abundance dramatically alter the composition of the tundra food web, the biomass and structure of vegetation, and the productivity of numerous other birds and mammals.
- Recovery of lemmings to high populations after low density years is most often associated with a period of successful breeding and recruitment of young under the winter snow.
- Regularity in cycle duration seems to be decaying in many Arctic regions, and cycle amplitude in some regions has collapsed to relatively low densities. More monitoring is required to clarify these patterns.
- Future field investigations and modeling are required to identify the primary factors influencing the lengthening period between cycle peaks, and the apparent disappearance of strong population increases to peak densities, because understanding and predicting future Arctic food web dynamics depends on our understanding of lemming abundance.
Lemming Population Dynamics
Brown lemmings (Lemmus spp.) and collared lemmings (Dicrostonyx spp.) are the only small rodents with natural distributions in high Arctic regions. They are also found throughout the low Arctic, often in conjunction with various vole species (genera Microtus and Myodes). The exact number of recognized species varies depending on interpretations of genetic data, especially from island populations (e.g., Wrangel Island). Species composition differs considerably between the Palearctic (Europe and Asia) and the Nearctic (North America and Greenland). Based on Jarrell and Fredga (1993), there are at least three Lemmus species in the Palearctic: the Siberian brown lemming L. sibiricus, the Norway lemming L. lemmus, and even the Nearctic brown lemming L. trimucronatus ranging into Siberia. Also, the Palearctic collared lemming, Dicrostonyx torquatus, is widespread through Eurasia. In the Nearctic, Lemmus is represented by only one species (L. trimucronatus), but Dicrostonyx by at least two species: the Nearctic collared lemming Dicrostonyx groenlandicus, and the Ungava collared lemming Dicrostonyx hudsonius.
Lemmings are of particular interest and ecological importance because they are prey for the majority of Arctic predators, and in many Arctic regions their populations follow multiannual population fluctuations of considerable amplitude (Stenseth and Ims, 1993). Lemmings can also affect the species composition and dynamics of tundra vegetation (Olofsson et al., 2011) - see the Vegetation essay for further information on tundra vegetation. When abundant, lemmings attract nomadic and migratory predators, support high reproductive success in these and resident predators, and indirectly influence the population dynamics of various alternative prey such as nesting shorebirds (see the Waders (Shorebirds) essay) and waterfowl (Gauthier et al., 2004; Ims and Fuglei, 2005; Gilg et al., 2006). Grazing impacts of lemmings during population peaks are so profound that they can be detected from satellite images (Olofsson et al., 2012). Changes in lemming abundance dramatically alter the composition of the tundra food web, and the productivity of numerous other birds and mammals, from year to year; see the Arctic Fox essay, for example.
The population fluctuations in lemmings have been characterized as cycles because peak (high density) populations often recur every three to five years (Stenseth and Ims, 1993). Peak populations generally last no longer than a year, suffering heavy mortality from the various predators they attract, especially during the Arctic summer. Their death rate exceeds production of young and the population declines into a period of low density that can last one or two years. During this low phase, the predators that specialize in lemmings (such as Snowy Owl Bubo scandiacus, and the stoat Mustela erminea, and least weasel M. nivalis) move elsewhere, become scarce, or suffer reduced reproduction of their own (MacLean et al., 1974; Gilg et al. 2006). Recovery of lemmings to high populations densities (the increase phase of the cycle) most often includes a period of successful breeding and recruitment of young under the winter snow (Ims et al., 2011), often in conjunction with a dearth of predators (Stenseth and Ims, 1993). Snow conditions seem particularly influential in this winter breeding because early and deep snow provides maximal insulation for the lemmings living in their winter nests on the ground surface (Duchesne et al., 2011; Reid et al., 2011). See the Snow essay for additional information on snow distribution and other characteristics.
Lemming abundance is monitored at Arctic sites using density of their winter nests, mark-recapture live trapping, or snap trapping. Although considerable variability in the amplitude and duration (period) of lemming cycles has been noted from site to site, including some regions with no strong cycles (e.g., western North American mainland, Krebs et al., 1995, 2002) or irregular outbreaks (e.g., low-arctic Fennoscandia; Ims et al., 2011), limited long-term data sets have often indicated remarkable regularity in duration within a site, with variable amplitude. For example, on Bylot Island, Nunavut, Canada, Nearctic collared lemmings and Nearctic brown lemmings fluctuate fairly synchronously, with much lower amplitude in the collared lemmings. The brown lemmings exhibit outbreaks, with highly variable amplitude, every 3 to 4 years (Fig. 4.4; Gruyer et al., 2008; G. Gauthier unpublished data), but there is little evidence of substantive shift in duration or amplitude during the past two decades. Although peaks in abundance tended to be lower from 2001 to 2009 than during the previous decade, the most recent peak (2011) was very high.
In some regions, however, the cyclic pattern is changing, especially the cycle duration. On Wrangel Island, northeast Russia, the period between years with peak densities has increased from five years in the 1970s to close to eight years in the 1990s and 2000s (Fig. 4.4; Menyushina et al., 2012). In Greenland, Nearctic collared lemming abundance is tracked using winter nest counts at Traill Island (~72°N) and Zackenberg (~74°N), both in high Arctic northeast Greenland. Until 2000, lemming dynamics on Traill Island were characterized by regular cycles of approximately 4 years (Fig. 4.4; Gilg et al., 2003; Schmidt et al., 2012). Given the high degree of correlation in abundance between the two localities (Schmidt et al., 2008), the dynamics at Zackenberg were most likely similar to those on Traill Island prior to 1996. Around 2000, the population dynamics changed simultaneously at both localities, and regular cycles were replaced by irregular, lower amplitude fluctuations at low densities, especially at Traill Island (Fig. 4.4; Schmidt et al., 2012). On southern Banks Island, in the western Canadian archipelago, outbreaks of Nearctic collared lemmings and Nearctic brown lemmings occurred every 3 to 4 years in the 1960s and 1990s (Maher, 1967; Larter, 1998). Further north on the island, the cyclic period seems to have increased to 5 years since the late 1990s (Parks Canada, 2009).
In some other regions the data are less clear, partly because of more intermittent monitoring. On the Taymyr Peninsula of north-central Russia, Siberian brown lemmings cycled with outbreaks every 3 to 4 years from the 1960s to 1990s (Kokorev & Kuksov, 2002). More recent monitoring suggests a more variable period (Fig. 4.4; Ebbinge & Mazurov, 2005; Popov 2009). In this region collared lemmings are less numerous but fluctuate in synchrony.
Interpretations of these trends towards longer cycle duration and cycles with noticeably lower amplitude are driven by correlations and models, and specifically those involving changes in snow conditions associated with a warming Arctic climate (Ims and Fuglei, 2005) - see the Snow essay for additional information on changing snow conditions. The idea that winter snow conditions could have a dramatic effect on dynamics by influencing winter reproduction was proposed by MacLean et al. (1974). Reduced cyclic amplitude suggests constraints on the winter breeding necessary for rapid population growth. Gilg et al. (2009) have modelled the effects of longer snow-free periods (earlier melt and later onset) and more thaw and freeze events in winter, and found them sufficient to explain the changing amplitudes in the Greenland data. Prolonged cycle duration could also result from a changing snow pack, including more frequent thaw-refreeze events causing ground icing and limiting food availability for winter reproduction, as suggested by Menyushina et al. (2012) for Wrangel Island. Also, recent studies on the Norwegian lemming from low-Arctic and sub-Arctic Fennoscandia provide a connection between dampened cycles and milder winters during which snow melt creates ice layers, which make it more difficult for lemmings to reach their food plants (Kausrud et al., 2008; Ims et al., 2011).
The processes described in the previous paragraph require further investigation, including attempts to refute alternative hypotheses for these changing dynamics, such as changing food quality or availability, changing temporal distributions of predators (e.g., the Arctic fox - see the Arctic Fox essay) associated with changing marine ice distribution and duration, and shifts in long-term climate phases such as the North Atlantic Oscillation (NAO) - see the essays on Air Temperature, Atmospheric Circulation and Clouds, Snow and Greenland Ice Sheet for information about the influence on the NAO on the physical environment. More monitoring, at a greater diversity of sites, is also required to search for replicable patterns. However, there is growing concern that changing winter conditions in the Arctic may be altering lemming population dynamics, with numerous, often dramatic and unpredictable direct and indirect effects on predators and alternative prey in the same food web (Schmidt et al. 2012). Moreover, the future fate of the lemming cycle may have dramatic implications for the development of tundra vegetation, in particular the rate of expansion of shrubs under climatic warming (Olofsson et al., 2009) - see the Vegetation essay for information on tundra vegetation and changing shrubs.
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