Of lemmings and snowshoe hares: the ecology of northern Canada

Abstract

Two population oscillations dominate terrestrial community dynamics in northern Canada. In the boreal forest, the snowshoe hare (Lepus americanus) fluctuates in cycles with an 8-10 year periodicity and in tundra regions lemmings typically fluctuate in cycles with a 3-4 year periodicity. I review 60 years of research that has uncovered many of the causes of these population cycles, outline areas of controversy that remain and suggest key questions to address. Lemmings are keystone herbivores in tundra ecosystems because they are a key food resource for many avian and mammalian predators and are a major consumer of plant production. There remains much controversy over the role of predation, food shortage and social interactions in causing lemming cycles. Predation is well documented as a significant mortality factor limiting numbers. Food shortage is less likely to be a major limiting factor on population growth in lemmings. Social interactions might play a critical role in reducing the rate of population growth as lemming density rises. Snowshoe hares across the boreal forest are a key food for many predators and their cycles have been the subject of large-scale field experiments that have pinpointed predation as the key limiting factor causing these fluctuations. Predators kill hares directly and indirectly stress them by unsuccessful pursuits. Stress reduces the reproductive rate of female hares and is transmitted to their offspring who also suffer reduced reproductive rates. The maternal effects produced by predation risk induce a time lag in the response of hare reproductive rate to density, aiding the cyclic dynamics.

Theoretical impacts of habitat loss and generalist predation on predator–prey cycles

“…With increasing human disturbances and as climate warming alters tundra landscapes (Krebs 2011), these answers will be important not just for understanding the causes for the absence of cycles in southern latitudes, but also for predicting future changes to high amplitude northern cycles. In the case of the Canada lynx, the "booms" of these northern cycles may be important for the persistence of southern populations, as the resulting dispersal maintains the geographical reach of the lynx, which is important for the viability of the species (Murray et al 2008, Squires et al 2013.…”

“…The multiannual population cycles of certain mammals and their predators in northern latitudes have been studied extensively over the last century (Elton and Nicholson 1942, Keith 1990, Akçakaya 1992, King and Schaffer 2001, Korpimäki et al 2004, Tyson et al 2010, Krebs 2011. The importance of cyclic herbivores, such as snowshoe hares (Lepus americanus) and brown lemmings (Lemmus trimucronatus), to tundra and boreal ecosystems lies at the heart of this interest in oscillatory population dynamics (Krebs 2011). …”

“…Hypotheses for the causes of population cycles have invoked sunspots, weather, forest fires, stress, food shortage, plant defenses, and parasites (Akçakaya 1992). While some of these features likely affect population densities, predation is now widely accepted as a key driver of cyclic fluctuations (Akçakaya 1992, Turchin 2003, Korpimäki et al 2004, Krebs 2011. However, less attention has been given to the cause of the observed reductions in amplitude and densities of cyclic species in their respective southern ranges (Akçakaya 1992), and it is this phenomenon to which this study is devoted.…”

Adaptation and habitat selection in the eco-evolutionary process

“…As we embark on developing these more complex models, we should ponder the remarkable fit that simple linear models seem to provide with highly complicated systems. Even the simplified Arctic foodweb in which we predict the eco-evolutionary future of brown lemmings has far more connections than we are likely ever to measure effectively [88]. Yet, amidst this complexity, brown-lemming habitat use on Herschel Island is revealed by a single-species linear isodar that is independent of the densities of two similar coexisting rodent species [87].…”

“…Selection gradients against occupation of dry habitats become progressively steeper, however, with increasing population size. The implication for species with wildly fluctuating population sizes, such as those that typify lemmings [88], are rather profound. When lemming populations exist at low density, almost any strategy of habitat selection yields similarly high fitness.…”