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animal species

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  Lemmings  
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Collared and Brown Lemmings
Two species of lemmings are found on Bylot Island: the Collared Lemming (Dicrostonyx groenlandicus) and the Brown Lemming (Lemmus sibiricus). The Brown Lemming is typically found in wetlands where it feeds on plants such as sedges and grasses. In contrast, the Collared Lemming prefers a dryer habitat (uplands) where it mainly feeds on forbs and shrubs. Collared Lemming variable © Gilles Gauthier

Both lemming species found on Bylot Island play major ecological roles in the terrestrial ecosystem. They are the main preys of many arctic predators such as Arctic Foxes, Weasels and Snowy Owls, which largely depend on them. Brown and Collared Lemmings also influence tundra vegetation by dispersing seeds or devastating plants when overgrazing them.

A particular characteristic of these two lemming species is the cyclic nature of their populations. Simply, this means the Brown and Collared Lemming populations go through phases of very low to very high densities, depending on food availability. If conditions are good, lemmings can reproduce and have many large litters every year. As a result, their population grows and grows until it reaches a point at which there are not enough plants anymore to sustain all the animals. At this point the population crashes, the vegetation regenerates and the cycle starts over again. On Bylot Island, 3 to 4 years can pass between two peaks in the lemming population.

Because lemmings have important ecological roles, cycles in their populations have large impacts on arctic ecosystems (see Arctic Fox and Snowy Owl sections). For this reason, the Bylot Island environmental monitoring project participates in the small-mammal survey coordinated across the Northwest Territories and Nunavut (Canada) by the Renewable Resources office in Yellowknife, a governmental agency.

Each year since 1994, lemming trapping-censuses are carried out in two study plots of the Qarlikturvik Valley (one in wetlands, one in dryer uplands). Since 1997, another census is carried out at a third study plot in an upland habitat nearby the main geese nesting site (see Snow Goose section). At each study plot, 50 snap-traps are baited during a period of 10 to 11 days, for a total of 500 to 550 trap-nights (50 traps x 10-11 nights) per plot. The lemming abundance index is determined by calculating the number of lemmings caught per 100 trap-nights.

In 2004, we initiated a new sampling program based on live-trapping of lemmings. This program will enable us to estimate seasonal and annual variations in abundance, reproductive activity, and survival of lemmings, which will improve our knowledge of their population dynamics. In order to do so, we are using 2 grids laid out in the Qarlikturvik Valley (one in wetlands and one in a drier habitat), each with 144 live traps baited with apples. We monitor these traps during 4 periods of 5 days from early June to mid-August. All trapped animals are identified, sexed, weighed, marked with electronic PIT tags (or checked for the presence of such tags) and released.

Results

Fluctuating Lemming Abundance (snap-traps survey)

In the Qarlikturvik Valley, we observed that Collared and Brown Lemmings are synchronised, but the Brown Lemming cycle is more accentuated than that of the Collared Lemming.

 

Considering both the Qarlikturvik Valley plots and the upland plot of the main geese colony, we observed a delay of one year between the population peaks of 2000 and 2004 (Qarlikturvik) and the peaks of 2001 and 2005 (main geese nesting site). Interestingly, the year of peak abundance usually followed the year of lowest abundance since the previous peak. Thus, abundance appears to build up abruptly (i.e. within one year) whereas the decline, though rapid in the year following the peak, was not complete until at least two years after the peak, and was thus more gradual.

Population Dynamics (live-trapping survey)*

Results from our 2004 and 2005 field season showed that the density of Brown Lemmings declined markedly between the two years while that of Collared Lemmings was relatively constant. For Brown Lemmings, 2004 appeared to be a peak year in their cycle and 2005 a decline phase.

The proportion of juvenile Brown Lemmings in the population was lower during the decline phase (2005) than in the peak phase (2004), but did not differ between years in the Collared Lemmings. The proportion of juveniles in the population increased seasonally in both species.

Based on the weight and age of Brown Lemmings captured during the summers 2004 and 2005, we were also able to infer the distribution of their birth seasons. Results showed that during the 2004 peak most births occurred during the summer while in 2005 (decline phase) most of the lemmings captured over the summer were born during the previous winter and spring.


* GRUYER, N. 2007. Étude comparative de la démographie de deux espèces de lemmings (Lemmus sibericus et Dicrostonyx groenlandicus), à l’Île Bylot, Nunavut, Canada. M.Sc. thesis, Université Laval, Québec, QC, Canada.