Lynx-Snow Hare Cycle in Canada

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Lynx-Snow Hare Cycle in Canada

CAT NEWS
Issue 20, Spring 1994

by Kim G. Poole *

Lynx Lynx canadensis numbers cycle in response to population levels of the snowshoe hare Lepus americanus, their main prey. Two methods have been suggested to manage lynx during their 8-11 year cyclic fluctuations. One strategy is to curtail or eliminate trapping for 3-4 years during snowshoe scarcity (a tracking harvesting strategy), when recruitment in the lynx population is absent or negligible. However, as has been noted, some trappers would face difficulties if required to forego selling valuable lynx pelts for several years. Other researchers, citing the high trapping mortality of lynx determined from radio-collaring studies, suggested that refugia or untrapped reservoirs of habitat should be maintained to sustain sufficient numbers of lynx to provide "seed stock" for the next increase in hare numbers and lynx production. Should a large portion of lynx habitat continuously remain untrapped, trapping restriction during the low phase of the cycle may not be necessary.

Most studies of lynx ecology have examined populations facing moderate to heavy trapping pressure. In this study, I examined the population dynamics of an unharvested population in the North West Territories (NWT), ultimately to determine whether a refugia management system may be a feasible management option.

Lynx are trapped for their pelts by many residents of the NWT, and provide an important source of income for northerners. Since a decline in hare numbers was imminent, I predicted that once hare numbers declined, a majority of the resident lynx would disperse or die, leaving a core of previously-resident individuals (perhaps five or 10-15% of the peak population) occupying enlarged home ranges within the study area. This prediction was similar to the "core population hypothesis" recently detailed by Breitenmoser et al. (1993). The objectives of this portion of the study were to examine the influence of a reduction in snowshoe hare abundance on lynx density, survival, kit production, and home range size.

To better manage lynx, it is necessary to understand the importance of untrapped areas to cyclic population fluctuations. Consequently, I examined an unharvested population in the NWT between 1989 and 1993 to determine the influence of a reduction in snowshoe hare densities on lynx population characteristics. Lynx densities peaked at about 30/100 km², and declined to about 3/100 km² the winter after the crash in hare numbers. Kitten production and survival were high during peak and declining hare densities, with kittens making up 45-49% of the population. No kitten recruitment occurred during years of low hare densities. Annual survival rates declined from about 0.90 before and during the decline in hare densities, to 0.25 (P 0.001) during the first year of low hare densities, and 0.37 during the second year of the low. All radio-collared lynx resident prior to or during the hare decline dispersed and/or died by the end of the first winter of low hare densities.

Natural causes accounted for 1/4 (25%) deaths during years with high and declining hare densities, compared with 19/27 (70%) deaths during years with low abundance of hares (P = 0.077). Most deaths from natural causes occurred during midwinter. Annual home range size was similar between sexes (P 0.46). and differed among years (P 0.01); the smallest ranges occurred during declining hare numbers. Trapping pressure in the region surrounding the study area was comparatively light: only 10-12% of the estimated lynx population was harvested annually, and 19-23% of the area was 10 km from any trapline.

This study is the first to document substantial natural lynx mortality during years of low hare densities. Results suggest that lynx trapping during at least the first two winters of hare scarcity may be partly compensatory. Some season restrictions may be desired to enhance the capability of lynx populations to recover once hare numbers begin to rebound.

(Update to February 1994)
Snowshoe hare numbers have been slightly higher this winter, the third winter of the low in hare densities. This slight increase in prey density over the last two winters may have been just enough for the Iynx: all nine animals being monitored by radio-telemetry have survived the winter thus far (a winter that has been characterized by seven weeks of less than -30 C and often less than -40 C weather). The lynx remaining in or near the study area are occupying expanded home ranges. To date we have captured 74 lynx since March 1989.

I hope to continue the study for several more years into the increase in hare and lynx densities, which I do not predict will occur until, possibly, 1996. With the current low pelt prices and low lynx densities, trapping effort in most of the western NWT is greatly reduced, placing less pressure on the Iynx populations.

Reference
Breitenmoser, U., Slough, B.G. and Ch. Breitenmoser-Würsten. 1993. Predators of cyclic prey: is the Canada lynx victim or profiteer of the snow shoe hare cycle? Oikov 66 issue 33:551-554

(Condensed from Poole, K.G. in press. Characteristics of an unharvested lynx population during a snowshoe hare decline. Journal of Wildlife Management.)

* Furbearer Biologist, Wildlife Management Division,
NWT Department of Renewable Resources, Canada.

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