What factors determine cyclic amplitude in the snowshoe hare (Lepusamericanus) cycle?

Charles Krebs, John Bryant, Knut Kielland, Mark O'Donoghue, Frank Doyle, Suzanne Carriere, Donna DiFolco, Nathan Berg, Rudy Boonstra, Stan Boutin, Alice Kenney, Donald Reid, Karin Bodony, Judy Putera, Henry Timm, Toby Burke, Julie Maier, Howard Golden

Research output: Contribution to journalArticlepeer-review

20 Citations (Scopus)


Snowshoe hares (Lepus americanus Erxleben, 1777) fluctuate in 9–10 year cycles throughout much of their North American range. These cycles show large variations in cyclic amplitude and we ask what factors could cause amplitude variation. We gathered data from 1976 to 2012 on hare numbers in the boreal forest of Alaska, Yukon, Northwest Territories, and northern British Columbia to describe the amplitude of hare fluctuations and to evaluate four possible causes. First, weather could cause variation in amplitude via hare reproduction or survival, but this mechanism does not fit our data. Second, bottom-up processes involving forest succession could explain amplitude variation through changes in winter forage availability, but succession is too slow a variable in our study areas. Third, plant defenses entrained by hare over-browsing in one cycle can produce variation in plant quality and quantity in subsequent cycles. A mathematical model suggests this is a possible explanation. Fourth, predator recovery following the cyclic low is inversely related to hare cyclic amplitude, and the existing data are consistent with this mechanism. A standardized regional monitoring program is needed to improve our understanding of cyclic amplitude variation in hares and the possible role of predators and winter foods in affecting amplitude.
Original languageEnglish
Pages (from-to)1039-1048
Number of pages10
JournalCanadian Journal of Zoology
Issue number12
Publication statusPublished - 20 Oct 2014
Externally publishedYes


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