This indicator tracks population numbers of small mammals and snowshoe hare, major prey species in NWT ecosystems.
The information for this indicator is obtained from the NWT small mammal survey1 and the NWT hare transect survey. The small mammal survey (SMS) is designed to monitor changes in vole, mouse, lemming, and shrew abundance across five ecozones in the NWT, and to link with similar a survey in Nunavut2. The hare transect survey (HTS) is designed to monitor changes in hare numbers across three forest ecozones in the NWT. This monitoring provides information on predator (furbearer) abundance to trappers. Lynx is the main predator of snowshoe hare.
Small mammals and hares play a keystone role in both Arctic and boreal ecosystems3. They are major prey species for foxes, marten, lynx, raptors, and other carnivores. Cyclic fluctuations in their abundance are reflected in similar fluctuations in the abundance of their predators4,3,5. The effects of low abundance in small mammal numbers can also affect alternative prey species such as duck eggs6. When hares are scarce, foxes and other predators will spend energy on alternative, harder to find food sources, such as waterfowl nests. A sharp decrease in waterfowl productivity has been measured in NWT taiga ecosystems in years of low hare density. A 10-year cycle in whooping crane productivity has been recorded in which years of low whooping crane productivity match years of low hare numbers7.
Small mammal/hare density and trend information is used in predicting population trends of economically important furbearers and in monitoring natural changes in predator/prey relationship in northern ecosystems. NWT residents have noticed that foxes are more likely to enter communities when hares and small mammals are low.
The small mammal species tracked in the NWT differ for each ecosystem. In forested regions, small mammals include deer mouse, meadow vole, southern red-backed vole, and northern red-backed vole. On the tundra, small mammals include collared lemming and brown lemming. Snowshoe hare are found in all forested regions of the NWT.
Current view: status and trend
Small mammal populations fluctuate in numbers at most sites surveyed. Peaks in small mammal numbers occur in different years in different regions. There is not enough information to determine if there are similar cycles in the Inuvik area (northern taiga plains). Small mammal fluctuations at the only site on the tundra in the southern Arctic (Daring Lake) do not appear to be in synchrony with any sites in forested NWT. Data from surveys performed in Nunavut in 1987-2001 suggest that the Daring Lake site matches fluctuations measured in other areas of the barrens (i.e., Baker Lake).
Hare: Number of pellets counted on permanent transects cleared every year in June (exceptions in timing exist). Pellet counts are transformed to an estimate of hare density - hare per hectares - using a regression developed over 20 years in the Yukon. Source: NWT Hare Survey.
Snowshoe hare can reach very high densities every 10 years or so. Trappers have reported that the past two peak densities in hares have not been as high as in the 1970s. Based on information from the past three cycles, it appears that peaks in numbers occur in slightly different years in different regions. Little evidence for hare population cycles has been detected in the extreme southwest portion of the NWT (site in Dehcho). Flooding in some years at this site may explain this.
Hare and Lynx: Hare density as in Hare graph. Number of lynx pelts sold at auctions. Source: NWT Hare Survey and ITI Fur Database.
The 10-year cycle in hare numbers is strongly reflected in lynx fur sale records. The strong relationship in the 10-year cycles in snowshoe hare and lynx has been studied for about 100 years. These reasons for the 10-cycles are proving more complex than anyone anticipated. The explanation8 that best fits with observations and ecological mechanisms involves a decline in food availability as hare populations go up. When hare density is very high, increased diseases and predation by lynx results in a rapid crash in hare numbers. The timing and synchronization of these cycles across large parts of North America are best explained by large weather fluctuations such as the North Atlantic Oscillation9,10 and the El Niño Southern Oscillation11. Weather patterns may influence these cycles in many ways. For example, the type and total amount of snowfall may affect the ability of lynx to successfully hunt hare12.
It is difficult to predict when peaks in numbers of small mammals will occur in the future. Snowshoe hare numbers peaked near 2010. An increase in hare numbers has been noted in most regions of the NWT, followed by a decline. This last peak in numbers again did not appear as large as those seen in the 1970s and earlier. The reasons for this are unknown.
Cycles or fluctuations in small mammal numbers have been recorded in most northern ecosystems, including in other jurisdictions in North America, Scandinavia, northern Europe, northern Russia8,3,19,26,38. The 10-year cycle in hare numbers is one of the best-known ecological phenomena in northern ecosystems13,14,8. These cycles provide a pulse of extremely high numbers of prey available to a small set of northern predators. There is evidence that the 10-year cycle in hare density decreases or disappears in regions where hare habitat has been severely fragmented15. As in the NWT, small mammal and hare cycles or fluctuations are often found to be synchronized over large regions16,10. A greater synchrony is found within regions experiencing the same climate variability16,14,17,8,9.
There is increasing evidence from northern Europe that small mammal cycles can collapse quickly and populations can reach permanently low levels if some climate conditions, such as the length of winter, change18. Long-term monitoring of small mammals and hare in the NWT is essential to detect whether or not similar collapses occur in the NWT.
- To find more on small mammal and hare survery in the NWT: small mammal and hare surveys in the NWT.
- To find more information on what drives the hare cycles see references below.
Other focal points
- See NATURAL CLIMATE FLUCTUATIONS and CLIMATE AND WEATHER for indicators on weather-climate events that may drive or influence population cycles in northern species.
- See indicators on USE OF RENEWABLE RESOURCES for more information on hunting statistics.
Ref. 1. Shank C.C. 1997. The Northwest Territories small Mammal Survey: 1990-1996. Department of Resources, Wildlife and Economic Development, Yellowknife, NT.
Ref 2. Krebs C.J. et al. 2002. Synchrony in lemming populations in the Canadian Arctic. Can. J. Zool. 80:1323-1333.
Ref 3. Krebs C.J.1996. Population cycles revisited. J. Mammalogy (8).
Ref 4. Finerty J.P. 1980. The population ecology of cycles in small mammals. Yale University Press, New Haven, CT. 234 pp.
Ref 5. Boonstra R., C.J. Krebs, and N.C. Stenseth.1998. Populations cycles in small mammals: the problem of explaining the low phase. 79:1479-1488.
Ref 6. Rodney W.B. et al. 2005. Effects of small mammal cycles on productivity of boreal ducks. Wildlife Biology 11:3-11.
Ref 7. Boyce M.S., and R.S. Miller. 1985. Ten-year periodicity in whooping crane census. Auk 102(3): 658. American Ornithologists Union.
Ref 8. Stenseth N.C. 2007. Canadian hare-lynx dynamics and climate variation: need for further interdisciplinary work on the interface between ecology and climate. Climate Research 34(2): 91-92.
Ref 9. Stenseth N.C. et al. 2004. The effect of climatic forcing on population synchrony and genetic structuring of the Canadian lynx. Proceedings of the National Academy of Sciences 101:6056-6061.
Ref 10. Stenseth N.C. et al.1999. Common Dynamic Structure of Canada Lynx Populations Within Three Climatic Regions. Science 285:1071.
Ref. 11. Zhibin Z., T. Yi, and L. Zhenqing. 2007. Factors affecting hare-lynx dynamics in the classic time series of the Hudson Bay Company, Canada. Climate Research 34:83-89.
Ref 12. Stenseth N.C. et al. 2004. Snow conditions may create an invisible barrier for lynx. Proceedings of the National Academy of Sciences 101:10632-10634.
Ref. 13. Krebs, C.J. 2001. What Drives the 10-year Cycle of Snowshoe Hares? BioScience 51:25-35.
Ref. 14. Krebs C.J., S. Boutin, and R. Boonstra. 2001. Ecosystem Dynamics of the Boreal Forest - The Kluane Project, Oxford University Press. 511 pp.
Ref. 15. Wirsing A.J., T.D. Steury, and D.L. Murray. 2002. A demographic analysis of a southern snowshoe hare population in a fragmented habitat: evaluating the refugium model. Canadian Journal of Zoology 80:169.
Ref. 16. Bowman J., R.D. Phoenix, A. Sugar, F. Neil Dawson, and G. Holborn. 2008. Spatial and temporal dynamics of small mammals at a regional scale in Canadian boreal forest. Journal of Mammalogy 89:381-387.
Ref 17. Post, E. 2005. Large-scale spatial gradients in herbivore population dynamics. Ecology 86:2320-2328.
Ref 18. Ims R.A., J.A. Henden, and S.T. Killengreen. 2008. Collapsing population cycles. Trends in Ecology & Evolution 23:79-86.