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What drives the 10-year cycle of snowshoe hares?

What Drives the 10-year Cycle of Snowshoe Hares? CHARLES J. KREBS, RUDY BOONSTRA, STAN BOUTIN AND A.R.E. SINCLAIR In 1831 the manager of a Hudson's Bay Company post in northern Ontario wrote to the head office in London.
THE TEN-YEAR CYCLE OF SNOWSHOE The local Ojibway Indians were starving, he reported, becauseof a scarcity of "rabbits," and they were unable to trap for furs HARES—ONE OF THE MOST STRIKING because they spent all their time fishing for food (Winter-halder 1980). These shortages of so-called rabbits, which ap- FEATURES OF THE BOREAL FOREST— parently occurred approximately every 10 years, are regularlymentioned in Canadian historical documents from the 18th IS A PRODUCT OF THE INTERACTION and 19th centuries. Those rabbits were in fact snowshoe hares BETWEEN PREDATION AND FOOD (Lepus americanus), and their 10-year cycle is one of the mostintriguing features of the ecology of the boreal forest.
SUPPLIES, AS LARGE-SCALE Ten-year cycles were first analyzed quantitatively when wildlife biologists began to plot the fur trading records of EXPERIMENTS IN THE YUKON HAVE Hudson's Bay Company during the early 1900s. The Hud-son's Bay Company, established in 1671, kept meticulous records of the numbers of furs traded from different postsspread across Canada. The most famous time series drawntogether from those records was that of Canada lynx (Eltonand Nicholson 1942; Figure 1). The lynx is a specialist preda- over the last 40 years ecologists working in Alberta, the tor of snowshoe hares, and the rise and fall in lynx numbers Yukon Territory, and Alaska have put together an array of mirrors, with a slight time lag, the rise and fall of snowshoe studies that have resolved most, but not all, of the enigmas hare populations across the boreal region.
behind these cycles (Keith 1990, Boutin et al. 1995).
The spectacular cycles of snowshoe hares and their preda- To understand any fluctuating population, one must first tors have captured the attention of biologists as well as his- know in detail the mechanisms of changes in births, deaths, torians. These cycles are highlighted in virtually all ecology and movements that are the proximate causes of the changes texts and are often cited as one of the few examples of in numbers. Before we describe these details, we should Lotka-Volterra predator–prey equations, a simple model note that these 10-year hare cycles tend to occur in synchrony which shows never-ending oscillations in the numbers of across broad regions. Indeed, hares across most of Canada predators and their prey. Cycles seem to violate the implicit and Alaska reached a peak in 1997–1999 during the most re- assumption of many ecologists that there is a balance in na- cent cycle. We explain the reasons behind this synchrony be- ture, and anyone living in the boreal forest would be hard low, but let us note here that movements of hares cannot pressed to recognize a balance among the boom and bust in explain these population changes via immigration or emi- nature's economy. The challenge to biologists has been to un- gration. Movements on a local level might be important, but derstand the mechanisms behind these cycles, which has not at the regional level all populations rise and fall in unison.
been easy. One cycle lasts 10 years, and few PhD students or Population changes must be driven by changes in births researchers wish to take 10 years to obtain n = 1. Fortunately, Charles J. Krebs (e-mail: [email protected]), an ecologist in the Department of Zoology at the University of British Columbia, Vancouver, B.C.
V6T 1Z4, has been studying population cycles for 41 years; A. R. E. Sinclair, also with the Department of Zoology at the University of BritishColumbia, is a population and community ecologist who works extensively in East Africa on ungulate and bird populations and in the Yukon onhare population cycles. Rudy Boonstra, a population biologist in the Division of Life Sciences, University of Toronto, Scarborough, Ontario M1C1A4, is interested in the role of stress in population dynamics of mammals. Stan Boutin is an ecologist in the Department of Biological Sciences,University of Alberta, Edmonton, Alberta T6G 2E9; he has been studying northern mammal populations for 21 years. 2001 American Instituteof Biological Sciences.
January 2001 / Vol. 51 No. 1 • BioScience 25

Figure 1. Canada lynx fur returns from the Northern Department of the Hudson's Bay Company from 1821 to 1910. The
Northern Department occupied most of western Canada. The cycle for these data averages 9.6 years. Data are from Elton
and Nicholson (1942). Photo: Mark O'Donoghue.
Reproductive and survival changes
ductive output occurred in 1973, 2 years after the density Snowshoe hares can have three or four litters over a summer, peak of 1971. We found a similar but not identical pattern with five leverets on average in each litter. All hares begin to of response in hares in the southwestern Yukon. The re- breed in spring when they are 1 year old, so age at sexual ma- productive rate at the cyclic peak in the Yukon was about two- turity is a constant. If reproductive rates are to vary, only lit- thirds of the maximum, compared with one-half of the ter size or the number of litters can change. Lloyd Keith and maximum in the Alberta peak. But reproduction in both ar- his students at the University of Wisconsin, working in cen- eas was minimal in the decline phase (about one-third of tral Alberta, supplied the first detailed description of the way maximum reproduction) and highest in the early increase in which hares change their reproductive rate over a 10-year phase of the 10-year cycle (Stefan 1998, Hodges 2000).
cycle (Cary and Keith 1979). Reproductive output reaches Changes in mortality rates are the other driver of changes its peak very early in the phase of population increase, when in hare numbers over the cycle. At Kluane Lake in the Yukon females are producing 16–18 young per summer. It then be- we measured survival rates of hares with radio collars dur- gins to fall rapidly while numbers are still rising; it reaches ing two population cycles. Additional data on survival of a nadir in the year of the density peak or 1–2 years thereafter, hares over two cycles in central Alberta was obtained through during the decline phase of the cycle. The response of re- mark-and-recapture methods and radiotelemetry (Keith productive output to hare density seems to lag 1–2 years, so 1990). The pattern of change for adult hare survival is that, as shown in Figure 2, for example, the lowest repro- shown in Figure 3. Adult survival rates begin to drop slowly Figure 2. Changes in the annual reproductive output of female snowshoe hares in the Rochester area of central Alberta,
1962–1976. Reproductive output was measured in autopsy samples. Data from Cary and Keith (1979). Photo: Alice Kenney.
26 BioScience • January 2001 / Vol. 51 No. 1 Figure 3. Changes in adult hare survival rates over the 10-year cycle at Kluane Lake, Yukon, from 1977 to 1996. Hare density
(histogram) in spring of year t is plotted along with survival rates averaged from spring of year t to t+1 for radio-collared
hares in control areas. Too few hares were captured in 1985–1987 to estimate survival accurately.
as the population increases to a peak but then drops dra- consistent. Lloyd Keith and colleagues found the same matically for 1 to 2 years, part of the cause of the collapse changes in central Alberta cycles that we found in south- in population numbers (Krebs et al. 1986, Hodges 2000a).
western Yukon cycles (Keith and Windberg 1978). The key Once low numbers are reached, adult survival rates im- finding is that both reproduction and survival rates begin prove slowly but do not reach the maximum until 4 to 5 years to decay in the increase phase of the cycle, 2 years before peak after the peak.
densities are reached. Maximal reproduction and highest sur- Juvenile survival can be broken down into preweaning sur- vival rates occur early in the increase phase of the cycle. By vival for the first 30 days of life, and postweaning survival the late increase phase, reproduction has already slowed from 30 days to 1 year of age the following April. Post- and survival rates of both adults and juveniles are falling.
weaning survival follows the pat-tern already illustrated in Figure 3for adult hares. Preweaning survivalis more difficult to measure, and wehave data only from the Yukon forthis stage of the life cycle. We cagedpregnant hares caught in the wildwhen they were near term and thenradio-tagged the leverets immedi-ately after birth when the cage wasremoved. Figure 4 shows that sur-vival is very poor in this early stage.
Relatively high survival occurs onlyin the first 2 to 3 years of the in-crease phase, and survival is alreadylow before peak hare density isreached. Preweaning survival re-mains low in the peak and at least 2years into the decline phase, and thelowest survival occurs near the endof the decline when hare density is Figure 4. Preweaning survival of snowshoe hares over a population cycle at Kluane
already very low (Stefan 1998).
Lake, Yukon Territority. Leverets were radio-tagged at birth and followed for 30
The demographic pattern of the days. It was impossible to obtain any juveniles in the summer of 1993. Hare density
hare cycle is remarkably clear and in spring is shown by the histogram.
January 2001 / Vol. 51 No. 1 • BioScience 27 Both reproduction and survival rates continue to fall for 2 Alternatively, food quality could change over the cycle.
to 3 years after the peak of the cycle, and over the low phase John Bryant at the University of Alaska suggested one at- they start to recover to high values.
tractive qualitative food hypothesis based on secondarychemicals: Shrubs and small trees can fight back against Causes of the cycle
browsers by increasing their content of secondary chemicals What causes these changes in reproduction and survival? such as tannins and resins, which deter digestion in herbi- There are three main factors that seem most likely to cause vores (Bryant 1981). Indeed, experimental browsing of hare cycles: food, predation, and social interactions. In ad- shrubs in Alaska has shown that the plants can respond to dition to these single-factor explanations, two multifactor damage by increasing their secondary chemical defenses explanations have been suggested, one involving food and (Bryant et al. 1985). The key question is whether these plant predation, and the other—the most complex hypothesis— changes can influence the hare cycle.
involving all three factors.
To answer this question, we conducted five food-addition Many other factors might affect snowshoe hare cycles, but experiments during two hare cycles in the southwestern these seem more likely to be modifying influences than pri- Yukon. In four we provided high-quality rabbit chow with- mary causes. Disease and parasitism are two ecological fac- out limit to hares; in the fifth experiment we added high- tors that might affect hare populations but do not seem to quality natural food to a declining hare population (Krebs be an essential cause of cycles. Parasite loads, for example, et al. 1985, 1995, Sinclair et al. 1988). The response of hares might cause hares to be in poor condition and therefore more to rabbit chow is classic: Hares move into the food-addition susceptible to predators. Lloyd Keith and his students sur- areas and their density increases approximately two- to veyed hare parasite loads for many years in Alberta and threefold in comparison with control areas. But once the den- concluded that none of the many parasites of the hares sity increases on the food-addition areas, the hare cycle caused much direct mortality (Keith et al. 1985). Experi- continues unchanged. Hares decline in number at the same mental work with antihelminthics in field populations of time and at the same rate on the food areas as on unma- hares either had no measurable impact on reproduction nipulated controls.
and survival (Sovell and Holmes 1996) or produced mini- Artificial food-addition experiments have been criticized mal effects (Murray et al. 1997). The conclusion is that dis- because the added food is high quality and not natural. We ease and parasites may affect some hare populations tried to address this criticism by supplying natural food to sporadically (see reports in Chitty 1948, 1950, for example), one declining hare population. Tony Sinclair and Jamie but they cannot be an essential cause of cycles.
Smith had shown that snowshoe hares largely avoided small The food hypothesis is attractive because it can explain white spruce trees because the needles contain camphor both why reproduction changes over the cycle and why (Sinclair and Smith 1984, Rodgers and Sinclair 1997). Con- survival might change as well. There are two variants of sequently, small spruce seedlings were the least preferred food the food hypothesis. First, hares may run out of food and in cafeteria trials with hares. But foliage from large white starve, or, second, the quality of the food may decline. Be- spruce trees with branches beyond the reach of hares con- cause hares eat a variety of green plants in the summer, no tain no camphor, and these branches become highly pre- one has considered food shortage in summer to be an im- ferred food when supplied in a cafeteria trial. This portant factor. Winter food plants are the small terminal observation was dramatically verified when a large white twigs of willow, birch, and small trees, as well as other spruce tree was blown over by a windstorm: Hares devoured shrubs; most studies have concentrated on the possibility that the fallen branches. We therefore decided to feed a popula- winter foods are limiting to hares (Keith et al. 1984). Like all tion of hares through a decline by cutting down white herbivores, snowshoe hares have preferred winter foods spruce trees and thus providing natural, highly preferred food and may browse a large fraction of these preferred plants at to a collapsing hare population.
the peak of the cycle.
Stan Boutin and Scott Gilbert did this experiment over There is little evidence from our Yukon studies that over- three winters, with the results shown in Figure 5. The extra all food quantity is limiting at any time. We measured food natural food produced no detectable effect on the rate of abundance over the cycle by quantifying edible forage and population collapse. The failure of this extra food to affect we assessed consumption rates of marked twigs. Con- the hare population decline was shown clearly on five areas sumption increased markedly during the peak (Smith et al.
in two cycles in the southwest Yukon. Such results imply that 1988, Hik 1995, Krebs et al. 2001), with 80% to 90% of the food shortage by itself is not the explanation for the hare cy- available bog birch being consumed. However, only 20% to cle. Whatever secondary chemical changes occur in winter 40% of the grey willow—the dominant shrub in the area— food plants, they are at most a contributing factor, not the was consumed, thus indicating that food was not limiting primary cause of the hare collapse.
at any time in the cycle. If the absolute abundance of food Another way of approaching the possible role of food in were limiting, we should have found hares that had starved, hare cycles is to improve the quality of the vegetation by but only about 3% of the hare mortalities could be directly adding nutrients in fertilizer. We ran this experiment from attributed to starvation.
1987 to 1996 on two areas in the southwestern Yukon, each 28 BioScience • January 2001 / Vol. 51 No. 1 Figure 5. Changes in snowshoe hare numbers on control (1050, red) and food-supplemented (blue) areas during the
population decline of 1981–1983 at Kluane, Yukon. The natural feeding experiment was begun in October 1981 (blue
triangle). Summer months are shaded yellow. Data are from Krebs et al. (1985).
1 km2. On each area we added NPK fertilizer each spring to rels and ground squirrels. Of the leverets with radio tags, 81% increase the availability of soil nutrients. Boreal forest soils died because of predation (O'Donoghue 1994). These ob- are typically impoverished in nutrients, particularly nitro- servations of natural history support the contention that pre- gen, and nutrients added through fertilizer are immedi- dation by a variety of birds and mammals plays an important ately taken up by plants. We measured plant responses to role in the hare cycle.
nutrient additions and found large increases in individual For mammalian predators in winter, we used snow track- plant growth in grasses, shrubs, and trees (Turkington et al.
ing to monitor density changes and kill rates of lynx and coy- 1998). None of this plant improvement resulted in more otes. All hare predators showed strong numerical changes that snowshoe hares on the fertilized areas, relative to the con- lagged behind the hare cycle 1–2 years (Boutin et al. 1995).
trols; we therefore concluded that the dynamics of the hare In addition, both lynx and coyotes killed more hares per day cycle cannot be changed by nutrient additions to the ecosys- in the peak and decline phases than during the increase.
tem. The message seemed to be repeated: The hare cycle is These kill rates were well above previous estimates and well not driven primarily by plant–herbivore interactions.
in excess of energy demands. Surplus killing seems to be a The predation hypothesis is the next most likely expla- characteristic feature of these predators.
nation for the hare cycle. In our studies with radio-collared To test the predation hypothesis, we excluded mammalian hares, the immediate cause of death of 95% of the hares was predators from two areas by constructing electric fences in predation by a variety of predators, the main ones in the the Yukon, each 1 km2. We also attempted to exclude avian Yukon being lynx, coyotes, goshawks, and great horned owls predators from smaller areas inside the electric fence by (Rohner and Krebs 1996, O'Donoghue et al. 1997). Few means of fish netting and monofilament fishing line strung hares died with signs of malnutrition, and those that did in trees, but these proved impractical and ineffective. The starve were found more often in the increase and peak fence was permeable to hares and other small mammals, phases rather than in the decline (Hodges 2000a). In con- which could move in and out at will. In one fenced area we trast to adult hares, leverets are killed by a variety of small also added food, so we had a combined treatment manip- raptors, such as boreal owls, red-tailed hawks, kestrels, and ulating predation pressure and food supplies (Krebs et al.
hawk owls, and by small mammals, particularly red squir- 1995). Because of the size of the exclosures and the main- January 2001 / Vol. 51 No. 1 • BioScience 29 Figure 6. Changes in survival rates of
snowshoe hare numbers on control
and fenced areas during the
population cycle of 1988–1996 at
Kluane, Yukon. The arrows show the
mean survival rate for each
treatment. The survival rate per 30
days is averaged over each year, with
90% confidence limits, for radio-
collared hares.
tenance they required, we were unable to replicate these food during the peak of the cycle in 1989 and 1990 had no fence treatments.
impact on reproductive output (O'Donoghue 1994). How- The main impact of the electric-fence predator exclo- ever, during the decline phase in 1991 and 1992, the preda- sures was to increase the survival rate of radio-collared tor exclosure plus food treatment caused a dramatic increase hares. Figure 6 shows that the collapse in survival that nor- in reproductive output over that found in control hares mally occurs in the peak and decline phases of the cycle was (Hik 1995, Stefan 1998). Because we were unable to obtain nearly eliminated by the exclusion of mammalian predators.
these reproductive data on food addition sites in 1991 and There is a slight decrease in survival during the decline 1992, we do not know if it is the additional food or the ex- phase with the combined predator exclosure and food treat- clusion of mammalian predators, or a combination of these ment, but the impact of additional food on survival during factors, that produced the results shown in Figure 7.
the decline is small. The mortality rate is almost entirely dri-ven by predation.
Two further conclusions follow from these experiments.
How might predator exclusion affect reproductive output? Inasmuch as avian predators had access to the two preda- There has been much interest in the indirect effects of preda- tor exclosures, avian predation by itself is not sufficient to tors on prey (Lima 1998). One way in which predators explain the changes in survival rates. There is a slight re- might cause a decline in reproductive output is by stressing duction in survival inside the predator exclosures during the hares through high rates of encounter and repeated unsuc- decline (Figure 6), and this is a measure of the impact of bird cessful attacks during the peak and decline phases (Boon- predation by itself on hare survival. Since many species of stra et al. 1998a). Chronic stress has many direct detrimental predators are involved in causing the collapse in prewean- effects on mammals, including reduction in reproductive ing, juvenile, and adult hare survival, we cannot pinpoint the rate, mobilization of energy reserves, and increased sus- role of any individual predator species. In particular, the ceptibility to diseases and parasites. Stress effects may also snowshoe hare cycle is not strictly a lynx–hare cycle, as be indirect and long term, affecting adult brain function many textbooks claim, and if the lynx is removed from the (Lupien and McEwen 1997) and offspring viability and be- predator community—as it is on Anticosti Island in the St.
havior (Matthews 2000). Figure 8 illustrates these ideas for Lawrence River in eastern Canada—the hare cycle contin- a hypothetical hare cycle in which predation pressure causes ues unchanged because of predator compensation (Stenseth chronic stress. Unfortunately, we have not yet been able to et al. 1998).
measure stress levels through an entire hare cycle, but the data If we can explain the changes in mortality rates by we have from declining and low populations is consistent predation, how can we explain the changes in reproduction with this hypothetical scheme.
that accompany the cycle? We have attacked this question What is the role of high-quality food in the hare cycle? experimentally by providing hares on two open areas with Population density increased on areas provided with sup- supplemental food year-round. We measured annual re- plemental food. Hares on the areas provided with supple- productive output for female hares by multiplying preg- mental food did not lose weight over winter and in general nancy rates and average litter sizes for each of the summer were in better body condition than control hares. Food litters and summing these for each breeding season. Figure quality clearly limits body condition and population den- 7 summarizes our data. Reproductive output at Kluane sity in hares. In addition to these direct effects, changes in Lake was severely reduced in the decline phase, as Lloyd Keith food quality could have subtle indirect effects on hares, (Cary and Keith 1979) found in Alberta (Figure 2). Adding contributing to chronic stress and making them more or less 30 BioScience • January 2001 / Vol. 51 No. 1 Figure 7. Reproductive output of female snowshoe hares over a population cycle at Kluane Lake, Yukon. The blue line shows
hare density in control populations. Supplemental food (green bars) does not affect reproductive output at the peak of the
cycle, but in the decline phase of 1991 and 1992 high reproductive output was sustained on the experimental area fenced
from mammalian predators and provided with supplemental food (red bars). We were unable to measure reproductive
output in every year for all treatments, and thus we do not know for the critical decline years of 1991 and 1992 whether
reduced reproductive output is caused by food or predation or both.
able to avoid predators, parasites, and disease (Hik 1995, (Krebs 1986). Hares are not territorial, and aggression seems Boonstra et al. 1998a, Hodges et al. 1999). Poorer body con- to be restricted to males competing in mating chases and to dition in the peak of the cycle is one consequence when hares females resisting attentive males. There are no records of in- overgraze their preferred winterfoods and suffer from chronicstress, causing them to mobilizeenergy. This loss of condition maylead to reduced ability to avoidpredators and thus to an increase inhares' stress levels. The impact ofextra food would therefore not di-rectly affect reproduction and mor-tality rates, but it would actindirectly to make hares less sus-ceptible to stress. Indirect effectshave limited power to affect popu-lation dynamics, however, and allour feeding experiments show thatextra food does not make hares lesssusceptible to predation and cannotprevent the hare population de-cline.
Figure 8. The chronic stress hypothesis as an explanation of the changes in reproduction
Another possible source of stress that accompany the snowshoe hare cycle. Chronic stress is postulated to arise from signs
for hares is social interactions.
of predator abundance (odor, visual sightings, tracks, unsuccessful chases), and is thus
Snowshoe hares are not particu- related to predator densities. According to this hypothesis, any physiological measure of
larly good candidates for social reg- chronic stress, such as cortisol levels, should be maximal late in the decline phase of the
ulation of population size, and hare cycle. The immune system would also be compromised because of chronic stress.
most hypotheses about hare cycles We do not yet know whether this hypothesis is the correct explanation for the reduced
have ignored the social dimension reproduction that occurs during the hare cycle.
January 2001 / Vol. 51 No. 1 • BioScience 31

Figure 9. The annual reproductive
output of snowshoe hare females
maintained in a colony at
Vancouver. Individual females were
taken from field populations and
classified by the state of the field
population at the time of initial
capture. For 4 years there were
enough females to compare
simultaneously the population of
females taken from the peak phase
of the cycle with those taken from
the low phase. Low-phase females
maintain a lifetime reproductive
output much greater than peak-
phase females; these lab results
mimic the observed changes in field
populations. Numbers above bars
are numbers of females.
fanticide in hares, and without territoriality and infanti- predation and food supplies, but of these two factors, pre- cide there would seem to be little room for social interactions dation is clearly the dominant process. The impact of food to play a role in population dynamics.
is felt largely in winter and it is mostly indirect. Hares do not Nevertheless, there remain some puzzles. One is the lack usually die directly of starvation or malnutrition—the im- of an explanation for the low phase, which follows the de- mediate cause of death is virtually always predation. But food cline and lasts 2–4 years (Boonstra et al. 1998b). A second quality and quantity affect body condition and in this way puzzle is evidence implicating the role of maternal effects in may predispose hares to predation, increased parasite loads, reproductive performance. Tony Sinclair maintained a lab- and higher levels of chronic stress. These indirect effects of oratory colony of hares for 14 years in Vancouver to test for predation and food are the probable cause of reduced re- impacts of secondary chemicals on food choice. Females were productive output. Hares in peak and declining popula- maintained in individual cages with high-quality food and tions must trade off safety and food, and these behavioral were treated for potential parasites. Hares in captivity reg- tradeoffs define the dynamics of the decline (Hik 1995, ularly live 5–7 years, much longer than they live in nature.
Hodges 2000b). The result is a time lag in both the indirect Therefore the colony at times consisted of a mixture of fe- effects and the direct effects of predation, which causes the males trapped live in the Yukon from peak populations and cyclicity. The low phase of the cycle is the combined result from low populations. Figure 9 compares the reproductive of continuing predation mortality and slowly recovering output in captivity of these two groups of hares (A.R.E.
Sinclair, personal communication, 1999). Hares taken fromlow populations maintained a lifetime reproductive output Synchrony in the hare cycle
more than double that of hares taken from peak populations, Not only do snowshoe hare numbers fluctuate in 10-year cy- suggesting that female hares are already programmed for cles, but these cycles tend to occur in synchrony across their reproductive success early in life, and no matter how much of the boreal forests of Canada and Alaska. Synchrony fine an environment is provided, they cannot change.
is not absolute, however; different regions within Canada can These results are completely serendipitous: This was not drift out of phase. Smith (1983) used questionnaire data on a planned comparison, females could not be matched for age, snowshoe hare abundance from 1931 to 1948 to measure and this reproductive measurement was not the reason the synchrony across Canada (Figure 10). He concluded that colony was maintained. There was no correlation in these there was a large area of cyclic synchrony in northern Man- colony data between the reproductive output of mothers and itoba and Saskatchewan that was 2 years ahead of "average," daughters, suggesting that this effect was not a genetic. A and areas in southern Ontario and Quebec that were up to planned comparison of colony hares should be carried out 2 years later than average for this time period. Populations to see whether the effect can be verified in another popula- in the Yukon were also up to 2 years later than average. In tion. At present, these results must be considered unproven, some sense, the peak of the hare cycle for this time period but tantalizing.
was like a traveling wave in a pond, with the center in the We are now close to understanding the snowshoe hare cy- middle of the boreal forest. The result of this kind of trav- cle. The 10-year cycle is a result of the interaction between eling wave is that synchrony falls off with distance, as shown 32 BioScience • January 2001 / Vol. 51 No. 1

larly structured (Figure 12). These three regions coincidedwith the climatic zones defined independently from theNorth Atlantic Oscillation (NAO), a climatic oscillationsimilar to the El Niño in the Pacific Ocean. The reasons forthese correlations are not clear, though Stenseth et al. (1999)suggest that the changing snow levels associated with theNAO might affect the hunting efficiency of lynx.
What ecological factors might cause synchrony across large geographic regions? Two general models for synchronycan be suggested, one driven by weather and one by dispersalmovements. The most famous model to synchronize the harecycle across North America is the weather model related tothe sunspot cycle (Sinclair et al. 1993). Sunspots and harenumbers are highly correlated for three time periods dur-ing the past 250 years: 1751–1787, 1838–1870, and1948–1986, which were all periods of high-amplitudesunspot fluctuations (Sinclair and Gosline 1997). Because Figure 10. Synchrony in snowshoe hare cycles across
sunspots affect broad weather patterns, it is possible that Canada, 1931–1948, as measured by questionnaires
sunspots might, through weather, entrain snowshoe hare cy- (Chitty 1948, 1950). The average peak phase across
cles across the continent when solar activity is unusually high.
Canada was scaled as 0.0, and the contour lines indicate
The main problem with this explanation is that there is at peaks occurring earlier than average (red, negative
present no mechanism for sunspot-affected weather patterns contours) or later than average (green, positive contours).
to translate into demographic impacts on hares. All our During this period, hare peaks were reached earliest in
studies in the Yukon suggest the predominant role of preda- the central boreal region of northern Saskatchewan and
tors in driving the hare cycle. As mentioned above, Stenseth Manitoba (Smith 1983).
et al. (1999) suggested an indirect impact of snow depth on in Figure 11 for lynx fur return data from the Canadian hunting success of mammalian predators as one way in provinces for the period 1920–1987 (Ranta et al. 1997).
which synchrony might be coordinated, but there are no data Because Canada lynx fur harvests are highly correlated to test this suggestion. There is also a problem in reconcil- with snowshoe hare numbers, the fur trading records of ing the global nature of sunspot activity with the local vari- the Hudson's Bay Company have been used extensively to ations in synchrony shown in Figure 10.
analyze synchrony in the 10-year cycle (Stenseth et al. 1999, Dispersal movements would appear to be the more likely Haydon and Greenwood 2000). Regions across Canada explanation for synchrony. Hares disperse on a local scale of showed a higher degree of synchrony in the 19th century than a few kilometers (Gillis 1997, Hodges 2000b), so hare move- during the 20th century, for reasons that are not clear.
ments seem incapable of affecting synchrony except on a lo- Within Canada, Stenseth et al. (1999) showed, there were cal scale. Predator movements are more likely the key element three broad regions within which the lynx cycle was simi- causing synchrony within the boreal region. Long-range Figure 11. Synchrony among pairs of
provinces in Canada for lynx fur
return data from 1920 to 1987. The
geographic midpoint of each province
was used to calculate the distance
between provinces. The synchrony
index is the simple correlation
coefficient between the pairs of
provinces. Synchrony falls off with
distance and then rises again because
British Columbia and the Yukon as
well as eastern Canada tend to peak
2–3 years later than the average of
central Canada. The dashed line is a
simple polynomial regression. Data
from Statistics Canada, as used by
Ranta et al. (1997).
January 2001 / Vol. 51 No. 1 • BioScience 33 Lotka and Volterra were partly correct when they guessed that the snowshoe by the North
hare cycle was a predator– prey oscillation, but they missed the critical point that large circles define
the cycle can be understood the three regions
only by analyzing three within which the
trophic levels rather than lynx cycle is most
two. The hare cycle is pro- duced by an interaction between predation and food regions fit within
supplies, and its biological impacts ripple across many species of predators and prey zones. Stenseth
in the boreal forest.
et al. (1999).
The boreal forest is one of the great ecosystems of theearth, and the 10-year snow- dispersal movements of radio-collared lynx have been de- shoe hare cycle is one of the most striking features of this scribed in several studies (Mowat et al. 2000). Documented ecosystem. After 70 years of questionnaire research, time dispersal movements up to 1100 km have been recorded, with series analyses, and field experiments, we have a good 15 movements greater than 500 km. These dispersal move- understanding of the dynamics behind the hare cycle and ments were detected only because the dispersing lynx were the importance of predation and food supplies in regulat- caught by fur trappers, and thus constitute a biased (trun- ing that cycle. The snowshoe hare is a critical species in the cated) set of true dispersal movements. Houston (1978) re- boreal forest: If it should disappear, many species of ported similar data for movements of great horned owls, with predators would go with it, and the structure of the plant 36 banded owls moving 265 to1415 km before being shot, community would be altered substantially.
trapped, or found dead. These data for great horned owls and The boreal forest ecosystem and the 10-year cycle in lynx indicate the potential for predator dispersal to syn- snowshoe hares are clearly resilient to a variety of natural chronize hare cycles over large areas of the boreal forest re- disturbances, from forest fires to short-term climatic fluc- gion (Ydenberg 1987, Ims and Steen 1990). If predators tuations, but it is unclear whether they can withstand the move from food-poor to food-rich regions on a scale of 1000 changes that humans impose without collapsing. The km, they could readily bring local regions into synchrony in impact of directional climate change on the boreal forest the general pattern shown in Figure 10. The predator- community will probably be less in the short term than the dispersal hypothesis for synchrony of hare cycles is attrac- impact of forestry and other human activities such as oil tive because it is parsimonious, since predation plays such and gas exploration. We should explore the limits of an important role in driving the hare cycle through its resilience of the snowshoe hare cycle as forest harvesting direct effects on hare mortality and its indirect effects on hare extends north into the boreal forest, lest we compromise this fascinating system.
Lynx, which have been declared a threatened species in the continental United States, and the dynamics of its interac- tion with snowshoe hares in states with southern borealforests, such as Montana, are in critical need of study (Rug- Boonstra R, Hik D, Singleton GR, Tinnikov A. 1998a. The impact of predator-induced stress on the snowshoe hare cycle. giero et al. 2000). If hare populations are synchronized re- gionally by predator movements, the fragmentation of hare Boonstra R, Krebs CJ, Stenseth NC. 1998b. Population cycles in mammals: and lynx habitat in the southern parts of Canada and in the The problem of explaining the low phase. northern United States could break the ebb and flow of Boutin S, et al. 1995. Population changes of the vertebrate community dur- these linkages. The present shortage of genetic informa- ing a snowshoe hare cycle in Canada's boreal forest. Bryant JP. 1981. Phytochemical deterrence of snowshoe hare browsing by ad- tion on the geographic structure of both snowshoe hare ventitious shoots of four Alaskan trees. Science 213: 889–890.
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