Snowshoe hares

The '10-year' hare and lynx cycles have also been examined in previous sections. We have seen, for example, from Stenseth et al.'s (1997) analysis of time series (see Section 14.5.2) that despite becoming a 'textbook' example of coupled predator-prey oscillations, the hare cycle appears in fact to be generated by interactions with both its food and its predators, both considered as guilds rather than as single species. The lynx cycle, on the other hand, does indeed appear to be generated by its interaction with the snowshoe hare.

This supports other results obtained by much more direct, experimental means, reviewed by Krebs et al. (2001). The demographic patterns underlying the hare cycle are relatively clear cut: both fecundity and survival begin to decline well before peak densities are reached, arriving at their minima around 2 years after density has started to decline (Figure 14.14).

First, we can ask: 'What role does the hares' interaction with their food play in these patterns?' A whole series of field experiments in which artificial food was added, or natural food was supplemented, or food quality was manipulated either by fertilizers or by cutting down trees to make high-quality twigs available, all pointed in the same direction. Food supplementation may improve individual condition and in some cases lead to higher densities, but food by itself seems to have no discernible influence on the cyclic pattern (Krebs et al., 2001).

On the other hand, experiments in which either predators were excluded, or they were excluded and food was also supplemented, had much more dramatic effects. In the study by Krebs et al. (1995) carried out at Kluane Lake in the Yukon, Canada (Figure 14.15a), the combination of the two treatments all but eliminated the pattern of decline in survival over the cycle from 1988 to 1996, and predation played by far the major role in this.

Furthermore, food supplementation reduced slightly the initial decline in fecundity prior to peak densities (Figure 14.15b), but the combination of food supplementation and predator exclusion brought fecundity up to almost maximum levels at the phase of field-scale manipulations of food and/or predators

Figure 14.14 (a) Variation in reproductive output per year (dots) as density (continuous line) changes over a snowshoe hare cycle in central Alberta, Canada. (b) Variation in survival over two snowshoe hare cycles at Kluane Lake, Yukon, Canada. Too few hares were caught to estimate survival between 1985 and 1987. (After Krebs et al., 2001; (a) following Cary & Keith, 1979.)

Figure 14.14 (a) Variation in reproductive output per year (dots) as density (continuous line) changes over a snowshoe hare cycle in central Alberta, Canada. (b) Variation in survival over two snowshoe hare cycles at Kluane Lake, Yukon, Canada. Too few hares were caught to estimate survival between 1985 and 1987. (After Krebs et al., 2001; (a) following Cary & Keith, 1979.)

Figure 14.15 (a) Survival of radio-collared hares (with 90% confidence intervals) over a hare cycle from 1988 to 1996 at Kluane Lake, Yukon, Canada. The bars are densities; lines show the survival in controls (•) with mammalian predators excluded (□) and with mammalian predators excluded and food supplemented (a). (b) Reproductive output over a hare cycle from 1988 to1995 at Kluane Lake (the line). It was possible to compare control values with those from treatments of food supplementation in 1989 and 1990, and with those where food was supplemented and mammalian predators excluded in 1991 and 1992. (After Krebs et al., 2001; (a) following Krebs et al., 1995.)

Figure 14.15 (a) Survival of radio-collared hares (with 90% confidence intervals) over a hare cycle from 1988 to 1996 at Kluane Lake, Yukon, Canada. The bars are densities; lines show the survival in controls (•) with mammalian predators excluded (□) and with mammalian predators excluded and food supplemented (a). (b) Reproductive output over a hare cycle from 1988 to1995 at Kluane Lake (the line). It was possible to compare control values with those from treatments of food supplementation in 1989 and 1990, and with those where food was supplemented and mammalian predators excluded in 1991 and 1992. (After Krebs et al., 2001; (a) following Krebs et al., 1995.)

lowest fecundity following the density peak. Unfortunately, it was not possible to measure fecundity in a treatment where only food was supplemented - an example of the disappointments that almost inevitably accompany large field experiments - so the effects of food and predators cannot be disentangled. Of these, any effects of food shortage on fecundity would be easy to understand. It is also possible, though, that an increased frequency of interaction with predators could reduce fecundity through its physiological effects on hares (reduced energy or increased levels of stress-associated hormones).

Thus, these hard-won results from field experiments and the analyses of time series essentially agree in suggesting that the snowshoe hare cycle results from interactions with both its food and its predators, with the latter playing the dominant role. It is also noteworthy that, at least over some periods, there has been a high correlation between the hare cycle and the 10-year cycle of sunspot activity, which is known to affect broad weather patterns (Sinclair & Gosline, 1997). This type of extrinsic, abiotic factor was initially a strong candidate for playing a major role in driving population sunspot cycles?

cycles generally (Elton, 1924). Subsequently, however, they have received little support. In the first place, many population cycles are of the wrong period and are also variable in period (see, for example, the microtine rodents in the next section). Second, the population cycles are often more pronounced than the extrinsic cycles that are proposed to be 'causing' them. Also, even when a correlation has been established, as in the present case, this simply begs the question of what links the two cycles: presumably it is climate acting on some combination of the factors we have already been considering - predators, food and intrinsic features of the population itself - although no mechanistic basis for such a link has been established.

Overall, then, the snowshoe hare work illustrates how a range of methodologies may come together in the search for an explanation of a cyclic pattern. It also provides a very sobering reminder of the logistical and practical difficulties - collecting long time series, undertaking large field experiments - that need to be accepted and overcome in order to build such explanations.

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