A classic set of data reconsidered

As described in the Chapter 10, the British ecologist Charles Elton proposed that the data on pelt returns of snowshoe hare (Lepus americanus) and lynx (Lynx canadensis) by the Hudson's Bay Company provided support for the theory that periodical oscillations in populations are inherent in predator-prey interactions as predicted by the Lotka-Volterra equations. Many factors have been proposed to explain the hare-lynx cycles, including forest fires (Fox 1978). However, Bryant and Kuropat (1980) and Bryant et al. (1983) proposed that the hare cycle was the result of induction of chemical defenses in the Lare's primary winter foods, willow (Salix spp.), alder (Alnus spp.), and birch (Betula spp.). The chemical pinosylvin methyl ether, a toxic phenolic, deters feeding by snowshoe hares on alder. The chemical is present in the foliar buds and catkins, but not in the internodes. The hares will eat the internodes, but they are higher in fiber and lower in nutrients and carbohydrates as compared to buds and catkins. Willow, alder, and birch all have inducible chemical defenses. When the snowshoe hare damages twigs or leaves, the re-growth contains high quantities of phenolic resins and terpenes. Bryant et al. proposed that the decreased amount of palatable browse provides the hares with little high-quality food and results in population declines. Once the hare populations are low, the amount of damage to twigs and leaves is reduced enough that the induced defenses decline. The browse becomes more palatable and the hare population increases again. According to this reinterpretation, the lynx feed well at the peak hare population levels, especially since the hare are in poor condition due to lack of quality food. Lynx populations go up, but the major effect of the lynx is to accelerate the decline in a hare population already on the downswing.

Recently, however, Karban and Baldwin (1997) reviewed the literature and concluded that induced resistance cannot provide the single factor that is necessary and sufficient to produce cyclic dynamics. As was outlined in the previous chapter, Krebs et al. (1995, 2001) found that both food availability and predation work synergistically to produce the hare cycles. Karban and Baldwin (1997) feel that it is unclear at this point what role induced resistance plays in driving population cycles of herbivorous species.

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