It is appropriate to begin with the well-known case of the snowshoe hare-lynx population cycles recorded in Canada and Alaska. These cycles were discovered by Elton while he was examining the hunting records from the Hudson Bay Company in Canada. He found that the hare and lynx pelt records as recorded by trappers exhibited strong periodic population fluctuations, and that the lynx populations seemed to track the hare densities, albeit with a significant time lag. The cycles are pronounced and occur over wide geographic areas; in 1997-99 hare population peaks were synchronous over large parts of Canada and Alaska. Since the cycles appear in the herbivore, the hares, and the predator, the lynx, the obvious causal mechanism seemed to be predation. So far, experimental data have supported this early hypothesis, with some caveats.
Although observing time-series data and making conclusions using models may give us insights into what causes the lynx-hare cycles, manipulative experiments are also necessary to determine what mechanisms are responsible for different parts of the cycles. Both the increases and decreases in herbivores may be caused by the same or different organisms. Although difficult to implement, some large-scale manipulations have helped ecologists examine individual portions of the hare-lynx cycles. The general strategy of these experiments is to try and change one major ecological factor in the hope that this manipulation will stop the cycles from occurring. These methods are very powerful, since a change in a single factor, such as food supply or predator abundance, may be shown to be responsible for the cycles.
Researchers have added food to hare populations in an effort to reverse the decline phases of a population cycle, with mixed results. When commercial-grade rabbit feed was added to large (1 km2) experimental areas the amplitude of the cycles increased, but the cycles continued unabated. The increases in hare populations with food addition were not the result of increased fecundity by itself, but by immigration into the experimental areas from outside of the plots. Thus, food itself was not enough to 'rescue' the populations from their decline phase. When more natural sources of food such as tree boughs were used to amend hare diets the same increase in population densities occurred during the increasing phase of the cycles, but this was again not enough to prevent the population crash portions of the cycles. Finally, even the addition of fertilizer to the extant vegetation cannot prevent the decline phases of cycles. On the other side of the cycles, the increases in population densities were also not significantly affected by food additions. Thus, it appears that food abundance is not solely responsible for either the increase or decrease portions of hare cycles.
In another large-scale manipulation with the hare-lynx system, researchers attempted to exclude large predators that were known to cause a great deal of mortality in hare populations. These included both lynx and coyotes, which caused very high proportions of all hare mortality in natural, unmanipulated populations. Hare populations in these exclosures did not show the marked decrease in survival rates seen in areas open to predators, but instead maintained relatively stable survival rates. Avian predators such as raptors did not seem to have a significant effect on survival rates, although it is important to realize that they still may be important in affecting certain parts of the cycle. Avian predators may, to some extent, be redundant with lynx and coyotes, replacing their numbers if they should decline.
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