Role of Food in Hare Population Cycles

Abstract

The snowshoe hare has a well-documented 10-yr cycle in North America's Boreal Forest; the Arctic hare probably has cyclic fluctuations of comparable periodicity, amplitude and geographic scope within the taiga of USSR. Arctic hare populations have a 3to 4-yr cycle in the forests of Norway and Sweden. Fluctuations of black-tailed jackrabbits in Utah, near the species' northern limit, exhibit an approximate 10-yr periodicity. Cyclic declines in snowshoe and Arctic hare populations having 10-yr cycles of abundance are likely initiated by food shortage over winter which lowers reproduction and juvenile survival. Declines in Arctic hares having a 3to 4-yr cycle in Scandinavia appear to be linked to declines in microtine rodents which cause markedly increased predation on hares and other alternative prey. Declines in the cyclic jackrabbit population of northern Utah are allegedly part of a predator-prey oscillation with coyotes. In neither of the latter two fluctuations is food shortage involved. Both cyclic and noncyclic hare populations may become food short at high densities. It is proposed that the 10-yr cycle is dampened or replaced by irregular fluctuations where snowshoe habitat is fragmented and insular because: (1) the greater structural (niche) diversity of fragmented habitats increases prey species diversity, and hence the stability and facultative nature of predator populations; and (2) the resulting sustained predation on dispersing hares blocks potential increases in distribution and density. Thus it may be the character of predation, as molded by habitat factors, that primarily determines the regularity of snowshoe hare fluctuations. This is likely also true for other species of hares. Because snow cover effectively protects vegetation from browsing by snowshoe and Arctic hares, the impact of varying snow depth on overwinter food supplies may be highly significant to hare populations at or near cyclic peaks.