In the natural world, two ecological tenets always seem to hold. First, an ecological phenomenon is a function of a complex set of factors. Second, most -- if not all -- phenomena are dynamic and unpredictable.
Population fluctuations, or cycles, are an ecological phenomenon that has intrigued scientists for many decades.
Population cycles in leporids (rabbits and hares) presumably occur in some hare species but not in rabbits. Thus, we can assure all the kids out there that the Easter Bunny is coming to town and doesn't exhibit population ups and downs (at least we think not). Although a cycle of eight to nine years may have existed at one time in our familiar eastern cottontail, it has likely been masked by population declines, massive introductions and changing land uses throughout the 20th century.
In more northerly latitudes, snowshoe hares undergo a "10-year cycle" -- population cycles of predators dependent on hares as the major food resource typically lag about two years behind those of hares. In some locations, such as Alberta, population densities vary 23-fold over the length of a cycle.
During a peak year of this Alberta cycle, population estimates were 2,300 hares/100 hectare, which is about 20 hares in an area equivalent to Beaver Stadium's football field. During low years of the snowshoe hare cycle, numbers can drop to less than one hare/ha. Fluctuations of hare populations in northern latitudes are not necessarily synchronous across the geographic range of the species but may vary with differences in weather and predator patterns.
Cycles in snowshoe hare populations of northern latitudes appear to be linked to winter food shortages. In winter, hares feed extensively on twigs and shoots of trees and other woody vegetation.
As populations of hares increase, the supply of this food becomes depleted. Therefore, populations of hares "crash," and predator abundance rather than food supply becomes the most important factor in regulating numbers of hares during decline and low phases of the cycle.
Another hypothesis is that predation instead of reduced food availability in winter is the key factor causing hare cycles.
A shortage of winter food during the snowshoe hare cycle has several immediate consequences to hares. Regardless of age, individuals experience increased over-winter weight loss during years of food scarcity (12 to 16 percent) compared to years with abundant food (5 percent). Adult females produce fewer young, ranging from 18 young/ female per year during the "highs" of the cycle to eight young/female during the "lows" of the cycle.
The fewer young born during the decline phase of the cycle gain weight slower with food-scarce mothers compared young born during the peak phase with food-abundant mothers; presumably because food-scarce mothers have less energy available for milk production for growing young.
In addition, lower weight gain in young translates to lower probability of them surviving the following winter.
Precipitous drops in snowshoe hare numbers over the course of the cycle can strongly affect predator populations. For instance, the lynx (a wild cat similar to the bobcat), which is a well known predator of snowshoe hares, feeds almost exclusively on hares during the peak years of the hare cycle.
As hare abundance begins to decline, lynx revert to other food items such as carrion and ruffed grouse.
Predators often exhibit a second response to low prey availability; for instance, lynx numbers in Alberta declined five-fold during years coinciding with low abundance of hares compared to years with high abundance.
Productivity of lynx also was markedly influenced by low hare abundance, with no kittens produced during years with scarce hares compared to two to three lynx litters of one to four kittens each raised in years with abundant hares.
In particular, yearling lynx seem to immediately curtain reproduction after hares decline, but adult lynx probably cease reproduction two years after the start of the decline.
Speaking of the Easter Bunny - did you ever wonder how it stays so healthy? It goes to the White Building on campus for eggsercise, particularly hareobics!
