Geoscience Reference
In-Depth Information
Some studies show that predators cannot remove prey fast enough to affect their population peaks, whereas
others indicate that the number of prey controls the predator population—either way, a feast-or-famine situ-
ation. Another theory posits that an internal physiological mechanism, not an extrinsic factor, kicks in to re-
store balance when a population gets too high or too low.
Perhaps the most fascinating and studied of these cycles is the one linking lynx and snowshoe hare numbers.
This predator-prey relationship was first documented in the 1930s, when numbers of pelts of snowshoe hare
and lynx received by the Hudson's Bay Company were analyzed. It was clear that hare populations built up to
a peak and then crashed, on average every ten years. The cycle of the hare's principal predator, the lynx,
lagged a year behind the hare's cycle, which also affected other species such as red fox and great horned owl.
Because fluctuations in its numbers affect the populations of several other species in the ecosystem, the snow-
shoe hare has been called a keystone species. This cycle is repeated in synchrony across the continental swath
of the boreal domain, although it lags by a year or two in Newfoundland, where the hare was introduced in the
late 19th century. Even here, however, the cycle began repeating itself shortly after the hare's introduction, as
it had, and does, elsewhere in the boreal world.
The persistence of this cycle is believed to be related to the simplicity of the northern ecosystems. In the
case of the hare and lynx, it is close to being a one-prey, one-predator system. Because there are few if any
other prey to support the lynx, once the hare population goes into decline, the lynx population must soon fol-
low. Since it takes three to four years for hare numbers to recover—approximately one quarter the length of the
whole cycle—the predator and prey populations do not reach equilibrium and the cycle is maintained. But the
primary causes of the crash and the lag time have been subject to a number of theories and are still open to de-
bate.
The interaction of food supply and predation appears to cause the die-off of hare populations, which is
worsened by weather factors. During the peak population, predation exacts a heavy toll—83 percent of mortal-
ity, in one study—even when food supplies are adequate, driving the hares into decline. Vegetation seems to
play a role as well. At peak population levels food resources are somewhat reduced, forcing hares to feed in
areas with less shrub cover and therefore making them more vulnerable to predation. At the same time, snow-
shoe hares become stressed by the failed attempts at predation by coyotes and lynx, which are successful only
30 to 40 percent of the time. Increases in the level of stress hormones in the hares' blood stream are thought to
permanently alter their reproductive physiology, with the consequence that litter size declines. These hormones
are then passed on to the next generation through the placenta of the stressed females. The lag of three to four
years in the cycle may be related to the fact that it takes that long for enough females without these stress-
altered physiologies to begin reproducing at normal levels.
ANYONE WHO HAS
spent time in the northern woods is only too aware of the torment of biting insects experi-
enced there. But this miasma of mosquitoes, blackflies, and other insect life, which drives warm-blooded
creatures to distraction, also attracts millions of feathered migrants, many of which winter in the neotropics of
the Americas. These insects provide the protein necessary for reproductive success. Among the long-distance
migrants are a broad range of bird species, including osprey, peregrine falcon, common nighthawk, alder and
least flycatchers, Swainson's thrush, red-eyed vireos, and more than twenty warbler species, including Ten-
nessee, yellow, chestnut-sided, Cape May, palm, and bay-breasted warblers. Density of breeding pairs is very
high, ranging from three hundred to six hundred pairs per square kilometer in balsam fir, the richest conifer
habitat in the boreal region. The obligate insect eaters spend only three months in the north hawking their air-
borne prey, whereas insectivores that also eat seeds and fruit might linger for five months. Of the three hun-
dred species that travel distances to reap the advantages of the boreal forest's smorgasbord, only one-tenth, or
thirty, winter there. These hardy winter birds include the raven, black-capped chickadee, blue jay, spruce and
ruffed grouse, woodpeckers, grosbeaks, and owls.
