Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 415

Migration
Migration allows some mobile species a way of utilizing the high mountain environment
during favorable periods and lowland or other mountain environments during more
stressful periods. Strictly speaking, migration is the “large scale shift of the population
twice each year between a restricted breeding area and a restricted wintering area”
(Lack 1954: 243). While some species practice a twice-yearly movement, others, like the
mountain sheep, may shift between as many as six seasonal home ranges (Geist 1971).
There may also be lesser movements in response to the sporadic occurrence of severe
weather, as well as poor or exceptionally favorable food conditions.
In mountain sheep, knowledge of the home range is passed on from the older to
the younger generation as a living tradition that eliminates the need for young sheep
to explore for suitable living space on their own. The latter is the common mode for
the young of many species to find a place to live. Because living space for sheep is re-
duced during interglacial times to widely spaced, tiny specks of grassland surrounded
by timber, young sheep exploring for living space on their own would be faced with a
daunting task with little payoff. It is more efficient to follow older individuals and thus
learn quickly where to feed or to flee from predators. Unfortunately, harassment can
make mountain sheep vacate home ranges and “forget” about them, leading to habit-
at loss at best and extinction at worst. A policy of aggressive reintroductions of sheep
to abandoned habitats increased their number continent-wide by almost 50 percent
(Toweill and Geist 1999). Similarly, the astute application of ecological and behavioral
knowledge acquired by field observations led to the reclamation of abandoned coal strip
mines in Alberta, Canada, into custom-built bighorn sheep habitat. It was searched out
eagerly by the sheep, and led to quick population growth and the largest-bodied big-
horns since Pleistocene times (MacCallum and Geist 1992, 1995).
Migration in middle and high latitudes generally coincides with the seasons, al-
though some birds migrate daily during summer. In lower latitudes, migration is less
important, but occurs to a certain extent (Moreau 1951, 1966). The island-like nature
of mountains greatly facilitates part-time, seasonal occupancy, since animals can reach
the area above timberline or escape it by relatively short vertical migrations. This is
a fundamental difference between alpine and arctic environments: Daily occupancy is
not possible in the Arctic, and migration is almost totally limited to birds since, with
the exception of the caribou, mammals make no attempt to move the great distances
required (Irving 1972). By contrast, mammals move relatively easily to and from the
alpine tundra, although few migrant mammals breed there. A number of herbivores
(e.g., rabbit, porcupine, deer, moose, caribou, and elk) and a variety of predators (e.g.,
weasel, badger, wolverine, fox, wolf, and bear) wander above timberline during sum-
mer. Some retreat to lower altitudes during winter, but late winter upward migrations
are practiced also by some, such as mountain caribou ( Rangifer tarandus caribou ) and
black-tailed deer ( Ococoileus hemionus columbianus ), which use the high snow levels
at upper elevations to access tree lichens in the subalpine forests. In late winter, hard
snow crusts and firm windblown snow may allow mountain goats and sheep to resume
roaming and to disperse to higher elevations. Here they move about unimpeded, feed-
ing on the sparse but nutrient-rich vegetation exposed by winds on very high mountain