Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 416

ridges. Wolves and wolverines ( Gulo luscus ) may also hunt at the highest elevations in
late winter.
The number of birds in the alpine tundra rises exponentially as they arrive for the
brief summer. Many are returning from overwintering in lower latitudes, while others
are simply migrating upward from the surrounding lowlands. In the passing parade
from early spring to autumn, the first birds in the high country are usually herbivores
with catholic food habits which harvest dead insects on the snow. As conditions improve
and insects become plentiful, insectivores become dominant. A fairly high percentage of
these bird herbivores and insectivores breed in the alpine tundra. Later in the summer,
raptors (birds of prey) move in to harvest the newborn crop of small rodents and birds.
The raptors seldom breed above timberline, but are mobile enough to migrate daily to
and from the alpine area. In the Beartooth Mountains of Wyoming, raptors usually do
not appear before the end of July, but are plentiful from then on (Pattie and Verbeek
1966).
Migration is primarily restricted to birds and mammals. Reptiles and amphibians are
not mobile enough, and barriers such as waterfalls and beaver dams generally prevent
mountain fishes from migrating. Although there are some remarkable movements of in-
sects in mountains, true insect migration is rare and takes place primarily among low-
land species. For example, insects exhibit a curious tendency to seek out the summits of
mountains, known as summit swarming or hill-hopping (Hudson 1905; Van Dyke 1919;
Chapman 1954; Edwards 1956, 1957; Shields 1967). This has been observed by many
people in many parts of the world, but the exact reasons for such behavior remain elu-
sive. Some consider the insects to be victims of the winds, blown there against their
will; others maintain that the wind may aid them but they move primarily of their own
volition. Both explanations appear to be true. A variety of lowland insects are occasion-
ally drawn up to high altitudes by ascending air currents, and if sufficient heights are
reached, they are frozen by low temperatures. The dead and stunned insects are then
deposited in vast quantities on glaciers and snowfields, where they serve as an import-
ant source of food for resident birds and nival insects (Swan 1961; Mani 1962; Papp
1978; Spalding 1979). Such movement is unintentional, however, and is clearly a one-
way trip.
The case of insects moving upward under their own volition is much closer to true
migration. The principal insect groups displaying this behavior are the Coleoptera
(beetles, weevils), Diptera (mosquitoes, flies), Hymenoptera (bees, ants), and Lepidop-
tera (butterflies, moths) (Shields 1967). Explanations include an innate urge to ascend
the highest point, the search for food, attraction to the heat and light, and the use of the
highest point as a meeting place for mating. There is no reason to choose among these,
since they may all operate at one time or another, but the last explanation seems best
borne out by the known facts. The tendency among many insects to seek out the highest
available point in order to mate has a selective advantage among sparse and isolated
populations, for it ensures the meeting of males and unmated females and helps to sta-
bilize the gene pool (Shields 1967).
A specialized example of insect migration is that of ladybird beetles ( Coccinellidae ).
Vast swarms of these colorful little creatures have been observed on mountain peaks
around the world (Edwards 1956, 1957; Mani 1962). They apparently swarm up from
the lowlands to certain peaks in summer and congregate in assemblages numbering up