winter. Only four North American species can withstand the cold to reside year
round in the Arctic: the raven (Corvus corax), the rock ptarmigan (Lagopus mutus),
the snowy owl (Nyctea scandiaca), and the hoary redpoll (Acanthis hornimanni) (Lavigne
et al. 1989). Amphibians and reptiles are particularly affected by temperature. The
American alligator (Alligator mississipiensis) cannot tolerate temperatures below 5°C.
Although several species of amphibians and reptiles tolerate freezing temperatures,
in general there is a negative relationship between the number of species and the lat-
itude. The direct effect of cold in limiting the distribution of these groups is prob-
ably less important than the availability of hibernation sites remaining above lethal
temperatures (Lavigne et al. 1989).
Movements of large mammals can be affected by temperature. In the Rocky
Mountains several ungulates, such as moose (Alces alces), elk, and deer, move down
hill for the winter. Sometimes a temperature inversion in winter positions a warmer
air layer above a colder one, and in these conditions Dall sheep (Ovis dalli) in the
Yukon climb higher rather than lower.
The limiting effects of temperature are demonstrated by changes in the range of
several species during historic times. Temperatures increased in the northern hemi-
sphere between 1880 and 1950. The breeding ranges of herring and black-headed
gulls (Larus argentatus, L.ridibundus) moved north into Iceland, and that of green
woodpeckers (Picus viridis) extended into Scotland. Temperatures have declined since
1950 and the breeding ranges of snowy owls and ospreys (Pandion haliaetus) have
moved south (Davis 1986). On the American prairies the warming period was asso-
ciated with severe droughts in the 1930s. As a result the cotton rat (Sigmodon hispidus)
has spread north (Davis 1986). Further changes in the distribution of these and many
other wildlife species are expected in the future, as a result of global warming.
Cold temperatures themselves may be less important than the consequent changes
in snow pack. Caribou must expend greater amounts of energy in exposing ground
lichens when snow develops a crust (Fancy and White 1985). Even further north on
Canada’s High Arctic Islands the warming temperatures of spring melt the surface
snow. As the water trickles through the snow pack it freezes when it hits the frozen
ground and forms an impenetrable layer. The caribou abandon feeding in those areas
and may migrate across the sea ice to areas where the wind has blown the shallow
snow away (Miller et al. 1982).
Deep snow limits other species also. North American mountain sheep (Ovis
canadensis,O.dalli) are usually found in winter on cold windswept ledges where there
is little snow. Deer (Odocoileusspecies) are limited by snow cover of moderate depths
(<60 cm) whereas moose can walk through meter-deep snow (Kelsall and Prescott
1971). Both move to coniferous forest in late winter because the snow is less deep
there (Telfer 1970; Rolley and Keith 1980).
The stress of cold temperature has resulted in various adaptations to conserve energy,
the most notable being the hibernation of ground squirrels during winter and the
dormancy and lowering of body temperature of bears. Hummingbirds also lower body
temperature overnight to about 15°C or when resting in cold conditions, a state called
torpor. The limiting effect of temperature on ground squirrels operates indirectly through
soil type, slope, and aspect. Squirrels need to dig burrows deep enough to avoid the
cold and this requires sandy, friable soil. They also need to avoid being swamped by
melt water in spring, so burrows are situated on slopes where water can drain away.
Similarly, in Australia, the distribution of rabbits within the 27°C isotherm is
brent
(Brent)
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