The Time Of Testing
Snow. Ice. Wind. Birches bare lines against the sky.
Conifers green and full on the white hills. Silence. Now wind again.
Hare tracks frozen across the trail. This is winter, the time of
testing.
Bringing cold, wind, snow, and reduced food and
cover, winter is indeed the most critical time of year for life on Isle
Royale. But the island's plants and animals have a long time to prepare
for it, since the slow-cooling water of Lake Superior warms the cold air
above it until ice forms on the lake.
Fall may be considered to begin in August, when
island birds begin migrating, and to end in late November, when snow
usually falls in earnest. In mid-September, maple leaves start to turn
red and yellow, and birches add their gold. Colors peak in the first
half of October, as aspens, too, turn yellow. Then, with the first good
wind, leaves fall. In October, brook trout, lake trout, and whitefish
spawn. Moose, too, end the year with reproductive activity. The bulls
have been getting more and more edgy since early September. They rub the
velvet off their antlers in early September, and track down cows through
late September and October. Their calls and grunts resound through the
fall forest. Meanwhile, insect numbers have been diminishing rapidly.
Mosquitoes have died out during August, flies in September. Most other
insects are killed or go into hibernation during the frosts of
September and October. Monarch butterflies, however, flutter south across
Lake Superior toward the Gulf of Mexico, obeying the yearly command that
pulses through their tiny nervous systems.
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The period of late autumn and winter brings strong winds as well as low
temperatures to Isle Royale. (Photo by Robt. G. Johnsson)
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By late November, when the white winter blanket has
settled, plants are dormant; insects, reptiles, and amphibians are
hibernating; and most birds have migrated. Most of the mammal
contingent, however, faces winter head-on.
Of Isle Royale's current 15 species of mammals, only
the bats hibernate or migrate to the south. To survive the winter, all
others must find food and must themselves avoid becoming food for
something else. Of Isle Royale's 120 or so species of summer birds,
however, only about 20, joined by a few species from the Arctic, stay
through the winter. These few birds and mammals are all that remains to
enliven the elemental scene at this difficult time of year.
The conditions these animals must face vary from
winter to winter, but always they are severe. Temperatures usually range
between -25deg;F and +40deg;F. Frequent strong winds, generally
from the west, add greatly to the chill of low temperatures. Snow depths
range from one to three feet, with less under dense conifers and
considerably more in drifts. Crust conditions within or on the snow
sometimes help but more often hinder animals. Taken together, all these
conditions make great demands on the energy of animals, at a time when
food is in shortest supply.
Over millenniums, northern wildlife has evolved
various strategies for survival. Birds, with the power of flight, can
go long distances to find food. In winters when birch seed is abundant,
redpolls, pine siskins, and goldfinches may remain on the island in
large numbers. In poor years they may wander in search of more
productive areas. The same strategy is used by other northern finches,
such as pine and evening grosbeaks, purple finches, and crossbills,
depending on the supply of mountain-ash fruits, pine seeds, and other
food. Birds that rely more on insect eggs and larvaesuch as
chickadees, blue and gray jays, and woodpeckershave a more dependable
source and consequently remain on the island in fairly stable numbers
from one winter to another. Along the south shore, where the lake is
usually open, a few mergansers and goldeneyes sometimes gather to feed on
fish and other aquatic life. The ducks thus hunt in an environment that
is usually considerably warmer than the air above. A few birds of prey,
such as goshawks, horned owls, and in some years the snowy owls of the
Arctic find enough birds and small mammals to tide them over.
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The fruits of mountain-ash provide a reserve of winter food for many
animals, including the red fox. (Photo by Wm. Dunmire)
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Plant-eating mammals, which are prey species, have
developed highly individualistic methods for coping with winter and its
predators. Deer mice and red squirrels lay up provisions during the
long fall in numerous cachesin trees, logs or underground. Mice
and squirrels store mostly seeds of various kinds, though squirrels may
rely on fungi and birch and alder catkins in years when
conifer seeds are scarce. For these little animals, the snow is a
blessing, providing them with covered runways hidden from most predators
and with insulation from the chill air above.
Beavers have a winter system of living that insulates
them almost completely from atmospheric rigors and the danger of
predators. Before ice forms, they cut branches of aspen, birch, and
other favored foods and stick them into or weight them with stones on
the bottom of their pond or lake, usually near the lodge. They plaster
their lodge with a new layer of mud, leaves, and sticks. When winter's
cold spreads a layer of ice over the water and freezes the mud on his
house, the beaver is effectively sealed in, and winter and predators are
sealed out. When he gets hungry, he leaves his lodge through an
underwater entrance and visits the food cache. Occasionally, however, he may
leave the confines of his home and pond to gnaw the bark from a tree
felled in the fall. At such times he risks being caught by the wolf pack
or perhaps by some half-starved lone wolf rejected by the pack and
unable to kill moose.
Muskrats lead a much more hazardous life. They do not
store food for the winter, and though they feed upon aquatic plants and
some animals beneath the ice, they also venture out for land plants.
While foraging, they are under threat from virtually every predator on
Isle Royalemink, otter, weasel, red fox, wolf, lynx, and large
hawks and owls. Perhaps it is only his caution and unspecialized food
tastes that allow any muskrat to survive the winter.
Cover is surely the prime winter concern of the
snowshoe hare, for in most areas the supply of woody stems and twigs is
adequate for his dietary needs.
When winter strips away the deciduous leaves and
green ground cover, the hare cannot safely wander as far as he did in
summer. White-cedar swamps become headquarters for many hares, since
they provide dense, low foliage that can also be eaten. On Isle Royale
the hare's chief enemy is the red fox; the lynx, its principal predator
through much of the north, is so rare here that it is hardly a threat;
and the wolves concentrate on moose. Hares rely on their speed and their
broad, snowshoe feet to escape foxes. If the snow is soft, foxes will
sink in farther than hares during a chase, but if a crust has formed the
chase is more even. Its change in winter to a camouflaging white coat
also aids the hare in its struggle to survive.
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For the red fox, winter is the critical season for survival on Isle
Royale.
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And what about the moose, that huge beast that eats
more and provides more food for predators and scavengers than any other
animal on the island? Snow depth seems to be the moose's main concern,
since it affects ease of travel. During winters with deep snow, moose
concentrate near shorelines, where the abundant conifers intercept much
of the falling snow and also provide browse. In these forests moose can
also feed on twigs and bark of aspen, birch, and mountain-ashamong
their favored foods. Frequent blowdowns put more food within reach.
Deep, soft snow is more easily navigated by adult moose than by wolves,
though calves are seriously hindered. A crust on or within the snow
probably aids pursuing wolves. It seems, then, that food sources and
snow depth regulate the moose's winter wandering, and that wolves are
accepted as an environmental hazard. Healthy adult moose, in fact,
seldom need to fear wolves; calves and infirm adults are the usual
victims.
Predators, too, are faced by a reduced food supply in
winter. Insects, reptiles, amphibians, and many binds have died,
hibernated, or gone south. The young of various mammals are not as
abundant as in summer, having been reduced by the many hazards of their
environment. The remaining prey animals spend more time in dens. Most
predators, therefore, are forced to hunt farther and longer for a meal.
The island's few otters, for instance, must follow streams for miles,
diving in where fast water keeps ice from forming, and searching for
fish and crustaceans as far under the ice as their lungs will allow.
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Redpolls and purple finches may remain on Isle Royale
through the winter when the supply of birch and other favored seeds is
good. (Photo by Durward L. Allen)
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In winter, red foxes depend heavily on hares; and,
as we have seen, catching a hare in the snow is no easy job. In years
of good mountain ash crops, foxes, as well as ravens and other wildlife,
eat many clusters of the orange-red fruit. As we will see shortly, foxes
are also inadvertently assisted through the winter by wolves.
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Timber-wolf tracks follow the shore of Siskiwit
Bay. (Photo by Jim Cole)
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The wolf's only real source of food in winter is the
moose herd. Hares and smaller animals are hardly big enough to justify
the energy expended in catching them, and beavers seldom venture ashore
in that season. Possibly, wolves have an easier time finding and
killing moose in winter than in any other season. Wolf packs hunt by
following the easiest routesusually on the wind swept ice along
shorelines, but sometimes inland along previously used routes. The
wolves trot along, usually in single file, until moose or fresh tracks
are found. If a discovered moose stands its ground they usually soon
leave, wary of the animal's dangerous hoofs. If the quarry runs, they
chase it, single file. If deep snow, thick cover, or a long head start
prevents them from catching up with the moose in a short time, they
abandon the pursuit. But somehow they recognize weakness, and keep after
infirm individuals. Attacking mainly the rump, they slow the animal
down, eventually bring it to the ground, and then quickly kill it. Some
victims, however, are merely wounded and left to weaken, and are killed
later. After a kill, the wolves gorge, nest, then usually feed
intermittently until the last bones are gnawed. A day or two after
killing a calf, or several days after killing an adult, they are on the
move again, seeking more fuel for their ever burning bodily
furnaces.
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The three scavenging ravens are members of a flock that follows the wolf
pack. (Photo by Durward L. Allen)
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A kill has ecological importance that goes far beyond
the survival of wolves. For when wolves leave a carcass, temporarily or
permanently, other animals come for a share. Red foxes, particularly,
congregate at a kill, for the moment forgetting their territorial
animosities. Ravens, it seems, make most of their winter livelihood from
wolves. They actually follow wolf packs on the daily rounds, picking
some sustenance from feces left by the wolves, and waiting for them to
make a kill, from which they later get a meal or two. Gray jays and blue
jays, smaller cousins of the raven, also visit moose carcasses for
scraps. Tiny chickadees pick at the bone marrow where the opening is too
small for ravens or jays. Deer mice undoubtedly gnaw the bone marrow,
and red squirrels may sometimes make use of the kill. Thus several kinds
of animals ride through winter partly on the coattails of the wolf.
Just as fall is prolonged by the warming effects of
Lake Superior, so spring is delayed by the water's slow adjustment to
the temperature of the air above. The patches and solid sheets of ice
that form on much of the lake are slow to break up. When they finally
do, the cold water continues to cool the spring air masses flowing over
the lake. The shore ice usually goes in April, although some years it
persists until May. On May 3, 1972, when Ranger III made its
first trip of the year, it had to break ice in Rock Harbor for several
hours before it could reach Mott Island. That day, the snow lay 5 to 8
feet deep behind the National park headquarters building.
During May, tree leaves begin unfurling; skunk
cabbage, hepatica, and other early flowers bloom; smelt and suckers
swarm up streams to spawn; birds return en masse from the south.
By mid-June, when the trees are usually fully leaved and many forest
flowers are blooming, summer can finally be acknowledged.
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