Wolves, Moose, And The Balance Of Nature
Through the cycle of the seasons, and face of the
landscape; and populations of living things fluctuate, sometimes
violently. Over centuries, slow climatic change brings new assortments
of plants and animals. Yet the island remains a green place teeming with
life, and the diversity of life on it perhaps continues to grow. How
does all this individual change result in a collective stability, and
can we find in this ecological drama some lessons for our own
species?
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Depending mainly on moose, Isle Royale's wolves also prey upon the
snowshoe hare. (Top photo by Greg Beaumont; bottom photo by Robt. G. Johnsson))
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The now-famous story of the island's wolves and moose
can begin to show us how the Isle Royale web of life hangs together.
Sometime in the first decade of this century, it seems, moose became
established (or re-established) on the island. This significant
happening was probably related to a regional change in the vegetation.
Between 1890 and 1910, logging and fires in the northern Great Lakes
area destroyed much of the reindeer lichen, on which caribou heavily
depend, and created open areas where tree seedlings and shrubsfood
for deer and moosecould flourish. Consequently, caribou decreased
and deer and moose increased. Isle Royale's moose immigrants, which
probably swam singly or in small groups from the Canadian mainland, were
perhaps pressured by a growing population to seek new feeding grounds.
As sometimes happens when a new species reaches an island with suitable
habitat, an irruption followed. By 1930, when Adolph Murie studied the
situation, the moose population had reached an estimated 1,000 to 3,000
animals, and a large part of the woody vegetation within their reach was
overbrowsed. Murie correctly predicted that starvation and disease
would soon decimate the herd.
The first large die-off occurred in the winter of
1933. By 1936, when fire burned a quarter of the island and further
reduced the browse supply, moose numbers had dropped to an estimated 400
to 500. From this low point, the herd increased again, aided by
luxuriant resprouting on the burned area. By the later 1940's there were
numerous signs that die-offs would again occur.
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A wolf pack withdraws after an unsuccessful
attack on a cow moose and a yearling.
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Boom-and-bust cycles might have continued had it not
been for the arrival, in the late 1940's, of the timber wolf. Through
the first half of the century its smaller cousin, the coyote, had been
present; but coyotes do not prey upon moose. By 1957, somewhere between
15 and 25 wolves inhabited the island. Coyotes, probably killed off by
the intolerant wolf, had disappeared. In 1960 David Mech, a Purdue
graduate student studying wolves for a doctoral dissertation,
determined after many hours of aerial observation that the wolves
numbered 21 or possibly 22. This population was composed of a large pack
of 15, which hunted mostly on the southwest two-thirds of the island,
and small groups of two or three that hunted mostly on the northeast
one-third and along the north shore.
From an aerial census made in March 1960, Mech
estimated the moose herd at 600. Apparently the wolves were controlling
the herd at a level below that at which the food supply would control
it, for browse species were growing in areas where they had not been
evident for decades. Furthermore, the calculated annual kill of adults
by wolves was nearly equal to the calculated number of yearlings
surviving to join the breeding population each spring. And apparently
the herd was healthy: the proportion of cows bearing twins as opposed to
a single calf was much higher than it had been in the 1930'sa
sign of good nutrition.
Research in the late 1960's and early 1970's
indicated that the moose herd may have increased. An aerial count in
mid-winter, 1972, produced an estimate of about 900 animals. Counts of
moose pellets by researchers from Purdue and Yale suggested that the
population in the early 1970's was 1,000 to 1,500. Wolf numbers remained
in the low 20's. Thus it appeared that a slow rise in the moose
population was underway, though wolves had not decreased.
Although wolves certainly have a damping effect, the
ultimate determinant of moose numbers is the island's food supply. That
supply depends on the stage of forest succession and the extent of
browsing by moose. The relationship between these two factors is
delicate and complex. In many parts of the northern forest, succession
proceeds to a stable state in which spruce and fir form a closed canopy.
In the course of this succession, food for moose declines as trees grow
above browsing height and some favored browse species are shaded out. On
Isle Royale, where moose densities are among the highest known and where
wind takes a large toll of trees, succession seldom proceeds to a pure
spruce-fir stand. The toppling of trees creates openings in which shrubs
and saplings spring up, producing much browse. Browsing is so heavy in
many of these areas that the young trees cannot grow up. This maintains
a good food supply for a long time but eventually can result in
elimination of the browse species and replacement by grass and
unpalatable shrubs or by spruce. If it continues, such heavy browsing
will cause certain tree species to drop out of the forest: mountain-ash
essentially everywhere, aspen and cherry almost everywhere, paper birch
and fir in many areas, and yellow birch in some areas. Only fire or a
major decline in moose numbers will break this trend: major fires create
so much new growth that moose cannot suppress it all; and a population
decline has a similar effect. Thus the moose population cannot be
expected to stay indefinitely at its present high level without the
advent of fire.
Just as available browse limits moose numbers,
available moose would ultimately limit wolf numbers. But there is
considerable evidence that social pressures may set an upper limit to
wolf density, even when the food supply goes on increasing. Within each
pack there is a "dominance order;" each animal knows its social standing
with respect to all the others. Furthermore, there are separate
dominance orders among males and females. Normally, the lead, or alpha,
male mates with the alpha female. Matings between subordinate animals
are often prevented by the lead pair. As a pack grows, sexual rivalries
tend to become more and more complicated, and stress increases.
Reproduction may also be inhibited by the dominance of large packs over
smaller ones. If the territories of such packs are not well separated,
the small pack probably has no chance of raising pups. Thus as wolf
numbers grow in an area, increasing social controls apparently restrict
reproduction.
Death of pups is another, perhaps greater, control on
numbers. On Isle Royale, between 2 and 5 females are fertilized every
year. This would produce a rapid population growth if it were not for
the high mortality rate of pups. Many pups die during the denning
period, for causes not yet well understood. Others succumb after
leaving the den. If a pup survives its first 6 months, or until fall,
its chances of living several more years are good.
Though wolves have the potential to reproduce
rapidly, and sometimes do so after heavy reductions, in most wolf
populations the proportion of pups is small. This is the case on Isle
Royale, where the density of wolves, about 1 per 10 square miles, is one
of the highest known. This is possible only because of a very high moose
density of 40 to 50 per 10 square miles. At present, the wolves
apparently are holding moose numbers steady or to a slow rise, and
environmental and social factors are keeping wolf numbers stable.
Among Isle Royale's other animals, there are examples
of both stable and unstable populations. The red squirrel population,
though high, does not change greatly. As we have seen, this species has
no really efficient predator, like the marten, on the island. Its
numbers are regulated largely through reproductive control. Females bear
only one litter, averaging three young, in a year, and many do not
reproduce at all in poor cone years. Thus the birth rate is low, the
survival rate is high, and the chatter of red squirrels remains
perennially ubiquitous.
Deer mice, though without competition from other
small mammals except red squirrels, remain rather thinly distributed
over the island. In favorable habitat, there are only 1 or 2 per acre.
This low density, which is comparable to that on the mainland, must
represent the limit allowed by available food and shelter, and suggests
that the island deer mice have not occupied the ecological niches of
other small rodents found on the mainland but not on the island. Though
deer mouse numbers in early spring may be only one-eighth of those in
late August, numbers from one spring to another do not vary greatly.
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The flicker and other woodpeckers help to keep insect populations within
bounds.
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On the other hand, snowshoe hares are famous for
their boom-and-bust population cycles. With peaks coming regularly at
about 10-year intervals, these cycles exhibit extreme swings in the far
north, less pronounced ones in the southern part of hare range (which
includes Isle Royale). Like rabbits, hares have a high reproductive
potential, and variations in the reproductive rate, influenced by
availability of nutrients, seem the prime reason for fluctuations in
the hare population. Diseases and predation are factors in the downward
part of the cycle.
Predators that depend on hares are forced to take the
same population roller-coaster ride. On Isle Royale, red fox numbers
seem to peak about one year behind hare peaks, though careful estimates
of the fox population have not been attempted. If the lynx, now rare or
absent on the island, were more common, its population swings could be
expected to parallel those of the hare.
Insect populations, also, sometimes explode and then
fade. The larch sawfly boomed early in this century, killed many larches
(tamaracks), and then dwindled. The spruce budworm multiplied fast in
the 1930's, causing concern for the firs, a chief winter food of moose,
then also overabundant. But since that time the insect has quietly
retreated. The outbreak of the large aspen tortrix in the 1970's may
give another example of the normally temporary nature of such
explosions. As with other animals, insect outbreaks are eventually
controlled by food supply, predators, disease, parasites, and weather,
and by aspects of the species' life history.
What determines the relative stability on instability
of populations? This is a complex question with no simple answer, but an
important part of it lies in the diversity of species present. The more
species present in an area, the more stable its populations are likely
to be. This is so because fluctuations within individual species tend to
cancel each other out, and because various species act as a check on on
a food supply for others. If, for instance, Isle Royale had more species
of small mammals, it probably could support more foxes, and more small
mammals might reduce the swings in the fox population by providing more
choice of food, and a more stable total supply, in the critical winter
season. If, say, voles were on the island, their numbers might be high
at a time when hares were scarce, thus allowing more foxes to survive
the winter. The relative stability of wolf and moose numbers on the
island, though the wolf is dependent on the moose and is its only
predator, is an interesting exception to the principle.
As a rule, islands have fewer species of animals than
do mainland areas of comparable size and habitats. This is true mainly
because for many species islands are difficult to reach. And if an
established species is somehow wiped out, it may be a long time before
new colonists arrive to try again. On the mainland, migration from one
area to another is much easier. A number of species of vertebrates, as
we have seen, and perhaps hundreds of invertebrates that live on the
north shore of Lake Superior are not found on Isle Royale. Perhaps if
all these were present, animal numbers on the island would be somewhat
more stable. Generally speaking, however, population fluctuations on
Isle Royale do not seem appreciably greater than they are on the
mainland.
However much or little they may fluctuate, the
populations on Isle Royale are determined largely by the quantity and
quality of vegetation. This is so because green plants, directly in the
case of herbivores, and indirectly in the case of carnivores, supply
virtually all the food for animals. Exactly which species and how many
of each are present at any given time depend on the species available in
the region, their particular food requirements, and a number of other
factors, some which we have just considered.
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Green plants, such as blueberry and bog rosemary, are the foundation of
terrestrial food chains. (Photos by Robt. G. Johnsson)
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Green plants, in turn, depend on water, carbon
dioxide, and the energy of sunlight to produce the sugar from which the
compounds necessary to sustain life are built. Plants also require
other mineral nutrients from the environment for there own maintenance.
So the potential amount of vegetation on Isle Royal is controlled by the
amount of incoming energy from the sun, the amount of moisture, and the
mineral nutrients in the soil and air. The nutrients further determine
the quality of the vegetation as food for animals. In northern
coniferous forests ??? the water percolating down through the soil carries
much of the needed nutrient below the reach of plant roots, and the low
rates of evaporation do not allow a counter-flow of minerals upward.
This mineral deficiency together with the lower amount of sunlight,
results in less production of plant ??? matter here than in the deciduous
forests to the south, and considerably less than in tropical forests.
The island's forests are, however, much more productive than arctic
tundra, which suffers from greater deficiencies. And current research
by a Yale University team suggests that Isle Royale supports more animal
life than do many other parts of the northern forest at similar
latitudes. They suspect that this can be attributed to the island's
underpinning of basalt and sedimentary rocks, which contribute more of
the essential minerals to the soil than does the granite that underlies
large areas elsewhere.
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The breeding habitats of great blue herons have been
greatly diminished except in protected environments such as Isle
Royale. (Photo by Robt. G. Johnsson)
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The relationships between incoming energy, minerals,
plants, and animals can be studied particularly well on islands because
the water prevents much interchange with the surrounding region. On
Isle Royale, for instance, the interdependencies of moose, wolf, and
vegetation are much easier to study than they are in mainland areas
because here the animals are virtually "penned in" by Lake Superior:
their numbers are little affected by immigration and emigration. The
added fact that nature is allowed to operate unhindered creates an
outdoor ecological laboratory of exceptional valueone that has
attracted scientists since the middle of the last century.
What does Isle Royale tell us about our own
relationships with nature? One clear message is that we should encourage
diversity. For instance, we create potentially very unstable situations
by devoting large areas to one crop, which can be devastated by a single
insect species or disease. Such monocultures limit the animal life that
can act as checks on exploding pest populations. And we never know when
some obscure plant or animal may be needed to provide something required
for the welfare of man or his environment. But the fundamental message
is that man, like other animals, is ultimately controlled by his
environment. If we do not stabilize our numbers, we face the unpleasant
alternative of starvation, disease, and warfare. And if we continue to
poison our environment, it will eventually poison us.
For the earth, too, is an islandan island in
the sea of space. All that we have is here, finite, wrapped within our
round shore.
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