Wolf pups like the two to the right are contributing
to the recovery among the historical predators of moose of Isle Royale National
Park, Mich., like the cow and calf on the ice at right.
by: Les Line
This winter was long, tough and tumultuous on Isle
Royale National Park in Lake Superior, scene of the worldís best
studied predator-prey relationship, and the news is what wolf watchers were
hoping to hear. A troubled wolf population has rebounded sharply and at
the same time moose numbers have crashed in spectacular fashion.
Studies of wolves and moose and how their populations
are linked began on the island in 1958 under the direction of Dr. Durward
L. Allen, a wildlife ecologist at Purdue University in Indiana. The result
is said to be the longest-running study of a wildlife population in the
For several years Dr. Rolf O. Peterson, who has
been studying the parkís wolf packs since 1970, has worried that
the wolves might be on a fast track to extinction. Dr. Peterson saw their
numbers crash from 50 in 1980 to 14 two years later, apparently from an
outbreak of canine parvovirus, a deadly new disease that had ravaged dogs
on the mainland and was brought to the island on hikers boots. He watched
the population stagger along at a dozen or so animals for more than a decade
and grow top-heavy with old wolves that had little success at replacing
By 1993 there were only three females, all of them
old, among the 13 surviving wolves. Just one of them had ever succeeded
in rearing pups, and biologists feared that inbreeding had caused the isolated
wolf population to stagnate.
Dr. Peterson, a wildlife ecologist at Michigan Technological
University in Houghton, also watched moose numbers on the 210-square-mile
wilderness island mount in the absence of the normally heavy wolf predation.
By the winter of 1994-95, the Isle Royale moose population stood at 2,400,
the biggest herd since the early 1930ís and far higher than any count
since 1949, when wandering wolves, perhaps only a pair, crossed a rare ice
bridge from the Ontario mainland and first came to the island. The wolf
population at the end of the winter of 1994-95 was 15. The balance between
wolves and moose, which was never quite as magical as some popular articles
portrayed it to be, was decidedly out of sync.
But Dr. Peterson recently returned from his annual,
seven-week-long survey of Isle Royaleís two pre-eminent species,
and he reported that the islandís wolf population had climbed back
up to 22, which was the average in the years before the buildup in the late
1970's to the 1980 peak. Seven pups survived from last summer's litters.
The Isle Royale wolves hunt in separate packs, and,
Dr. Peterson said: there are pups in all three territorial packs for the
first time in nine years. The wolves appear to be making a strong comeback.
But he said, they are not out of danger. The virus disappeared a few years
ago, but we havenít resolved the main scientific question. That,
he said is: does the lack of genetic diversity pose a problem?
The Isle Royale moose population, meantime, has plunged to an estimated
1,200 animals, half the previous year's count. It was a matter of time until
something broke, and it broke in a big way, Dr. Peterson said. Everything
turned against the moose at once. It was the severest winter Iíve
seen on the island. The temperature fell to 43 degrees below zero, the coldest
on record. Three feet of snow aggravated a extreme shortage of winter browse,
and the moose also suffered from a heavy winter tick infestation that caused
substantial hair loss and left them in a weakened state.
Dr. Peterson described starving moose falling into
Lake Superior when they tried to reach the last tidbit of browse on trees,
leaning from the tops of steep cliffs. I doubt that any calves, which are
weaker than adult moose, will survive the winter, he said. By February,
the proportion of calves in the herd already was down to 5 percent, lower
than in any of the last 38 years. Calves and elderly animals are the main
winter prey of the wolves.
The die-off will make it easier for the wolves to
bring the moose numbers back under control so the islandís overbrowsed
forest can begin to recover, Dr. Peterson said. The wolves couldn't have
done it by themselves. He said that a major forest regeneration would occur
if the herd were reduced to between 500 and 700 animals.
The sudden crash of the islandís moose population
was no surprise to experienced wildlife biologists. It had to happen, given
a tough winter and a forage base that was in such poor shape, said Dr. James
Peek, professor of wildlife resources at the University of Idaho in Moscow.
The Isle Royale experience tells us that the balance
of nature is a dynamic process, not the status quo, Dr. Peek said. What
weíre seeing in the way of predator-prey-vegetation interactions
is a constantly changing kaleidoscope of events. The moose herd probably
will never increase to that level again. But it will increase if there are
mild winters and the wolves get on top and suppress the moose so the forage
can improve, or if a big forest fire creates a lot of new habitat, he said.
In his recent book The Wolves of Isle Royale: A
Broken Balance, published by Willow Creek Press, Dr. Peterson emphasized
that ìthe long record of wolf and moose fluctuations at Isle Royale
bears no resemblance to a static balance between predator and prey. From
1959 to the crash of 1980, he wrote, wolf and moose populations appeared
to cycle in tandem, with wolves peaking about a decade after moose. Wolves
simply followed trends in their primary prey, moose over 10 years old. He
Neither moose nor wolves lived on Isle Royale until
this century, at lease in historic times. Indian tribes hunted, fished and
mined copper on the island for 4,000 years but archeologist have found no
trace of moose bones at their campsites. Dr. Peterson said that moose, which
are strong swimmers but rarely venture out on open ice, first crossed the
20-mile-wide channel from the mainland about 1900.
Changing Fortunes of Wolf And Moose
Moose and wolf population fluctuations on Isle Royale depend on factors
like weather, disease and possibly genetic problems. Scientists are trying
to sort out reasons for the current wolf resurgence.
On Isle Royale, moose discovered a haven from
predators and a virgin food supply, he wrote. For almost 30 years, nothing
held back their increase in numbers. The population bomb exploded in the
1920ís and the herd swelled to several thousand. Biologists reported
that balsam fir, the main winter food for moose, was in desperate shape
and predicted a catastrophe. By 1935, only a few hundred starving moose
The herd eventually recovered because of the fire
in 1936 that burned 20 percent of the island and created a new forest, Dr.
Peterson said. Then in the winter of 1949, nine years after Isle Royale
became a national park, wolves made what was apparently a one-time run from
Lake Superior rarely freezes over, said Dr. David
Mech, a research biologist with the National Biological Survey in St. Paul,
and a wolf expert. It has to be cold enough and calm enough for a bridge
to form, and then you need a pack of wolves predisposed to make a long trek
over the ice. The chances of that happening are pretty remote or Isle Royale
would have had wolves long ago.
In fact, genetic studies using mitochondrial DNA
indicate a single ancestor for all of Isle Royalíes wolves. Scientists
tested almost 250 wolves from the Ontario mainland before finding the genotype
in an animal taken by a trapper 50 miles north of Lake Superior, Dr. Peterson
reported. DNA fingerprinting showed the wolves to be highly inbred, comparable
to members of a single family. Further analysis revealed that wolves on
Isle Royale had lost roughly 50 percent of the genetic variability of mainland
wolves. Isle Royale, he said, would provide an acid test for the notion
that inbred populations are not viable.
Scientists point to the cheetah, which has become
so inbred from a long-term population decline in Africa that it has less
than one-tenth of the genetic variety of domestic cats. Cheetah cubs in
zoos and wildlife parks often die before reaching maturity and they are
vulnerable to disease. Moreover, studies have found 70 percent of cheetah
sperm to be abnormally formed.
While genetic decay may have prevented the wolf population from rebounding,
Dr. Peterson believes food was a limiting factor. Few moose from the ages
of two to eight are killed by wolves, he noted. Prime-age moose are too
dangerous to approach, he said. Moose commonly stand and pugnaciously face
the wolves, which take the cue and leave. To my knowledge, no one has ever
observed wolves killing a moose that did not run when first confronted by
its predators. The front legs of a 900-pound moose are formidable weapons.
Dr. Peterson described an old and blind moose that stood its ground against
a pack of wolves for three days until the predators decided to look elsewhere.
moose are the wolves bread and butter, he said, and in recent years the
Isle Royale herd has been dominated by vigorous young adult moose. In the
late 1990ís, he wrote, alpha positions in wolf packs will be filled
by a new generation of young wolves, disease-free and well-fed throughout
their lives. Their reproductive success should shed light on the important
issue of genetic variability in small, isolated populations.
Dr. Mech, who was Dr. Allenís first wolf
student almost 40 years ago, said Isle Royale ìrepresents the best
experiment to determine the effects of inbreeding on a wild population of
canids, and it is showing wolves to be much more resistant to inbreeding
depression than one might expect. But Dr. Robert Lacy, a population geneticist
at the Brookfield Zoo in Chicago, believes that view is too optimistic.
Thereís a lot of chance involved in how inbreeding
affects any population, Dr. Lacy said. A number of studies have shown that
one small group of animals will get lucky and do well and another group
will get into a lot of trouble with developmental defects and high infant
mortality. Maybe the Isle Royale wolves just got lucky.
A classic case, Dr. Lacy notes, is the small surviving
population of endangered Florida panthers. The effects of inbreeding on
the panthers are severe. Most of the males born in recent years have either
one testicle or none, their sperm is poor and many of the kits show congenital
heart defects, he said. Biologists have introduced cougars from east Texas
into the Everglades to enhance the panther gene pool.
Another reason Iím pessimistic is that the
effects of inbreeding may not show up until the population is stressed by
food shortages or a new disease hits, Dr. Lacey said. The news is hopeful
but Iím still worried about the future of the Isle Royale wolves.