Download Lesson 4 - Frontenac Secondary School

Grade 9 Academic Science – Ecology
Wolves and Yellowstone
Hamann, Jack. 1997. Wolves Rreturn to Yellowstone Sparks Controversy. CNN.
http://www.cnn.com/EARTH/9711/12/yellowstone.wolves/
The meals are catered, but the menu lacks variety -- cold carcasses of deer and elk road-kill served up to a
wolfpack on probation, locked in a pen.
The Nez Perce Pack is at the centre of a firestorm over which wolves should live and which should die at
Yellowstone National Park. Named for a tribe of American Indians who fled through the park hoping for
sanctuary in Canada, the pack of wolves is also looking for a safe haven in this remote wilderness. It is
been almost three years since the federal government began the controversial measure of reintroducing
wolves into Yellowstone. While conservationists supported the move, ranchers were wary.
Breeding program successful
Some 80 wolves now live in or near the park, many of them pups. Wolves are held in "acclimation pens"
where, it is hoped, they will get used to their surroundings and form packs. Packs, it is believed, are less
likely to stray from the park than stragglers.
"In general, the acclimation has worked," says Doug Smith of Yellowstone Park. "Of the 31 wolves brought
from Canada, there was really only one wolf, Number 27, where we feel acclimation did not work." Number
27 killed almost 50 sheep a few weeks ago, and had to be destroyed. The rancher who lost sheep was
compensated with money from Defenders of Wildlife, a pro-wolf group.
How great a threat?
The 50 lost sheep attracted attention, but were hardly a rarity. In Montana alone, tens of thousands of freerange livestock die of various causes every year. "Predators are just a minute portion of that," says Ed
Bangs of the National Fish and Wildlife Service. "Wolves are an immeasurable portion of that. On average,
wolves kill about four or five a year, so industry
wise, wolf predation means nothing to the industry
or the economy of this area."
"Some ranchers say, 'Look, it's people or wolves,
not both,'" Smith says. "I think we're at point now
with wildlife and biodiversity and conservation
biology in the United States that we can do better. It
is not black or white. We can have both."
While the wolves pose some threat to humans' domesticated animals, there is little risk to people. While
humans have killed an estimated two million wolves in this century, there is not a single documented case of
a human being killed by a healthy wild wolf.
A pack of trouble
As biologists and rangers approach the Nez Perce pack's chain-link pen deep in the park's interior, they
hear a surprising sound -- a wolf barking from outside. Somehow the alpha male, the leader of the pack, has
escaped, only two weeks after he was recaptured. In his last escape, he dug under the fence and freed the
rest of the pack. "I think one's out," says biologist Carrie Shaefer. "That bark is an aggressive, defensive
behavior."
If the pack escapes before spring when they are scheduled for release, they will likely threaten livestock and
will have to be destroyed.
The Nez Perce pack was released in April 1996, and immediately lost "pack unity," the National Fish and
Wildlife Service said. Individuals strayed into areas with little wild prey and plenty of livestock, a handful of
sheep and cattle were killed and two pack members were shot dead.
Since most of the Nez Perce pack are still in the pen, recapturing the stray is not Bangs' top priority. "I think
the main thing is to keep the fence running, because he's not going anywhere," he says. "Not with his
woman or buddies in the pen. He is a family-oriented animal."
In the cramped offices of the Wolf Recovery Program, Smith and Bangs struggle over the fate of fugitive
wolves. The return of the wolves to Yellowstone requires a balancing act among ranchers and
conservationists, a dispute with these men in the middle. "I am getting a call from a rancher, yelling at me
because the wolves are out and the killed some sheep," Bangs says. "The next phone call is from an animal
rights person saying I am a bastard who should burn in Hell for murdering babies. Then the next call is
another rancher."
The wolves already in Yellowstone have bred so successfully that plans to transplant other wolves into the
area are on hold. That is a success story to some, but how success will be defined -- and what it will cost -is the ongoing debate.
Yellowstone and the Gray Wolf
American Farm Bureau Federation. 1998. AFBF Responds to Distortions by
Defenders of Wildlife. http://www.fb.org/news/graywolf.html
Defenders of Wildlife, in its attempt to persuade the American Farm Bureau Federation to drop its lawsuit
against the Interior Department’s Canadian Gray Wolf Introduction Program in the Yellowstone and central
Idaho region, has distorted the nation's largest farm organization's position in the case. The American Farm
Bureau Federation's arguments, which are fully supported by Judge William Downes, have been made
clear. The following points are intended to clarify some of the distortions generated by Defenders of Wildlife
and other groups.

The American Farm Bureau Federation filed its lawsuit, in conjunction with the Montana, Idaho and
Wyoming Farm Bureaus, because the Interior Department broke the law in launching the wolf
introduction program in 1994. Interior Secretary Bruce Babbitt violated provisions of the Endangered
Species Act when he brought the gray wolves into the Yellowstone and central Idaho region.

The gray wolf is not threatened by extinction. Bringing them into the Yellowstone and central Idaho
region under the guise of the Endangered Species Act was illegal.

The American Farm Bureau Federation never has advocated the killing of the Canadian gray wolves as
a means of their removal from the Yellowstone and central Idaho region. The Interior Department
assured Judge Downes that it could recapture and relocate the wolves from the Yellowstone and central
Idaho region. It is Farm Bureau’s hope that the federal government backs up its assertion that it could
easily capture and remove all Canadian wolves from the Yellowstone and central Idaho region.
Defenders of Wildlife -- not Farm Bureau -- is the only organization that continues to raise the issue of
killing wolves as a means of removal.

In a Jan. 16, 1998 follow-up ruling to his Dec. 12, 1997 decision, Judge William Downes wrote, “The
evidence before this court revealed that the Defendants (the Interior Department) could remove nonnative wolves from the experimental population areas and transplant them elsewhere within the territory
of the United States. The record suggests that if non-native wolves can be humanely captured and
transferred to the experimental population areas, they can also be humanely captured and removed.
The order to remove the wolves is not intended to serve as a license to euthanize wolves and any
interpretation that it does so is misplaced.”

This issue is about the rule of law. It’s a matter of what’s right -- and legal. Interior Secretary Bruce
Babbitt ignored the implications the Wolf Introduction Program would have on native wolves, ranchers
and their livestock when he launched the program.

Because of this overly ambitious program, farmers and ranchers were forced into a position of
defending their property and rights.

The American Farm Bureau Federation tried to stop the introduction of the Canadian gray wolves to the
Yellowstone and central Idaho region before the Gray Wolf Reintroduction Program was launched.
Farm Bureau’s suit was filed before any Canadian wolves were even captured. Yet, in the face of our
lawsuit, the Interior Department, knowing that the court could find the program illegal and order the
Canadian gray wolves’ removal, brought the Canadian wolves into the United States before the court
could decide the legal issues.
Wolf Restoration to Yellowstone
USA National Park Service - Yellowstone National Park. 2006. Wolf Restoration to Yellowstone.
http://www.nps.gov/yell/nature/animals/wolf/wolfrest.html
Northern Rocky Mountain wolves, a subspecies of the gray wolf (Canis lupus), were native to Yellowstone
when the park was established in 1872. Predator control was practiced here in the late 1800s and early
1900s. Between 1914 and 1926, at least 136 wolves were killed in the park; by the 1940s, wolf packs were
rarely reported. By the 1970s, scientists found no evidence of a wolf population in Yellowstone; wolves
persisted in the lower 48 states only in northern Minnesota and on Isle Royale in Michigan. An occasional
wolf likely wandered into the Yellowstone area; however, no verifiable evidence of a breeding pair of wolves
existed through the mid 1990s. In the early 1980s, wolves began to reestablish themselves near Glacier
National Park in northern Montana; an estimated 75 wolves inhabited Montana in 1996. At the same time,
wolf reports were increasing in central and north-central Idaho, and wolves were occasionally reported in the
state of Washington. The wolf is listed as "endangered" throughout its historic range in the lower 48 states
except in Minnesota, where it is "threatened."
National Park Service (NPS) policy calls for restoring native species when: a) sufficient habitat exists to
support a self-perpetuating population, b) management can prevent serious threats to outside interests, c)
the restored subspecies most nearly resembles the extirpated subspecies, and d) extirpation resulted from
human activities.
The U.S. Fish & Wildlife Service 1987 Northern Rocky Mountain Wolf Recovery Plan proposed
reintroduction of an "experimental population" of wolves into Yellowstone. In a report to Congress, scientists
from the University of Wyoming predicted reductions of elk (15%-25%), bison (5%-15%), moose, and mule
deer could result from wolf restoration in Yellowstone. A separate panel of 15 experts predicted decreases in
moose (10%-15%) and mule deer (20%-30%). Minor effects were predicted for grizzly bears and mountain
lions. Coyotes probably would decline and red foxes probably would increase.
In October 1991, Congress provided funds to the U.S Fish & Wildlife Service (USFWS) to prepare, in
consultation with the NPS and the U.S. Forest Service, an Environmental Impact Statement (EIS) on
restoring wolves to Yellowstone and central Idaho. After several years and a near-record number of public
comments, the Secretary of Interior signed the Record of Decision on the Final Environmental Impact
Statement (FEIS) for reintroduction of gray wolves to both areas. Staff from Yellowstone, the USFWS, and
participating states prepared to implement wolf restoration. The USFWS prepared special regulations
outlining how wolves would be managed as a nonessential experimental population under section 10(j) of
the Endangered Species Act. These regulations took effect in November 1994. As outlined in the Record of
Decision, the states and tribes would implement and lead wolf management outside the boundaries of
national parks and wildlife refuges, within federal guidelines. The states of Idaho, Wyoming, and Montana
have begun preparation of wolf management plans.
Park staff assisted with planning for a soft release of wolves in Yellowstone. This technique has been used
to restore red wolves in the southeastern United States and swift fox in the Great Plains and involves
holding animals temporarily in areas of suitable habitat. Penning of the animals is intended to discourage
immediate long-distance dispersal. In contrast, a hard release allows animals to disperse immediately
wherever they choose, and has been used in Idaho where there is limited access to the central Idaho
wilderness.
In the autumn of 1995 at three sites in the Lamar Valley, park staff completed site plans, and archaeological
and sensitive plant surveys. Approximately 1 acre was enclosed at each site with 9-gauge chain link fence in
10' x 10' panels. These enclosures could be dismantled and reconstructed at other sites if necessary. The
fences had a 2' overhang and a 4' skirt at the bottom to discourage climbing over or digging under the
enclosure. Each pen had a small holding area attached, to allow a wolf to be separated from the group for
medical treatment. Inside each pen were several plywood security boxes to provide shelter. For the 1996
release, one pen was relocated to Blacktail Plateau and another was constructed in the Firehole Valley in
central Yellowstone. Subsequently pens have been relocated from Lamar to other areas in the park interior
to facilitate releases into other geographic areas or the park or special circumstances that require the
temporary penning of wolves.
USFWS and Canadian wildlife biologists captured wolves in Canada and released them in both recovery
areas in 1995 and 1996. As planned, wolves of dispersal age (1-2 years old) were released in Idaho, while
Yellowstone released pups of the year (7+ months old), together with one or more of the alpha pair
(breeding adults). Young pups weigh about 75 lbs. and are less likely to have established a home range.
The goal was to have 5-7 wolves from one social group together in each release pen.
Each wolf was radio-collared when captured in Canada. For about 8-10 weeks while temporarily penned, the
wolves experienced minimal human contact. Approximately once each week, they were fed road-kills. They
were guarded by rangers and other volunteers who minimized the amount of visual contact between wolves
and humans. The pen sites and surrounding areas were closed and marked to prevent unauthorized entry.
Biologists used radio-telemetry to check on the welfare of wolves.
Although concern was expressed about the wolves becoming habituated to humans or to the captive
conditions, the temporary holding period was not long in the life of a wolf. In Alaska and Canada, wolves are
seldom known to develop the habituated behaviors seen more commonly in grizzly bears. Wolves, while
social among their own kind, typically avoid human contact. They are highly efficient predators with welldeveloped predatory instincts. Their social structure and pack behavior minimizes their need to scavenge
food or garbage available from human sources. Compared to bears, whose diet is predominantly vegetarian,
wolves have less specific habitat requirements. The wolves' primary need is for prey, which is most likely to
be elk, deer, and other ungulates in these recovery areas.
In 1995, fourteen wolves were released into Yellowstone National Park. In 1996, seventeen more wolves
were brought from Canada and released. After release, several thousand visitors were lucky to view wolves
chasing and killing elk or interacting with bears during spring. A park ranger and a group of visitors watched
a most exciting encounter between two packs which likely resulted in one young wolf's death. This was not
the first fatal encounter between wolves, although human-caused mortalities still outnumber inter-pack strife
as a cause of wolf deaths.
Yellowstone's first fourteen wolves bore two litters totaling nine pups. In 1996, four packs produced fourteen
pups. After the wolves’ release in 1996, plans to transplant additional wolves were terminated due to
reduced funding and due to the wolves' unexpected early reproductive success.
In early 1997, ten young wolves, orphaned when their parents were involved in livestock depredation on the
Rocky Mountain Front in northwestern Montana, were released into the park. In the spring of the wolf
restoration project's third year, nine packs of wolves produced 13 litters of 64 pups. Three of the packs
produced multiple litters which, while documented in the literature, is still unusual. Alpha male wolves
generally do not breed with their own offspring, possibly to prevent inbreeding. However, as wolves were
matched up during temporary periods of penning and as pack members shifted or were killed and replaced
by other dispersing wolves, the occasional result has been packs in which one or both of the alpha pair were
not the parents of subordinate pack members. Consequently, the alpha males probably had less incentive to
breed with only one female, especially since food was abundant and the packs were still in the early stages
of establishing their territories. Lone wolves continued to roam widely, but most of the wolves remained
primarily within the boundaries of Yellowstone National Park
An estimated 100,000 park visitors have observed wolves since their return in 1995. The program's visibility
has resulted in opportunities to educate audiences about predator-prey relationships, endangered species
restoration, and the importance of maintaining intact ecosystems. The program has also generated
numerous partnerships with private groups and individuals who generously donated their time and money—
critical in an era of reduced budgets and staff downsizing.
For both Idaho and Yellowstone, wolf population recovery is defined as having about 100 wolves, or
approximately 10 breeding pairs, established in each area for 3 successive years. The goal to restore
wolves and begin delisting them by approximately 2002 appears within reach. The return of the only species
known to be missing from the world's first national park for the past half-century has been a milestone in
ecological restoration. It has not only restored the wildlife complement of greater Yellowstone; it has been a
symbolic victory for conservationists who patiently and persistently reversed the once-dominant attitude
against predators to one of acceptance. We believe that Aldo Leopold would be proud that so many humans
have come to respect even these "killer creatures" with whom we share the Earth.
Yellowstone Park and Wolf Reintroduction
Ives. Sarah. 2004. Wolves Reshape Yellowstone National Park. National Geographic.
http://news.nationalgeographic.com/kids/2004/03/wolvesyellowstone.html
In the late 1990s scientists began to notice changes in Yellowstone National Park, in Wyoming. Trees that
had stopped growing decades ago began to grow again. Animals started behaving differently. What caused
these mysterious changes? Scientists believe they have found the answer: Wolves used to live in
Yellowstone, but many people thought wolves were pests that killed livestock and harmed crops. In 1926
the last wolf in Yellowstone was shot and killed. For the next 70 years Yellowstone did not have any wolves.
Scientists decided to bring them back to the area in 1995. The U.S. Fish and Wildlife Service brought in 15
gray wolves from Canada.
"Wolves were reintroduced because historically wolves lived in Yellowstone," explained William Ripple, a
forest ecologist at Oregon State University. Ripple studies wolves' effects on Yellowstone. "Now with the
reintroduction of wolves, Yellowstone has all of the top predators it has had for thousands of years."
Today between 250 and 300 wolves live in Yellowstone, and they have already left their mark. "The effect
has been dramatic," said Douglas Smith, leader of the Yellowstone Wolf Project. For example, wolves have
been hunting and eating elk, a member of the deer family. The elk leftovers provide food for animals such as
ravens, eagles, and bears. Wolves also scare elk from streams. With fewer elk near the water, plants that
grow there, such as willows, can grow taller. "Benefits of this new plant growth include more habitat for birds
and more plant food for beaver," Ripple explained. "The number of beaver in northern Yellowstone has
increased dramatically since wolves were reintroduced."
Wolves have such a big effect on Yellowstone because scientists believe wolves are a keystone species.
Keystone species are species on which a large number of other plants and animals depend. "The removal
of a keystone species can lead to the extinction of other species," Ripple said. According to Smith, the
reintroduction of wolves has affected more than 25 species in Yellowstone. "We think that Yellowstone will
be a different place in 20 years because of wolves," Smith said.
Not everyone is happy with the wolves' return, however. Some farmers say that wolves kill their animals.
"Ranchers want to limit the number of wolves in the area, because if there are too many wolves, they always
get in trouble," said Jennifer Ellis of the Idaho Cattle Association. "Many people who live in wolf-inhabited
areas have had not only their cows, calves, and sheep killed by wolves, but also their cats, dogs, and
horses."
Ripple noted that there is a program in place to pay farmers for any money they lose due to wolves killing
cattle. Some farmers may not be happy, but many scientists believe that Yellowstone is starting to look
more like it did before people began to interfere with the animals there.
Reintroduction of Wolves to Yellowstone
2003. Wolves Are Rebalancing Yellowstone Ecosystem. Science Daily.
http://www.sciencedaily.com/releases/2003/10/031029064909.htm
The reintroduction of wolves into Yellowstone National Park may be the key to maintaining groves of
cottonwood trees that were well on their way to localized extinction, and is working to rebalance a stream
ecosystem in the park for the first time in seven decades, Oregon State University scientists say in two new
studies.
The data show a clear and remarkable linkage between the presence of wolves and the health of an entire
streamside ecosystem, including two species of cottonwoods and the myriad of roles they play in erosion
control, stream health, and nurturing diverse plant and animal life. The findings of these studies were
recently published in Ecological Applications, a journal of the Ecological Society of America, and the journal
Forest Ecology and Management. "In one portion of the elk's winter range along the Lamar River of
Yellowstone National Park, we found that there were thousands of small cottonwood seedlings," said Robert
Beschta, professor emeritus in the College of Forestry at OSU and an expert on streams and riparian
systems. "There should also have been hundreds of young trees, but there were none. Long-term elk
browsing had been preventing any seedlings from getting taller."
That pattern was common throughout the study area - lots of seedlings in combination with large cottonwood
trees generally more than 70 years old, but little or nothing in between. Young cottonwoods, willows, and
other streamside woody species are a preferred food for browsing elk during the harsh winters in northern
Yellowstone, when much of the other forage is buried under snow. But when packs of wolves historically
roamed the area, food was not the only consideration for elk, which had to be very careful and apparently
avoided browsing in high-risk areas with low visibility or escape barriers.
Wolves were systematically killed in the Yellowstone region and many other areas of the West beginning in
the late 1800s. A concentrated effort between 1914 and 1926 finished the job - the last known wolf pack
disappeared in 1926. "I considered a variety of potential reasons that might explain the historical decline of
cottonwoods that began in the 1920s and have continued up to the last couple of years," said Beschta. "I
looked at climate change, lack of floods, fire suppression, natural stand dynamics, and numbers of elk. But
none of those factors really explained the problem. "Ultimately, it became clear that wolves were the
answer."
While elk populations fluctuated over the decades when wolves were absent, browsing behavior appears to
represent an important factor related to streamside impacts. With no fear of wolves, the elk could graze
anywhere they liked and for decades have been able to kill, by browsing, nearly all the young cottonwoods.
Other streamside species such as willows and berry-producing shrubs also suffered. That in turn began to
play havoc with an entire streamside ecosystem and associated wildlife, including birds, insects, fish and
others. Trees and shrubs were lost that could have helped control stream erosion. Food webs broke down.
"Before the wolves came back, it was pretty clear that in some areas we were heading towards an outright
extinction of cottonwoods," Beschta said.
Now, with the recent reintroduction of wolves back into Yellowstone in 1995, streamside shrubs and
cottonwoods within the Lamar Valley are beginning to become more prevalent and taller, and were the focus
of a second study in the same area. That study outlines how the fear of attack by wolves apparently
prevents browsing elk from eating young cottonwood and willows in some streamside zones. With the
renewed presence of wolves, young cottonwoods and willows have been growing taller each year over the
last four years on "high-risk" sites, where elk apparently feel vulnerable due to terrain or other conditions that
might prevent escape. In contrast, on "low-risk" sites, they are still being browsed by elk and show little
increase in height.
"In one case where a gully formed an escape barrier for elk, the tree height went up proportionally as the
gully deepened and formed an increasing barrier to escape," said William Ripple, a professor with the
College of Forestry at OSU. "Where the fear factor of wolves is high, the young trees and willows are doing
much better and growing taller."
Traditionally, "keystone" predators such as wolves were known to influence the population of other animals
that they preyed on directly, such as elk or antelope. What researchers are now coming to better understand
is the "trophic effect," or cascade of changes that can take place in an ecosystem when an important part is
removed, Ripple said. The comparatively pristine conditions of a national park allowed this type of research
to make "cause and effect" studies more feasible, the scientists point out. "The removal of wolves for 70
years - and then their return - actually set the stage for a scientific experiment with fairly compelling results,"
Beschta said.
In a larger context, the studies also raise valid questions about other complex and poorly understood
interactions between plants, animals, and wildlife in disturbed ecosystems across much of the American
West, and perhaps elsewhere in the world, the scientists say. In some areas of the West, the disappearance
of up to 90 percent of the aspen trees has been documented - another species of plant that is also highly
vulnerable to animal browsing when it is young. "The last period when aspen trees in Yellowstone escaped
the effects of elk browsing to generate trees into the forest overstory was the 1920s," Ripple said, "which is
also when wolves were removed from the park." In at least one place - America's first national park - there
is now cause for hope. While it is too early to confirm the widespread recovery of cottonwoods and willows,
the reintroduction of wolves appears to have put a stop to major declines in the survival of these plants, the
researchers found.
"One point that should not be missed is this is actually great news for the potential recovery of cottonwood
trees and mature willows in Yellowstone National Park," Ripple said. "We now have a pretty good idea why
they were in decline and the return of wolves should help pave the way for their recovery. Even though it
may take a very long time, for a change it looks like we're headed in the right direction."
Yellowstone Wolves - Data
Table 1. Yellowstone National Park Wolf Pack Sizes October – December 2003
Pack Name
Pup Count
Summer
Pup Count
Fall
Adults
Yearlings
Total Pack
Group 1
5
0
2
2
Agate Creek
8
4
5
9
Bechler
4
4
4
8
Buffalo Fork
2
0
3
3
Chief Joseph
5
5
2
7
Cougar Creek
4
4
6
10
Druid Creek
13
9
8
17
Geode Creek
8
2
5
7
Gibbon*
5
5
-
-
Leopold
8
8
11
19
Mollie
2
2
5
7
Nez Perce
2
2
13
15
Rose Creek
4
4
2
6
Slough Creek
6
6
9
15
Swan Lake
6
6
14
20
Yellowstone
Delta
4
4
13
17
Loners
-
-
2
2
TOTAL
79
60
109
169
*Breeding female died June 2003
Source – Defenders of Wildlife. 2004. Wolves at Yellowstone.
http://www.defenders.org/wildlife/wolf/ynp.html
The current status of the 1995 wolves by individual wolf. January 2003
In March 1995, 14 wolves were released in Yellowstone National Park. When the last of the
original 14 died, the wolf population was 148 wolves within the Park and 270 wolves in the
Greater Yellowstone ecosystem population (includes the Park).
Table 2. Current status of 1995 wolves in Yellowstone
Location
Number
Age
Weight
(pounds)
Crystal Creek
Rose Creek
Soda Butte
1
Current Status
pup
77
Alpha male Leopold Pack.
1996-2002, killed by rival pack
2
pup
80
Killed 1996
3
adult male
98
Was alpha male Crystal Creek Pack
killed by the
Druids 1996
4
adult
female
100
Alpha female of the Crystal Creek
Pack until early 2000. Became a lone
wolf. Her
radio-collar failed. As of 2002
presumed to have died, but her body
never found
5
pup
75
Was alpha male of the Crystal Creek
Pack killed by an elk 1998
6
pup
72
Alpha male Rose Creek Pack.
Natural mortality July 2000.
1
pup
77
Alpha female of the naturally formed
Leopold
Pack, until late spring of 2002 when
she was killed by another wolf pack.
2
adult
female
98
Alpha female of the Rose Creek Pack
from 199599. Later she became a
beta
member of the new Beartooth Pack
east of Yellowstone. Most famous
wolf in the Yellowstone
recovery area. She presumably died
sometime after fall 2001
3
adult
122
Illegally shot spring of 1995
1
adult
92
Illegally shot spring of 1996
2
adult
113
Illegally shot spring of 1995
3
adult
113
Was alpha male Soda Butte Pack
died of old age March 1997
4
adult
89
Alpha female Soda Butte Pack 1995
to April 2000.
Probably killed by a moose.
5
pup
75
Alpha male of the naturally-formed
Washakie Pack Control kill 1998
Source – Defenders of Wildlife. 2004. Wolves at Yellowstone.
http://www.defenders.org/wildlife/wolf/ynp.html
DelGiudice, Glenn D. 1998. The Ecological Relationship of Gray Wolves
and White-Tailed Deer in Minnesota. Forest Wildlife Populations & Research Group - Minnesota
Department of Natural Resources. http://www.wolf.org/wolves/learn/scientific/delguidice.asp
INTRODUCTION
To best understand and appreciate the relationship of gray wolves (Canis lupus) and white-tailed deer
(Odocoileus virginianus) in Minnesota today, information concerning their evolutionary backgrounds is
helpful. Modern white-tailed deer have been evolving for about 1 million years (since the Pleistocene epoch).
However, artiodactyls (even-toed ungulates or hooved animals) from which they evolved have existed for at
least 58 million years (Kulp 1961). Similarly, wolves have existed in their present form for 1-2 million years,
but evolved from flesh-eating ancestors that occurred some 45-55 million year ago (Matthew 1930).
Because wolves have ranged over most of the Northern Hemisphere, a greater area than inhabited by any
of their prey, primary species in the wolf's diet, has and still, depends on geographic location (Mech 1970).
Today in the Great Lakes region, and in Minnesota specifically, white-tailed deer are the primary prey of
wolves, with moose (Alces alc es) typically being of secondary importance where they both exist (Stenlund
1955, Mech and Frenzel 1971, Van Ballenberghe et al. 1975, Fritts and Mech 1981, Fuller 1989). Clearly,
white-tailed deer and gray wolves have helped hone morphological and behavioral adaptations in each other
for at least 1 million years of coevolution (Nelson and Mech 1981).
Herein, the objectives of the Minnesota Department of Natural Resources (MNDNR) are to (1) provide a
general historical perspective for gray wolves and white-tailed deer in Minnesota and (2) highlight research
findings specifically concerning wolf-deer interactions and aspects of each species' ecology pertinent to
those interactions.
WOLF AND DEER POPULATIONS
Historical Background
White-tailed deer.--White-tailed deer range throughout Minnesota and are considered the state's most
valued big game animal. Recent estimates of the state's pre-fawning (spring) population have exceeded
600,000 deer, with fall estimates reaching as high as 1 million deer (MNDNR 1990). White-tailed deer were
not always so abundant. Prior to European settlement (before 1860), deer were most common in the
hardwood forests of central and southeastern Minnesota and were relatively rare in the pine (Pinus spp.)
forests east and north of the Mississippi River (MNDNR 1990). Two principal factors positively impacting the
growth of the deer population have been (1) alterations of their habitat by timber harvesting, fire, and
agriculture; and (2) increasingly intense management directly by manipulation of the population's sex and
age composition (through hunting) and indirectly via habitat management programs.
The first big game law affecting deer in Minnesota was established in 1858, although enforcement was likely
difficult (MNDNR 1974). Initial management attempts at estimating the annual deer harvest and written
records date back to 1918. The methodology for estimating annual harvests and hunter success rate
gradually improved, so that by 1956 a statistically valid sampling method was adopted (MNDNR 1974).
Over the years, deer population trends have varied around the state. By 1920, deer had become fairly
common in northern Minnesota, but they were nearly eliminated from the prairies due to farming and
subsistence hunting (MNDNR 1990). Since the 1950s, documented fluctuations in the state's deer
populations have been attributable to changes in winter severity, habitat, hunting, and predation primarily by
wolves. A pronounced decline in deer numbers and the harvest in northern Minnesota occurred in response
to a series of severe winters, over-harvests (either sex seasons), and overly mature habitat during the late
1960s and early 1970s (MNDNR 1974, 1990; Mech and Karns 1977). Deer populations across eastern
North America similarly experienced dramatic decreases, irrespective of whether the deer were impacted by
wolf predation or hunting (Voigt 1990). Minnesota's 1971 deer hunting season was closed, a practice which
had been exercised intermittently by the state in the past.
Minnesota was divided into Deer Management Units and Permit Areas (subunits) following the closed
season of 1971. The Permit Areas allow a more refined level of local deer management, specifically with
respect to setting goals and managing the annual harvest. Further, the MNDNR changed from earlier eithersex hunting to a limited harvest of females. From 1976 to 1995, the state's deer population increased rather
steadily with record harvests being documented (MNDNR 1974, 1997). Relatively high losses of deer were
observed in the state's northern forests during the historically severe winter of 1995-96, and evidence
indicated low reproductive success during the subsequent spring (DelGiudice 1996, 1998; Nelson, personal
communication). Mortality rate was markedly lower during winter 1996-97, despite weather conditions nearly
as severe (G. D. DelGiudice, unpublished data; M. E. Nelson, personal communication). Followed by a
historically mild winter (1997-98) and an expect ed relatively high reproductive success during the 1998
fawning season, initiation of recovery of the northern deer population is imminent.
Timber Wolves.--Today, wolves do not range statewide, and their numbers have followed a trend different
from that of deer. Further, the factors directly responsible for the changes in their numbers are somewhat
different from those affecting deer. Historically, wolves occupied all habitats of Minnesota, ranging from
prairies to forests (Young and Goldman 1944, Mech and Frenzel 1971). As early as 1849, the state began
paying bounties for wolves, and by 1900, wolves were rare in southern and western Minnesota (Herrick
1892, Surber 1932, Fuller et al. 1992). As both forests and wolves were harvested aggressively, wolf
numbers and range continued to diminish, despite increases in some of the local deer populations. By 1941,
the highest wolf densities (estimated 39 wolves/1,000 km 2 [386 mi2], Olson 1938) occurred in the northern
third of Minnesota (Surber 1932, Fuller et al. 1992). Based on the field research of Stenlund (1955) in
northeastern Minnesota and MNDNR records of bounty rates in northwestern Minnesota, Fuller et al. (1992)
estimated that during 1950-1952, the wolf population was roughly 430-636 for the 31,080 km2 (12,000 mi2)
of "major" wolf range and 450-700 wolves when areas of occasional wolf observations were included. The
average number of wolves submitted annually for bounty was 253, not including individuals collected by
MNDNR personnel (MNDNR files, Fuller et al. 1992). The average annual number of wolves submitted for
bounty decreased to 186-189 during 1953-1965, and a gross estimate of the population was 350-700
wolves, restricted primarily to the northeastern portion of the state (MNDNR files, Cahalane 1964).
Bounties for wolves and all predator species were discontinued in 1965, and in 1966, wolves were classified
by the federal government as an "endangered species," but legal harvests continued through 1973.
Harvests included wolves trapped by a MNDNR Directed Predator Control Program which concentrated
efforts in areas of livestock depredation (Fritts 1982, Fuller et al. 1992). By 1970, the estimate of wolves was
750, changing little since the 1953-1965 estimate (Leirfallom 1970, Nelson 1971, Fuller et al. 1992), and
they were assigned full protection in the Superior National Forest (Van Ballenberghe 1974).
In 1974, wolves were fully protected statewide by the Endangered Species Act of 1973. Subsequent
estimates of wolf numbers were 500-1,000; 1,000-1,200; and 1,235 in 1973, 1976, and 1979, respectively
(Mech and Rausch 1975, Bailey 1978, Berg and Kuehn 1982). The latter estimate was associated with a
major or "primary" wolf range of 36,500 km2 (14,093 mi2) and a "peripheral" range of 55,600 km2 (21,467
mi2) (Berg and Kuehn 1982).
Wolves in Minnesota were reclassified as "threatened" in 1978 (U.S. Fish and Wildlife Service 1978 from
Fuller et al. 1992), and occupied wolf range had increased to 57,050 km 2 (22,027 mi2), nearly double the
estimate for 1950-1952 (Fuller et al. 1992). Most wolves occurred in areas where there was less than 0.70
km roads/km2 and less than 4 humans/km2 (Mech et al. 1988b). An extensive survey during winter 1988-89,
estimated an occupied wolf range of at least 53,100 km 2 (20,502 mi2), excluding approximately 8,000 km2
(3,089 mi2) within this contiguous range (Fuller et al. 1992). These authors identified an additional 11,500
km2 (4,440 mi2) of potential range. Us ing two different approaches, Fuller et al. (1992) estimated the
population of winter 1988-89 at 1,521 and 1,710 wolves. The survey of 1988-89 was repeated during winter
1997-98 with far more observers and technology, and although the data are not yet analyzed, preliminary
evidence indicates that wolf numbers have increased and range expanded (Paul 1997).
Wolf Territories, Deer Movements and Distribution, and Predation RisK
Olson (1938), Murie (1944), and Young and Goldman (1944) pioneered ecological study of wolves and their
prey. The earliest studies of the relationship between wolves and white-tailed deer in Minnesota and
Wisconsin were conducted by Olson (1938), Thompson (1952), and Stenlund (1955). Including and since
that time, most of the research information concerning wolf-deer interactions has been generated from
studies conducted in what is considered primary wolf range of northern Minnesota. The most long-term and
diverse ecological data have been generated from research conducted in northeastern Minnesota (Mech
and Frenzel 1971; Hoskinson and Mech 1976; Mech 1977a,b,c, 1994; Mech and Karns 1977; Rogers et al.
1980; Nelson and Mech 1981, 1986a,b, 1991; DelGiudice et al. 1991; Kunkel and Mech 1994), followed by
northcentral (Berg and Kuehn 1980, 1982; Fuller and Snow 1988; Fuller 1989, 1990, 1991; DelGiudice
1995, 1996, 1998; DelGiudice and Riggs 1996) and northwestern Minnesota (Fritts and Mech 1981).
Overall, these regional studies have illuminated both strong similarities, as well as quantitative variation of
different aspects of wolf and white-tailed deer ecology and the interactions of these 2 species.
The territorial nature of wolves has been thoroughly documented. In Minnesota, their territories range from
52 to 555 km2 (20-214 mi2) and remain relatively stable throughout the year with no distinctly different winter
and summer ranges (Mech 1972, 1973, 1974, 1977a; Van Ballenberghe et al. 1975; Fritts and Mech 1981;
Fuller 1989). The wide variation in territory sizes is in part linked to prey density. Wolf pack territories are
smaller as deer become more numerous (Fritts and Mech 1981, Fuller 1989). Further, territory size of a
newly formed pack may be large, but decreases have been observed once such packs become established
in an area (Fritts and Mech 1981). Wolf packs tend to be discriminatory in the use of their territory, using
some portions more than others; this appears to be related to differences in physiography and deer
densities, as well as proximity of neighboring wolf pack territories (Ho skinson and Mech 1976; Mech
1977a,b,c; 1994; Fritts and Mech 1981). With respect to the latter, evidence has indicated that boundaries of
territories of neighboring packs commonly overlap by an estimated 6.4 km (4.0 mi) (3.2 km inside and
outside estimated territorial borders, Mech 1994). This area, referred to as a "buffer zone," may be
contested by neighboring packs, and consequently receives less use by the wolves of these packs (Peters
and Mech 1975; Hoskinson and Mech 1976; Mech 1977a,c, 1994; Fritts and Mech 1981; Nelson and Mech
1981). Indeed, the inter-pack strife associated with buffer zones may be so serious that a wolf's "risk of a
fatal encounter" with neighboring packs may be notably increased (Mech 1994).
In contrast to wolves, most white-tailed deer in northern Minnesota, up to 85%, are seasonal migrators with
a high fidelity for distinctly different winter and spring-summer-fall home ranges (Hoskinson and Mech 1976;
Nelson and Mech 1981,1987; Fuller 1990; DelGiudice 1993). Observed spring and fall migration distances
of deer have ranged from 3 to 37 km (2-23 mi), with most being 8-16 km (5-10 mi) (Hoskinson and Mech
1976, Nelson and Mech 1981, DelGiudice 1993). Long-term research collected in northeastern Minnesota
has shown that deer are particularly vulnerable to predation by wolves during fall migration to winter yards
(Nelson and Mech 1991). Duration of this migration is typically brief (average of 4.5 days), but daily mortality
of deer has been disproportionately high. Specifically, study deer spent approximately 1% of the year
migrating, but deer killed by wolves during this time comprised 21% of all kills. The reasons for this
increased vulnerability are n ot fully understood, but changing weather conditions, less familiar terrain, and
other factors have been proposed as contributing factors that warrant further study. Aggregations of deer
and extensive trail networks once in the winter yard assist them in their defense against wolf predation
(Nelson and Mech 1981, 1991). Higher winter mortality for male and female yearlings and adult females that
did not yard compared to those that did has been reported (Fritts and Mech 1981, Nelson and Mech 1981).
Further, several studies have observed a preponderence of deer-kills by wolves along the edges of wintering
yards and more adult male than female deer killed by wolves during winter, which may be related to
differences in the spatial distribution of the adults of the sexes in the yards (Stenlund 1955, Pimlott et al.
1969, Mech and Frenzel 1971, Mech and Karns 1977, Fritts and Mech 1981, Nelson and Mech 1986a). All
of this suggests that yarding is, at least in part, an antipredato ry behavior not simply a strategy strictly linked
to nutritional and thermal benefits.
Size of seasonal home ranges of deer can vary markedly (Kohn and Mooty 1971, Hoskinson and Mech
1976, Nelson and Mech 1981). Winter home ranges are the smallest with reported average sizes ranging
between 18 and 36 ha (44-89 ac) for all sex and age classes (Nelson and Mech 1981; G. D. DelGiudice,
unpublished data). Following spring migration, widely dispersed summer home ranges have averaged 83,
109, and 319 ha (205, 269, and 788 ac) for adult females, yearling bucks, and adult bucks, respectively
(Nelson and Mech 1981). Does with fawns reduce their home range size by as much as 65% after giving
birth (e.g., from 108 to 38 ha [267 to 94 ac], Nelson and Mech 1981). The wide dispersion of does prior to
fawning and the reduced home range sizes during the fawning season appear to be antipredatory
strategies---newborn fawns attempt to avoid predation by hiding not fleeing. In northeastern Minnesota, fall
home ranges of bucks increased significantly to 225 and 749 ha (556 and 1,850 ac) for yearlings and adults;
ranges of adult females tended to increase, but differences were not significant (Nelson and Mech 1981).
Interestingly, preliminary evidence has suggested that the territorial buffer zones of wolf packs may have
real significance for the long-term persistence of deer populations in Minnesota's forests. Studies of radio-
collared deer in primary wolf range have shown that most of their seasonal home ranges were located at the
boundaries of juxtaposed territories of wolf packs, where, as discussed above, inter-pack strife is greatest
and wolf activity tends to be less (Peters and Mech 1975; Hoskinson and Mech 1976; Mech 1977a,b, 1994;
Fritts and Mech 1981; Nelson and Mech 1981). Deer home ranges in these buffer zones may serve as
"reservoirs of deer" when the combined influence of several factors (e.g., severe winters, wolf predation,
hunting, maturing habitat--Mech and Karns 1977) causes the decline of a local deer population.
Wolf Predation of Deer and Food Habits of Wolves
Across northern Minnesota, hunter harvest and wolf predation are the primary causes of mortality of whitetailed deer (Hoskinson and Mech 1976; Nelson and Mech 1981, 1986a,b; Fuller 1990; DelGiudice 1994,
1998; DelGiudice and Riggs 1996). However, it is important to note that the relative importance of these 2
causes of mortality vary among years and local deer populations, dependent upon differences in hunter
pressure, winter severity, wolf and deer densities, and sex and age composition of the deer population. It
has been well documented in studies across northern Minnesota that wolves primarily kill young of the year
and older deer in the population, and these individuals are often compromised by deterioration of their
overall physical condition or a specific abnormality (Mech and Frenzel 1971, Fritts and Mech 1981,
DelGiudice 1996, DelGiudice and Riggs 1996). In northeastern Minnesota, the average age of deer (both
sexes) killed by hunters was 2.6 years compared to 4.7 years for deer killed by wolves during DecemberMarch (1966-1969) (Mech and Frenzel 1971). Similarly, in northwestern Minnesota, the average ages of
deer (both sexes) killed by hunters and wolves were 3.3 and 7.6 years, respectively (Fritts and Mech 1981).
In northcentral Minnesota, the median ages (half the cause-specific mortality occurred by this age) of female
deer killed by hunters and wolves were 2.5 and 6.5 years, respectively (DelGiudice 1996).
Wolf predation on deer is greatest during mid-late winter, coinciding with the period of poorest condition and
deepest snow (Mech and Frenzel 1971, Mech 1977b, Moen and Severinghaus 1981, DelGiudice et al. 1992,
DelGiudice 1998), then again during the fawning period of early summer when neonates are vulnerable prey
to wolves (Pimlott et al. 1969; Mech et al. 1971; Fritts and Mech 1981; Nelson and Mech 1981, 1986a,b;
Fuller 1989; Kunkel and Mech 1994) and other predators (see Fuller 1990; Dickson 1992; L. L. Rogers,
unpublished data). For the period 1973-1984, Nelson and Mech (1986a) reported an annual mortality rate of
radio-collared deer (both sexes) attributable to wolves of 0.16-0.19 (i.e., 16-19%), except for yearling
females which experienced a rate of 0.05. In a study of wolves and deer in northcentral Minnesota (Bearville
study area north of Grand Rapids), Fuller (1990) documented an annual wolf-caused mortality rate of 0.04
for radio-collared dee r >/=1.0 year old (both sexes) during the period 1981-1986. From 1991 to 1997,
DelGiudice (1996) observed annual, wolf-caused mortality rates of 0.09-0.26 for radio-collared, female deer
(primarily >/=1 yr old). In eastern Ontario, Kolenosky (1972) documented a wolf-caused mortality rate of
0.09-0.11 when snow depths were fairly deep. The variation of mortality rates of deer from area to area and
year to year are primarily attributable to variations in deer:wolf ratios, winter severity, presence of alternate
prey (e.g., moose), and the sex and age composition of the deer populations. Of note, wolf predation rates
on deer appear to be higher in areas with lower deer:wolf ratios (Kolenosky 1972, Nelson and Mech 1981,
Fuller 1990). Moreover, there is good evidence that pairs of wolves in a given area have higher winter kill
rates of deer per wolf than packs of wolves (>/=3 members), suggesting a low optimum pack size for
maximizing energy to individual members (Fritts and Mech 1981, Schmidt and Mech 1997).
Severe winter weather negatively impacts survival of northern white-tailed deer either through nutritional
restriction when predators are scarce or by predation where wolves or other predators are common
(Severinghaus 1947; Erickson et al. 1961; Mech et al. 1971; Verme and Ozoga 1971; Nelson and Mech
1986b; DelGiudice 1996, 1998). Several studies have demonstrated that snow depth (and density)
specifically has a strong, direct influence on wolf predation of deer in Minnesota and elsewhere (Pimlott et
al. 1969; Mech et al. 1971; Nelson and Mech 1986b; DelGiudice 1996, 1998). In the extreme case, during
severe winters of exceptionally deep snow (>/=70 cm), excessive or surplus killing of deer by wolves,
characterized as multiple kills made over short distances and brief periods of time with less than complete or
no consumption, has been documented (Mech et al. 1971, DelGiudice 1998). Deteriorating condition of deer
as winter progresses and impedence of deer move ments by deep snow that is less supportive of deer than
wolves have been implicated as primary factors contributing to this phenomenon (Mech et al. 1971,
DelGiudice 1998). Surplus killing of prey under certain environmental conditions has been documented for a
wide variety of predators (Kruuk 1972).
Scat analyses have revealed much about the year-round food habits of wolves, including that deer are their
single most important food, except during winter in the Boundary Waters Canoe Area where they rely
primarily on moose (Frenzel 1974, Van Ballenberghe et al. 1975, Fritts and Mech 1981, Fuller 1989). Both in
northwestern (i.e., Beltrami State Forest) and northeastern Minnesota, deer occurred in the scats of wolves
more in winter than in the summer (Thompson 1952, Pimlott et al. 1969, Van Ballenberghe et al. 1975,
Theberge et al. 1978, Fritts and Mech 1981); this is congruent with the evidence from monitoring causespecific mortality of radio-collared deer. Fawns are an important food item in the summer diet of wolves in
the Lake Superior region (Thompson et al. 1952, Pimlott et al. 1969, Frenzel 1974, Van Ballenberghe et al.
1975, Fritts and Mech 1981, Nelson and Mech 1986a, Kunkel and Mech 1994). Indeed, evidence indicates
that fawns probably account for most of the individual deer consumed by wolves during June-July, as well as
a higher proportion of the overall biomass consumed (Fritts and Mech 1981). Vulnerability of fawns to
predation by wolves and black bears (Ursus americanus) is at least partly related to their nutritional status,
which may be directly influenced by the severity of the previous winter and its effect on prenatal nutrition
(Verme 1962, Mech and Karns 1977, Mech et al. 1987, Kunkel and Mech 1994). Annual percentages of
young fawns taken by wolves (and other predators) are still relatively unknown, and additional study is
warranted.
In some areas such as northwestern Minnesota, where deer are largely the most important year-round food
for wolves, there are brief periods (April-May) when wolves rely more on moose. Certainly, moose are the
second most important food item in this area during summer and winter, with wolves consuming more
individuals and biomass during summer than winter (Fritts and Mech 1981). The relative importance of
moose in the diet of wolves that depend primarily on deer year-round may vary, dependent upon deer
densities and the degree with which factors such as winter severity and disease or parasites (e.g., winter tick
infestation, cerebrospinal nematodiasis) predispose moose to predation or scavenging by wolves.
Snowshoe hares (Lepus americanus) are consumed by wolves year-round, and although the number of
individuals consumed may be second only to deer, the biomass contributed to the diet is rather insignificant
(< 1%) (Fritts and Mech 1981, Fuller 1989). Beaver (Castor canadensis) have been found in the diet of
wolves in spring and summer (Byman, unpublished thesis, Frenzel 1974, Fritts and Mech 1981, Fuller
1989); data indicate that its importance may be greater in northeastern than northwestern Minnesota. An
increased consumption of beaver accompanied a decline in white-tailed deer populations in southern
Ontario (Voigt et al. 1976, Theberge et al. 1978). Other mammals documented in the diet of wolves include
bog lemming (Synaptomys sp.), fox squirrel (Sciurus niger), jumping mouse (Zapus hudsonius), meadow
vole (Microtus pennsylvanicus), white-footed mouse (Peromyscus sp.), and woodchuck (Marmota monax)
(Young and Goldman 1944, Fritts and Mech 1981). Additional miscellaneous foods have included black
bear, striped skunk (Mephitis mephitis), wolf, unidentified Canis, various bird species, duck and duck egg
shells, and insects. Wolves have also consumed fruits such as blueberries (Vaccinium sp.) and strawberries
(Fragaria sp.) during summer and early fall (Van Ballenberghe et al. 1975, Fritts and Mech 1981).
Livestock Depredation by Wolves
Where they coexist, wolves will occasionally prey on livestock, including cattle (primarily calves), sheep,
turkeys, swine, and goats, but generally, they occur in the wolf's diet in low amounts, both with respect to the
number of individuals and biomass consumed (Fritts and Mech 1981, Fuller 1989, Fritts et al. 1992). There
is substantial temporal and spatial variation concerning livestock depredation by wolves, with wolves in
some areas relying on domestic prey more heavily than others in certain years (Fritts et al. 1992). Most
depredations occur in northcentral and northwestern counties where farm and livestock densities are highest
within wolf range (Fritts et al. 1992). Research has indicated an inverse relationship between the severity of
the previous winter, and thus the vulnerability of newborn fawns, and the amount of depredation on livestock
(and pets) (Mech et al. 1988a, Fritts et al. 1992). Li vestock depredation by wolves tends to be higher in
some areas following a less severe winter when the nutritional status of deer fawns is better and they are
less vulnerable.
During the period 1975-1986, the wolf depredation control program verified an average 30 complaints per
year involving an average 21 farms. However, for the shorter, more recent interval 1979-1986, these figures
were 33 verified complaints and 24 farms (0.33% of farms in wolf range). During 1979-1986, average
annual, verified losses of 23 cattle, 49 sheep, and 173 turkeys were reported; most complaints occurred
during May-September. Numbers of animals wounded or killed peaked in May, July-August, and AugustSeptember for cattle, sheep, and turkeys, respectively (Fritts et al. 1992). The area affected by verified
depredations expanded 48.7% from 38,228 km 2 (14,760 mi2) in 1975-1980 to 56,827 km2 (21,941 mi2) in
1981-1986; the expansion occurred in all directions, but north (Fritts et al. 1992). When adult and yearling
wolves were removed by the contro l program, additional losses did not occur in 55% of the cases, 22% of
the cases when young of the year were removed. Removal of breeding versus nonbreeding members of
packs did not have any greater impact on subsequent losses of livestock.
From 1987 to 1997, verified complaints of wolf depredation on livestock and the number of farms involved
has tripled (Paul 1997). Specifically, in 1997, there were 109 verified complaints at 93 farms. Also during this
period, the number of wolves captured and removed from the population has increased from 30-35 per year
to just over 200 per year; the average has been 158 wolves captured per year during the past 5 years (Paul
1997). A state program which compensates farmers for livestock lost to wolves, paid an average $37,299
per year over the past 5 years. Greater detail of wolf depredation of livestock and the depredation control
program in Minnesota can be obtained elsewhere (Fritts et al.1992, Paul 1997).
Wolves and Deer---Influences at the Population Level
As is evident from the above, wolf and white-tailed deer populations are intricately linked, each having a
potentially strong influence on the other's population performance (i.e., survival rates, reproductive success).
The specific type and strength of the influences or interactions between wolf and deer populations are a
function of factors such as individual densities, sex and age structures, human-related activities (e.g.,
hunting, poaching, dogs, timber harvesting, road development, supplemental feeding), winter severity,
presence of alternate prey, and habitat quality (Mech 1986, Mech et al. 1988b). Clearly, nutrition is the basis
of this link in both directions. Wolves have a direct impact on white-tailed deer populations in being the
primary cause of natural mortality, again, with the most vulnerable individuals being newborn fawns and
fawns (>/=0.6 yr old) and older adults during winter (Mech and Frenzel 1971; Nelson and Mech 1986a,b;
DelGiudice and Riggs 1996; DelGiudice 1996). Thus far, there is little evidence to show that wolves are a
limiting factor on deer, or alone, have caused population declines (Voigt 1990). However, in combination
with severe winter conditions, overly mature habitat, and/or human exploitation, wolf predation has
contributed to locally declining deer populations such as occurred in the central Superior National Forest in
the late 1960s-early 1970s and elsewhere during winter 1995-96 (Mech and Karns 1977; DelGiudice 1996,
1998). In the central Superior National Forest and areas further north and northeast, wolves may be
affecting the recovery of the deer population (Nelson and Mech 1981). Wolf predation may represent
compensatory or additive mortality. For example, if wolves kill deer during winter that ultimately would have
died of starvation, the predation is compensatory mortality. However, if in very severe winters, wolves are
able to kill some relatively healthy deer that would have survived t he winter nutritional stress, then that
predation is additive. Generally, based upon ages of deer killed by hunters and wolves (discussed earlier), it
is believed that most deer deaths from wolves in winter are a combination of compensatory and additive
mortality, whereas, the hunter-kill tends to be more additive (i.e., prime age classes). A recent study reports
that during 1975-1997, numbers of wolves in the east-central Superior National Forest accounted for only
23-38% of the inter-year variation in buck harvest for areas in and near the 2,060 km 2 (795 mi2) wolf census
area (Mech and Nelson 1998). Year-to-year analysis of annual changes in numbers of wolves and harvested
bucks revealed inconsistent associations. Increases in annual deer harvests in much of Minnesota wolf
range, even as it expanded, indicates that generally, the wolves' effect on deer harvests has been minimal.
Presently, too little is known about t he mortality of deer neonates to speculate whether its effects are
compensatory or additive; however, preliminary evidence has indicated that some newborns killed by wolves
and bears may be compromised nutritionally (Kunkel and Mech 1994).
Because deer are the wolf's primary food across most of their range in Minnesota, it is easy to understand
the eventual, nutritional consequences for wolves of a local deer decline. The estimated minimum
maintenance requirement of food for free-ranging wolves is 1.7 kg per day of deer-equivalent biomass
(assuming an average 54.5 kg of food provided from an adult deer [portions of this can be from beaver,
snowshoe hares, moose, etc...]) (Mech 1970). Estimated daily consumption rates per wolf have ranged from
0.2 to 7.3 kg (Mech 1966, Mech and Frenzel 1971, Kolenosky 1972, Kuyt 1972, Fritts and Mech 1981, Fuller
1989). It has been estimated that a wolf requires the equivalent of 15-20 adult-sized deer per year (Mech
1971, Kolenosky 1972, Fuller 1989). However, low deer numbers; fewer old, vulnerable deer (i.e., a young
age structure); or a mild winter in a given area will influence the number of deer or biomass each wolf
actually consumes. Consequence s of submaintenance consumption and nutritional stress for wolves during
the breeding (February-March) and denning seasons (late April-May) can be increased adult mortality
(undernutrition, more trespassing and inter-pack strife), small litters, and decreased pup survival. In the past
in the central Superior National Forest, the collective effect of such dynamics involved a local wolf population
that decreased 50% in 4 years (Seal et al. 1975; Van Ballenberghe and Mech 1975; Mech 1977a,b, 1986).
This type of predator-prey relationship---a predator population contributing to the decline of its prey
population, followed by a lag period and subsequent decline in the predator population---is classical, but it is
not simple. As mentioned above, many other factors will interact, influence the specific dynamics of the
relationship (what changes occur and the magnitude of those changes) and determine the temporal scale
over which they occur.
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Grade 9 Academic Science – Ecology
Wolves – Yellowstone and Isle Royale
Smith, D.W., R.O. Peterson and D.B. Houston. 2003. Yellowstone after Wolves. Journal of Bioscience.
American Institute of Biological Sciences.
http://www.wolf.org/wolves/learn/intermed/inter_mgmt/ystone_wolves.asp
Abstract: With gray wolves restored to Yellowstone National Park, this ecosystem once again supports the
full native array of large ungulates and the attendant large carnivores. We consider the possible ecological
implications of wolf restoration in the context of another national park, Isle Royale, where wolves restored
themselves a half-century ago. At Isle Royale, with relatively few resident mammals, wolves completely
eliminated coyotes and went on to control moose population dynamics, with implications for forest growth
and composition. At Yellowstone, we predict that, to a degree, wolf restoration will have similar effects,
reducing elk and coyote density. As at Isle Royale, Yellowstone plant communities will be affected, as well
as mesocarnivores, but presently there is great uncertainty. At Yellowstone, ecosystem response to the
arrival of the wolf will take decades to unfold, and we argue that comprehensive ecological research and
monitoring should be an essential long-term component of the management of Yellowstone National Park.
Reintroduction of gray wolves to Yellowstone National Park surely ranks, symbolically and ecologically, as
one of the most important overt acts of wildlife conservation in the 20th century. Once again, Yellowstone
harbors all native species of large carnivores - grizzly and black bears, mountain lions, and wolves. Before
wolf reintroduction, there was a concerted effort to predict the ecological effects of wolves in Yellowstone
(Cook 1993). Has reality, so far, met expectations? And has what we learned in Isle Royale National Park,
where wolves introduced themselves over 50 years ago, relevance for Yellowstone in the future?
Gray wolves were restored to Yellowstone National Park during 1995-96, with the release of 31 wolves
captured in western Canada (Bangs and Fritts 1996, Phillips and Smith 1996). In the 7 years following their
initial release wolves have recolonized the 8991-km2 park and several adjacent portions of the 72,800 km2
"greater Yellowstone ecosystem (GYE)." We use initial studies and field observations to determine to what
extent wolves may have already begun to restructure the Yellowstone ecosystem. Although we consider
wolves park-wide, we focus on the 1,530 km2 "northern Yellowstone winter range," an area dominated by
steppe and shrub steppe vegetation that supports 7 species of native ungulates -- elk, bison, mule deer,
white-tailed deer, moose, pronghorn antelope, and bighorn sheep, one non-native ungulate --mountain goat,
and 5 species of native large carnivores -- gray wolf, coyote, grizzly bear, black bear, cougar. Only about
65% of the northern range is inside the park; the remaining 35% occurs on public and private lands north of
the park along the Yellowstone River (Lemke et al 1998). Since many of these wildlife species are hunted
outside the park, we include technological humans as additional, formidable, predators in the system. While
the National Park Service manages Yellowstone with an overall goal of minimal human intervention allowing natural ecological processes to prevail inside park boundaries—its wildlife populations may be
profoundly altered by human actions, including hunting, outside the park.
Simplicity and Complexity - Isle Royale and Yellowstone
We find it useful to contrast the Yellowstone system with that of Isle Royale National Park, a less complex
ecosystem renowned for long-term studies of the interaction of gray wolves with moose conducted there
(Peterson 1995, Peterson et al. 1998). Amid the complexity of Yellowstone, where might we expect to find
the ecological footprints of wolves, and where might science make its greatest gains? We anticipate that
long-term studies similar to those of Isle Royale will be required to understand the effects of wolves in
Yellowstone. We could have picked other parks - Riding Mountain in Manitoba or Denali in Alaska which are
both multi-carnivore/prey systems like Yellowstone- but long term data (turn of the century to present) on
willdife population sizes from these other areas were lacking, and we do not have intimmate experience with
these parks. Often what is important is subtle and detailed which can be the difference between an informed
conclusion and one that is not. Where appropriate we make comparisons to other wolf-prey systems.
Isle Royale and Yellowstone provide opposite extremes in faunal and food web complexity. Isle Royale is a
closed system with fewer species (one-third the species found on the adjacent mainland), and Yellowstone
is an open system with greater diversity of both predators and prey (Fig. 1). Thus, Isle Royale should be
more amenable to scientific scrutiny, with clearer cause-and-effect relationships among a few key species, a
good starting point and example for interpreting Yellowstone.
- There are surprising parallels in the history of Isle Royale and Yellowstone during the past century,
particularly in concerns raised over too many ungulates and their effects on their habitat. During a wolf-free
period, both ecosystems saw ungulates increase to levels that alarmed some knowledgeable observers, and
coyotes were numerous in both areas.
It is not only ecology that is complex at Yellowstone. Its bureaucratic history as the nation's "first" national
park (Haines 1977) is long and rich. Management of Yellowstone's wildlife, particularly on the northern
range, has a history of concern and controversy dating from the establishment of the park in 1872 (Pritchard
1999). Early on, extirpation of many native species was feared because of intense hide and market hunting.
Understandably, this period was followed by one of progressively increased husbandry of native ungulates,
which eventually involved winter-feeding and predator control. Gray wolves were effectively eliminated by
the 1930's (Weaver 1978). During the extended drought of the 1930s some ungulate species, particularly
elk, were considered to be "overabundant," and "range deterioration" became an issue. This led, in turn, to
intense and highly controversial reductions of elk, bison, and pronghorn populations by field shooting and
trapping, aimed at testing the effects of reduced ungulate densities on vegetation conditions. By the late
1960s elk numbers had been reduced by perhaps 75%, to around 4,000 animals (Houston 1982). In 1969, a
moratorium on reductions was instituted, as attempts were made to rely more upon natural regulation of
ungulate numbers within the park and to restore hunting opportunities outside (reductions inside the park
had essentially eliminated elk hunting outside). Efforts to rely on more natural processes have, in one sense,
culminated in restoration of the wolf. This brief outline of management history is treated in detail by Meagher
(1973), Houston (1982), and Pritchard (1999).
Like Yellowstone, Isle Royale had a wolf-free era that also resulted in an over-abundant moose population
(Allen 1979). Instead of artificial reductions to control moose, the Park Service unsuccessfully tried to
reintroduce zoo-raised wolves in 1952 (Allen 1979). But unlike Yellowstone, wolves reintroduced themselves
to Isle Royale in the late 1940s by crossing the ice of Lake Superior (Allen 1979). What did the arrival of the
wolf mean for the Isle Royale ecosystem? While scientific uncertainty continues on the relative roles of
bottom-up (nutrition/vegetation) and top-down (wolf predation) influences on moose population dynamics
(Messier 1994, Peterson 1995), the historic chronology of moose numbers indicates that wolf predation
tends to cap moose density (Fig. 2). The peak in moose numbers in the early 1970s ended when severe
winters affected vulnerability (Peterson 1977), and the resulting increase in wolves kept the moose
population low for many years. More wolves indirectly allowed forest recovery via less moose browsing (topdown; McLaren and Peterson 1994). However, when wolves crashed in the 1980s, from 50 to 14 in two
years, and were limited because of a canine parvovirus, an accidentally and human introduced disease
(Peterson 1995), moose increased until catastrophic starvation hit in 1996 (one of the most severe winters
on record; Peterson et al. 1998).
The rise and fall of Isle Royale's wolf population can be read in growth rings of balsam fir trees - trees
flourish when wolf numbers increase and moose are reduced (McLaren and Peterson 1994, McLaren 1996).
The relative abundance of coniferous and deciduous trees, strongly influenced by moose browsing, further
affects litter composition and nutrient cycling in the soil, so the ripple effect beginning with the arrival of
wolves extends far and wide (Pastor et al. 1993). But it is not that simple. On one-third of Isle Royale, fir
trees are able to escape moose browsing (because of thick, high density stands) and grow into the canopy,
but on the majority of the island, balsam fir trees are unable to grow out of the reach of moose (McLaren and
Janke 1996). Hence, moose remain a powerful force shaping forest succession, even with intense wolf
predation. Variations in soil types, disturbance history (fire and wind) and light intensity complicate a system
that, in comparison to Yellowstone, is easily understood. Even after a century with moose, the forest of Isle
Royale has not yet reached equilibrium. One needs a long-term perspective and study to understand
completely the dynamics of long-lived plants and animals. In the public perception, however, the arrival of
wolves solved the problem of an overpopulation of moose.
Another Look at Predictions
- Will wolves stabilize prey fluctuations in Yellowstone, especially those of elk (Boyce 1993), or will wolves
destabilize elk fluctuations, exacerbating population fluctuations (National Research Council 2002)? How far
will the ecological ripple extend? Moose no longer number 3,000 on Isle Royale like they did before wolves
(Allen 1979), so will elk ever exceed 19,000 again like they did before wolves and after the artificial
reductions in Yellowstone?
- Before wolf introduction, several studies used modeling to predict future impacts of wolves on the
Yellowstone ecosystem (Yellowstone National Park et al. 1990, Varley and Brewster 1992, Cook 1993, U.S.
Fish and Wildlife Service 1994). These were comprehensive efforts, prepared for Congress and the general
public, which focused on the interaction of wolves with native ungulates, livestock, and grizzly bears.
Simulations predicted between 50-120 resident wolves in YNP, with packs on the northern range, MadisonFirehole, and possibly the Gallatin and Thorofare areas (Fig. 3; Cook 1993:254-256).
All models suggested that elk would constitute the primary prey for Yellowstone wolves. Four models dealt
with the impact of wolves on native ungulates (Vales and Peek 1990, Garton et al. 1992, Boyce 1993, Mack
and Singer 1992, 1993); no simulation predicted large declines in ungulates following wolf restoration. The
northern Yellowstone elk population was predicted to decline 5-30% over the long term, with levels of
decline contingent on the extent of hunter harvest of female elk outside the park (Mack and Singer 1993,
Boyce 1993). Boyce (1993) suggested that some reduction of hunter kill of cow elk outside the park might be
necessary over time, but restrictions on bull harvests would be unnecessary. Significant effects on other
prey species (bison, moose, and mule deer) were not anticipated.
In contrast to most predictions based on modeling, Messier et al. (1995) suggested that elk might decline
substantially following wolf recovery because of the number of predator species involved. In boreal
ecosystems where moose deal with multiple predators, moose density typically declines with each additional
carnivore species (including human hunters; Gasaway et al. 1992). According to this thinking, the
exceptionally high density of moose at Isle Royale (averaging about 2 per km2) occurs because there is only
one predator - the wolf. Where wolves and bears coexist, calf survival is consistently reduced, and moose
density is always less than 1 per km2, usually less than 0.4 per km2 (Messier 1994). The only geographic
region where moose density is comparable to that of Isle Royale is Fennoscandia, where humans are the
predominant predator species (bears have a minor presence), or the Gaspe Peninsula in New Brunswick,
where there are black bears but no wolves and no hunting is allowed.
Messier et al. (1995) believed that Yellowstone elk would decline significantly, more than the 5-30%
predicted by Boyce (1993) and Mack and Singer (1993), especially where human hunting of cow elk was
permitted. Focusing on the northern Yellowstone elk herd, at a pre-wolf winter elk density of >10 per km2,
they anticipated that elk numbers would decline during the inevitable severe winters and would not rebound
because of relatively low calf survival. What will be critical for elk recovery after declines will be the level of
human hunting of elk outside the park, the only mortality factor that can be completely managed.
Both the historical record at Isle Royale and the predictions of Boyce (1993) for the northern Yellowstone elk
underscore the dynamic future that will follow wolf recovery. Fluctuations in wildlife populations are normal;
the renowned "balance of nature" at Isle Royale is decidedly dynamic. Wolf peaks lag behind those of prey,
and wolf declines follow prey declines. In the past four decades, two major declines in moose at Isle Royale
have occurred when severe winters coincided with high moose density (> 3 per km2; Peterson 1995).
Predictions for the wolf-prey system at YNP were similarly variable over time (Boyce 1993).
Media attention and scientific debate have focused heavily on population size for northern Yellowstone elk.
Average population size is an interesting statistic, but no one should expect elk to spend any time there. At
most times, they will either be increasing or decreasing, and at any given time wolves and elk will probably
show opposite trends.
Isle Royale moose have spent more time below the population mean, probably because of suppression by
wolves. Possibly this reflects the resilience of wolves in the face of prey decline, and the anti-regulatory
(inversely density-dependent) influence of wolf predation that has been noted by wildlife managers in Alaska
(Gasaway et al. 1992). An important question for Yellowstone, however, is to what extent will wolves prey on
bison, a more formidable prey species that is more difficult to kill (Smith et al. 2000). If wolves do utilize
bison, which are widespread and abundant at 4,000 animals, that will certainly change predictions of wolf
impacts on elk.
For the threatened grizzly bear population of GYE, wolf restoration was predicted to have either no impact or
a slightly positive impact (Servheen and Knight 1993). Wolf predation on bear cubs was expected to be
offset by better feeding conditions as bears usurp wolf-kills (Servheen and Knight 1993). Carcasses would
be more evenly distributed for bears throughout their seasons of activity, rather than coming as a pulse in
late winter and early spring—the pre-wolf condition. Bears would not have to risk killing elk themselves, but
could scavenge kills of wolves well distributed in space and time.
Although there was a general awareness of interspecific competition among native canids when the effects
of wolf reintroduction were being assessed a decade ago, there were few predictions about exactly what
wolf recovery would mean for coyotes, which on the northern range existed at one of the highest densities
known for the species (Crabtree and Sheldon 1999). Some predicted wolves would reduce coyotes and that
coyote reduction would affect other species (YNP et al. 1990, Varley and Brewster 1992). On Isle Royale,
where wolves and coyotes competed for all the same prey species, wolves eliminated coyotes in about 8
years (Mech 1966).
Prior to wolf reintroduction in Yellowstone, there were no predictions about possible responses in northern
range vegetation caused by changes in distribution or density of ungulates, particularly elk. The forage for
most ungulates wintering on the northern range--elk, bison, mule deer, bighorn sheep, pronghorn--is
produced primarily in the extensive grasslands and shrub steppes. Grasslands are dominated by native
species, although several alien grasses have been introduced (both accidentally and deliberately) and
dominate local sites (Yellowstone National Park 1997, Stohlgren et al. 1999). A series of studies suggests
that this grazing system is stable and highly productive; ungulate herbivory accelerates nutrient cycling and
actually enhances productivity of the range (Houston 1982). Long-term changes in the vegetation (increased
distribution and density of coniferous forests, increased abundance of big sage, decline in aspen and willow
communities) seem to be associated with herbivory and suppression of natural fires, which occurred during
a shift to a warmer, dryer climate (Meagher and Houston 1998). It's worth noting, however, that aspen and
willow are minor components of northern range vegetation (less than 1 or 2 %); the difficulty of basing
management of the larger grazing system on minor components of the vegetation is explored by Meagher
and Houston (1998:247).
Another unresolved point, far too complex to realistically simulate, is the productivity of the northern range,
which nourishes the elk in winter. This is a unique north temperate grassland, one that has been compared
to Africa's Serengeti. A much higher proportion of plant biomass can be consumed by ungulate grazers than
ungulate browsers, which depend on annual growth of twigs and buds of woody shrubs. It is possible that
the bottom-up stimulation of productivity from this grassland system will sustain elk at high density with a full
suite of predators, both wild and human. A review committee of eminent scientists recently focused on the
condition of the northern range (National Research Council 2002), concluding that high ungulate density was
not causing irreversible damage to this ecosystem. Now that wolves are present, this committee firmly
endorsed the scientific imperative to monitor ecosystem status closely.
The Unfolding Yellowstone Story
Wolf numbers. As we write (summer 2002), at least 216 free-ranging wolves (pre-pup 2002) can be found in
the GYE, with about 14 packs (132 wolves) holding territories in or mostly within YNP, and 14 packs (84
wolves) outside (Fig.4). About 77 wolves (in 8 packs) occur on the northern range (very close to the number
predicted for this area (Boyce 1993, Mack and Singer 1993)). Initial rate of increase for the wolf population
was very high (Fig. 5), but now population growth within YNP has slowed, and most recent increases have
occurred outside the park. Here we summarize the current status of wolves and their primary prey, the
northern Yellowstone elk, and note some preliminary observations of other selected species affected by wolf
recovery.
Wolf territories. The northern range, targeted during reintroduction, is well-occupied by wolves, and virtually
all potential wolf habitat in the park is occupied to some extent, including several areas that may not prove
suitable for long-term occupancy (Fig. 4). Wolf packs have established year-round territories, despite the
seasonally migratory nature of their ungulate prey. This was an important uncertainty before wolf
introduction (Boyce 1993). The territories have been quite labile, and further subdivision seems likely,
especially for the very large Druid Pack (37 wolves in August 2000, split into 4 packs April 2002), which now
dominates much of the northern range in the park and has forced some packs into peripheral areas. Across
the park, wolf packs exist approximately in the places predicted by Boyce (1993; Fig. 3).
Wolf-prey relationships. As expected, elk are the primary prey for wolves in the park year round,
representing 92% of 1,582 wolf kills recorded from 1995-2001. As elsewhere, wolf predation in winter has
been highly selective; calves represent about 43% of wolf-killed elk, cows 36 %, and bulls 21 % (compared
to the approximate winter population proportion of 14 % calves, 60 % cows, and 23% bulls). The adult elk
killed by wolves have been very old, with a mean age of 14 years for wolf-killed cow elk (Mech et al. 2001).
In contrast, human hunters outside the park kill female elk in their reproductive prime, at an average age of 6
years. Bull elk killed by wolves are taken primarily in late winter and average 5 years old, which is the same
average age for hunter-killed bull elk. Examinations of femur marrow from the wolf-killed elk on the northern
range indicate that 34% (N = 494) had exhausted all fat reserves.
Although elk represent the primary prey for wolves throughout the park, bison are taken during late winter in
interior portions of YNP (Smith et al. 2000) and moose are important along the southern boundary. Yet
neither of these species represents more than 2% of wolf diet in winter (although in some areas during late
winter it is higher). While wolves have killed some bison (Smith et al. 2000), so far most Yellowstone packs
are supported almost entirely by elk.
Coyotes: Before wolf reintroduction, coyote population density on the northern range was about 0.45 per
km2, organized as packs with well-established borders (Crabtree and Sheldon 1999). Wolves began to kill
coyotes soon after they were released in YNP. During 1996-1998 wolf aggression toward coyotes resulted in
a 50 percent decline in coyote density (up to 90% decline in core areas occupied by wolf packs) and
reduced coyote pack size on the northern range (Crabtree and Sheldon 1999). In the Lamar Valley of the
northern range the coyote population declined from 80 to 36 animals from 1995 to 1998, and average pack
size dropped from 6 to 3.8 animals (Crabtree and Shelton 1999). With lower coyote density, litter size
increased, but the increased production of pups has been insufficient to offset the effects of wolves.
Although data are preliminary, pronghorn fawn survival seems positively correlated with wolf density and
inversely correlated with coyote density, as most fawn mortality is caused from coyote predation (J. Byers
personal communication).
In about 84% of 145 wolf-coyote interactions observed at wolf kills, wolves prevailed over coyotes. Wolf kills
clearly provide food for coyotes (virtually all winter kills are visited by coyotes), but coyotes that scavenge
wolf-kills risk death from wolves.
Scavengers. Besides coyotes, nine other scavenger species have been observed using wolf kills. All wolf
kills are visited by ravens, magpies, and eagles. Many kills in the non-winter months are visited by both
species of bears. In winter wolf kills are tremendous centers of activity for scavengers, and small packs of
wolves lose large amounts of food to scavengers (Hayes et al. 2000). Kills are especially important to ravens
-- the average number of ravens per wolf kill was 29 and the most recorded was 135, a record in the
literature (Stahler et al. 2002). Ravens follow wolves and discover carcasses immediately, or even before,
flying overhead as wolves pursue and kill prey (Stahler et al. 2002).
Grizzly bears. The grizzly bear population in the GYE has increased dramatically since the 1970's, although
the bears are still listed as "threatened" under provisions of the Endangered Species Act. The population
was estimated at 354 bears, including 35 sows with cubs at heel during 2001 (Haroldson and Frey 2001).
Fifty-eight wolf-bear interactions have been recorded in YNP. Most interactions occur at wolf kill sites, where
control of the carcass is hotly contested; typically bears win the encounter even though outnumbered by
wolves. In one case a bear held 24 wolves at bay. Although fully capable of killing ungulates, especially in
spring, grizzly bears now appear to seek out wolf kills and are often successful at driving wolves from
carcasses.
Cougars. The cougar population on the northern range has been monitored intensively through most of the
1990s (Murphy 1998). The present population on the northern range, roughly 25 animals, appears to have
slowly increased during the 1990s and in the presence of wolves (T. Ruth, personal communication).
Documented interactions between wolves and cougars have been rare, seemingly because of separation in
habitat used by the species (cougars inhabit rock outcrops and cliffs along rivers). Field observations
suggest that cougars avoid wolves, are subordinate at carcasses, and are at risk of predation. In one
incident four cougar kittens were killed by wolves (T. Ruth, personal communication).
Mesocarnivores. The effect of wolves on these animals has yet to be documented; we indulge in some
speculation, however. Yellowstone has robust populations of some mid-sized carnivores (weasels, marten,
badger), but low populations of others (fishers, wolverines, red fox, lynx, bobcat, otter). Several species may
benefit from the advent of wolves. The red fox, for example, which competes more closely with coyotes than
wolves, may increase because of lower numbers of coyotes. Wolverines, which scavenge carcasses, may
also increase.
Elk. From 1981-82 through 1994-95, winter numbers for the northern Yellowstone herd averaged 15,520+
2,324 (SD) elk (Lemke et al 1998). Annual hunter harvests out side the park during the same 14 year period
were variable but averaged 1,823+ 1,022 elk, including 1,192+ 661 animals taken during the late hunt (65%
of the total harvest; Fig. 6). The late hunt targets "antlerless" elk (females and calves of the year) that
migrate from YNP; most elk harvested each year are females, followed by calves, with a quota limiting bull
harvests to around 100 animals (Lemke et al 1998).
No elk counts were made during the 1995-96 or 1996-97 winters, just after wolves were released on the
northern range (Fig. 6). The winter of 1996-97 was severe; rain on deep snow during December and
January was followed by subzero temperatures, sealing off the supply of winter forage. Record ungulate
migrations from YNP were documented and large numbers perished. A count in the following January, 1998,
tallied 11,736 elk in the northern herd, lower even than after the drought, fires, and severe winter of 19881989. Counts for the next 4 winters, 1999 through 2002, ranged from 11,742 to 14,539 (Fig. 6). While the elk
population appeared to rebound slightly after the severe winter of 1996-1997, the pace of recovery was
evidently very slow compared to the one that followed the dieoff in 1989. Already, in the media, attention has
abruptly switched from concern about too many elk for the northern range to too few elk for human hunters
outside the park.
It's worth noting that elk are the main prey for cougars, and cougars have a greater per capita kill rate than
do wolves (Murphy 1998). Cougars kill an elk about every 9 days throughout the year (Murphy 1998); in
winter a wolf kills an elk about every 15 days on average (Mech et al. 2001; summer wolf kill rates are
unknown). Elk calves are also seasonally important in the diet of coyotes and grizzly and black bears (Clark
et al. 1999). Grizzly bears also kill adult elk and bison. Coyotes were estimated to take over 1,200 elk
annually from the northern herd (about the same number as grizzly bears and cougars combined) before
wolf restoration (Crabtree and Sheldon 1999). For all these predators, elk calves are a major dietary
component. Thus, in a very real sense, the abundance and survival of cow elk, through their annual
production of young, support major links in the Yellowstone food web and will determine the trajectory of the
elk population in the future.
Population data for the non-migratory Madison-Firehole elk in YNP, which numbers about 500 animals,
suggests that their numbers have been relatively stable since wolf reintroduction (Eberhardt et al. 1998).
This herd, surviving near warm geothermal "oases" in a region with very deep snow, could be at risk from
wolf predation if packs hunt more intensively in that area. Data are sparse for the other 6 migratory herds
that occupy YNP during summer (total summer elk population is approximately 30-35,000 elk) but do not
suggest herd declines after wolf arrival.
Bison: About 4,000 bison occur park wide, with about 600-700 wintering on the northern range. Bison
management has been controversial because some animals harbor the bacterial disease brucellosis, and
there is a remote chance the disease could be transmitted to livestock when bison migrate from the park
(U.S. Department of the Interior-National Park Service 2000). Wolves kill bison (Smith et al. 2000), but
predation on bison is not yet widespread. One pack in Pelican Valley utilizes bison during late winter when
bison are vulnerable and migratory elk are unavailable (Smith et al. 2000). Wolves are much less successful
killing bison than killing elk; most bison stand their ground when confronted and this behavior seems to pose
great difficulty for the attacking wolves. Bison carrion has been important wolf food during summers in the
Lamar and Hayden Valleys whenever bulls die from injuries received during the rut. Bison carcasses attract
concentrations of carnivores; including wolves, which scavenge extensively. Two grizzly bear cubs have
probably been killed by wolves at these bison carcasses.
Moose: The moose population on the northern range, numbering only a few hundred, declined precipitously
following the 1988 fires (Tyers and Irby 1995). This occurred because subalpine fir forests burned
extensively, eliminating for many decades—if not centuries--- these high-elevation moose habitats used in
winter. Only 26 instances of wolf predation on moose have been recorded since wolf restoration.
Bighorn Sheep. We do not expect wolves to affect the small population of about 175 bighorn sheep. Only
one kill has been recorded since wolf reintroduction, and wolves spend very little time in the steep terrain
commonly frequented by sheep.
Deer. We also do not expect wolves to significantly affect deer populations. The park does not contain good
white-tailed deer habitat, and the low numbers have not changed following wolf reintroduction. Wolves are
not known to have killed any in the park. Mule deer are abundant numbering about 2-3,000 but migrate out
of the park in winter, escaping some of the most intense wolf predation. Additionally, many mule deer winter
in close association with humans, largely avoided by wolves.
Beaver. Beaver are widely but patchily distributed in Yellowstone; most areas have few to none, although
they are abundant in the Yellowstone River delta (south of Yellowstone Lake) and along the boundary of the
park north of West Yellowstone, Montana. Systematic ground surveys began in 1988 and have continued at
5-year intervals; aerial surveys began in 1996 and have continued in alternate years. The 2001 aerial survey
revealed 77 colonies distributed across the park.
During 1996, shortly following wolf reintroduction, there were no documented beaver colonies on the
northern range. Since then, beavers have established 4 colonies in this portion of YNP, following recent
beaver reintroductions on the adjacent Gallatin National Forest (D. Tyers, U.S. Forest Service personal
communication). Only two wolf pack territories contain substantial populations of beaver, the Yellowstone
Delta and Cougar Creek packs (Fig. 4). Wolves likely prey on beavers, as elsewhere, but we have
documented only one beaver kill, and have only rarely found beaver remains in wolf scats (but these two
packs are not well sampled). Beavers are also closely associated with willow communities, a situation with
relevance to wolves (see below).
Vegetation: Interesting changes in willow and aspen growth have occurred in the late 1990s. Increased
height of some aspen stands has been attributed to elk redistribution following the arrival of wolves (Ripple
et al. 2001), but the initial trend has ceased. Some stands of willow have also increased in stature, but it is
still too early to know if this is due to wolves. Recovery of woody plants would be consistent with the
speculation that some cottonwood stands dating from the 1880s could not recover until elk numbers were
reduced or displaced, although this may be too simple an interpretation as there has been strong debate on
this issue (Meagher and Houston 1998, National Research Council 2002). No large stands of cottonwoods
have been established on the northern range in the past 120 years. Currently very rare, but in some places
increasing since wolf reintroduction, willow and aspen are important for many bird species, small mammals,
beavers and moose. Ongoing research will continue to sort through this very complex issue, one that could
be the most important impact of wolf reintroduction to Yellowstone.
What Next for Yellowstone? Science Amid Management and Controversy
Predicting the future of the Yellowstone ecosystem is professionally hazardous--some would say foolhardy.
Nevertheless, some initial patterns have emerged, and we deem this preliminary look ahead worthwhile.
Predictions help frame research agendas and prepare the public, perhaps lessening some of the
controversy that seems to engulf the management of Yellowstone.
At both Yellowstone and Isle Royale, before the arrival of wolves, the condition of the vegetation was a
source of concern and disagreement (Mech 1966, Pritchard 1999, National Research Council 2002). The
controversy evaporated at Isle Royale when wolves were established and a natural "balance" was assumed
to exist, although some people feared the wolves would kill all the moose and then start in on the people
(Isle Royale National Park files, letter to the Superintendent, 1956). Similar letters in numerous newspapers
in the Yellowstone area have appeared regularly. Based on the Isle Royale experience, we anticipate that
the condition of the vegetation on the northern range will subside as a popular debate topic in the media and
that, in time, the fear that wolves will kill all the elk will also be put to rest.
Much will depend on the population trajectory for Yellowstone wolves. At what point will wolves have
saturated YNP, particularly the northern range, and how will we know when that point is reached? There are
indications that rapid population growth for wolves on the northern range has ceased, but wolves should
continue to increase until chronic food limitation is evidenced through declining numbers and weights of
pups and intraspecific killing. While this is hardly the case at present, body weight for young wolves born in
the Druid pack was lower in 2002 than in previous years. Field evidence that wolves are approaching their
carrying capacity at Yellowstone is also suggested by increased levels of intraspecific strife in 2001 and
2002 and by a plot of wolf locations that indicates little vacant territory (Fig. 4). Isle Royale history and other
studies indicates that there will be continual flux in pack territorial relationships (Fritts and Mech 1981, Mech
et al. 1998).
Wolf-prey ratios may help indicate when wolves have saturated their Yellowstone niche. After wolf recovery
the average predicted elk:wolf ratio was 166, based on specific examples illustrated by Boyce (1993) for
100-year simulations, very similar to the elk:wolf ratio of 154 actually observed in 2002. At Isle Royale,
during 1959-2002, predator and prey have fluctuated around a mean moose/wolf ratio of 46, but this figure
has fluctuated from 18 to 145 (Peterson et al. 1998). We speculate that the difference in prey-to-predator
ratios between Isle Royale and Yellowstone may arise from the differences in body size (moose are twice as
large as elk) and social behavior (elk live in groups, moose live singly). We stress that it will be some time
before anything resembling a dynamic equilibrium at Yellowstone can be actually documented.
Before 1995, Isle Royale supported one of the highest year-round densities of wolves, 42 wolves/1000 km2
in the world. By 2002, wolves on Yellowstone's northern range had already reached a density of 50
wolves/1000 km2. Boyce (1993) predicted a mean population size of 76 wolves in YNP, primarily in the
northern range, and a range from 50-120 under most management scenarios; in 2002 there were 77 wolves
using the northern range and 132 wolves within YNP. The wolf population might overshoot equilibrium levels
in a manner analogous to the irruptive pattern commonly observed for ungulate populations colonizing from
low densities. The density reached by wolves in their initial period of rapid growth may indeed be very high,
but this says little about the nature of the equilibrium which will be attained, perhaps decades hence.
We are confident that form and function of the Yellowstone ecosystem will change because of wolf recovery.
Reductions in the coyote population have already occurred, and elk numbers rebounded less after the
severe winter of 1996-1997 but not to a degree that threatens the survival of either species in Yellowstone.
A resulting trophic cascade will reverberate through the ecosystem. From the complicated food web that
exists inYellowstone (Fig. 1), it is not hard to imagine that indirect effects of wolf recovery will be substantial.
While riparian willow habitats form a very small part of the northern range, any reestablishment of these
woody shrub communities would increase biodiversity.
Although our expectations for wolf effects in Yellowstone are based on inferences from other studies, and
may seem self-evident, we realize that specific predictions may be wrong. Even in a system as simple as
Isle Royale, predictability has been poor following four decades of scientific scrutiny; none of the
expectations for the moose herd, voiced in turn by Mech (1966), and Peterson (1977) actually happened.
Rather, external forces such as severe winters, summer heat and outbreaks of winter ticks (driven by warm,
dry spring weather) have caused the moose population to decline (DelGiudice et al. 1997). Surprises, as in
the arrival of exotic disease which caused a wolf crash at Isle Royale in the early 1980s, are virtually
guaranteed in the long term, and they will assuredly influence, and possibly determine, the outcome of the
great natural experiment in wolf-elk dynamics now launched at Yellowstone.
Science will be challenged to clarify exactly which changes in Yellowstone have been prompted by the
addition of wolves. No wildlife population response at Yellowstone can be attributed to the actions of just one
species (although coyotes may be an exception) or just one external event - such simplification of cause and
effect is rarely possible in the science of ecology. Large perturbations, as with unique weather-driven events,
will loom large in the future of Yellowstone. The 1988 fires burned about 36% of the land area of the park,
affecting forage supplies for native ungulates (positively and negatively), but there is plenty of room for
future fires in a climate that seems more conducive to large conflagrations. Given time, the severe winter of
1996-1997 will be matched and exceeded. Climate change will magnify the scientific challenge (National
Research Council 2002). The danger we perceive is that all changes to the system, now and in the future,
will be attributed entirely to restoration of the wolf. Testing research hypotheses is particularly difficult in
natural areas, where experimental manipulations are limited and controls are either absent or difficult to
establish. In particular, the problem of multiple causation has plagued the testing of hypotheses in ecology
and has frequently confounded inferences derived from field studies (National Research Council 1997). This
stumbling block may be particularly troublesome in the complex Yellowstone ecosystem; the need to design
research hypotheses that discriminate among potential competing causes is very real.
As in the past, elk management decisions outside the park will influence future population levels of elk inside
the park. Some curtailment of midwinter shooting of cow elk outside the park might be necessary because
wolves and humans, with very different hunting strategies, nevertheless compete over common prey.
Successful coexistence of wolves and human hunters is a management conundrum that will test wildlife
managers and challenge long-held beliefs. This is not the first time that the Yellowstone ecosystem has led
us into uncharted waters, and our responses to this latest natural experiment will be no less interesting than
the welter of ecological effects.
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