With gray wolves restored to Yellowstone National Park, this ecosystem once again supports the full native array of large ungulates and their 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, where resident mammals are relatively few, wolves completely eliminated coyotes and went on to influence moose population dynamics, which had implications for forest growth and composition. At Yellowstone, we predict that wolf restoration will have similar effects to a degree, reducing elk and coyote density. As at Isle Royale, Yellowstone plant communities will be affected, as will mesocarnivores, but to what degree is as yet undetermined. 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.
The reintroduction of gray wolves to Yellowstone National Park (YNP) surely ranks, symbolically and ecologically, among the most important 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 does what we have learned in Isle Royale National Park, where wolves introduced themselves over 50 years ago, have relevance for Yellowstone in the future?
Gray wolves were restored to Yellowstone National Park in 1995-1996 with the release of 31 wolves captured in western Canada. In the 7 years following their initial release, wolves have recolonized the 8991-square-kilometer park and several adjacent portions of the 72,800 km greater Yellowstone ecosystem (GYE). We use initial studies and field observations to determine the extent to which wolves may have already begun to restructure the Yellowstone ecosystem.
Although we consider wolves throughout the park, we focus on the 1530 km northern Yellowstone winter range, an area dominated by steppe and shrub steppe vegetation that supports seven species of native ungulates (elk, bison, mule deer, white-tailed deer, moose, pronghorn antelope, and bighorn sheep), one nonnative ungulate (mountain goat), and five species of native large carnivores (gray wolf, coyote, grizzly bear, black bear, and cougar). Only about 65% of the northern range is inside the park; the remaining 35% is on public and private lands north of the park along the Yellowstone River.
Because many of the wildlife species on the northern range are hunted outside the park, we include humans as additional, formidable predators in the system. Although the National Park Service manages Yellowstone with an overall goal of minimal human intervention, allowing natural ecological processes to prevail inside park boundaries, 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. 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.....Where appropriate, we make comparisons to other wolf-prey systems....
There are surprising parallels in the histories 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 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. 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 increasing husbandry of native ungulates, which eventually involved winter feeding and predator control. Gray wolves were effectively eliminated by the 1930s. 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 4000 animals (Houston 1982). In 1969 a moratorium on reductions was instituted in an attempt to rely more on natural regulation of ungulate numbers within the park and to restore hunting opportunities outside (reductions of elk within the park had essentially eliminated elk hunting outside). Those efforts to rely on more natural processes have, in one sense, culminated in restoration of the wolf....
Like Yellowstone, Isle Royale had a wolf-free era, which resulted in an overabundant moose population. Instead of artificial reductions to control moose, the Park Service tried unsuccessfully to reintroduce zoo-raised wolves in 1952. But unlike in Yellowstone, wolves reintroduced themselves to Isle Royale in the late 1940s by crossing the ice of Lake Superior. What did the arrival of the wolf mean for the Isle Royale ecosystem? Although the relative roles of bottom-up (nutrition and vegetation) and top-down (wolf predation) influences on moose population dynamics are not fully understood, the historic chronology of moose numbers indicates that wolf predation tends to cap moose density. The growth in moose numbers peaked in the early 1970s and ended when severe winters affected vulnerability, and the resulting increase in wolves kept the moose population low for many years. The greater number of wolves indirectly allowed forest recovery by reducing browsing by moose. However, when wolves crashed in the 1980s -- from 50 to 14 in 2 years -- and were limited because of a canine parvovirus, a disease accidentally introduced by humans, moose numbers grew until catastrophic starvation hit in 1996 (one of the most severe winters on record).
The rise and fall of Isle Royale's wolf population can be read in the growth rings of balsam fir trees-trees flourish when wolf numbers increase and moose are reduced. The relative abundance of coniferous and deciduous trees, which is 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....
....Variations in soil types, disturbance history (fire and wind), and light intensity complicate a system that, in comparison with Yellowstone, is easily understood. Even after a century with moose, the forest of Isle Royale has not reached equilibrium. One needs a long-term perspective and study to completely understand 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, or will wolves destabilize elk fluctuations, exacerbating population fluctuations?....
Before wolf reintroduction, several studies used modeling to predict the future impacts of wolves on the Yellowstone ecosystem....All models suggested that elk would constitute the primary prey for Yellowstone wolves. Four models dealt with the impact of wolves on native ungulates (Carton et al. 1990, Vales and Peek 1990, Mack and Singer 1992, 1993, Boyce 1993); no simulation predicted large declines in ungulates following wolf restoration. The northern Yellowstone elk population was predicted to decline 5% to 30% over the long term, with levels of decline contingent on the extent of hunter harvest of female elk outside the park. Boyce (1993) suggested that some reduction in the number of cow elk killed by hunters 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 and colleagues (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 km) 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 km and usually less than 0.4 per km.....
Messier and colleagues (1995) believed that Yellowstone elk would decline significantly, more than the 5% to 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 prewolf winter elk density of more than 10 per km, 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. 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 antiregulatory (inversely density-dependent) influence of wolf predation that wildlife managers in Alaska have noted. An important question for Yellowstone, however, is to what extent wolves will prey on bison, a more formidable-and more difficult to kill-prey species. If wolves do prey on bison, which are widespread and abundant (4000 animals), predictions of wolf impacts on elk will certainly change.
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. 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 prewolf condition. Bears would not have to risk killing elk themselves but could scavenge wolf kills, which are 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. Some predicted that wolves would reduce coyotes and that the coyote reduction would affect other species. On Isle Royale, where wolves and coyotes competed for all the same prey species, wolves eliminated coyotes in about 8 years.
Before the reintroduction of wolves 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. A series of studies suggests that this grazing system is stable and highly productive....
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 by ungulate browsers, which depend on the 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....
The unfolding Yellowstone story
In the summer of 2002, at least 216 free-ranging wolves (before the 2002 birth of pups) could be found in the GYE, with about 14 packs (132 wolves) holding territories in or mostly within YNP and 14 packs (84 wolves) outside. About 77 wolves (in 8 packs) occur on the northern range (very close to the number predicted for this area). The initial rate of increase for the wolf population was very high but population growth within YNP has slowed now, 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 wolf reintroduction, is well-occupied by wolves: 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. Wolf packs have established year-round territories, despite the seasonally migratory nature of their ungulate prey....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 four packs as of April 2002), which now dominates much of the northern range in the park and has forced some packs into peripheral areas....
Wolf-prey relationships. As expected, elk are the primary prey for wolves in the park year-round, representing 92% of 1582 wolf kills recorded from 1995 to 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 with the approximate winter population proportion of 15% calves, 60% cows, and 25% bulls). The adult elk killed by wolves have been very old, with a mean age of 14 years for wolf-killed cow elk. 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 as 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 the wolf diet in winter, though the figure is higher in some areas during late winter. Although wolves have killed some bison, 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 km^, organized as packs with well-established borders. Wolves began to kill coyotes soon after they were released in YNP. During 1996-1998, wolf aggression toward coyotes resulted in a 50% decline in coyote density (up to a 90% decline in core areas occupied by wolf packs) and reduced coyote pack size on the northern range ....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 by coyote predation....
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 nonwinter months are visited by both species of bears (grizzlies and black bears). In winter, wolf kills are tremendous centers of activity for scavengers, and small packs of wolves lose large amounts of food to scavengers. Kills are especially important to ravens -- the average number of ravens per wolf kill was 29 and the largest number recorded was 135, a record in the literature (Stahler et al. 2002). Ravens follow wolves and discover wolf kills immediately, or even before the kill as they fly overhead while wolves pursue their prey.
Grizzly bears. The grizzly bear population in the GYE has increased dramatically since the 1970s, although the bears are still listed as threatened under provisions of the Endangered Species Act. In 2001 the population was estimated at 354 bears, including 35 sows with cubs at heel. 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 prevail in the encounter even though wolves outnumber them. 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. The present population on the northern range, roughly 25 animals, appears to have slowly increased during the 1990s and in the presence of wolves. Documented interactions between wolves and cougars have been rare, seemingly because of separation of the habitats used by the two species (cougars inhabit rock outcrops and cliffs along rivers). Field observations suggest that cougars avoid wolves, are subordinate at kill sites, and are at risk of predation. In one incident, four cougar kittens were killed by wolves.
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 midsized 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 with wolves, may increase because of lower numbers of coyotes. Wolverines, which scavenge carcasses, also may increase.
Elk. From 1981-1982 through 1994-1995, winter numbers for the northern Yellowstone elk herd averaged 15,520 (+ or - 2324, standard deviation). Annual hunter harvests outside the park during the same 14-year period were variable but averaged 1823 elk, including 1192 animals taken during the late hunt (65% of the total harvest). 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.
No elk counts were made during the winters of 1995-1996 and 1996-1997, just after wolves were released on the northern range (figure 6). The winter of 1996-1997 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....While the elk population appeared to rebound slightly after the severe winter of 1996-1997, the pace of recovery was evidently very slow compared with the one that followed the die-off in 1989. Already media attention has abruptly switched from concern about too many elk for the northern range to concern about too few elk for human hunters outside the park.
It is 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. In winter, a wolf kills an elk about every 15 days on average; summer wolf kill rates are unknown. Elk calves are also seasonally important in the diet of coyotes and grizzly and black bears. Grizzly bears also kill adult elk and bison. Coyotes were estimated to take more than 1200 elk annually from the northern herd (about the same number as grizzly bears and cougars combined) before wolf restoration. 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 approximately 500 nonmigratory Madison-Firehole elk in YNP suggest that their numbers have been relatively stable since wolf reintroduction....Data are sparse for the other six migratory herds that occupy YNP during summer (the total summer elk population is approximately 30,000 to 35,000), but they do not suggest that herds have declined since the arrival of wolves.
Bison. About 4000 bison occur throughout the park, with about 600 to 700 wintering on the northern range....Although wolves do kill bison (Smith et al. 2000), predation on bison is not yet widespread....Wolves are much less successful at killing bison than at killing elk; most bison stand their ground when confronted and this behavior seems to pose great difficulty for the attacking wolves....
Moose. The moose population on the northern range, numbering only a few hundred, declined precipitously following the 1988 fires....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 affect deer populations significantly. The park does not contain good habitat for white-tailed deer, and the low deer numbers have not changed following wolf reintroduction. Wolves are not known to have killed any in the park. Mule deer are abundant, numbering about 2000 or 3000, but they 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 in areas largely avoided by wolves.
Beaver. Beaver are widely but patchily distributed in Yellowstone....The 2001 aerial survey revealed 77 colonies distributed across the park.
During 1996, shortly after wolf reintroduction, there were no documented beaver colonies on the northern range. Since then, beavers have established four colonies in this portion of YNP, following recent beaver reintroductions on the adjacent Gallatin National Forest. Only two wolf pack territories-the Yellowstone Delta and Cougar Creek-contain substantial populations of beaver. Wolves most likely prey on beavers in YNP as they do elsewhere, but we have documented only one beaver kill and have only rarely found beaver remains in wolf scats. Beavers are also closely associated with willow communities, a situation with relevance to wolves, as discussed below.
Vegetation. Interesting changes in willow and aspen growth occurred in the late 1990s. Increased height of some aspen stands has been attributed to elk redistribution following the arrival of wolves, 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 attributable 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 -- the debate on this issue is intense (Meagher and Houston 1998, NRC 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, which could well turn out to be the most important aspect of wolf reintroduction to Yellowstone.
What next for Yellowstone? Science amid management and controversy
....At both Yellowstone and Isle Royale before the arrival of wolves, the condition of the vegetation was a source of concern and disagreement. 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....Based on the Isle Royale experience, we anticipate that the condition of the vegetation on tghe northern range will subside as a popular topic of debate 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 supported by increased levels of intraspecific strife in 2001 and 2002 and by a plot of wolf locations that indicates little vacant territory. Isle Royale history and other studies indicate that there will be continual flux in pack territorial relationships.
....Before 1995, Isle Royale supported one of the highest year-round densities of wolves-42 per 1000 km in the world. By 2002, wolves on Yellowstone's northern range had already reached a density of 50 wolves per 1000 km....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 that 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, it is not hard to imagine that indirect effects of wolf recovery will be substantial. Although riparian willow habitats form a very small part of the northern range, any reestablishment of these woody shrub communities would increase biodiversity.
....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 to 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....Climate change will magnify the scientific challenge. The danger we perceive is that all changes to the system, now and in the future, will be attributed solely to the restoration of the wolf....[T]he need to design research hypotheses that discriminate among potential competing causes is therefore very real.
As in the past, elk management decisions for areas 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, though their hunting strategies are very different, 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.
Photo: Wolf pack with grizzly bear sow and cubs.
Photo: Wolf feeding on elk carcass.
Photo: Three wolves traveling.
Photo: Wolf chasing coyote.
Figure 1: Yellowstone has more interacting species than does Isle Royale, which leads to greater complexity and makes scientific study and understanding more challenging, (a) Isle Royale; (b) Yellowstone.
Figure 2: Fluctuations in the numbers of wolves and moose in Isle Royale National Park, 1958-2002.
Figure 3: Before wolves were reintroduced, researchers predicted the wolves would settle in the four sites shown above.
Figure 4: Wolf pack territories in Yellowstone National Park in 2001.
Figure 5. Population growth of wolves in Yellowstone National Park and on the northern range of the park, 1995-2001.
Figure 6. Elk population fluctuations and hunter harvests, 1965-2001.