Yellowstone Wolves Case Study
In the half moonlight at dawn on a sharply cold January morning, they looked like small ponies galloping beside the old railroad at the northern entrance of Yellowstone National Park. They weren't ponies. This was the “Eight Mile” wolf pack, each member huge, healthy, and vigorous, romping through the light snow on a morning quest for elk, bison, or anyone too slow to get out of their way. It was an incredible moment, one that evoked feelings shared by the hundreds of wolf watchers who come to Yellowstone every month of the year hoping to experience even a glimpse of the wolves.
The enthusiasm of the wolf watchers is almost totally reversed by the many local ranchers who live outside the park and regard wolves as varmints, best used for target practice. The pack that I heard howling outside the cabin every night was quickly dispatched by the local rancher soon after I left; their pelts could be found for sale in one of the souvenir stores at the entrance to Yellowstone.
Scientists initially appear as polarized in their opinions of the role of wolves and large predators in ecosystems as the wolf watchers and ranchers are about their value to the local economy. Wolves were introduced back into Yellowstone following the development of a huge environmental impact assessment (EIA) that attempted to predict the outcome of their reintroduction. The EIA, a 4-ft-deep pile of documents, provided solid testimony to the need for a deeper empirical and theoretical understanding of how ecological food webs respond to species additions and losses. At the time, even the suggestion of introducing wolves created huge discord in the ranching community surrounding Yellowstone; most ranchers (and some ecologists) were convinced wolves would feed exclusively on cattle and sheep; the ranching industry was dead set against reintroduction. A curious event then occurred: photographers started getting photographs of wolves that had naturally colonized the park. As any natural colonization would provide the wolves with full legal protection under the United States Endangered Species Act, the Ranchers Association hastily made a U-turn and supported introduction on the grounds that experimentally introduced wolves were nonnative and could be shot if they left the park. I know of no better environmental example of nonlinear political expediency.
Once introduced in 1995 and 1996, the wolf population grew rapidly. At the time, the elk population was declining from an all-time high and provided a large supply of prey to fuel wolf reproduction; the population increased at close to the maximum rate ever recorded . As the wolf numbers increased, the elk numbers decreased, but at a rate that was more parsimoniously explained by a prolonged drought and levels of human harvest, the decline in abundance far exceeding that which could be accounted for purely in terms of elk consumed by wolves ,. Significant evidence does suggest that the elk had changed their feeding habits in the presence of wolves, avoiding areas where they could readily be ambushed –. This allowed vegetation in riparian areas to recover; photographs taken at a variety of locations showed considerable recovery of aspen in areas where it had become overgrazed in the years when elk were abundant ,. Although these riparian areas cover only a small area of the ecosystem (<2%), the park was witnessing the first significant growth of aspen for over half a century. More recent data suggest that similar recoveries are being seen in cottonwoods and willows ; this in turn has led to an increase in the abundance and diversity of riparian bird species . All of this evidence suggests that wolves have a strong top-down effect on trophic structure of the ecosystem (Fig. 1).
Alternatively, climate has been argued to be the principal driver of ecosystem change, not wolves; changes in vegetation may have been driven by bottom-up changes in water availability due to changes in snow melt patterns . Wolf population expansion occurred at a time when the Yellowstone region was entering a prolonged drought that also reduced forage available to elk; this combined with human harvest contributed significantly to the declines in ungulate abundance. Furthermore, climate change has lengthened the growing season for willows and aspen by around 27 days in the last couple of decades , while the vegetation in many areas of the park is dominated by conifer forest that has simultaneously been recovering from the fires of 1989. Thus, it is not straightforward to differentiate between postfire recovery and the indirect effects of carnivores on vegetation regeneration.
Concomitant to wolf introduction, the grizzly bear population was increasing, creating the potential for indirect competition between bears and wolves as the latter selectively prey on old or injured elk in the winter. This predation reduces the number of elk that would otherwise die and become available for grizzlies emerging from hibernation in the spring. This absence of “frozen meals” caused grizzlies to switch to feeding on elk calves as an alternative spring food source when recovering from their long winter fast . As elk numbers declined following the triple assault of drought, wolves, and bears, both grizzlies and some wolf packs switched their attention to bison , which require larger packs to make an effective kill but ultimately provide a larger meal. All of the extra carcasses have provided a new bounty of food for ravens and golden eagles, both of which have increased in abundance .
Less well understood is the impact of wolves on coyotes, the numbers of which may have declined since wolves were reintroduced ; carnivores are aggressive to other carnivores of similar but slightly smaller body size. As coyotes were the primary predators of sheep, you would think that the sheep ranchers would applaud wolves for the reduced loss of stock to predators; they have been noticeably silent on this front. More subtly, the presence of wolves may help reduce the threat posed by chronic wasting disease (CWD), an emerging prion pathogen that is spreading from elk and deer to cattle and is arguably the biggest biological threat to ranching in the region . Unfortunately, the ranching community does not recognize that the wolves may be doing them a huge favor by removing sick elk and mule deer infected with CWD (and elk and bison infected with brucellosis) from the wild reservoir of infection. If CWD or Brucella enters cattle herds in the states bordering Yellowstone, then federal mandates will hugely restrict movement of cattle in and out of these states.
We may have to wait at least another ten years before the impact of wolves on the Yellowstone ecosystem is fully quantified. Although many strong patterns are observed, several of these may be correlation without causation (for example, the increase in beaver abundance is more likely to be a consequence of beaver introductions to the north of Yellowstone National Park [NP] ). Furthermore, although there is considerable pressure from the conservation community to sanctify the wolves as bringing only benefits to the ecosystem , there is still a need for stronger data to support some of the beneficial claims made for wolf reintroduction. Some of this will come from Yellowstone, but this needs to be combined with studies of wolf reintroduction, or natural reestablishment, in other ecosystems. If the patterns observed in Yellowstone are repeated, as preliminary evidence suggests, then hard-core wolf haters are going to need to reconsider the labelling of wolves as varmints.
The research and debates surrounding the role of wolves in modifying the behavior and abundance of species on multiple trophic levels in Yellowstone illustrates the complex interactions between the forces that structure patterns of abundance in natural ecosystems. The debate gets to the heart of one of the central scientific challenge of ecology: how can we understand the structure of food webs? Central to any discussion of food-web dynamics and ecosystem management is the relative importance of top-down roles played by large predators and pathogens and bottom-up forces driven by the climatological processes that determine plant growth. All of the work from Yellowstone cries out for the development of next-generation, population-based ecosystem models that focus on interactions between climate, vegetation, and the dominant herbivore and carnivore species in the park. In particular, food-web ecologists need to more aggressively move beyond descriptions of the network geometry of food webs and grasp the thistle of food-web dynamics. More generally, we cannot afford for this debate to become polarized; that simply suggests to funding agencies and the general public that ecologists do not know how ecological systems function. Instead, we need to frame the discussion as a major scientific challenge that requires significant international and national funding.
There are curious and unexplored parallels between work on food webs and trophic interactions and that of physicists who are trying to understand the forces that determine the way the universe is structured at either the atomic or astronomical level. At both scales, a series of nested forces hold increasingly large particles together using a mixture of centripetal and gravitational forces, which operate essentially as bottom-up forces (although this is almost a metaphysical debating point!). Seen from this perspective, the current controversy about ecosystem-level effects of wolf reintroduction to Yellowstone NP is every bit as scientifically exciting as the recent discovery of the Higgs boson. Determining the strength of the forces produced by the loss or addition of particles or species to these very different systems are key scientific questions for the 21st century. Although each discipline uses very different types of equipment, budgets, and collaborations to undertake experiments that generate data for subsequent analysis, they each seek the answer to the same questions: “What are the fundamental forces that structure the universe in which we live, how do they operate, and how can we measure them?”
The hunt for the Higgs boson—an infinitely tiny particle whose energy is required to hold the interior particles of atoms in orbit—was an international research collaboration with a budget that exceeded all funding for ecology over the last ten years, perhaps even over the last century! In contrast, funding for work on natural ecological systems is usually cobbled together from a mixture of government and individual research funds; it is rarely clear from year to year when, or if, funds will appear for the next year's salaries. One benefit of working in national parks is that management occasionally allows experimental introductions, or removals, of species that permit investigation of the impact of these changes at ecosystem-level scales. However, the results of management experiments are rarely clear-cut and often ambiguous. It is all too easy to be critical about lack of controls and absence of replication (which is nontrivially a function of trivial budgets!), but understanding how food webs in national parks react to the addition and loss of species is as scientifically challenging as searching for tiny particles using very expensive particle accelerators. The central problem is ecological budgets are tiny compared to those for “big science,” so we need to use all sources of information that are available, including management exercises, to interpret findings at the appropriate ecosystem-level scale.
If we agree that physicists and ecologists are both trying to understand the forces that determine the structure of the universe, what are the major scientific differences between their approaches? Ecologists are focusing on understanding these forces at the spatial and temporal scales intermediate to that of physicists—less heroic, perhaps, but the scale that is directly relevant to humans. Less heroic or not, from the perspective of systems with interacting components, ecosystems and their constituent species will always be as complicated as those exhibited by atoms and bosons or galaxies and planets, perhaps more so; food webs have many different types of “particles” (species) that interact, evolve, and behave nonlinearly in a huge variety of time, and spatial, scales. Ultimately, we need to arrive at a realization that the mathematics of food webs and ecosystems is as complicated as that found in any of the problems of atomic and galactic structure studied in physics. Increasingly, we are realizing that the quality of human life on the planet depends on a deep functional understanding of the forces that structure the dynamics of food webs and the ecosystem services they provide to the human economy. We may even need new mathematics to deal with these levels and layers of complexity.
The current controversy about the role that wolves play in modifying the behavior and dynamics of other species in Yellowstone is a classic case study in this broader class of problems: it is about understanding how to measure forces and processes that act between operators at a variety of different spatial and temporal rates within a natural ecosystem that contains a diversity of natural heterogeneities (that initially appear to confound the search for broad patterns). If we re-pose ecology as the science that examines the forces that structure the central part of the universe in which we live, then more funding might be available to address these complexities; we would also simultaneously attract more bright minds willing to grapple with complexity.
From a much broader perspective, we need many more ecosystem-level studies of how species interactions between predators, parasites, and prey change the patterns of spatial heterogeneity in vegetation that ultimately drive levels of biodiversity at higher trophic levels. This is an exercise that requires a new generation of spatial, multispecies, multitrophic models and many more debates such as the current one about the role of wolves in Yellowstone. Resolving these discussions will allow ecologists to present a much stronger case to funding agencies and the general public for ecology to be recognized as the central scientific discipline of the 21st century. Ecology's mathematical problems are as complex as anything in physics, and their solutions are required with increasing urgency, particularly if we want to test these assumptions and predictions against viable natural ecosystems.
Time spent at the Santa Fe Institute constantly stimulates me to think about the need to understand problems of “complexity.” The ideas described here were developed from many hours spent in Yellowstone in conversation with Peter Hudson, Emily Almberg, Tim Coulson, Paul Cross, Mary Meagher, Doug Smith, Dan McNulty, Dan Stahler, Bridgett van Holdt, and Robert Wayne. Anieke van Leeuwen, Annarie Lyles, and Mercedes Pascual commented extensively on an earlier draft. The opinions expressed here are purely my own.
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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 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 (Bangs and Fritts 1996, Phillips and Smith 1996). In the 7 years following their initial release, wolves have recolonized the 8991-square-kilometer (km2) park and several adjacent portions of the 72,800 km2 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 km2 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 (Lemke et al 1998).
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 (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 multicarnivore–multiprey systems like Yellowstone—but long-term data (from the turn of the century to the present) on willdife population sizes from these other areas were lacking, and we do not have intimate experience with these parks. Often, what is important is subtle and detailed yet can account for 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 (figure 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 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 (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 increasing husbandry of native ungulates, which eventually involved winter feeding and predator control. Gray wolves were effectively eliminated by the 1930s (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 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. 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, which resulted in an overabundant moose population (Allen 1979). Instead of artificial reductions to control moose, the Park Service tried unsuccessfully to reintroduce zoo-raised wolves in 1952 (Allen 1979). But unlike in 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? Although the relative roles of bottom-up (nutrition and vegetation) and top-down (wolf predation) influences on moose population dynamics are not fully understood (Messier 1994, Peterson 1995), the historic chronology of moose numbers indicates that wolf predation tends to cap moose density (figure 2). The growth in moose numbers peaked in the early 1970s and ended when severe winters affected vulnerability (Peterson 1977), 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 (top-down; McLaren and Peterson 1994). 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 (Peterson 1995), moose numbers grew 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 the 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, 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 (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 most 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 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 (Boyce 1993), or will wolves destabilize elk fluctuations, exacerbating population fluctuations (NRC 2002)? How far will the ecological ripple extend? Moose no longer number 3000 on Isle Royale, as they did before wolves (Allen 1979), so will elk ever exceed 19,000, as they did before wolves and after the artificial reductions in Yellowstone?
Before wolf introduction, several studies used modeling to predict the future impacts of wolves on the Yellowstone ecosystem (YNP et al. 1990, Varley and Brewster 1992, Cook 1993, USFWS 1994). These were comprehensive efforts, prepared for Congress and the general public, that focused on the interaction of wolves with native ungulates, livestock, and grizzly bears. Simulations predicted between 50 and 120 resident wolves in YNP, with packs on the northern range, Madison-Firehole, and possibly the Gallatin and Thorofare areas (figure 3; Cook 1993).
All models suggested that elk would constitute the primary prey for Yellowstone wolves. Four models dealt with the impact of wolves on native ungulates (Garton 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, Mack and Singer 1993). 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 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 and 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 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 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 antiregulatory (inversely density-dependent) influence of wolf predation that wildlife managers in Alaska have noted (Gasaway et al. 1992). 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 (Smith et al. 2000). 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 (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 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 (Crabtree and Sheldon 1999). Some predicted that wolves would reduce coyotes and that the 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).
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 (YNP 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 is worth noting, however, that aspen and willow are minor components of northern range vegetation (less than 1% or 2%); Meagher and Houston (1998) explore the difficulty of basing management of the larger grazing system on minor components of the vegetation.
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. A review committee of eminent scientists recently focused on the condition of the northern range (NRC 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
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 (figure 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). The initial rate of increase for the wolf population was very high (figure 5), 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.
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 (figure 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 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. Across the park, wolf packs exist approximately in the places predicted by Boyce (1993; figure 3).
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 (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 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 (Smith et al. 2000), so far most Yellowstone packs are supported almost entirely by elk.
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% 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 (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 Sheldon 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 by coyote predation (John Byers, University of Idaho, Moscow, ID, personal communication, October 2003).
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.
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 (Hayes et al. 2000). 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 (Stahler et al. 2002).
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 (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 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.
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 (Toni K. Ruth, Wildlife Conservation Society, Bozeman, MT, personal communication, October 2002). 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 (Toni K. Ruth, Wildlife Conservation Society, Bozeman, MT, personal communication, October 2002).