CH A P T E R 6
Restoring Functionality in Yellowstone with Recovering Carnivores: Gains and Uncertainties Joel Berger and Douglas W. Smith
For those interested in the conservation of biological diversity, the word “natural” often conjures up systems that may be remote and also are likely to be pristine. To many, such regions exist where human influences are minimal and ecosystem processes operate today as they have in the past. To ecologists, however, such systems are rare or nonexistent. Nevertheless, these are precisely the images that jump to the forefront of the public mind when natural ecosystems are mentioned. In reality, no one really knows how best to creatively define “natural”—a difficulty that arises in part because of variation in dimensions that include time and space. If one were to imagine a vast boreal landscape devoid of modern human activity, the claim could easily be made that the system is more natural than one filled with gas pads, dammed rivers, or clear-cuts. But the conundrum is obvious when dealing with large carnivores, whether wolves (Canis lupus), bears (Ursus spp.), or lynx (Lynx canadensis), which are still abundant at some sites but not others. So could “natural” systems be those with a continued presence of top predators (Anderson 1991; Pritchard 1999)? The area in and around Yellowstone National Park provides a useful place to explore the relationship between carnivores and the definition of “natural.” The last wolves from the area outside the park were removed in the 1930s and from Yellowstone in 1926 (Weaver 1978), and wolves were not reestablished until 1995 and 1996 (Bangs and Fritts 1995; Phillips and Smith 1996). In this chapter we summarize results on how the return of wolves is reshaping the 8991 km2 Yellowstone National Park (YNP), and, in particular, how these apex carnivores are altering biological diversity (Smith et al. 2003). Continuous monitoring over periods of time when wolves were both present and absent of-
100
Restoring Functionality in Yellowstone with Recovering Carnivores
101
fers a unique opportunity to examine the link between large carnivores and biodiversity in this North American temperate ecosystem. YNP is part of a broader ~60,000 km2 ecosystem, so we also describe what changes wolves have effected beyond the park’s borders. This latter point is relevant scientifically and from a conservation perspective, particularly because there are many areas in the United States and other parts of the world where wolf or other large carnivore reintroductions will never be possible. In this case, however, gauging how wolves modify landscapes lends itself to an understanding of what can and cannot be achieved for the conservation of biological diversity, especially when predation is lost from other systems and where ecological dynamics can unfold with minimal current intervention by humans. From a conservation perspective it is also necessary to know how important large carnivores can be, especially wolves, in ecosystem functioning. Some say they are not important at all, that ecosystems can do without them, that their return only makes things seem “perceptually” healthier (Theberge 2000). But on the other hand, we know that wolves bring about dramatic change to the animals and plants present in the system. So is it really true that we don’t need wolves? Or have we lived long enough without them to know the long range impact of not having them? We also know that human hunters do not serve as surrogate wolf hunters (Wilmers et al. 2003b; Berger, this volume), even though this has been one rationale for massive global predator extermination. So where are we? Do wolves matter? Is carnivore conservation even relevant to anything of importance to human beings or ecosystems?
Ecology in Yellowstone National Park with and without Wolves An understanding of the prior management of YNP is fundamental to interpreting the role of wolves in restructuring ecological communities. Volumes have been written about the history of YNP (Haines 1977; Schullery 1997; Pritchard 1999). Here we draw only the most rudimentary historical sketch of species and processes relevant to assessing wolf-related effects. During the latter part of the 19th century, just after YNP had been established, poaching resulted in the killing of large numbers of elk (Cervus elaphus) and other
102
The Scientific Context for Understanding the Role of Predation
big game (Haines 1977). Bison (Bison bison) dropped to fewer than 50 animals (Meagher 1973). Subsequently, the existing herd was supplemented by the introduction of bison from elsewhere, and, within Yellowstone, all were fed and virtually farmed (Pritchard 1999). The population increased and such practices terminated in the 1960s (Meagher 1973). Elk within and beyond YNP were also managed. From the 1930s to 1968, elk from the northern Yellowstone herd, the park’s largest, were controlled by the Park Service. In 1968 the herd numbered 4000 animals. On the northern range of YNP animals were reduced by some 75% (Houston 1982), and the animal populations within the northern YNP boundaries were limited to around 4000. Once human harvest terminated, by the late 1980s, and then again after a crash, in the early 1990s the numbers within the park grew to approximately 19,000. Like bison (which increased to about 4000 animals in 1997, crashed, and then increased again to 4000 in 2004), there was little apparent significant predation on adults of either species (Houston 1982). Since animals move beyond park boundaries to nonprotected areas, they can be shot. Control actions beyond park boundaries limit numbers within the park because the removed animals cannot, obviously, return (Meagher 1989; Lemke et al. 1998; Smith et al. 2003). Although other species, including cougars (Puma concolor) and white-tailed deer (Odocoileus virginianus), have also changed in population size and distribution within and beyond the park, much attention has focused on abundance of the large herbivores such as elk and bison. During the 70-year period between wolf extirpation and reintroduction, strikingly different viewpoints have been expressed about changes in native biodiversity, often centering on what is natural and what is not. Among notable changes in wildlife since the establishment of the park has been colonization by moose (Alces alces) (Tyers 2003); the virtual elimination of beavers (Castor canadensis)(from the northern range; YNP, unpubl. data) and whitetailed deer; lack of recruitment in, and reduction of, aspen and cottonwood communities; diminished willow communities; and reduction of song birds (Hansen and Rotello 2002). Such modifications have variously been attributed to human mismanagement (Chase 1987), loss of indigenous human hunters (Kay 1994), extirpation of wolves, weather cycles and fire, erosion, and complexities that have yet to be unraveled (Singer et al. 2002, 2003). Although undoubtedly there is much merit in both the empirical support and the opinions that underlie these varied po-
Restoring Functionality in Yellowstone with Recovering Carnivores
Golden eagle
Bald eagle
Coyote
Wolf
Cougar
Raven
Grizzly bear
103
Magpie
Black bear
Human Cutthroat trout
Pronghorn
Mule deer
Other coniferous species
Other deciduous species
Moose
Grasses
Elk
Bison
Willows and aspens
Bighorn sheep
Forbs
Figure 6.1 Overview of simplified trophic level interactions involving scavengers, terrestrial carnivores, fish, ungulates, and plants in Yellowstone National Park. Dotted lines indicate possible competitive pathways. (Modified from Smith et al. 2003).
sitions, what is now clear is that, since their reintroduction within the park, wolves have mediated a ripple of effects throughout the food chain (Fig. 6.1). Among the major changes are both direct and indirect effects on processes, some with visible influences on species and landscapes.
Predator–Prey Relationships Of the seven native ungulates within the Yellowstone ecosystem—mule deer (Odocoileus hemionus), pronghorn (Antilocapra americana), white-tailed deer, elk, bison, bighorn sheep (Ovis canadensis), and moose—all but white-tailed deer have been killed by wolves within YNP; and this may be due to the fact that their populations are low due to restricted habitat. The major prey of YNP wolves both numerically and by biomass is elk, representing 92% of 1582 kills between 1995 and 2001. Calves are taken disproportionately relative to their availability (43% vs. 15%), adult females reflect the opposite (28% killed, 60% available), and adult
104
The Scientific Context for Understanding the Role of Predation
males are killed in proportion to their abundance (21% vs. 25%, respectively; Smith et al. 2004). Condition of elk as determined by marrow fat indicated that wolves may be selecting for elk in poor condition. An increasing number in early winter killed by wolves are in poor shape and virtually all of them in late winter show marrow fat depletion, the last fat reserve utilized by ungulates (YNP, unpubl. data). Among other large herbivores, moose and bison combined represented less than 2% of the total kills, although bison calves, like elk, were killed in numbers disproportionate to their abundance (Smith et al. 2000). Other species killed by wolves include beavers, coyotes (Canis latrans), badgers (Taxidea taxus), Canada geese (Branta canadensis), ground squirrels (Spermophilus spp.), and other small mammals (Ballard et al. 2003).
Relationships Involving Individual Species At least 16 vertebrates have been shown to be affected in ecologically important ways by wolf predation (see Fig. 6.1). For instance, not only have prey species such as elk been reduced in total population size (and for reasons not entirely due to wolves; other factors, such as female harvest of prime-aged elk, multiple carnivores, and multiyear drought are also important), but coyote densities have also been diminished at local scales by at least 50%, a change brought about through both spatial avoidance and intraguild predation (Crabtree and Sheldon 1999). For example, coyote population size in one region of the park declined from 80 to 36 individuals (Crabtree and Sheldon 1999). Wolves have also facilitated the availability of carrion for grizzly bears, not simply because wolves kill large items such as elk but also because grizzly bears now have more carrion available (Ballard et al. 2003). Indeed, grizzly bears tend to dominate wolves at carcasses (Smith et al. 2003). Influences of wolf populations on other carnivores have yet to be documented, although cougars tend to be driven from their kills by wolves more than the converse, and several cougar kittens plus one adult female were killed by wolves (Ruth et al. 2003; Smith et al. 2003). On the other hand, cougars have also killed wolves (YNP, unpubl. data). In addition to possible or demonstrated effects on fish, carnivores, and ungulates, the carrion made available by wolves also attracts at least 12 vertebrate scavengers, 5 of which visit nearly every wolf kill: bald (Haliaeetus leucocephalus)
Restoring Functionality in Yellowstone with Recovering Carnivores
105
and golden (Aquila chrysaetos) eagles, coyotes, ravens (Corvus corax), and magpies (Pica pica) (Wilmers et al. 2003a). For at least one of these species, ravens, a change in their patterns of food detection and acquisition has been noted since the return of wolves; ravens arrive at wolf-killed meals on average within one minute of death and actually follow wolves to locate food before they make a kill (Stahler et al. 2002). The greatest number of ravens ever recorded on a wolf kill, 135, comes from a report in YNP (Stahler et al. 2002), which recorded an average number of 29 ravens per wolf kill. It is hard to overstate the importance of wolf-provided carrion to the scavenger guild; indeed, this may be one of the most significant impacts of wolf recovery because no other animal provides the food to this group in the manner that wolves do. Cougars, the other primary large carnivore in YNP, cover their kills, making them unavailable to scavengers. Wolves, on the other hand, try to out-eat the masses of scavengers and can lose significant amounts of food to them (Vucetich et al. 2004).
Landscape-Level Change As is well appreciated from theory and a plethora of ecological studies, biological communities are often dynamic. Such dynamism has characterized a 100,000 ha region of northern YNP where aspen (Populus tremuloides) recruitment has declined during the past 100 years—a reduction that coincides with decades of high elk densities (Houston 1982; Ripple et al. 2001; Larsen 2002). However, abiotic and biotic factors have also been implicated in the decline of aspens and other mesic or riparian plant communities (Romme and Despain 1989; Kay 1994; Singer et al. 2002, 2003). Regardless of the cause of the decline, aspen stands are biodiversity hotspots across the West, especially for songbirds (Turchi et al. 1995). If wolves affect the foraging behavior and/or distribution of elk (as is suggested for numerous vertebrate predators; Brown et al. 1999), then, assuming that elk are structuring agents themselves, wolves may indirectly affect plant growth and recruitment. Elsewhere in this book, evidence is presented that large herbivores, especially at relatively high densities, have strong ecological impacts (see McShea; Terborgh; Berger, this volume). In the Greater Yellowstone Ecosystem, some evidence of a wolf-mediated pathway with ultimate effects on plants is based on work on aspen sucker growth. Ripple et al. (2001) found sucker height to be
106
The Scientific Context for Understanding the Role of Predation
significantly greater in areas with high wolf densities and where elk dung counts were lowest. These results, however, were ephemeral and not long term, suggesting multiple causes for the reported response (YNP, unpubl. data). Willow growth is perhaps a better example, but is equally complicated and also likely to be interactive between factors. Although willow growth is significant in some places, it is important to note that this response has not been uniform across the northern range (YNP, unpubl. data). However, we feel the most parsimonious explanation is that elk have reduced their use of areas where predation pressure was high and, as a consequence, the intensity of browsing on aspens and willows was relaxed (Ripple and Larsen 2000; Ripple et al. 2001). Beyond vegetation and elk, wolves may possibly affect diversity at other levels. For example, the nesting density of songbirds such as the common yellowthroat (Geothlypis trichas), Lincoln’s sparrow (Melospiza lincolnii), warbling vireo (Vireo gilvus), Wilson’s warbler (Wilsonia pusilla), and yellow warbler (Dendroica petechia) in willow stands varied in a nonlinear fashion with the severity of browsing, suggesting a threshold effect ( Jackson 1992). Similarly, where both grizzly bears and wolves were extirpated to the south of YNP, avian diversity was less than in areas where humans controlled another large cervid, moose (Berger et al. 2001a). Although not completely tested in YNP, both to the south (Grand Teton National Park; Berger et al. 2001a) and to the north (Banff National Park; Nietvelt 2001), a link between wolves, ungulates, vegetation, and songbirds has been established. We expect that, with further examination, the same will hold true in YNP. Evidence to include beavers in this mix is also accumulating: their return to the northern range in the past six years is correlated with wolf recovery and appears to be linked with increased availability of willow (Nietvelt 2001; Smith, unpubl. data). Hence, if the threat of predation by wolves on elk or moose redistributes these large-bodied ungulates away from willow communities, then herbivory is likely to be reduced, which may subsequently influence biological diversity (Berger et al. 2003). However, the relationship may not be so straightforward since many factors other than herbivory per se also affect the density and growth of willows and avian reliance upon these plant communities (Cody 1981; Finch 1989; Singer et al. 2000). It is important to note that grasses drive the northern Yellowstone system (and are not suppressed by ungulate grazing; Frank and McNaughton 1992) be-
Restoring Functionality in Yellowstone with Recovering Carnivores
107
cause only 2 to 4% of the entire area is deciduous woody vegetation (Smith et al. 2003). But there is evidence that the importance of woody vegetation is disproportionate to its abundance. Therefore, any increase may increase biodiversity; for example, as we are beginning to see with beavers responding to increased willow, which in turn provides aquatic habitat for other plants and animals (Baker and Hill 2003; YNP, unpubl. data).
Ecology beyond Yellowstone National Park with and without Wolves Although YNP is the one of the world’s premier locations, both for understanding ecological processes and because it has galvanized carnivore restoration globally (Clark et al. 1999), the park itself represents only ~15% of the entire Greater Yellowstone Ecosystem. As a consequence there is much to learn not only by temporal contrasts of processes prior to and after wolf reintroduction but also spatially through assessment of ecological effects of variation in wolf densities within and beyond park boundaries. Such contrasts of course are never as clean as purists hope because in addition to carnivore densities, sites vary in topography, history, structural community components, human management, and many other factors. Additionally, there have been only limited scientific study of areas beyond (or even within) YNP prior to wolf extirpation. Nevertheless, much can be gleaned by contrasts between wolf-related effects in the park itself relative to those outside the protected boundaries. Wolf densities on the northern range of YNP (~50 per 1000 km2) are some of the highest known in North America (Smith et al. 2003), and these are supported by a prey biomass that is similarly high (Singer and Mack 1999). Importantly, effects of current wolf densities can be contrasted between the park and regions beyond where densities are much lower due to conflicts with livestock. As apex carnivores, wolves would be expected to exert effects outside the park that might be similar to those within, assuming other factors to be equal. Given that lands outside YNP receive differing levels of protection, none as strict as within (where there is no hunting and grazing of livestock), opportunities exist to determine what lack of effects may be occurring because wolves are at reduced densities. For instance, in lands adjacent to the park, about 100 wolves have been killed
108
The Scientific Context for Understanding the Role of Predation
due to conflicts with people and livestock. Indeed, wolf densities in some areas of the Greater Yellowstone ecosystem may be 1% or less than those within YNP (see Fig. 16.4 in Berger, this volume). Human tolerance in the Yellowstone ecosystem, despite relatively higher economic well-being, is lower than tolerance for carnivores in other areas (e.g., Africa or India) where human population density is higher and economic well-being orders of magnitude lower (Creel and Creel 2002; Jhala and Giles 1991). This disparity is interesting and counterintuitive. Clearly, where there are no or few wolves, they will play no or little role in directly shaping ecosystems and biodiversity, except perhaps through a series of cascading processes where ungulate densities are unregulated or modified by humans. Understanding why wolves are not playing an ecologically functional role beyond YNP boundaries but still within the much larger Greater Yellowstone Ecosystem is relevant more from a conservation than an ecological perspective. This is because wolves in this region of the United States are soon to be de-listed from federal protection since population sizes have achieved a level at which demographic viability is expected (Smith et al. 1999). Once this occurs, the American public may believe that wolves are playing functional ecological roles. However, this is unlikely to be the case because wolves are killed beyond protected boundaries, and ungulates and other ecological responses may be derived more as a consequence of the human milieu than human hunting or wolves (Pyare and Berger 2003; Soulé et al. 2003). Nevertheless, the fact that wolves are currently playing an apex role in YNP offers unprecedented opportunities to understand basic ecology while minimizing the role of anthropogenic disturbance that characterizes so much of the terrestrial world, and allows us to better answer the question, Do wolves and other large carnivores matter?
Summary Following a 70-year absence, wolves were reintroduced to Yellowstone National Park (YNP) in 1995 and 1996 where they now play a key role as apex predators. Continuous monitoring of this north temperate ecosystem during the absence and return of wolves has provided an opportunity to examine the relationship between wolves and biodiversity. Not only do wolves define ecological relationships
Restoring Functionality in Yellowstone with Recovering Carnivores
109
with ungulates and vegetation but they are initiating subsequent effects on biodiversity through interactions with other predators and scavengers. Although wolf density on the northern range of YNP is one of the highest reported, effects of wolves on ecosystem processes and biological diversity beyond park boundaries are likely to remain less obvious. This is because beyond YNP boundaries there is a confounding array of other processes that include landscape-level changes and associated human activities, and in general human tolerance for wolves is low. Although the goal of wolf reintroduction into YNP was to support a self-sustaining population throughout the entire region, less attention was initially focused on functional ecological relationships. The YNP portion of the overarching U.S. Rocky Mountain wolf population therefore enables a fundamental understanding of how wolves affect biodiversity unfettered by direct anthropogenic manipulations. Beyond park boundaries, however, wolves are not likely to play critical longterm ecological roles.