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Ecological Dynamics on Yellowstone’s Northern Range 5 Conclusions and Recommendations NATURE IS DYNAMIC A PERVASIVE THEME in this report is the dynamic nature of the northern Yellowstone ecosystem. Over long periods, a changing climate and major geological processes have resulted in dramatic restructuring of the landscape and associated plant and animal communities. The Greater Yellowstone Ecosystem (GYE) has experienced large-scale disturbances including fire, floods, blow downs, ungulate and predator population fluctuations, and outbreaks of diseases and insects that affect plants and animals. In addition, during the late 1800s, intense reduction of carnivores and ungulates diminished or eliminated populations of key species. Furthermore, the northern range is part of a larger system where human activities are steadily increasing. Thus, we probably cannot ever manage Yellowstone National Park (YNP) to maintain some agreed-upon stable condition, if that were to become a management objective. We lack sufficient knowledge, resources, and capability to sustain any environmental state through active management. Given the ever-changing nature of the northern range on both temporal and spatial scales, can we determine which of the changes we observe in ungulate numbers and range, forest conditions, and riparian conditions are within the bounds of natural variation and which, if any, are caused by human activities?
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Ecological Dynamics on Yellowstone’s Northern Range To answer the question we must assess ecosystem resilience, resistance, and stability. Is the system easily modified? Does it readily recover from perturbation? Are there thresholds that result in major or irreversible changes in processes, ecosystem conditions, or population numbers? Theory and field studies have shown that some ecological systems change abruptly from one relatively stable state to another. In these situations, simply removing the factor or factors that caused change may not return the system to its previous state. For example, sustained, heavy livestock grazing in arid grasslands of the western United States, in the absence of fire, has led to invasion and establishment of shrubs and trees (Archer 1994). Once trees gained sufficient stature to capture much of the moisture supply, elimination of grazing did not result in reestablishment of grassland (Glendening 1952). Such a process is consistent with “state and transition” models and with the existence of multiple stable states (Allen-Diaz and Bartolome 1998). These conceptual models help us to appreciate the complexity of ecosystem relationships and processes and should be used to evaluate management of the northern range. How do these concepts help us to evaluate changes in the GYE? Many aspects of the northern range have been intensively studied, but it has not been experimentally shown, for example, how large a reduction in the consumption of aspen by ungulates would be required to permit their “recovery.” Consequently we do not know whether changes in plant communities during the 1900s indicate that a new state, characterized by fewer communities dominated by willows and aspen, is likely to persist. Research outside the park, however, does not support the hypothesis that a new state has become established (Kay 1990). To evaluate whether the northern range is approaching a threshold, beyond which willow and aspen communities will be unable to reestablish themselves, we must have some idea of the range of natural variation (Landres et al. 1999). Are changes on the northern range within limits to be expected since Europeans arrived? How important are rare events? The “natural” interval between large fires is thought to be on the order of 200 to 300 years—can we realistically expect to manage such events? Despite claims to the contrary, we found no evidence that the northern range is approaching a threshold after which we would observe irreversible changes, such as loss of local reproductive potential of key plant species (e.g., sagebrush or aspen), that would not have occurred if the park actively controlled ungulates. This finding results, in part, because much of the evidence of dramatic changes comes from communities that are successional or the result of disturbance (e.g., aspen and riparian communities) (Houston 1982). However, changes in
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Ecological Dynamics on Yellowstone’s Northern Range sagebrush cover and grassland composition, vegetation types that are neither successional nor the result of disturbance, have also occurred. In view of the profound changes that have occurred around the GYE, it is no longer possible to have an ecosystem that is identical to the natural state that existed there before European settlement—that is, containing about the same numbers and distributions of all the species of plants and animals. YNP still has all the species present there 150 years ago, but many of the large mammals can no longer respond to change as they used to—through migration or dispersal (Wambolt and Sherwood 1999). No aspect of the ecosystem can be considered “natural” in that sense. The question is whether the ecosystem appears to be headed for some state that is very different from any previous state that we know about in the past few thousand years. We do not think it is. Vegetation changes observed in the past 130 years or so appear to have been influenced more by ungulate browsing than by climate change. MANAGEMENT FOR ECOSYSTEM STATE OR ECOSYSTEM PROCESS? Natural resource managers typically try to reduce variation around some desirable ecosystem state. For wildlife managers, a desirable state usually is defined by a consistent harvest of the target species, stable vegetation communities, and a small loss of the target animals to severe weather. Restoration ecologists, on the other hand, try to achieve desired ecosystem dynamics by reducing or eliminating human perturbations and restoring natural ecosystem processes and the ecosystem components that drive these processes. Given the inherently dynamic state of most ecosystems, Boyce (1991, 1998) and others have suggested that a more appropriate management goal for YNP is to follow the “restoration” approach and maintain or restore ecological processes rather than try to maintain a particular ecological state. Management for processes would include maintaining or restoring the spatial and temporal variation that characterizes the natural ecosystem. Holling and Meffe (1996) persuasively argued that maintenance of natural variation is critical to the functioning of ecosystems and runs counter to most traditional management prescriptions. Because Yellowstone is influenced by periodic major events, both natural and human caused, it is probably impossible to maintain a particular state by active intervention. For example, the fires of 1988 resulted in substantial changes in the mosaic of vegetation communities, but these changes
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Ecological Dynamics on Yellowstone’s Northern Range appear to be an integral component of the system and within the bounds of disturbances that periodically occurred in YNP (Romme and Despain 1989). Also, 1996 and 1997 floods throughout the GYE altered riparian communities and triggered new riparian recruitment, as expected from low-frequency, high-magnitude hydrological events (Skidmore et al. 1999). If natural processes in YNP are to be managed or restored, we must change our focus from an emphasis on specific outcomes (the presence or absence of a species or state) to an emphasis on rates and variation. Ecological processes include production of crowd-pleasing cohorts of elk and bison calves in spring, but they also include the interrelationships between all species, including competition, predation, winter starvation, and changes in vegetation communities. Because ecological processes are dynamic, ecological communities change in time and space, with or without human intervention. The need to understand and permit the full range of ecological processes is emphasized by interactions between disparate elements of the northern range. Frank et al. (1998) compared the grassy rangeland of the northern range to the Serengeti ecosystem in Kenya and Tanzania, an area that supports a higher diversity of large herbivores than the northern range. Nonetheless, herbivores have a key role in altering the transformation of materials in the functioning of both systems. Nutrient turnover rates are high in herbivore-dominated systems (including Yellowstone), and these grassland systems have rapid cycling of nutrients driven by high harvesting rates by herbivores. Removal of herbivores would transform the system into one dominated by detritivores, with slower cycling of nutrients. Hobbs (1996) identified two major challenges to fully integrating the role of ungulates into ecosystem science. First, we need to better integrate the behavior of animals into ecosystem models. Many of the links between ungulates and ecosystem processes are the result of choices made by individual animals, such as selection of feeding sites, choice of forage items, and migration in response to climate, food availability, and other external pressures (e.g., hunting). Decisions about selection of habitats, feeding patches, and diets occur at a variety of scales (Senft et al. 1987, Bailey et al. 1996) and they have a profound influence on patterns of interaction between herbivores and ecosystems. Second, we need to better understand the interactions between population dynamics of animals and plants and ecosystem processes. Few studies have examined large-scale responses of ecosystems, including the response of animal and plant populations, to changes in herbivore density. Yellowstone’s northern range may offer us an unusual opportunity for such
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Ecological Dynamics on Yellowstone’s Northern Range studies. If the recent past is an indication of the future, we can expect large fluctuations in herbivore density and thus in their influence on ecosystem processes such as recycling and redistribution of materials, and successional dynamics. Management for ecosystem processes remains a challenge for the future, and currently is more a conceptual guide than a prescription for immediate action. In their plea for more enlightened management of large systems, Holling and Meffe (1996) noted the following: Our advice to ‘retain critical types and ranges of natural variation’ must remain for the present as a management goal to which to aspire, as a conceptual underpinning for management, rather than an operational dictum. In practice this translates to adopting a conservative approach to changing parameters of systems we understand poorly but that we wish to manage. It means that the default condition, unless clearly proven otherwise, should be retention of the natural state rather than manipulation of system components or dynamics. It argues for humility when managing large systems (Stanley 1995). The northern range’s natural state is a dynamic one. Retention of natural processes is as close as we can come to this recommendation. Despite our inability to manage natural processes, general guidelines are emerging for designing programs to monitor and detect environmental trends, and this remains an area of intensive research and evaluation (e.g., Dixon et al. 1998 and accompanying papers). It will be a challenge for YNP to look to opportunities of the future, without forgetting lessons from the past. Public education also is important. The National Park Service (NPS) should explain the importance of ecosystem processes, trophic level relationships among species, primary production, and nutrient cycling. Although emphasis on biodiversity is certainly justified, the role of the area’s landscape, climate, and history in maintaining the biodiversity of the area and its dynamic nature should be explained. That implies a focus on the web of life and its complexity in the lands under NPS jurisdiction and the change over time that characterizes natural systems, rather than on preconceptions about “the balance of nature” or the desirability of having many large, “charismatic” animals visible. NPS would do well to consider YNP a natural laboratory for public education, increasing public appreciation with an enhanced understanding gained through a park visit.
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Ecological Dynamics on Yellowstone’s Northern Range LARGE-SCALE INTERACTIONS AND PATTERNS A second recurrent theme in our report has been the importance of spatial scale. The northern range is an incomplete ecosystem for large herbivores that rely on heterogeneity in the distribution of foods that vary seasonally in abundance, quality, and availability. A large spatial extent provides reserve areas that may not be preferred during normal years but can be used during times of shortage. The importance of the heterogeneity that normally accompanies a large spatial extent was emphasized by Walker et al. (1987), who examined drought-caused mortality of ungulates in African reserves that varied in size from 442 to 19,000 km2. Mortality was relatively low in the large reserves because animals expanded their normal range and used reserve areas that were far from normal water sources during droughts. Walker et al. (1987) concluded that culling was unnecessary if there was sufficient spatial heterogeneity to provide reserve forage. Similarly, during severe winters ungulates in the northern range use areas outside park boundaries. However, many of these key areas are no longer accessible because of human activity and habitat fragmentation. A large spatial extent is also important to preserve key ecosystem processes in the face of disturbances that recur over periods of centuries and affect areas of tens to thousands of square kilometers. The large fires that burned much of YNP in 1988 are the most obvious example of such a phenomenon; other examples include the eruption of Mount St. Helens in Washington and major floods. These major events create patchiness in the environment, and they may provide for the simultaneous occurrence of a critical set of characteristics that permit, for example, the establishment of, or change in, plant communities (Coughenour 1991, Turner et al. 1997, Foster et al. 1998). In addition to providing forage reserves for ungulates, a large spatial extent allows animals to spread out the effects of their consumption. In YNP, ungulates use some areas heavily but others only lightly. In spring and early summer, the ungulates follow the emergence and greening-up of actively growing, nutritious plants, grazing intensively in a limited area for a period, then moving on, allowing the plants to recover. Thus, spatiotemporal heterogeneity is key to maintaining nutritious forages over an extended period, and the sequential greening of vegetation provides the impetus for herbivores to move on and allow the plants time to recover. These interactions should permit long-term sustainability of the system; however, intensive long-term use during extreme winter conditions may not permit some communities of woody plants to persist.
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Ecological Dynamics on Yellowstone’s Northern Range WEATHER, WOLVES, AND ASPEN IN YELLOWSTONE Major controversy focuses on the causes of the virtual absence of recruitment of tree-sized aspen on the northern range since 1920 (Romme et al. 1995, Ripple and Larsen 2000b). Not all the circumstances that permitted aspen to recruit before 1920 are known, but the most important factor currently preventing recruitment of tree-sized aspen is heavy browsing by elk. If browsing by elk were greatly reduced or eliminated for a long enough time, as seems to have happened during the market hunting period of the 1880s (Romme et al. 1995), recruitment of tree-sized aspen would be likely under the current climate. What circumstances that previously existed, but are no longer present, might have permitted recruitment? The most obvious is that elk did not use aspen for winter survival because they migrated to lower areas with alternative winter food sources. Another possibility is that a combination of severe winters and a healthy predator population greatly reduced elk numbers or their distribution. Weather during the 1800s—the end of the Little Ice Age—was consistently cooler and wetter than that of the 1900s (Chapter 2). This factor alone could account for smaller elk populations wintering on the northern range. In addition, wolves were present during the 1800s and they likely influenced the density and distribution of elk. Ripple and Larsen (2000b) suggested that wolves played a key role in the recruitment of aspen. They reviewed evidence showing that wolves can limit herbivore population size, but more importantly, wolves modify the location and feeding behaviors of ungulates that feed on aspen, thereby leading to localized recruitment of tree-sized aspen. Ripple and Larsen’s hypothesis can account for small-scale recruitment of aspen, and with the addition of severe weather it can also account for synchronized, large-scale episodes of aspen recruitment. For this to occur may require the simultaneous effects of weather, and predation. Severe weather during the winter following the 1988 fires resulted in the death of about 25% of the northern range elk population (Singer et al. 1989). Similar events occurred throughout the 1900s, most recently in 1996–1997. Elk populations have been subjected to annual harvest outside YNP ever since its establishment, and late-season hunting was initiated in 1968. Thus, the condition of the animals and the winter range is likely to have been better since 1968 than if the population size had been regulated solely by natural factors, including competition for forage and starvation. When subjected to a severe winter, a population strongly regulated by food supply and with limited ability to mi-
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Ecological Dynamics on Yellowstone’s Northern Range grate out of the area is likely to experience high rates of mortality, resulting in a population considerably smaller than its prewinter size. If the elk population declined to a small fraction of what the northern range could support, then predation by wolves, whose numbers appear to be largely independent of elk population density, could prevent rapid recovery. A low enough density of elk would allow some aspen to grow tall, and sustained predation by wolves and other predators could maintain the elk population at a low density for long enough to permit recruitment of tree-sized aspen. In this scenario, establishment of tree-sized aspen requires (a) the elk population to decline rapidly after it has achieved a size too large to be maintained by the food available, (b) migration to be restricted, (c) a severe winter that causes starvation, and (d) a vigorous predator population that can keep the elk population from rapidly recovering. These conditions have been absent from the northern range since at least the late 1800s, when most (but not all) of the present tree-sized aspen stands were formed. Such a scenario is not greatly different from that which explains recruitment of fir on Isle Royale (Post et al. 1999), an island where long-range moose migration is prevented. Wolves hunted moose more efficiently during winters with heavy snowfall, thereby depressing moose populations and releasing fir from heavy browsing. Several types of interactions have been proposed to account for predatorprey systems in which predation can maintain low densities of prey, but food limitations prevail at high densities (e.g., Walker and Noy-Meir 1982, Sinclair 1989, Boutin 1992). In general, theory suggests that prey populations are kept at lower levels only until predator populations decline or food sources increase. If this situation were to occur on the northern range, aspen recruitment would be episodic and occur at unpredictable and infrequent intervals. INDICATORS OF UNACCEPTABLE CHANGE If YNP continues to follow a policy that permits the natural range of variation, it will need to monitor ecosystem attributes that might indicate unacceptable change. Research in the park is only decades old, but some insights into past conditions are provided by analyses of lake sediments, tree rings, pollen profiles, and floodplain sediment profiles. These analyses of long-term trends identify the dynamic processes that led to current conditions of the Yellowstone ecosystem, but the linkages between past and present processes in the northern range have not been clearly demonstrated by research. Modification
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Ecological Dynamics on Yellowstone’s Northern Range of the Yellowstone ecosystem through reintroduction of wolves, expansion of wintering areas for ungulates north of the park, and continued implementation of external hunting of ungulates, in the context of a changing climate, creates a degree of complexity that makes projection of long-term conditions in the park and northern range difficult. The committee consequently recommends that a comprehensive, integrated program of research and monitoring be established to measure the consequences of current and future changes in the external and internal driving variables. This program should include continued studies of animal and plant populations and their interactions, studies of predator-prey relationships, and studies of changes in the behavior of ungulates and predators as the system adjusts to the reestablishment of wolves. Concurrent studies of riparian and aspen recruitment; sagebrush communities; stream fluvial geomorphic processes in relation to riparian vegetation dynamics; rain, snow, surface flows, and groundwater levels; and other ecosystem components are also needed. UNDERSTANDING THE CONSEQUENCES OF ALTERNATIVE MANAGEMENT APPROACHES Resource managers at YNP use natural regulation as the management approach for the biota of the northern range for scientific reasons and to meet public expectations. In any natural resource management context, the selection of an approach is inevitably in part a value judgment. What is our intent? What do we, as a society, or other decision-making level, want from or for the resource? Although managers generally strive to design multiple-use management approaches, in fact there often is an underlying policy purpose. In Yellowstone, managers could manage the system primarily to facilitate visitor interaction with animals, for ecosystem diversity, for scenic values, or for a combination of those values. The current decision to use natural regulation as opposed to management that actively reduces ungulate populations and thus decreases grazing and browsing pressure on the northern range is based in part on science (i.e., the determination that ungulate populations are at sustainable levels given the productivity of the range’s vegetation), and in part it is a value judgment (based on the goals humans have set for the system). The committee was asked to evaluate NPS’s natural regulation management approach. It was not asked or appropriately constituted to look in depth at alternative management approaches. But in studying the dynamics of
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Ecological Dynamics on Yellowstone’s Northern Range ungulate-ecosystem interactions in the northern range of the Yellowstone region and researching the impacts of natural regulation on the ecosystem, the committee gained insights about other approaches and learned lessons about associated scientific advantages and disadvantages. These insights may help YNP resource managers plan future actions for the northern range, use adaptive management principles, learn from the information generated, and change management approaches as needed, as more information becomes available. Adaptive management requires clearly defined goals, and it is predicated on use of a scientifically sound, comprehensive, integrated research program and long-term monitoring to determine the successes and/or consequences of management decisions. The following text explores the scientific lessons that might be learned from various management approaches, including natural regulation. The committee recognizes that NPS managers must balance many factors beyond science in its decision making, but we can assist that process by projecting some of the possible ecological consequences of those decisions. Reduction of Elk and Bison Populations Within the Park Although the committee concludes that the number of ungulates in the northern range is less than the number at which density-dependent factors would cause it to decline (Chapter 4), experience from population reductions conducted in the 1950s and 1960s and from elk density/vegetation response studies elsewhere in the Rockies supports the view that a smaller population might allow recovery of some plant communities now degraded or unable to establish new recruits (e.g., woody riparian species including willows, aspen, and sagebrush communities). The likelihood that ungulate populations will be less than they have been recently is greater now that wolves are present Experimental management to reduce ungulate populations, especially elk, and perhaps bison, could test the hypothesis that lower densities of these animals would allow increased recruitment of tree-sized aspen, expansion of willow communities, and growth of sagebrush to large sizes. The most effective way to reduce elk numbers in YNP would be to shoot them, but doing so might be contrary to the desires and values of the public. Visitors would see fewer of them, and shooting is likely to arouse strong public reaction. In addition, reducing ungulate numbers at this time would confound our ability to understand the effects of wolf reintroduction on ungulates. Finally, there is
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Ecological Dynamics on Yellowstone’s Northern Range concern that a reduced ungulate population might disrupt food availability for the several wolf packs (and other predators) that now have a satisfactory food base within the park and lead the wolves to seek a domestic food base outside the park. Reduction of Elk and Bison Populations Outside the Park To test the hypothesis that reduced ungulate populations might allow recovery of woody plant communities, resource managers might experimentally reduce populations outside the park by working with the multiagency Northern Range Coordinating Committee to increase hunter harvest. This approach might partially test the concept that reduced elk numbers can enhance conditions of several northern range ecosystems (e.g., aspen, riparian, and sagebrush communities). An indirect social effect might be benefits to the local economy through increased outfitter clientele. However, this management approach also might confound our ability to understand the effects of wolf reintroduction, and the key disadvantage of the approach is that hunting success cannot be assured because elk might remain within the park, even during severe weather. Improve Opportunities for Increased Out-Migration Because lower elevation winter range outside the park has been greatly reduced, YNP resource managers could work with other state and federal agencies and land owners adjacent to the park to add more lands at lower elevations for winter use by ungulates. Elk herds throughout the northern Rockies tend to migrate from high to lower elevations as winter develops; the intensity of winter conditions usually influences the distance they move. Although hunting pressure at the park boundary may reduce migration seasonally, lack of open migration routes and land available for foraging at lower elevations also may influence migration. Lack of low-elevation winter range may eventually create an elk population that does not migrate outside the park but uses only the in-park northern range and higher elevation summer ranges. Already there are non-migratory elk populations within inner basins of YNP. Continued efforts to increase land available for elk winter range might reduce ungulate effects on ecosystems within YNP during harsh winters or permit a large ungulate herd to be sustained within the northern range area
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Ecological Dynamics on Yellowstone’s Northern Range with less damage to woody vegetation. Increasing the amount of winter habitat available also might prevent the transition of some of the northern range herd from migratory to nonmigratory, a phenomenon that over time could have long-term effects on the conditions of the northern range. This approach has numerous social and economic implications beyond the scope of this scientific assessment. For example, lands north of YNP in the Paradise Valley of the Yellowstone River have been used for ranching for decades, and many areas are fenced. At the same time, the human population of the Paradise Valley is increasing rapidly, giving rise to increased boundary controls and diverse opinions about wildlife use of private property. Finally, national forest lands in the mountains bordering the valley already have elk, and these animals usually move to lower elevations in limited areas in the valley in winter. Natural Regulation YNP resource managers consider the northern range to be in acceptable condition and the role and numbers of ungulates and other wildlife appropriate for a national park, and the best available scientific evidence does not indicate that ungulate populations are irreversibly damaging the northern range (Chapter 4). In addition, several significant changes have been made in the northern range in recent years, including the reintroduction of wolves and expansion of the winter range outside the park; the long-term influence of these changes cannot yet be determined. Thus, YNP resource managers could continue to manage the northern range as they are now. That is, YNP managers would continue to let the populations of elk, bison, and other ungulates fluctuate without any direct (inside Yellowstone) controls, letting a combination of weather, wolves, range conditions, and external controls (e.g., outside-the-park hunting, land uses, and population reduction by state agencies, such as the Montana Department of Livestock’s program for bison) influence the population numbers. Experimentation with continued use of natural regulation within YNP, recognizing the many external influences, would test whether the elk population has reached a dynamic equilibrium since the low numbers of the 1950s and 1960s. It would also allow time to observe the influences of the addition of a top predator and more available winter range. It will require careful monitoring to obtain full value from the experiment and to detect potentially serious changes in the ecosystem before they become severe or even irreversible.
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Ecological Dynamics on Yellowstone’s Northern Range CONCLUSIONS Animal Populations Density-dependent and density-independent factors interact to regulate the elk and bison populations in the northern range. Responses of elk and bison to potential regulatory factors are different: bison tend to expand their range when their populations exceed roughly 2,500, whereas reproductive rates in elk decline when their populations exceed roughly 15,000. Despite the density-dependent factors that affect elk and bison, their populations have fluctuated for a variety of reasons, including variation in weather and because ungulates and their food do not always vary in a synchronized way. Without rigorous management intervention, and perhaps even with it, ungulate populations will continue to fluctuate. The pronghorn population has fluctuated widely and has been declining recently. Adverse factors include coyote predation and hunting on private land outside the park. Pronghorn may be affected by competition with elk, mule deer, and bison during severe winters. Bighorn sheep also may be responding adversely to many of these same factors. Wolves will affect the population dynamics of ungulates as well as those of other predators in YNP, as they do elsewhere. The nature and magnitude of the effects are not predictable at present, although it is likely that wolves will reduce elk numbers. They might increase the magnitude or frequency of elk population fluctuations and might cause changes in the behavior of ungulates, especially elk, including changes in areas where they forage and spend time. The effect of wolves on bison is likely to be less variable and dramatic than their effect on elk, their primary prey in YNP. Ungulates and Vegetation Tree-sized aspen have not been added to the population in the northern range since about 1920. Currently, herbivory by elk is high enough to prevent any such recruitment, and apparently it has been since 1920. Although there have been fluctuations in climate since then, none has been large enough or persistent enough to account for the failure of aspen recruitment. Two untested hypotheses, working independently or in conjunction, could explain past recruitment. One is that enough elk migrated out of the park in severe winters to greatly reduce browsing pressure on aspen. The other is that wolves, be-
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Ecological Dynamics on Yellowstone’s Northern Range fore their extirpation, affected the distribution and abundance of elk so that at least some recruitment of tree-sized aspen and willows occurred even when elk were moderately abundant. If the latter were the case, then the wolves recently reintroduced into Yellowstone, including those in the northern range, could promote the recruitment of adult aspen and willows. All tree-sized aspen in the northern range are more than 80 years old, and in the absence of recruitment, they will die out. Species associated with aspen will likely decline as well. Elk also are reducing the size and areal coverage of willows. Not enough is known about groundwater fluctuations or the role of secondary chemicals in herbivory to determine whether they are also affecting willow abundance. Plant architecture and areal coverage of sagebrush has decreased during recent decades through browsing by elk, pronghorn, bison, and mule deer. In addition, herbivory has altered community composition, size, and recruitment. The effects are more significant at lower than at higher elevations in the northern range. The composition and productivity of grassland communities in the northern range show little change with increasing grazing intensity. Humans, however, have changed the grasslands substantially by introducing exotic grasses and by other actions, many of which began before thorough inventories were initiated. Although conifer forests are used by ungulates, ungulates have little effect on conifer distribution and recruitment except for localized hedging of young conifers invading shrub and grassland areas. The summer range does not seem to be limiting to the ungulate populations. Densities are relatively low on the summer range because the animals are spread out over larger areas than during winter-range use. Ungulates apparently have little effect on summer range communities, with the exception of young aspen, which are severely browsed. The Northern Range The condition of the northern range is different today than when Europeans first arrived in the area. The committee judges that the changes are the result of the larger numbers of elk and bison in the area, combined with human development and possibly climatic variability. The committee concludes, based on the best available evidence, that no major ecosystem component is likely to be eliminated from the northern range in the near or intermediate term. Further, although we recognize that the current balance between ungulates and
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Ecological Dynamics on Yellowstone’s Northern Range vegetation does not satisfy everyone—there are fewer aspen and willows than in some similar ecosystems elsewhere—the committee concludes that the northern range is not on the verge of crossing some ecological threshold beyond which conditions might be irreversible. The same is true of the region’s sagebrush ecosystems, despite reductions in the number and size of plants in some lower-elevation areas. Natural Regulation True natural regulation (i.e., letting nature take its course with no human intervention) has not been possible for more than a century, nor is it likely to become possible in Yellowstone’s foreseeable future. Because of development on the park’s borders, ungulates do not have free access to areas outside YNP that they formerly used during times of environmentally imposed stress. Because ungulate populations are influenced by activities both inside and outside the park, the conclusions in this report should not be interpreted as either vindication or criticism of YNP’s natural regulation policy. YNP’s practice of intervening as little as possible is as likely to lead to the maintenance of the northern range ecosystem and its major components as any other practice. If the park decides that it needs to intervene to enhance declining species like aspen, the smaller the intervention, the less likely it is to do unintended damage. For example, if YNP decided to maintain tree-sized aspen in the park, putting exclosures around some stands would be an intervention much less likely to trigger unanticipated processes than an attempt to eliminate or greatly reduce populations of ungulates. Large ecosystems in general and YNP’s northern range in particular are dynamic. They change in sometimes unpredictable ways. The recent reintroduction of wolves, which has restored an important component of this ecosystem, adds to the dynamism, complexity, and uncertainty, especially in the short term. The near future promises to be most instructive about how elk and other ungulates interact with a complete community of predators. RECOMMENDATIONS Given the complexities involved in managing Yellowstone’s dynamic ecosystems, there is a continuing need for rigorous research and public education. The committee offers the following recommendations designed to enhance
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Ecological Dynamics on Yellowstone’s Northern Range understanding of key processes affecting Yellowstone’s ungulate populations, vegetation, and ecological processes. Park Management and Interpretation To the degree possible, all management at YNP should be done as adaptive management. This means that actions should be designed to maximize their ability to generate useful, scientifically defensible information, including quantitative models, and that the results of actions must be adequately monitored and interpreted to provide information about their consequences to guide subsequent actions. There is insufficient scientific knowledge available to enable us to predict the consequences of different management approaches. Thus, long-term scientific investigations and experiments are needed to provide solid scientific evidence for evaluating management options. The NPS educational and outreach program can play an important role in fostering public understanding of the complex and dynamic nature of ungulate ecology in the GYE, which is an essential adjunct to effective management of northern Yellowstone ungulates. Therefore, we encourage the NPS to increase its focus on entire ecosystem relationships, processes, and dynamics of the GYE, especially emphasizing the importance of primary production and trophic-level relationships. Vegetation A rigorous study focusing on aspen populations throughout the GYE should be undertaken to quantify the relative importance of the factors known or hypothesized to influence aspen stand structure. It should include establishing an increased number of large exclosures with a long-term commitment to monitoring the effects of restricting herbivory by ungulates. The study sites should be discussed in the NPS ecosystem interpretive program. A careful examination of the variables that are most strongly affecting the riparian ecosystems on the northern range is needed, especially the relationship between herbivory and groundwater availability. This should include an understanding of fluvial processes, surface and groundwater hydrology, and biotic processes.
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Ecological Dynamics on Yellowstone’s Northern Range Research should continue on northern range sagebrush/grassland communities. Research is needed to determine whether it is possible to differentiate ungulate use of tall and short willows based on both the food-deprivation levels of the ungulates (i.e., winter starvation) and the levels of secondary chemicals in the plants. Animal Populations The behavioral adaptations of elk and other ungulates, and the changes in patterns of habitat use as a consequence of the presence of the wolf as a large predator newly restored to the system, should be closely monitored as a basis for understanding the dynamic changes that are taking place within the system. The changes taking place in the interactions among the large predators of YNP and their effects on the trophic dynamics of the ecosystem should be closely monitored as wolves become an established component of the system. A thorough study of current and likely future trajectories of the pronghorn population and the role of human effects on this population is needed, including the influence of disturbance by visitors and the Stevens Creek bison facility. The study should evaluate the likely consequences of a full range of potential management options, from doing nothing to actively controlling predators and providing artificial winter feed. Periodic surveillance for pathogens (including brucellosis) in wild ruminants in the northern range should be continued, and a more thorough understanding of population-level threshold dynamics gained. Samples could routinely be obtained from animals immobilized for research, found dead, or killed by hunters. Biodiversity Aperiodic (every 10–15 years) and comprehensive biodiversity assessment is needed on the northern range to evaluate potential direct and indirect impacts of ungulate grazing, both of terrestrial and aquatic environments. Initially, species should be identified as consistent indicators of habitat change. These species should then be monitored intensively during periods between comprehensive assessments.
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Ecological Dynamics on Yellowstone’s Northern Range Human Influences A comprehensive research effort is needed to assess the influence of seasonal densities, distribution, movements, and activities of people within YNP and adjacent areas on wildlife species, their habitat use patterns, behavior, foraging efficiency, vegetation impacts, and other aspects of their ecosystem relationships. The effects of changing land-use patterns in the landscape surrounding Yellowstone on the park’s biota and natural processes, such as fire, need to be investigated. EPILOGUE GYE is dynamic, and change is a normal part of the system as far back as we have records or can determine from physical evidence. Based on that record of change, it is certain that sooner or later the environment of the GYE will change in ways that cause the loss of some species and changes in community structure. Human-induced changes, both within the GYE and globally, are likely to accelerate these changes. Although dramatic ecological change does not appear to be imminent, it is not too soon for the managers of YNP and others to start thinking about how to deal with potential changes. Before humans modified the landscape of the GYE—limiting access to much of it and interrupting migration routes—animals could respond to environmental changes by moving to alternative locations. To a lesser degree, and over longer time frames, plants could adapt as well, especially in places with significant topographic relief. But many options that organisms formerly had for dealing with environmental changes have been foreclosed because of human development of the region. Human-induced climate change is expected to be yet another long-term influence on the ecosystem. Reconciling the laudable goals of preserving ecosystem processes and associated ecosystem components with human interests and influences on wildlands will be a growing challenge in the future, not only in the GYE. That reconciliation will involve conflicting policy goals, incomplete scientific information, and management challenges. Resolving these conflicts will require all the vision, intellectual capacity, financial resources, and goodwill that can be brought to bear on them.
Representative terms from entire chapter: