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Ecological Dynamics on Yellowstone’s Northern Range Summary THE GREATER YELLOWSTONE ECOSYSTEM (GYE) is a large and rich temperate ecosystem that includes a variety of natural landscapes and supports diverse human activities. It provides economic, recreational, educational, and aesthetic benefits and has a growing resident human population. At the heart of the ecosystem is the 8,991-km2 Yellowstone National Park (YNP)—declared the world’s first national park in 1872, made a biosphere reserve in 1976, and added to the World Heritage List in 1978. Seven ungulate1 species are native to the region: elk, mule deer, bison, moose, bighorn sheep, pronghorn, and white-tailed deer. All the native large predators are present: grizzly bear, black bear, coyote, mountain lion, and the reintroduced gray wolf. YNP faces peculiar and complex management challenges. One of the most contentious issues is the management approach in place since the late 1960s called “natural regulation.” Under natural regulation, ecological processes within the park generally are left to function free of direct human interventions, or, as described by the National Park Service (NPS), “natural environments evolving through natural processes minimally influenced by human actions.” Concern has centered on the ecosystem of the northern range of YNP, especially about the effects of natural regulation on ungulate populations and subsequently their effects on vegetation. 1 Hoofed mammals.
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Ecological Dynamics on Yellowstone’s Northern Range Ungulates that graze within YNP for much of the year often winter in the northern range in and adjacent to the park. Two-thirds of the northern winter range is within YNP; one-third is north of the park boundary on public and private lands (Figures 1–1 and 1–2 in Chapter 1). Elk and bison populations in the northern range have increased dramatically in recent years, particularly in years with mild winters, leading some scientists and members of the public to question the appropriateness of the park’s natural-regulation policy. These critics believe the northern winter range is overgrazed and that woody vegetation and riparian areas are being damaged, mainly by elk. Further, they see overgrazing by elk and bison as contributing to serious erosion and stream degradation. However, other scientists and resource managers note that ungulates have influenced vegetation on the northern range for thousands of years and believe that natural density-dependent factors such as forage availability, predation, and disease are regulating population dynamics so that current conditions fall within the natural range of variability. In recent years, the controversy over natural regulation has heightened, especially in the northern range—wintering range of Yellowstone’s elk herds. In 1998, the U.S. Congress directed the NPS “to initiate a National Academy of Sciences review of all available science related to the management of ungulates and the ecological effects of ungulates on the range land of Yellowstone National Park and to provide recommendations for implementation by the Service.” In response to that mandate, the National Research Council convened the Committee on Ungulate Management in Yellowstone National Park. This committee of experts was charged to review the scientific literature and other information related to ungulate populations in the Yellowstone northern range and to attempt to clarify what is known and not known about natural regulation and the ecological effects of elk and bison populations on the landscape.2 The committee’s geographic focus has been Yellowstone’s northern range. The committee’s evaluation addresses the issue from a scientific perspective, which deals with only part of a multifaceted problem that includes sociological, economic, aesthetic, and other important dimensions beyond the scope of this study. 2 See Chapter 1 for the committee’s full statement of task and a description of its methods.
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Ecological Dynamics on Yellowstone’s Northern Range NATURAL REGULATION Natural regulation is the current NPS policy for managing ungulates and other ecosystem components within YNP. In theory, natural regulation means simply “free of direct human manipulation.” The intent is to allow the biological and physical processes within the park to function without direct human intervention. A more accurate definition of natural regulation as practiced by NPS in YNP is that it attempts to minimize human impacts on the natural systems of the park. Implementing natural regulation is difficult because the NPS must accommodate the millions of visitors to YNP annually and control naturally caused fires that threaten human life and buildings. Although YNP’s natural regulation policy involves little intervention within the park, ecological processes in the region are profoundly influenced by human activities outside the park. The underlying belief that national parks should, to the maximum extent possible, harbor natural ecosystems has fostered extensive debates about how to react to ecosystem change in the parks and how to determine when such change is caused by humans. In Yellowstone, the controversy concerns whether human activities have caused ungulate populations to grow too large for the ecosystem, or whether observed changes in the ecosystem are due to natural variability in factors such as climate. If the changes are caused by humans, YNP’s natural regulation policy presumably would allow intervention to mediate the effects. If they are not caused by humans, intervention would be inappropriate. The problem of differentiating human-caused from natural change is complex because no ecosystem on earth is entirely unaffected by human activity. Thus, defining “natural” is difficult. In view of the profound changes that have occurred within the GYE, such as increased development of roads and housing in areas adjacent to the park, it is no longer possible to have an ecosystem that is truly natural—that is, containing the same numbers and distributions of all the species of plants and animals that were there before European settlement, let alone before Native American populations arrived. YNP may contain many of the same species, but they can no longer respond to change as they used to by dispersal and migration. To understand the controversy about ungulate management in YNP and evaluate management options, it is also important to understand the distinction between policy and practice. A policy is formal and is used to determine present and future decisions. A practice, which is less formal and more flexi-
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Ecological Dynamics on Yellowstone’s Northern Range ble, is the actual action of fulfilling a management concept. Natural regulation clearly is regarded as a policy by YNP resource managers. As a result, if a change in the ecosystem is natural, then management intervention is contrary to park policy, whereas if change results from an action implemented by the park or other human causes, then management is considered appropriate. If natural regulation were only YNP’s practice, perhaps the debates could focus more on the actions and their outcomes and less on whether they were consistent with a policy. Also, adaptive management would be easier to pursue. The controversy over natural regulation in Yellowstone revolves primarily around the question of whether ecological processes within the ecosystem are seriously disrupted. Because some component of the ecosystem may appear to be disrupted—e.g., effects of heavy grazing in the northern range and degradation of its riparian areas—some people criticize natural regulation. Supporters of natural regulation argue that, in the face of constantly changing biotic and abiotic environments, current conditions are within the range of natural variation and that Yellowstone is not in ecological trouble. Therefore, one of this committee’s tasks was to evaluate whether conditions in the northern range ecosystem are outside the range of what might be expected based on comparisons with similar ecosystems elsewhere and historical information about the GYE. The committee also addressed whether current conditions if allowed to continue, are likely to lead to substantial and rapid change in any major ecosystem components or processes. PERSPECTIVE: FROM PREHISTORY TO THE PRESENT Abiotic Factors In addition to recent human-caused changes, the ecological processes of YNP have been profoundly influenced over the long term by changes in the physical environment. Climate change has been important during the past 10,000 years. Average temperature and precipitation have changed substantially over this period and continue to fluctuate over periods as brief as a few decades. The Little Ice Age, which ended in the late 1800s, was a cooler and wetter climate than that of the 1900s, during which no substantial trend in either temperature or precipitation is evident. There have been drier and wetter times, and the past century has been characterized by somewhat less snowpack and fewer very snowy years than the previous century.
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Ecological Dynamics on Yellowstone’s Northern Range Over millennia, geological events, including mudslides, erosion, and destructive earthquakes such as the one that struck in 1959, have recurred over the centuries and continue today. In the shorter term, on the order of centuries, the influence of geological events is not usually enormous (except for cataclysmic earthquakes or eruptions), but it is noticeable and is perhaps the largest factor influencing the valley floors. The major floods of 1996 and 1997, which caused changes in the park’s streams and their associated riparian communities, are examples of rapid geomorphological change. Fires also have been part of the GYE for millennia. Major fires appear to occur every few hundred years, usually during particularly dry periods. Smaller fires occurred approximately every 20–25 years in the northern portion of YNP before the initiation of fire-control measures in the late 1800s. Natural fires were suppressed in YNP through most of the twentieth century until that policy changed in 1972. The major fires of 1988, which occurred in a very dry period, affected about 36% of YNP. Biotic Factors Plants Although the northern range is primarily a shrub-steppe interspersed with Douglas-fir and lodgepole-pine forests, willows and aspen, which occupy only a small percentage of the range, have been the focus of scientific and public attention. Aspen spread clonally and typically regenerate by root sprouting with occasional episodes of seedling establishment. Thus, clones may be much older than the age of the oldest tree-sized stem. Recruitment (i.e., entry into the population) appears to have occurred through about 1920. Since 1920, however, recruitment of tree-sized aspen has been almost nonexistent. In most parts of the northern range, the sizes and aereal coverage of riparian willows and cottonwoods have decreased since the early 1900s. Most tree species that burned in the fires of 1988 successfully reseeded and regrew. Aspen vigorously reproduced by sprouting and seed germination, but browsing has prevented recruitment of tree-sized individuals. No systematic changes in abundance of other tree species have been documented, although there is evidence of expansion of coniferous forests into shrub and grassland areas. Sagebrush and grasses have changed little at higher elevations in the northern range, locations not heavily used by elk in winter. But the cover and den-
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Ecological Dynamics on Yellowstone’s Northern Range sity of sagebrush and other shrubs has been greatly reduced at some lower elevation sites, especially areas near the northern border of YNP. Introduced timothy grass has spread widely in the northern range’s Lamar Valley, and elsewhere in YNP. Animals Humans have used the area that is now YNP since the end of the last glaciation, about 10,000 years ago. This long history of human use has helped investigators to identify other species that were once present because humans (along with some other predators) tend to deposit animal and plant remains in concentrated areas. Based on evidence from this prehistoric period, it is clear that all the major ungulates and predators now occupying YNP, as well as many of the smaller animals, have been present for at least the past 10,000 years. However, it is impossible to estimate historical abundances of those species; all that can be stated with confidence is that they were present in large enough numbers that their remains can be found. Reliable population estimates for elk in the northern range became available only with the initiation of aerial surveys in 1952. Historical records since the 1860s report encounters with elk, bison, grizzly bears, and other mammals, but they do not permit estimates of the densities of those mammals in and around YNP. Thus, it is not possible—and probably never will be possible—to have good estimates of the populations of elk and bison in the northern range, annually and seasonally, between about 1870 and 1920, a focal period for the controversy over aspen and other vegetation. From the 1930s through the 1960s the NPS hunted and trapped elk to reduce their populations because park scientists during this period considered the northern range highly degraded by an excessive population of elk. By the late 1960s, when YNP adopted natural regulation, the northern range elk population had been reduced from some 10,000 animals to fewer than 5,000. But by the late 1980s, as many as 20,000 elk were on the northern range. Approximately 12,000 elk were on the northern range in 1999, with approximately 120,000 in the GYE as a whole. As of the most recent census (2000), roughly 2,500 free-ranging bison were also present in the GYE. Other ungulate populations in the GYE have also fluctuated over time; pronghorn apparently are declining, with only about 200 remaining in the late 1990s. During various periods after the park was established in 1872, ungulates
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Ecological Dynamics on Yellowstone’s Northern Range were fed in winter, hunting was restricted, and predators were killed. Predator populations (e.g., mountain lions) were reduced and wolves were extirpated. Of the major predators, the best current population estimates are for wolves, which were reintroduced to YNP in 1995. Today (2001) about 160 wolves reside in YNP, 50 of which are on the northern range. There are perhaps 300 grizzly bears, between 1,000 and 2,000 black bears, and a few hundred mountain lions in YNP and environs. Coyote populations have been reduced by wolves; for example, the coyote population in the Lamar Valley dropped from 80 in 1995 to 36 in 1998. Although beavers in the northern range have never been accurately counted, it is clear that their populations have declined dramatically since the 1920s. The reasons for and the effects of that decline remain uncertain, although they include commercial trapping in the decade after the park’s establishment. There is controversy about whether the decline in beaver populations is related to changes in aspen recruitment and changes in riparian vegetation communities. CONCLUSIONS Animal Populations Factors whose influences are related to population density (called density dependent) interact with factors whose influences are not (called density independent) to regulate elk and bison populations in the northern range. There is a strong density-dependent signal in northern range elk and bison population dynamics, but their responses differ: bison tend to expand their range to areas outside YNP when their population exceeds roughly 2,500, whereas reproductive rates in elk decline when their populations exceed roughly 15,000. In addition to density-dependent factors, elk and bison populations also are affected by density-independent factors such as weather and because ungulates and their food do not always vary in a synchronous way. Thus, some ungulate populations tend to fluctuate regardless of human management intervention. The pronghorn population has fluctuated widely during the past century and has been declining recently. Adverse factors include coyote predation and hunting on private land outside the park. Also, pronghorn may be affected by competition for food with elk, mule deer, and bison during severe winters. Wolves also affect the population dynamics of ungulates as well as those
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Ecological Dynamics on Yellowstone’s Northern Range of other predators in YNP. The nature and magnitude of the effects are not predictable at present, because the reintroduction was so recent (1995), but it is likely that wolves will reduce elk numbers. They almost certainly will cause changes in the behavior of ungulates, especially elk, including changes in areas where the elk spend time. The effect of wolves on bison is likely to be less variable and dramatic than their effect on elk, because elk are their primary prey in YNP. Ungulates and Vegetation Currently, in the northern range, herbivory by elk on young aspen is intense and has probably prohibited recruitment since 1920. Although there have been fluctuations in climate since 1920, none has been large enough or persistent enough to account for the failure of aspen recruitment. A plausible hypothesis—and it is no more than that at present—is that wolves, before their extirpation, affected the distribution and abundance of elk so that at least some recruitment of tree-sized aspen and tall growth of willows could occur. Recent restoration of wolves to YNP may allow evaluation of their role in aspen and willow recruitment and maintenance, but scientific information is lacking to understand the role of past development and hunting outside the park on elk behavior and migration patterns. All tree-sized aspen in the northern range are now more than 80 years old, and in the absence of recruitment their abundance will continue to decline. Species associated with aspen will likely decline along with tree-sized stems. Elk also are reducing the size and areal coverage of willows. Defensive chemicals3 in riparian woody vegetation may influence herbivory; however, their role in the decline in stature or loss of willows and riparian vegetation in the northern range during the past century is not known. The architecture, size, recruitment, and coverage of sagebrush have been changed by elk, pronghorn, bison, and mule deer. The effects are more significant at lower than at higher elevations in the northern range. 3 Plants defend themselves from predation in two main ways: (1) defense structures, such as thorns, and (2) toxicity and unpalatability caused by so-called “secondary chemicals.” These compounds may be directly toxic or they may reduce the food value of the plant—for example, by reducing the availability of the leaf tissue protein to the animal gut.
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Ecological Dynamics on Yellowstone’s Northern Range Composition and productivity of grassland communities in the northern range have not changed much with increases in herbivory. Humans have caused some changes (e.g., the introduction of timothy grass and other exotics). Other changes might have occurred before careful inventories were taken and may not be detectable. Although conifer forests are used by ungulates, there is no evidence that ungulates affect their species composition. Summer range does not seem to be limiting to the ungulate populations in YNP. Densities on the summer range are relatively low because the animals spread out over larger areas than in the winter range. There is little evidence for an ungulate effect on the summer range communities, with the exception of young aspen, which are heavily browsed. The Northern Range The condition of the northern range is different today than when Europeans first arrived in the area. This has led some people to conclude that something is “wrong” with YNP’s northern range. Such conclusions reflect subjective value judgments in addition to objective observations. For example, some people compare the northern range unfavorably with nearby ranches, but that reflects a mixing of values. Ranching seeks high production for human uses, but YNP seeks to preserve a natural environment and the species and ecological processes within it. The committee recognizes that such value judgments influence debates about YNP but has focused this report on scientific information and conclusions. The committee judges that the changes in the northern range are the result of the number of ungulates in the area combined with biophysical factors such as climatic variability, but current methods do not allow us to separate the relative contributions of each of these effects. However, the committee concludes, based on the best available evidence, that no major ecosystem component is likely to be eliminated in the near or intermediate term. Further, although we recognize that the current balance between ungulates and 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 at some lower elevations.
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Ecological Dynamics on Yellowstone’s Northern Range Natural Regulation The conclusions in this report should not be interpreted either as a criticism or as a vindication of YNP’s natural regulation policy other than to say that it has not been associated with ecological disaster. “Natural” cannot be unambiguously or objectively defined. In addition, human activities adjacent to YNP have large effects on the animals present at least seasonally within the park. The animals do not have free access to the adjacent areas that formerly were available to them as migration corridors and winter range. For these reasons, true natural regulation in YNP—that is, really letting nature take its course with no human intervention—is not possible. YNP’s practice of intervening as little as possible in the ecology of ungulates within YNP will likely allow the persistence of the northern range ecosystem and its major components as long as there is no large change in climate. If the NPS decided that it needed to intervene to protect species like aspen and the species that depend on tree-sized aspen stems, localized interventions would be prudent. For example, if YNP decided to maintain tree-sized aspen in the park, putting exclosures around some stands would be less potentially disruptive than eliminating ungulates or reducing their numbers. The most effective way to reduce ungulate numbers in YNP would be to shoot them (unless wolves have a larger effect than currently expected). Earlier shooting of elk in YNP provoked strong public protest. Without strong scientific justification for doing so, which the committee cannot provide, future shooting of elk in YNP would provoke strong public protest again, and its benefit would be unclear at best. We emphasize again that large ecosystems in general and YNP’s northern range in particular are dynamic. Ecosystems change in unpredictable ways. The recent addition of wolves, which has restored an important component of this ecosystem, adds to the dynamism and uncertainty, especially in the short term. Whether viewed as an experiment or not, 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.
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Ecological Dynamics on Yellowstone’s Northern Range The committee offers the following recommendations designed to enhance understanding of the key processes that affect 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 objective comparison of different management approaches and understanding of the consequences of management choices. Thus, long-term scientific investigations and experiments to provide solid scientific evidence for evaluating management options are needed. 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 the northern Yellowstone ungulates. In this regard, we encourage the NPS to increase their focus on entire ecosystem relationships, processes, and dynamics of the GYE, especially emphasizing the importance of primary production (conversion of sunlight to stored carbon by plants) and trophic-level (i.e., hierarchical levels in the food web) relationships. Vegetation A rigorous study focusing on current 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 the use of an increased number of large exclosures with a long-term commitment to monitoring the effects of restricting herbivory by ungulates. The study sites could be discussed in the NPS ecosystem interpretive program. The most important driving variables that affect the modified riparian ecosystems in these areas today, especially the relationship between herbivory
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Ecological Dynamics on Yellowstone’s Northern Range and groundwater availability, need to be carefully examined. This should include an understanding of fluvial processes, surface and groundwater hydrology, and biotic processes. Research and monitoring should continue on northern-range sage-brush-grassland communities. Research to determine whether it is possible to differentiate ungulate use of tall and short willows on the basis of both the food-deprivation levels of the ungulates (i.e., winter starvation) and levels of defensive chemicals in the plants is needed. Animal Populations The behavioral adaptations of elk and other ungulates as well as changes in their 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 the reintroduced wolves become an established component of the system. Thorough study of current and likely future trajectories of the pronghorn population and the role of human impacts on this population, including disturbance by visitors and the Stevens Creek bison facility, is needed. The study should evaluate the likely consequences of a full range of potential management options from doing nothing to actively controlling predators and providing 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 A periodic and comprehensive biodiversity assessment every 10 to 15 years is needed on the northern range to evaluate potential direct and indirect
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Ecological Dynamics on Yellowstone’s Northern Range impacts of ungulate grazing, for both terrestrial and aquatic environments. Indicator species that reflect habitat change should be identified. Those species should be monitored intensively between comprehensive assessments. Human Influence 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, effects on vegetation, and other aspects of their ecosystem relationships. The effects of changing land-use patterns in the landscape surrounding Yellowstone must be understood with regard to its expected influence on the park’s biota and natural processes, such as fire. 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 GYE will change in ways that cause the loss of some species and changes in community structure. If human-induced changes are taken into account, both within GYE and globally, that circumstance is likely to be sooner than would otherwise occur. 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 lower elevation wintering areas and interrupting migration routes—animals could respond to environmental changes by moving to alternative locations. Over a longer time frame, plants could adapt as well, although to a lesser degree, 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 in the region. Human-induced climate change is expected to be yet another long-term influence on the ecosystem. A future challenge for the GYE area and other wildlands will be reconciling the laudable goals of preserving ecosystem processes with human interests and influences. That reconciliation will
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Ecological Dynamics on Yellowstone’s Northern Range require resolving conflicting policy goals, bolstering incomplete scientific information, and overcoming management challenges. Doing so will require all the vision, intellectual capacity, financial resources, and goodwill that can be brought to bear on them.
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