Nature

YELLOWSTONE BISON conserving an american icon in modern society edited by P.J. White, Rick L. Wallen, and David E. Hallac YELLOWSTONE BISON Yellowstone Bison: Conserving an American Icon in Modern Society Editors P.J. White, Rick L. Wallen, and David E. Hallac Contributing Authors Katrina L. Auttelet, Douglas W. Blanton, Amanda M. Bramblett, Chris Geremia, Tim C. Reid, Jessica M. Richards, Tobin W. Roop, Dylan R. Schneider, Angela J. Stewart, John J. Treanor, and Jesse R. White Contributing Editor Jennifer A. Jerrett Yellowstone Association Yellowstone National Park, USA, 2015 P.J. White is the Chief of Wildlife and Aquatic Resources at Yellowstone National Park. Rick L. Wallen is the Bison Project Leader at Yellowstone National Park. David E. Hallac was the Division Chief of the Yellowstone Center for Resources at Yellowstone National Park between 2011-2014. The Yellowstone Association, Yellowstone National Park 82190 Published 2015 Printed in the United States of America All chapters are prepared solely by officers or employees of the United States government as part of their official duties and are not subject to copyright protection in the United States. Foreign copyrights may apply. National Park Service (NPS) photographs are not subject to copyright protection in the United States. Foreign copyrights may apply. However, because this work may contain other copyrighted images or other incorporated material, permission from the copyright holder may be necessary. Cover image: NPS/Neal Herbert. Half title image: NPS/Jacob W. Frank. Library of Congress Cataloging-in-Publication Data Yellowstone bison : conserving an American icon in modern society / edited by P.J. White, Rick L. Wallen, and David E. Hallac ; contributing authors, Katrina L. Auttelet [and 10 others] ; contributing editor, Jennifer A. Jerrett. pages cm Includes bibliographical references and index. ISBN 978-0-934948-30-2 (pbk. : alk. paper) 1. American bison--Conservation--Yellowstone National Park. 2. American bison-Conservation--United States. 3. Wildlife conservation--Yellowstone National Park. 4. Wildlife conservation--United States. 5. Wildlife management--Yellowstone National Park. I. White, P. J. (Patrick James) II. Wallen, Rick L. III. Hallac, David E. IV. Auttelet, Katrina L. V. Jerrett, Jennifer A. QL737.U53Y45 2015 599.64’30978752--dc23 2015004628 The authors gratefully acknowledge the generous contribution of the Yellowstone Association, whose publication grant enabled the production of this book. Contents Prefaceix Daniel N. Wenk, Superintendent, Yellowstone National Park Introductionxiii P.J. White, Rick L. Wallen, and David E. Hallac Chapter 1: The Population1 Douglas W. Blanton, P.J. White, Rick L. Wallen, Katrina L. Auttelet, Angela J. Stewart, and Amanda M. Bramblett Chapter 2: Brucellosis19 P.J. White, David E. Hallac, Rick L. Wallen, and Jesse R. White Chapter 3: Historical Perspective45 Rick L. Wallen, P.J. White, and Chris Geremia Chapter 4: Seasonal Distributions and Movements67 Chris Geremia, P.J. White, Rick L. Wallen, and Douglas W. Blanton Chapter 5: Reproduction and Survival83 Chris Geremia, P.J. White, Rick L. Wallen, and Douglas W. Blanton Chapter 6: Nutritional Ecology97 John J. Treanor, Jessica M. Richards, and Dylan R. Schneider Chapter 7: Ecological Role107 Rick L. Wallen, P.J. White, and Chris Geremia Chapter 8: Adaptive Capabilities and Genetics119 Rick L. Wallen and P.J. White Chapter 9: Cultural Importance131 Rick L. Wallen, P.J. White, and Tobin W. Roop Chapter 10: Current Management141 P.J. White, Rick L. Wallen, David E. Hallac, Chris Geremia, John J. Treanor, Douglas W. Blanton, and Tim C. Reid Chapter 11: The Future159 P.J. White, Rick L. Wallen, Chris Geremia, John J. Treanor, and David E. Hallac Acknowledgments179 Glossary of Terms 181 References197 Index243 Abbreviations used in citations: APHIS Animal and Plant Health Inspection Service MDOL Montana Department of Livestock MFWP Montana Fish, Wildlife & Parks NPS National Park Service USDA United States [U.S.] Department of Agriculture USDI U.S. Department of the Interior USFS U.S. Forest Service YNP Yellowstone National Park Recommended citation format: Entire book.—White, P. J., R. L. Wallen, D. E. Hallac, and J. A. Jerrett, editors. 2015. Yellowstone bison—Conserving an American icon in modern society. Yellowstone Association, Yellowstone National Park, Wyoming. Individual chapter.—Wallen, R. L., P. J. White, and C. Geremia. 2015. Historical perspective. Pages 45-65 in P. J. White, R. L. Wallen, D. E. Hallac, and J. A. Jerrett, editors. Yellowstone bison—Conserving an American icon in modern society. Yellowstone Association, Yellowstone National Park, Wyoming. Photograph courtesy of National Geographic by Michael Nichols Bison in winter, Yellowstone National Park. Preface Daniel N. Wenk, Superintendent, Yellowstone National Park My association with bison management in Yellowstone National Park began in the early 1980s when I worked for Superintendents John Townsley and Bob Barbee. Back then, there were fewer than 2,500 bison and the central herd was the largest herd in the park. Also, bison were not yet making seasonal migrations past the north and west boundaries of the national park. Because bison were not leaving the park, brucellosis within the population was less of a concern. Bison contributed greatly to the enjoyment of the park by visitors as part of the incredible wildlife display. Much has changed. About three decades later, when I was preparing to return to the park as Superintendent beginning February of 2011, I was made aware of a court-mediated settlement regarding the management of bison signed by the Secretaries of Agriculture and the Interior and the Governor of Montana. But my knowledge came from a distance, and I did not yet fully understand or appreciate the complexity of management or the intensity of emotion and conflict that was occurring on the boundaries of Yellowstone National Park. x Yellowstone Bison: Conserving an American Icon in Modern Society My education came quick. The winter of 2010-2011 was harsh and bison were moving into the Gardiner basin of Montana in large numbers. Within two weeks of my arrival in the park, I was in Montana Governor Brian Schweitzer’s office in the state capital to discuss bison management. The Governor suggested methods to control population numbers and brucellosis, including a proposal to hunt bison within the national park. I rejected this proposal because it was contrary to law, regulation, and policy of the National Park Service and Yellowstone. It was also unnecessary; the park could ecologically support the bison population and we were successfully managing the population without hunting in the park. In an unexpected turn of events, and what would become my first lesson in the politics of bison management, Governor Schweitzer issued an Executive Order prohibiting the shipment of bison on state highways. The hands of the National Park Service were tied. That winter, approximately 850 bison were held in capture facilities until early May when they migrated back into the park. What I learned in my first weeks as Superintendent is that the science, so well-highlighted in this book, must be joined with the strength and commitment of people to successfully manage a wide-roaming bison population and prevent brucellosis transmission to cattle. Many of these individuals are the authors of this book: wildlife biologists, lawenforcement officers, and others who are the experts in the field of bison management. Herein, they clearly articulate not only the science, but also, the cultural and political significance of bison management. During my time in Yellowstone, I have watched with great interest — and some amazement— that bison are vilified as the primary threat or vector for brucellosis transmission in the ecosystem. There is an illusory belief that if brucellosis were eliminated in bison it would be eliminated from the ecosystem. The authors show clearly that this scenario is unlikely, and that bison make up a small portion of the overall risk for brucellosis transmission to cattle. Bison are a wildlife icon in America, and Yellowstone bison represent one of the greatest wildlife conservation stories in our nation’s history. Preface The authors provide a compelling history of the conservation of bison from the early 1900s to the management of bison in modern society. This book is based on the best available science, understanding the importance of bison in the American Indian culture, understanding brucellosis and the role bison play in the ecology of the Greater Yellowstone Area, and an understanding of the stakeholders and local community issues. It informs our collective management and commitment to wild bison on the landscape. The iconic bison deserves our best efforts to assure its place on the American landscape. I am grateful to the authors for clearly articulating the issues we face as we collectively determine the future of these animals. The authors have given us a chance to advance our discussions based on a common understanding of the science, culture, and politics surrounding bison. xi NPS/Neal Herbert Clouds and bison across Hayden Valley, Yellowstone National Park. Introduction P.J. White, Rick L. Wallen, and David E. Hallac The plains bison (Bison bison or Bos bison), also commonly known as buffalo, once numbered in the tens of millions and ranged across much of North America, from arid grasslands in northern Mexico, through the Great Plains and Rocky Mountains into southern Canada, and eastward to the western Appalachian Mountains (Lott 2002; Gates et al. 2010; Bailey 2013). Plains bison are symbolic of the American experience because they are an inherent part of the cultural heritage of many American Indian tribes and were central to national expansion and development (Plumb and Sucec 2006). Only a few hundred plains bison survived commercial hunting and slaughter during the middle to late 1800s, with the newly established (1872) Yellowstone National Park providing refuge to a relict, wild, and free-ranging herd of less than 25 animals (Meagher 1973). This predicament led to one of the first movements to save a species in peril and develop a national conservation ethic by a few visionary individuals, American Indian tribes, the American Bison Society, the Bronx Zoo, and federal and state governments (Plumb and Sucec 2006). Bison numbers increased rapidly after protection from poaching, reintroduction to various xiv Yellowstone Bison: Conserving an American Icon in Modern Society locations, and husbandry (see Glossary of Terms). Today, more than 400,000 plains bison live in conservation and commercial herds across North America (Coder 1975; Boyd 2003; Plumb and Sucec 2006; Freese et al. 2007; Hedrick 2009). Despite this success, several scientists recently concluded that plains bison are ecologically extinct because less than 4 percent (20,000) are in herds managed for conservation and less than 2 percent (7,500) have no evidence of genes from inter-breeding with cattle (Freese et al. 2007). Most bison are raised for meat production, mixed with cattle genes, protected from predators, and fenced in pastures (McDonald 2001; Lott 2002; Freese et al. 2007; Sanderson et al. 2008; Gates et al. 2010; Bailey 2013). As a result, wild bison no longer influence the landscape on the vast scale of historical times by enhancing nutrient cycling, competing with other ungulates, creating wallows and small wetlands, converting grass to animal matter, and providing sustenance for predators, scavengers, and decomposers (Knapp et al. 1999; Lott 2002; Freese et al. 2007; Sanderson et al. 2008; Bailey 2013). The restoration of wild bison has advanced more slowly, and with much greater debate, than nearly all other wildlife species over the past 150 years (Lott 2002; Bailey 2013). Bison are massive animals that compete directly with humans and livestock for use of the landscape (Boyd 2003). Their preferred habitats include nutrient-rich valley bottoms where agricultural and residential developments occupy most of the land, while public lands are more likely to encompass mountainous areas (Scott et al. 2001; Becker et al. 2013). Given existing habitat loss and the constraints modern society has placed on the distribution of wild bison, it is unlikely many additional populations will be established and allowed to roam widely across the landscape (Lott 2002; Boyd 2003). Thus, the few remaining wild and wide-ranging populations of plains bison in the Greater Yellowstone Area (Jackson and Yellowstone populations), Canada (Pink Mountain, British Columbia and Prince Albert National Park), and Utah (Book Cliffs and Henry Mountains) are very important (Keiter and Boyce 1991; Franke 2005; Plumb et al. 2009; White and Wallen 2012; Bailey 2013). Introduction In 1973, Dr. Mary Meagher, a research biologist working in Yellowstone National Park, released a scientific monograph that provided insightful information on the life history, behaviors, and ecology of Yellowstone bison. This work was invaluable to biologists that managed bison during subsequent decades, and much of it is still pertinent and has been referenced in this book. During the past 40 years, however, there have been significant advances in understanding bison ecology, and also, substantial changes in the abundance, distribution, movements, and management of Yellowstone bison (Plumb et al. 2009; Gates and Broberg 2011; White et al. 2011, 2013b; White and Gunther 2013). In addition, there are biological, political, and social threats to Yellowstone bison that hinder their conservation and the recovery of the species elsewhere (Franke 2005; Plumb et al. 2009; Bailey 2013; Treanor et al. 2013; White et al. 2013a). This book provides updated information on Yellowstone bison. We compiled information from numerous published and unpublished sources (e.g., articles, environmental compliance documents, newspapers, reports, websites) not readily available to many stakeholders. We reorganized this information into a more concise and readable format, without detailed data collection methods and statistical analyses. However, the original sources of information are cited and the wording is often similar to preserve original intent and avoid misrepresentation. In the book, we discuss opportunities for bison conservation in the Yellowstone area, misconceptions and competing social values that prevent an easy path forward for bison conservation, and the potential for Yellowstone bison to contribute to the conservation of plains bison across their historic range. Our objectives are to communicate this information to natural resource managers, wildlife ecologists, and anyone interested in plains bison so they can work together to enhance the conservation of this species in modern society. Also, we hope this information will benefit the millions of people that visit Yellowstone National Park each year or monitor the condition and management of the park’s resources via the Internet or other outreach avenues. xv Photograph courtesy of Daniel Stahler Bull bison in the Pelican Valley of Yellowstone National Park. Chapter 1 THE POPULATION — ATTRIBUTES, BEHAVIOR, DISTRIBUTION, RESOURCE USE, AND TRENDS Douglas W. Blanton, P.J. White, Rick L. Wallen, Katrina L. Auttelet, Angela J. Stewart, and Amanda M. Bramblett Yellowstone bison are noteworthy in modern times because, unlike most other conservation herds, this population has thousands of individuals that roam relatively freely over an expansive landscape (Franke 2005; Freese et al. 2007; Bailey 2013). They also exhibit wild behaviors reminiscent of prehistoric populations, with large congregations of individuals during the breeding season to compete for mates, as well as migration and pioneering movements to explore new areas (Plumb et al. 2009; Gates and Broberg 2011; Geremia et al. 2011, 2014b). These behaviors contributed to the successful restoration of a population that was on the brink of extinction just over a century ago (Plumb and Sucec 2006; Plumb et al. 2009). 2 Yellowstone Bison: Conserving an American Icon in Modern Society Attributes Plains bison are massive animals, with males (900 kilograms or 1,985 pounds) having larger maximum weights than females (500 kilograms or 1,100 pounds; Meagher 1986). Males are full-grown by 5 to 6 years of age, while females mature near 3 years of age (Meagher 1986). Yearlings weigh 225 to 320 kilograms (500 to 700 pounds), while calves 8 to 9 months old weigh 135 to 180 kilograms (300 to 400 pounds; Meagher 1973; Gogan et al. 2010). Adult bison are dark chocolate-brown in color, with long hair (15 centimeters or 6 inches) on their forelegs, head, and shoulders, but short dense hair (3 centimeters or 1 inch) on their flanks and hindquarters (Meagher 1986). The fur of newborn calves is reddish tan in color, but begins turning brown at about 2.5 months (Meagher 1986). Both sexes have two horns that curve upward from their head and are retained for their lifespan. Horns of adult males in Yellowstone average about 36 centimeters (14 inches) in length, while those of adult females average about 31 centimeters (12 inches).1 All plains bison have a shoulder hump that extends to about 1.8 meters (6 feet) above-ground in adult males and 1.5 meters (5 feet) above-ground in adult females (Meagher 1986). Bison use their large shoulder and neck muscles to swing their heads from side-to-side to clear snow from foraging patches (Picton 2005). Bison get their first permanent incisor during their third year of life and gain teeth every year thereafter until they have a full set of permanent teeth at age five (Fuller 1959). Bison are agile, strong swimmers, and can run 55 kilometers (35 miles) per hour (Meagher 1973, 1986). They can jump over objects about 1.8 meters (6 feet) tall and have excellent hearing, vision, and sense of smell (Meagher 1973; Lott 2002). Social Behavior Yellowstone bison are gregarious and often form female-led groups consisting of females of all ages and young males less than 4 years old. Group formation and dispersion is flexible, with the exception of a mother and her calf (Lott and Minta 1983; Lott 2002). Group sizes in Yellowstone 1 Portions of this chapter and other portions of the book have been included in and/or adapted from briefs, letters, websites, the Yellowstone Resources and Issues Handbook, and other National Park Service documents. The Population 3 NPS/Jacob W. Frank Bison near Wraith Falls in the northern region of Yellowstone National Park. 4 Yellowstone Bison: Conserving an American Icon in Modern Society National Park average about 20 bison during winter (maximum = 175) and 200 bison (maximum = 1,000) during the summer breeding season from mid-July to mid-August. Group sizes decrease through autumn (average = 50 to 100 bison; maximum = 250 to 450) and reach their lowest level during winter in March and April (U.S. Department of the Interior [USDI2], National Park Service [NPS] 2010). The gregarious nature of bison tends to hold groups together, but their movements result in groups frequently encountering each other and intermixing to some extent (Lott 2002). Yellowstone bison are noteworthy in modern times because, unlike most other conservation herds, this population has thousands of individuals that roam relatively freely over an expansive landscape. The strongest relationships in plains bison society are between adult females and their calves, which are dependent on their mothers for food and security during their first three months of life (Lott 2002). Thereafter, calves suckle less and begin to procure food by grazing (Green 1992). When breeding begins, calves spend more time away from their mothers interacting with other bison approximately their own age (Green et al. 1989; Green 1992). However, most calves stay near their mothers for about one year (Lott 2002). Older and larger female bison often use threatening postures and pushing to establish dominance over unrelated younger or smaller animals, but actual fights are rare. Dominant females often displace other bison from feeding craters dug into the snow pack when food is limited during winter. Adult bison form courtship groups in the Hayden and Lamar valleys of Yellowstone National Park during July and August (Meagher 1973). Plains bison reach sexual maturity at 2 to 4 years of age, but males usually do not successfully breed until about 6 years due to the presence of larger, older males (Meagher 1986; Berger and Cunningham 1994). Mature males fight to determine dominance, with competitive interactions including threatening postures, growling-type vocalizations, and sometimes, violent head-to-head clashes with opponents pushing to displace each other (McHugh 1958; Berger and Cunningham 1994). Winners have an opportunity to copulate with receptive females, and as few as 10 percent of the males in the population may complete 50 percent of the breeding during a given year (McHugh 1958, 1972; Berger and Cunningham 2 Abbreviations for government agencies frequently referenced in citations are provided after the Table of Contents The Population 1994; Halbert et al. 2004). Following breeding, mature males segregate and spend the rest of the year alone or in small groups (McHugh 1972; Meagher 1973; Lott 2002). More information on bison reproduction can be found in Chapter 5. Yellowstone bison employ a predator defense strategy whereby bison in a group cooperate to defend themselves and their young (Smith et al. 2000; MacNulty et al. 2007; Becker et al. 2009a). When threatened by predators such as wolves (Canis lupus), bison often gather together around young animals. Older males and females may challenge the predator(s), with their heads down and horns ready to hook their opponents. If one bison becomes vulnerable or is attacked, other bison may engage the predator(s) from a different direction. Bison usually prevail against one or a few predators when they employ this group defense strategy (MacNulty et al. 2007). More information on predation and mortality of bison can be found in Chapter 5. Habitats and Distribution Historically, bison occupied about 20,000 square kilometers (7,720 square miles) near the sources of the Yellowstone and Madison rivers (Schullery and Whittlesey 2006). Today, this range is restricted primarily to the northern and central regions of Yellowstone National Park and adjacent areas in Montana (Figure 1.1). Few bison currently migrate from the park into adjacent areas of Idaho and Wyoming. Within this range, Yellowstone bison use a variety of habitats, including sedge meadow, upland, burned and unburned forest, sub-alpine, creek/river riparian areas, willow, and agricultural land outside the park (Meagher 1973; Jerde et al. 2001; Gross et al. 2010). Also, much of central Yellowstone is influenced by heat flowing to the surface from the interior of the Earth (Watson et al. 2009). This heat produces hydrothermal features such as geysers, hot springs, fumaroles, and mud pots in some areas, but also warms more extensive portions of the landscape where snow pack is reduced or eliminated (Watson et al. 2009). These geothermally influenced areas are often used by bison during winter (Meagher 1973; Bruggeman et al. 2009c). 5 6 Yellowstone Bison: Conserving an American Icon in Modern Society Figure 1.1. Map depicting Yellowstone National Park and the pre-settlement, mid-20th century, and 2014 distribution of Yellowstone bison (adapted from Plumb et al. 2009). This figure does not depict the historic or current distribution of plains bison in and near Grand Teton National Park, the National Elk Refuge, or Jackson Hole, Wyoming. The Population Bison in central Yellowstone occupy the central plateau, extending from the Pelican and Hayden valley areas (Figure 1.2) with a maximum elevation of 2,500 meters (8,200 feet) in the east to the lower-elevation (2,000 meters [6,570 feet]) and geothermally influenced Madison headwaters area in the west (Meagher 1973; Bruggeman 2006). Winters are often severe, with temperatures reaching -42 degrees Celsius (-44 degrees Fahrenheit) and snow pack exceeding 1.8 meters (6 feet) in some areas. Bison in central Yellowstone congregate in the Hayden Valley for breeding (Meagher 1973; Geremia et al. 2011, 2014b). Afterwards, most bison move between the Madison, Firehole, Hayden, and Pelican valleys, but some travel to the northern region of the park before returning to the Hayden Valley for the subsequent breeding season (Geremia et al. 2011, 2014b; White and Wallen 2012). Bison in northern Yellowstone primarily occupy the Yellowstone River drainage and surrounding mountains between the Lamar Valley and Mirror Plateau in the east (maximum elevation = 2,740 meters [9,000 feet]) and the lower-elevation Gardiner basin in the west (1,615 meters [5,300 feet]) (Meagher 1973; Houston 1982; Barmore 2003; Geremia et al. 2011, 2014b). The northern region of Yellowstone is drier and warmer than the rest of the park, with average snow depths ranging from about 1 meter (3.5 feet) at higher elevations to less than 0.3 meter (1 foot) at lower elevations. Bison in northern Yellowstone congregate in the Lamar Valley and on adjacent plateaus during the breeding season (Meagher 1973; Geremia et al. 2011, 2014b). More information and maps of the distribution and movements of Yellowstone bison are provided in Chapter 4. Feeding Bison are ruminants with a large, multiple-chambered stomach containing microorganisms such as bacteria and protozoa that facilitate the breakdown of plant material (Van Soest 1994; Feist 2000). Grasses, sedges, and other grass-like plants comprise more than 90 percent of the diets of Yellowstone bison through the year (Meagher 1973; Singer and Norland 1994; Barmore 2003). Forbs and the leaves of woody plants comprise less than 5 percent of their diets (Meagher 1973). 7 8 Yellowstone Bison: Conserving an American Icon in Modern Society Figure 1.2. Names of various places and areas used by bison in and near Yellowstone National Park. Darker shading indicates areas used more frequently by 66 adult female bison fit with radio collars during 2004 through 2012 (Geremia et al. 2014b). The Population Yellowstone bison make foraging decisions at multiple spatial scales, including the selection of foraging areas across the landscape, foraging patches within an area, and plant species and individual plants within a foraging patch (Bruggeman 2006). The length of time bison spend foraging in an area before moving is affected by the perceived value of the area compared to other recently visited areas — including the quantity and quality of forage, amount of snow, previous foraging experiences (learning), and competition with other bison and/or other ungulates (Bruggeman 2006). During winter, forage for bison in the Yellowstone area is mostly dead and of low quality. Thus, vegetation quality has little influence on the selection of foraging patches, and factors such as snow pack and competition that influence the availability of forage are more important (Wallace et al. 1995; Fortin et al. 2003; Bruggeman 2006). Bison tend to select foraging patches in areas with less snow because displacing snow reduces efficiency and contributes to increased energetic costs (Bjornlie and Garrott 2001). As an area becomes covered by deeper snow or occupied by numerous animals competing for forage, bison will eventually search for another area with less snow or fewer animals (Bruggeman 2006). As a result, large shifts in bison distribution may occur to lower-elevation meadows with more energy efficient foraging during severe winters (Bruggeman 2006). Furthermore, bison in central Yellowstone may choose to feed in geothermally influenced areas where the time and energetic costs of displacing snow are minimal, but the quantity of forage is relatively low and there are other costs to feeding (e.g., faster wear of teeth due to silica in the soil; high arsenic and fluoride concentrations in water and plants; Garrott et al. 2009b; Geremia et al. 2009). During summer, bison tend to repeatedly graze productive areas, selecting grasses from dry uplands and sedges from moist sites (Wallace et al. 1995; Olenicki and Irby 2003). High densities of bison can deplete high-quality forage patches, resulting in frequent movements and substantial variation in grazing intensity across the landscape (Gates and Broberg 2011; Kohl et al. 2013). A study during summer and autumn 9 10 Yellowstone Bison: Conserving an American Icon in Modern Society of 1998 through 2000 found bison in the Hayden Valley foraged in upland grasslands until they had eaten 50 to 60 percent of the grasses and foraging efficiency decreased (Olenicki and Irby 2003). Bison then began moving across the valley in search of ungrazed patches or grazed patches with regrowth (Olenicki and Irby 2003). They also used moist communities adjacent to grasslands, and ate sedges and other grass-like plants after they had depleted preferred grasses in upland communities (Olenicki and Irby 2003). Scientists in Yellowstone National Park are currently studying the effects of predators such as wolves on bison distribution, habitat selection, and foraging patterns. In Prince Albert National Park, Canada, the selection of foraging meadows by bison was primarily influenced by forage availability and quality, and secondarily by the risk of encountering wolves (Fortin and Fortin 2009; Fortin et al. 2009; Harvey and Fortin 2013). Bison were more vulnerable to predation during winter, and as a result, groups avoided areas with deep snow and ate less at each site when risk was greater (Fortin and Fortin 2009; Harvey and Fortin 2013). More information on feeding by Yellowstone bison and their ecological role can be found in Chapters 6 and 7. Energetics and Nutritional Condition Metabolic rates for plains bison are about 0.12 to 0.16 megajoules per kilogram of body mass per day in winter and about 0.21 to 0.24 megajoules per kilogram per day in summer (Christopherson et al. 1979; Feist 2000). Bison reduce their metabolic rates during winter via hormonal changes in response to shorter daylight periods and colder temperatures (Christopherson et al. 1979; Feist 2000). Thus, there is a reduction in energy costs and forage intake during the time of year when prolonged under-nutrition lowers body condition (Feist 2000). Digestible energy intake for adult female bison in Yellowstone during winter ranged from 115 to 155 kilocalories per kilogram0.75 of body mass per day, and was greater in northern than central Yellowstone (DelGiudice et al. 2001). Within central Yellowstone, intake of metabolizable energy by bison during winter was lower for bison in the Hayden Valley with deep snows The Population 11 NPS/Jim Peaco Bison in a thermally influenced area near Obsidian Creek in the northern region of Yellowstone National Park during winter. 12 Yellowstone Bison: Conserving an American Icon in Modern Society than for bison in the Madison headwaters area where geothermally warmed basins reduced snow accumulation and afforded easier access to vegetation (Bruggeman et al. 2009c). Bison gain weight during the spring and summer and lose weight during autumn and winter (Feist 2000). Yellowstone bison experience progressive nutritional deprivation, low dietary intake of minerals (e.g., sodium, phosphorus), and increased break-down of muscle mass for energy during winter (DelGiudice et al. 1994). Bison calves typically have 40 to 50 percent lower fat reserves and higher winter mortality rates than adults, especially during severe winters (DelGiudice et al. 2001). Under-nutrition