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Ecological Dynamics on Yellowstone’s Northern Range 4 Present Conditions: Animals POPULATION DYNAMICS OF NORTHERN RANGE UNGULATES YELLOWSTONE’S NORTHERN RANGE supports a rich community of native ungulates. Ungulates in the park were subject to market hunting until the 1880s, and the park’s wildlife was not seriously protected until the U.S. Army was assigned administration of the park in 1886 (YNP 1997). By this time, most ungulate populations had been greatly reduced. The Army (and later NPS) managed and protected the resident ungulates and diligently controlled predators, which resulted in greatly reduced populations of coyotes, bears, and mountain lions, and the extirpation of wolves from Yellowstone National Park (YNP). YNP adopted a policy of “natural regulation” in 1968, which led to increased populations of elk and bison. Throughout the twentieth century, management of ungulates has been controversial and great concern has been expressed by the public and park officials about the “correct” management objectives and the actions needed to achieve them. Density Dependence and Natural Regulation The concept of density-dependent regulation of population sizes has figured prominently in the controversy over management of elk and bison in YNP
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Ecological Dynamics on Yellowstone’s Northern Range (Houston 1982, YNP 1997). For a population to be regulated by density-dependent factors, some combination of the following processes must operate. As population density increases, mortality and emigration rates increase and the rate of reproduction decreases. Increases in mortality can result from depletion of food supplies because individuals find it increasingly difficult to obtain adequate nutrition. Diseases, whose transmission is facilitated by high population densities, and predators also can increase mortality (Sinclair 1989, Royama 1992, Begon et al. 1996). Rates of reproduction may decrease because females cannot obtain enough food to support high rates of pregnancy and because offspring may be born at lower weights and less appropriate times than when food supplies are good. These rates may change gradually with population density, or there may be thresholds at which major changes occur (Fowler 1987; McCullough 1990, 1992). The combination of these processes tends to cause population densities to decline when they are high and to increase when they are low. However, this does not guarantee that population densities will stabilize or reach some equilibrium because changes in rainfall, snow accumulation, fires, and other abiotic events may cause large fluctuations in the capacity of the landscape to support the population (Soether et al. 1997). In other words, because the environmental conditions in the landscape may vary considerably, the magnitude of variation in the density of a population by itself cannot be used to assess the importance of density-dependent factors in regulating the size of a population. Most populations of larger herbivores are subject to a combination of stochastic and density-dependent processes that lead to large variation in rates of juvenile survival and subsequent changes in population growth rates (McCullough 1990, Sinclair and Arcese 1995, Soether 1997, Gaillard et al. 1998). The conceptual basis of density-dependent population regulation is simple, and there are many examples of ungulate populations in which fecundity declines or mortality increases as population density increases. However, no single statistical method identifying density dependence has emerged, despite vigorous discussion (Strong 1986, Pollard et al 1987, Turchin 1990, Dennis and Taper 1994, Soether 1997, White and Bartmann 1997, Shenk et al. 1998, Bjornstad et al. 1999). Many unharvested ungulates are regulated, at some point, by density dependence (McCullough 1979, Sinclair 1979, Fowler 1981, Gaillard et al. 1998), but populations are always subject to a multitude of factors and it can be difficult to distinguish the effects of density from those of other influences. The best evidence for density dependence comes from direct measures of changes in population processes such as mortality, fecundity, and migration (Shenk et al. 1998).
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Ecological Dynamics on Yellowstone’s Northern Range Elk The intense public debate since the 1920s over management of the northern range elk population has stimulated numerous studies of the Yellowstone elk population (Barmore 1980; Houston 1982; Chase 1986; Merrill and Boyce 1991; Coughenour and Singer 1996a; Singer et al. 1997, 1998). Nevertheless, reliable data on population size and distribution exist for only the past several decades, and population size estimates in published reports differ (e.g., compare Houston  with Lemke et al. ). Elk are highly mobile. Inmild winters, they are widely dispersed and make extensive use of forested habitats where they are difficult to count; aerial counts during harsh winters, when elk move to lower elevations, are therefore more reliable than counts during mild winters. Hunting also alters distribution and the number of animals in the population. Aerial surveys of elk initiated in 1956 marked the beginning of reasonably reliable estimates of elk in YNP (Houston 1982). Comparisons of population estimates are confounded by changes in survey technique, differences in the time of survey (e.g., before or after harvest), and vagaries of weather that influence animal movements and visibility. Harvest and Movement Historical records of northern range elk illustrate the dominant influence of intense management before 1968 and the effects of management and natural processes since 1968 (Figure 4–1). Public concerns about overgrazing of the northern range resulted in herd reduction by the park at rates that kept the elk population relatively low and stable from the 1920s until 1968 (Houston 1982). After 1967, when elk harvest stopped, the elk population increased (Figure 4–1). From 1968 through 1975, hunter harvests outside the park from the northern herd dropped from about 1,500 elk per year to fewer than 200 (Houston 1982). The winter late hunt north of YNP resumed in 1967 amid concerns that disturbances due to hunting would inhibit movements of elk from YNP to the historical winter range north of park boundaries. Until the severe winter of 1988, relatively few elk were observed north of Dome Mountain (approximately 16 km north of YNP) (Lemke et al. 1998). The northern range elk population, which had expanded to about 20,000 animals, responded to heavy snows in the winter of 1988–1989 by moving to a lower-elevation winter range en masse. More than 3,000 elk were observed in the area of Dome Mountain,
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Ecological Dynamics on Yellowstone’s Northern Range FIGURE 4–1 Elk population counts and harvest of northern range elk. Vertical bars show the number of elk harvested. Filled circles are counts from aerial surveys; crosses are ground counts. Sources: YNP 1997, Lemke et al. 1998, Lemke 1999. a substantial proportion of the 7,000 to 8,000 elk that migrated north out of YNP that year (Figure 4–2) (Lemke et al. 1998). This event marked a major change, or restoration, in behavior of northern range elk. A significant proportion of the population has consistently migrated to the area of Dome Mountain in subsequent years. From 1975 to 1988, an average of about 200 elk per year wintered north of Dome Mountain; the average increased to about 2,800 per yearfrom 1989 to 2001 (Lemke et al. 1998; Lemke 1999; T.Lemke, Montana Fish, Wildlife and Parks, personal communication, 2001). From 1989 to 1999, an average of 5,600 elk (range, 1,533 to 8,626) wintered outside the northern border of the park, including the area of Dome Mountain (Lemke et al. 1998; Lemke, personal communication, 2001). The many elk wintering outside YNP boundaries are using an expanded winter range. Houston (1982) estimated that during the 1970s the elk winter range consisted of 109,000 ha (as measured by Lemke et al. ), whereas current winter distribution typically includes around 153,000 ha of winter range, an increase of 41% (Lemke et al. 1998). Most of the increase in winter range is outside YNP, where the area utilized increased from 22,000 to 53,000 ha, including 9,200 ha north of Dome
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Ecological Dynamics on Yellowstone’s Northern Range FIGURE 4–2 Number of elk counted north of Dome Mountain, Montana. Sources: Lemke et al. 1998, Lemke 1999. Mountain. In response to the apparent need for a lower-elevation winter range, Montana Fish, Wildlife and Parks and the Rocky Mountain Elk Foundation collaboratively acquired 3,500 ha of key winter range on Dome Mountain (Lemke et al. 1998). Grazing by domestic livestock was discontinued on this land to provide an enhanced supply of winter forage for wildlife (Lemke et al. 1998). Regulation of Elk Populations The northern range elk herd is strongly influenced by both density-dependent and density-independent factors. Mortality of juveniles varies widely from year to year and is positively correlated with population density (Barmore 1980, Houston 1982, Merrill and Boyce 1991, Coughenour and Singer 1996a, Singer et al. 1997, Taper and Gogan 2002). Increased juvenile mortality rates at high elk densities are caused by grizzly bears, black bears, and coyotes (Singer et al. 1997), and, recently, wolves (Smith et al. 1999a). As elk density increases, pregnancy rates (Houston 1982) decline, and a larger proportion of elk calves are born later and at a lower birth weight. These calves survive less well
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Ecological Dynamics on Yellowstone’s Northern Range (Singer et al. 1993), which reduces the rate of population growth (Houston 1982, Merrill and Boyce 1991, Coughenour and Singer 1996a, Singer et al. 1997, NRC 1998). Together, these factors produce a negative correlation between population growth and population size (NRC 1998, Taper and Gogan 2002), a clear indication of density dependence. Weather has also had a major influence on the current migratory pattern of northern range elk. Deep snow restricts the area available for feeding by ungulates, and their response is to seek better foraging conditions at lower elevations (Coughenour and Singer 1996b). One result of the large migration of elk in response to the severe winter of 1988 was that some elk learned the landscape and continued to leave the park with greater frequency after 1989 (Figure 4–2). Elk migrations out of YNP were clearly influenced by weather. The size of the migration out of YNP is correlated with snow water equivalent (SWE),1 a rough measure of snow depth (NRC 1998). With more recent data (1989 to 1999), the correlation between the number of elk migrating from the northern range and SWE remains significant (Y =-2579.1+380.8; r2=0.41, p=0.03). Similarly, the number of elk killed in the late Gardiner hunt from 1976 to 1999 is correlated with SWE (Y=-333.3+71.5; r2=0.54, p=0.012). Neither the number of elk leaving YNP nor the take from the Gardiner hunt was correlated with population size (p>0.3). Although regression equations suggest that no elk will leave the park when SWE is less than 5 or 7 inches (12.7 to 17.8 cm), the lowest SWE recorded since 1949 was 10 inches (25.4 cm). Therefore, elk are likely to migrate from YNP in even the mildest of winters. The postulated effects of severe winter weather on elk have been corroborated by simulation modeling. Landscape-scale simulations of northern range elk identified winter severity as the primary cause of major mortality events (Turner et al. 1994b, Coughenour and Singer 1996b, Wu et al. 1996). Thus, one of the most important factors determining variation in elk population size is one over which managers have no control. The current pattern of winter use of Dome Mountain may be influenced by artificial feeding. In 1989, Montana Fish, Wildlife and Parks leased agricultural property on Dome Mountain that was used to produce alfalfa. The lease stipulated that only the first cutting of hay would be removed; all subsequent 1 SWE is the sum of snow water equivalent (inches) measured at Lupine Creek, YNP (elevation 2,249 m) and Crevice Mountain, YNP (elevation 2,560 m). SWE data from Farnes (1996) and Farnes et al. (1999).
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Ecological Dynamics on Yellowstone’s Northern Range growth would be left as winter forage for wildlife. From 1992 through 1996, an estimated 209 tons of hay per year was produced on this property (Montana Fish, Wildlife and Parks 1997 lease2), a desirable resource within easy reach of northern range elk. Although severe weather may have initially motivated elk movements out of YNP, the existence of high-quality and abundant forage has likely encouraged them to return to the area. A change in the relationship between SWE and elk north of Dome Mountain in winter is apparent from a comparison of data before and after 1989 (Figure 4–3). A key issue surrounding the natural regulation policy is the size around which the population will fluctuate and the impact of those populations on the northern range. In the absence of wolves, recent estimates of the largest elk populations the winter range can support range from 16,000 to 22,000 animals (Coughenour and Singer 1996a, NRC 1998, Taper and Gogan 2002), a large increase from the early estimates of 5,000 to 11,000 elk (Grimm 1938, 1939; Cooper 1963; Cole 1969). The increases resulted from a better understanding of elk dynamics and from a large increase in the area of available winter range (Lemke et al. 1998). Because only a fraction of the total maximum population is actually observed, these estimates translate to a total population of 20,000 to 22,000 elk that could be supported under the environmental conditions of the past few decades. Natural Variation in Elk Population Size Wildlife managers typically prescribe actions that reduce variation in population size or resources. Management of northern range elk has been no different, and observed population fluctuations would likely have been greater in the absence of annual herd reductions, acquisition of winter range, and provision of supplemental forage. Annual harvests prevented elk from attaining high densities that could have exacerbated intraspecific competition, led to more severe nutritional deprivation and adversely affected the range. DelGiudice et al. (1991) found that, during the relatively mild winter of 1987, elk on the northern range showed signs of hunger by midwinter. Nutritional deprivation was significantly associated with declines in cow/calf ratios. During the harsh winter of 1988, winter mortality of elk was severe, and NPS 2 Lease for area designated as Dome Mountain Wildlife Management Area, 1997.
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Ecological Dynamics on Yellowstone’s Northern Range FIGURE 4–3 Number of elk counted north of Dome Mountain, Montana, as a function of SWE. Open circles are data from 1969 to 1988; filled circles are data from 1989 to 1999. Sources: Farnes et al. 1999, Lemke et al. 1998, Lemke 1999. estimated that more than 4,000 elk died (Singer et al. 1989). Mortality during the 1988–1989 winter may have been increased by the previous summer’s drought combined with the large elk population, although a similarly severe winter die-off was reported in 1919 (reviewed by Houston 1982) and less severe die-offs occurred in 1974 and 1996 (YNP 1997; T.Lemke, Montana Fish, Wildlife and Parks, personal communication, January 18, 2000). Large mortality events may be relatively common among mammals, and their periodic occurrence is expected for Yellowstone’s ungulates (Young 1994, Erb and Boyce 1999). Potential Impact of Wolves on Elk Reintroduction of wolves to YNP in 1995 marked the restoration of the primary predator in the system. Wolves are probably a keystone species (i.e., a species that influences community structure out of proportion to their numbers) (Paine 1966) in the northern range and their activities could touch virtually every aspect of northern range ecology. Wolves regulate herbivore popu-
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Ecological Dynamics on Yellowstone’s Northern Range lations in other systems, with consequent effects on landscape and ecosystem processes (McLaren and Peterson 1994, Messier 1994, NRC 1997). The ability of wolves to regulate Yellowstone’s elk population depends on how wolves’ consumption of prey changes with prey availability and whether the elk killed by wolves would have died from other causes anyway. Most predators increase their consumption of prey as food becomes more available, thereby reducing the population growth rate of the prey and stabilizing population fluctuations. However, all predators exhibit satiation at some point, and if wolves become satiated when elk are highly abundant, then wolves are likely to have a destabilizing effect, exacerbating population fluctuations caused by severe winters or other factors. An additional, critical consideration is whether wolves kill animals that otherwise would not have died. If wolves kill animals that otherwise would have died from old age or starvation, they add little to the rate of mortality. Data necessary to develop detailed models of wolf-elk dynamics are unavailable, but simple models have been constructed to evaluate the likely range of effects of wolves on elk, and to a lesser extent, on the northern range ecosystem. Based on early models of wolf-elk dynamics, we could reasonably expect wolves to reduce the elk population by 5–20% (Boyce and Gaillard 1992, Mack and Singer 1993). So far, only Boyce and Anderson’s (1999) model includes stochastic variation due to predator behavior and vegetation dynamics. Boyce and Anderson’s (1999) model was intentionally simplistic in an effort to make the results interpretable, so their results are most useful for identifying qualitative trends in system responses. Boyce and Anderson compared effects of variation introduced at the bottom (vegetation) or top (predator functional response) of the system. When stochasticity was introduced by varying vegetation production, 95% of the variance in herbivore numbers was explained by vegetation alone. For this model, only 28% of the variance in population dynamics was explained by the number of predators. When variation entered the system at the top, via variation in the functional response of wolves, vegetation and predators alone accounted for 21% and 75% of the variation in herbivore numbers, respectively. These results identify a key problem for future research on interaction of wolves and herbivores: the source of variation in the system can have a profound effect on evaluation of the relative role of regulating factors. Weather has had a very large impact on dynamics of elk in Yellowstone, and wolves are unlikely to change this. On the other hand, a bad year for elk is likely to be a good year for wolves; therefore, variation in elk-wolf dynamics will almost certainly result from variation due to both changes in forage pro-
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Ecological Dynamics on Yellowstone’s Northern Range duction and in predation by wolves. Severe weather per se is unlikely to have any direct effect on wolves, but severe weather may make it easier for wolves to kill elk, thereby driving greater variation in prey populations (Post et al. 1999). Bison Management of bison in and around YNP also has been controversial, but for different reasons. Bison in YNP are infected with the bacterium Brucella abortus, the causative agent of brucellosis (Meagher and Meyer 1994), a disease that causes abortion in cattle and is of significant economic and political interest to the livestock industry. Efforts to prevent transmission of brucellosis from free-ranging bison to nearby cattle have resulted in the slaughter of more than 2,000 bison on the boundaries of YNP, which has created a public outcry. The Yellowstone herd is the only free-ranging bison herd that avoided extermination in the late 1800s, and for many people, bison are a symbol of the American West. Americans continue to care passionately about management of Yellowstone’s bison. A recent draft management plan (NPS 1998) evoked more than 60,000 written comments from the public. Bison were widely distributed in North America before YNP was created in 1872, but by 1900, only about two dozen free-roaming bison survived in YNP (Meagher 1973). Bison taxonomy remains controversial, but according to Reynolds et al. (1982) and Meagher (1973) plains bison (Bison bison bison) inhabited North America east of the Rocky Mountains; the wood or mountain bison (Bison bison athabascae) lived in grasslands in mountain valleys, parks, and northern boreal woodlands and tundra (Reynolds et al. 1982). Yellowstone’s remnant bison herd consisted of mountain bison. Park management of wildlife in the early 1900s involved supplemental feeding in winter, protection of bison within enclosures, and culling of weak individuals (Meagher 1973). Bison from domestic herds were transported to YNP to supplement the size of the tiny herd of wild bison, but many of those domestic animals were plains bison. The total number of bison in YNP in 1902 was 44 (Figure 4–4) (YNP 1997). With protection and intensive management, the bison population increased to more than 1,000 animals by the mid-1920s. Harvests were initiated then and conducted most years to keep the bison population at about 1,500 animals until the 1960s. By 1968 the population was 400 (Figure 4–4), after which harvest was stopped (Meagher 1973). In the absence of harvest after 1968, the bison population rapidly and consistently
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Ecological Dynamics on Yellowstone’s Northern Range FIGURE 4–4 Total bison count (filled circles) and removals (vertical bars) in Yellowstone National Park. Sources: Dobson and Meagher 1996, YNP1997, Taper et al. 2000. increased until the severe winter of 1988, by which time about 3,000 bison lived in YNP. Concerns over transmission of brucellosis from bison to domestic cattle had heightened by 1988, and more than 500 bison were slaughtered as they migrated out of YNP seeking suitable winter range. In the winter of 1996–1997, which had deep snows, more than 1,000 bison were slaughtered when they migrated outside the park. Bison are gregarious and naturally live in nomadic herds. As the bison population in YNP expanded, bison eventually formed more or less discrete herds. Through 1968, when there were about 400 bison in YNP, bison formed three winter herd subunits and two summer breeding populations (Meagher 1973). Major wintering areas were the northern range, central YNP (Hayden Valley/Mary Mountain), and the Madison/Firehole area in western YNP. Natural Regulation of YNP Bison Does the YNP bison population exhibit density dependence, and if so, how many bison is the park likely to support? Relevant data are available for only two periods of protracted growth of the YNP bison population. The first was
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Ecological Dynamics on Yellowstone’s Northern Range FIGURE 4–8 Bighorn sheep counts in the northern range. Filled circles are aerial counts and crosses are ground counts. Note that estimates from the methods differ, and the apparent large changes in population size are partly due to different survey methods and area surveyed. Sources: YNP (1997) for details of surveys; data from YNP (1997) and Lemke (1999). regulation by predation (Wehausen 1996, Ross et al. 1997), and ongoing studies of mountain lions may provide a better indication of the relative importance of factors that may control the bighorn sheep population. MAJOR PREDATORS IN THE YELLOWSTONE ECOSYSTEM Four large terrestrial carnivores prey on Yellowstone’s ungulates: grizzly and black bears (Ursus arctos and U. americanus), mountain lions (Puma concolor), and wolves (Canis lupus). Coyotes (Canis latrans), medium-sized carnivores (Buskirk 1999), also prey on ungulates. Humans also kill substantial numbers of ungulates and may be considered a sixth major predator in the GYE. Large terrestrial carnivores have large home ranges and move long distances, often ignoring political boundaries. For example, the range of Yellowstone’s grizzly bears “links most of the habitats, and associated species of the GYE” (Clark et al. 1999), and mountain lions radiocollared in the northern
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Ecological Dynamics on Yellowstone’s Northern Range GYE have dispersed to distant areas in Idaho, Wyoming, and Montana (Murphy 1998). As with most other mammals (Greenwood 1980), young males of these terrestrial carnivores typically disperse farther than females (Craighead et al. 1999). Large carnivores typically utilize a variety of habitat types, and although they may specialize on particular species or sizes of prey, they consume a variety of prey and other foods as the opportunity or need arises (Johnson and Crabtree 1999). Bears eat primarily plant material, although Yellowstone’s grizzlies consume relatively more ungulates than most other grizzly populations (Jonkel 1987, Mattson et al. 1991, Knight et al. 1999). Large carnivores and their prey are intelligent and adaptable animals (Berger et al. 2001), which makes their interactions complex. Among the influences on populations of large carnivores are changes in the abundance and relative abundance of different prey species; natural forces such as fire and weather, which influence the abundance, distribution, and availability of prey; human activities such as habitat alteration and hunting of prey populations; and changes in the abundance and behavior of other prey and predators. Large carnivores inhabited Yellowstone long before the first European explorers arrived. Bones of wolves, coyotes, and grizzly bears were found in various strata during excavations at Lamar Cave, whose strata extend back about 3,000 years. Multiple Euro-American observers reported wolves, coyotes, grizzly and black bears, and mountain lions in the GYE before 1882 (Schullery and Whittlesey 1999). Historical Human Impacts on Carnivore Populations Predator eradication was a major goal in late nineteenth- and early twentieth-century America. Predators were easily poisoned with strychninelaced carcasses, and by 1880, wolf, coyote, and mountain lion populations in the GYE were greatly reduced (Schullery and Whittlesey 1999). When the U.S. Cavalry arrived in 1886, predators were at first protected along with other animals. However, to protect game species, poisoning of coyotes was resumed in 1898, and in 1907 army personnel were directed to kill coyotes, wolves, and mountain lions (Schullery and Whittlesey 1999). From 1904 to 1935, predator control in Yellowstone resulted in the killing of at least 4,352 coyotes, 136 wolves, and 121 mountain lions (Schullery and Whittlesey 1999). However, by the 1930s, there was mounting opposition among ecologists and
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Ecological Dynamics on Yellowstone’s Northern Range conservationists to predator control in national parks. In 1936, official park policy changed to protect native predators, including coyotes, although park managers could still kill individual predators deemed “harmful” (Schullery and Whittlesey 1999). However, wolves were effectively extinct in Yellowstone by then (Smith et al. 1999b) and predator control probably had eradicated mountain lions as well (Craighead et al. 1999). Bears fared better than wolves and mountain lions. Bear populations were probably reduced by widespread poisoning of ungulate carcasses and by reductions in ungulate populations due to uncontrolled market hunting in the 1870s (Schullery and Whittlesey 1992). However, once they received army protection in 1886, bears began to feed at garbage dumps near hotels, where they soon became a popular tourist attraction (Schullery 1992, Knight et al. 1999). YNP personnel supported and expanded the bear feeding program and bears ultimately were fed at numerous garbage dumps as well as along roadsides. Grizzlies dominated the dumps, although male black bears used them when grizzlies were not present; female and subadult black bears tended to beg for food from tourists on park roads (Knight et al. 1999). Large numbers of bears became habituated to humans, and they injured people and damaged property. These “problem” bears were then removed from the park. From 1930 to 1969, 46 people were injured by black bears, and an average of 24 black bears were removed from the park per year (Schullery 1992). Although the viewing of bears feeding at dumps was immensely popular with tourists, opposition to the practice grew and the park responded by closing the last public-viewing area at a dump during World War II, although it did not actually close the dumps until the late 1960s and early 1970s (Knight et al. 1999). Closing the dumps led to very high grizzly bear mortality; 229 grizzly bears were removed from the GYE between 1967 and 1972 as bears that previously fed in dumps began to seek food in campgrounds and threatened human safety (Knight et al. 1999). Black bears begging for food remained a common sight along roads until the late 1960s. By 1975, park managers had effectively eliminated this sight by improving sanitation, enforcing a no-feeding policy, and removing begging bears (Knight et al. 1999). The states surrounding Yellowstone gradually reduced or prohibited hunting of grizzly bears, beginning with a prohibition by Idaho in 1946. However, a complete moratorium on hunting grizzly bears anywhere in the GYE was not imposed until 1974. The GYE grizzly bear was listed as threatened under the Endangered Species Act in 1975 (Knight et al. 1999).
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Ecological Dynamics on Yellowstone’s Northern Range Current Population Dynamics of Large Carnivores Grizzly Bears Yellowstone’s grizzly bears are the best-studied bears in the world, yet their status and future prospects continue to be subjects of vigorous controversy. Obtaining accurate estimates of the size and dynamics of a grizzly bear population is inherently difficult. Grizzlies are long-lived and females do not reproduce until they are about 6 years old. Grizzly bears are not easily observed as they are mostly solitary and travel over large home ranges in remote, mountainous, and largely forested country. There are additional logistical difficulties in reaching many parts of the study area, recapturing animals, and maintaining operative radiocollars on individual females to obtain long-term reproductive data. Grizzly bear research began in YNP in 1959. An interagency Grizzly Bear Team was established in the early 1970s. At first, park authorities added to natural logistical difficulties by prohibiting radiocollaring of bears until 1975; then they allowed only one or two bears per year to be collared (Knight et al. 1999). By 1982 there were enough data for researchers to conclude that adult female mortality was high and that the reproductive rates were so low that the population was declining at about 2% per year (Knight and Eberhardt 1985). In 1983, an Interagency Grizzly Bear Committee was formed and charged with devising management strategies to reverse the population decline. Agencies began to manage habitat for grizzlies by eliminating sheep allotments within grizzly areas, increasing efforts to prevent illegal killing of bears, and changing policies such as food storage, garbage disposal, and removal of problem bears to minimize the need to legally kill bears (Knight et al. 1999). Population analyses suggest that the Yellowstone grizzly bear population was relatively stable from about 1959 to 1993, with periods of slight increase or decrease (Eberhardt et al. 1994, Pease and Mattson 1999, Boyce et al. 2001). Pease and Mattson (1999) predicted a large (~15%) decline in the size of the grizzly bear population from 1993 to 1996 because of widespread failure of whitebark pine, a key food resource. Other models were less sensitive to this factor and suggested a slight increase in the size of the grizzly bear population over the same period. Counts of individual females with young cubs and survival rates also indicated a positive trend in grizzly bear numbers (Knight et al. 1999). All recent estimates of the size of the Yellowstone grizzly bear population are in the low hundreds (Craighead et al. 1999). These include an estimate of
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Ecological Dynamics on Yellowstone’s Northern Range 390 based on marked females; an estimate of 339 based on known families of bears; a statistical (bootstrapped) estimate of 344 with a 90% confidence interval of 280 to 610 bears (Eberhardt 1995); and an interagency review committee estimate of at least 245 bears, of which 67 were adult females (Eberhardt and Knight 1996). The Yellowstone grizzly bear population is currently isolated from other grizzly populations and is not large enough to avoid loss of genetic variation in the short term (Harris and Allendorf 1989). The current genetic effective population size is only 13 to 65 bears (Paetkau et al. 1998). The genetic effective population size of a wild population, which is generally much less than the census size of the wild population, is defined as the size of a hypothetical population that would have the same rate of decrease in genetic diversity by genetic drift (or increase in inbreeding) as the focal wild population (Hedrick 1983). Loss of genetic diversity reduces population fitness and the probability of long-term survival; thus YNP’s grizzly bears probably need more protected habitat and dispersal corridors to preserve genetic diversity (Craighead et al. 1999). The best habitat for possible future population expansion appears to be in Wyoming. The greatest single threat to the population is increasing development of private lands, which not only decreases habitat but also greatly increases the potential for human-bear conflicts and consequent death of the bears involved (Knight et al. 1999). Grizzly bears require a diverse habitat with minimal human disturbance to cope with climatic changes, alterations in the availability of different foods, human impacts, and changes in the abundance of other wildlife populations. Grizzlies can exploit marginal habitat to some degree but they require time to learn new habitat-use patterns when conditions change (Jonkel 1987). In Yellowstone, grizzlies feed on weakened and winter-killed elk and bison from March through May, and they kill newborn elk calves during May and June (Singer et al. 1997, Knight et al. 1999). A few individual bears kill healthy elk during the summer, and bull elk become more susceptible during the fall rut. Seeds of whitebark pine (Pinus albicaulis), currently threatened by white-pine blister rust (Cronartium rubicola) in the GYE, are an important food for Yellowstone’s grizzly bears. About 30% of the most productive whitebark pine areas burned during the 1988 fires (Knight et al. 1999). Black Bears Very little is known about the numbers and population dynamics of Yel-
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Ecological Dynamics on Yellowstone’s Northern Range lowstone’s black bears; research has been directed disproportionately at grizzly bears. The population has probably decreased since dumps were closed and feeding bears along roadsides was stopped. However, the only study of Yellowstone’s black bears was conducted more than 30 years ago. Barnes and Bray (1967) estimated a minimum density of 0.07 bear per km2 in their Gallatin Mountain study area. Estimates of black bear densities in other areas of North America range from 0.1 to 1.3 bears per km2; however, densities are generally lower at higher altitudes (i.e., YNP) because of the shorter foraging season and poorer soils (Kolenosky and Strathearn 1987). Cole (1976) estimated that there were 650 black bears within YNP by extrapolating the Barnes and Bray estimate over a larger area. Craighead et al. (1999) estimated that there are currently fewer than 2,000 black bears in the GYE. The status of the population is uncertain because of the lack of data, although the population is generally presumed to be stable. About 1,000 black bears are legally killed in Montana each year (Craighead et al. 1999). The ecology of black bears is known from many studies in other areas (Kolenosky and Strathearn 1987). Black bears are habitat and feeding generalists (Johnson and Crabtree 1999). They are usually forest dwellers, and the best black bear habitat is mixed forests that contain a variety of tree and shrub species of different ages (Kolenosky and Strathearn 1987). Black bears in Yellowstone typically prefer spruce-fir habitats and adjacent meadows but were often observed in lodgepole-pine forests along roadsides during the era of roadside feeding (Barnes and Bray 1967). Black bears are basically vegetarians and their diet appears to be largely determined by local food availability. Bears adapt to new sources of food and change their foraging habits accordingly. Black bears can kill young ungulates, which are vulnerable for a 2 to 4-week period after birth (Kolenosky and Strathearn 1987), and also eat carrion and insects. Killing young ungulates appears to be a learned behavior, and once learned, may continue to be part of an individual’s foraging routine (Kolenosky and Strathearn 1987). Carrion provided by cougars and other predators may be a significant food source for black bears in Yellowstone (Crabtree and Sheldon 1999a, Murphy et al. 1999). Ecological conflicts exist between black and grizzly bears and may be depressing populations of both species (Jonkel 1987). However, interactions between Yellowstone’s black and grizzly bear populations have not been studied. Where both species coexist in Alaska, grizzly bears dominate black bears (Miller et al. 1997). In interior Alaska, grizzlies are most commonly associated with alpine tundra habitats, whereas black bears frequent forest and lowland areas (Klein et al. 1998). In the absence of grizzly bears in northern Quebec,
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Ecological Dynamics on Yellowstone’s Northern Range black bears have moved into tundra habitats normally occupied by barren ground grizzlies west of Hudson Bay (J.Huot, Faculty of Science and Genetics, University of Laval, personal communication, January 18, 2001). Mountain Lions Mountain lions have excellent long-distance dispersal abilities (Murphy et al. 1999). Young males have dispersed as far as 480 km from their natal home range (Craighead et al. 1999). Presumably, these dispersal abilities enabled mountain lions to recolonize YNP after they were locally eradicated by predator control programs. The GYE mountain lion population ranges over some 2,200 ha of relatively contiguous habitat (Murphy et al. 1999). Craighead et al. (1999) estimated that there are fewer than 500 adult mountain lions in the GYE, but they gave no basis for this estimate. Because of their high female survivorship and fecundity (litters of up to six kittens), mountain lion populations are more resilient than those of some other large predators, such as bears (Weaver et al. 1996). Human-caused mortality is an important influence on most mountain lion populations, including those in the GYE outside the boundaries of YNP (Murphy et al. 1999). Most of this mortality is due to legal hunting: 48% of mortality among adult and subadult radiocollared mountain lions in the northern Yellowstone ecosystem was due to hunting; another 48% was attributed to natural causes (Murphy et al. 1999). Approximately 500 mountain lions were killed by hunters in Montana in 1998 (Craighead et al. 1999). However, mountain lion populations appear to have stabilized or even increased in many areas of the northern Rockies, including in the northern range area (Craighead et al. 1999), despite hunting pressure (Murphy et al. 1999). Mountain lions occupy a wide range of habitats, although they prefer areas of steep and rugged topography (Lindzey 1987). Abundant cover is important to them as it provides security from enemies, including other predators and humans, and increases hunting success as mountain lions typically hide and ambush their prey. Thus, the structural characteristics of the local vegetation appear to be more important than the dominant plant species (Lindzey 1987). Mountain lions are almost totally carnivorous and can kill all the YNP ungulates except adult bison. The diet of mountain lions varies with the abundance and availability of prey seasonally and geographically. Deer compose a major portion of their diet in most areas, although mountain lions also kill large numbers of small prey, such as snowshoe hares (Lepus americanus)
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Ecological Dynamics on Yellowstone’s Northern Range when they are abundant (Lindzey 1987). In the northern GYE, elk calves are a major source of food for mountain lions (Murphy 1998). However, because there are relatively few mountain lions and they kill far fewer ungulates than human hunters, Murphy (1998) concluded that they have little direct effect on the size of the elk and deer populations. Mountain lions are responsible for about 3% of the elk and 4% of the mule deer deaths in the northern GYE (Murphy 1998). Mountain lions kill about 12% of the buck mule deer, 9% of the elk calves, and 1% of the bull elk, but less than 5% of the other age-sex classes of elk and deer. Wolves Wolves in the northern Rocky Mountains were listed as endangered under the federal Endangered Species Act in 1974. Many scientists favored reintroduction over natural recolonization as a means of restoring wolves to the GYE. However, because of extensive controversy, wolves were not reintroduced to YNP until more than 20 years after they were listed as endangered (Bangs et al. 1998). In 1995, 14 wolves in three packs captured in Alberta, Canada, were introduced to YNP. In 1996, another 17 wolves in four packs captured in British Columbia, Canada, were introduced to YNP. In 1997, 10 pups and 3 adults from a pack captured in northwestern Montana (because they were chasing livestock) were released in the park (Bangs et al. 1998). The wolves have thrived and are being extensively monitored. Ten packs in 1997 and seven packs in 1998 produced pups; by fall 1998, the population estimate was 116 wolves (Bangs et al. 1998). According to the Yellowstone wolf project report for 1998 (Smith 1998), litter size in 1998 averaged 5.5 pups and 81% of the 44 pups born survived to the end of 1998. Fifteen wolves died in 1998: five pups, four yearlings, and six adults. About half this mortality was due to natural causes, including wolves killed by other wolves, avalanches, and elk. The remaining mortality was due to human activities, including wolves killed by control actions, illegal shooting, and vehicles. Some assumptions made before the initiation of the wolf reintroduction program have been validated (Smith et al. 1999b): no preexisting wolves were found in the GYE and it was necessary to impose land-use restrictions around wolf dens. However, wolves have killed more than the predicted 120 ungulates per year and fewer than the predicted 19 cattle and 15 sheep per year. Many more visitors than expected have seen wolves. It was predicted that wolves would travel outside the experimental area of
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Ecological Dynamics on Yellowstone’s Northern Range YNP and immediately adjacent neighboring GYE lands, but this had not occurred through the end of 1998 (Smith et al. 1999b). Wolves regularly disperse long distances: male wolves disperse a mean of 85 km and a maximum of 917 km from their natal home ranges (Craighead et al. 1999). Some wolves had dispersed out of the park as far south as the Jackson area (which is still part of the primary wolf recovery area) (Smith 1998), but none had dispersed from the area designated as the “Yellowstone Nonessential Experimental Population Area,” which includes all of Wyoming, Montana as far north as the Missouri River, and part of Idaho (Bangs et al. 1998). Wolves in different geographical locations rely on different species of prey. Wolves in North America prey largely on ungulates and beavers, but also take other types of prey (Carbyn 1987). Elk have been the wolves’ major prey in the GYE to date, which is not surprising given the abundance of elk. However, the wolves are known to have killed every ungulate species present except for bighorn sheep (Smith et al. 1999b). In 1998, wolf researchers found 109 definite and 121 probable wolf kills (Smith 1998). Eighty-six percent of these kills were elk (198), followed by 3% each of mule deer (7), pronghorn (6), and coyotes (7); 2% of bison (5); and 1% each of moose (3), wolves, and unidentified prey. The age composition of the elk kill was 43% calves, 21 % cows, 19% bulls, and 16% unknown. Packs on the northern winter range killed an average of one ungulate every two to three days during March and one every three to four days during November and December. Wolves can adjust to changing prey abundance and vulnerability (Messier 1995), although in multiple prey systems they may be slow to change their favored prey species (Dale et al. 1995). In some instances, wolves have switched to different age classes as their proportion in the prey population changed as a consequence of previous predation (Dekker et al. 1995) or winters with deep snow (Mech et al. 1995). Wolves have also learned to kill new types of prey (Klein 1995). However, it is difficult to predict how or when there may be substantial changes in the prey taken by Yellowstone’s wolves. The Yellowstone wolves have been returned to an ecosystem that existed without them for much of the past century. Adjustment of the system to the presence of wolves is likely to take many years, as the wolves and the other components of the ecosystem adjust to one another (Klein 1995, Berger et al. 2001). Wolves will probably alter their patterns of prey selection, pack structure, movements, and population dynamics as the density, distribution, predation avoidance behavior, and population structure of their prey species change. A similar adjustment can be expected between wolves and other carnivores. A return to relative stability of predator-prey relationships within the Yellowstone
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Ecological Dynamics on Yellowstone’s Northern Range ecosystem may take longer than the period that wolves were missing from the system. Long-term monitoring of the predators and ungulates within the Yellowstone ecosystem provides a unique opportunity to greatly expand knowledge about interspecies relationships among upper trophic levels. Coyotes Coyotes are the smallest and most numerous of Yellowstone’s major predators. Although they were the first of the park’s predators to be intensively studied (Schullery and Whittlesey 1999), there was a long gap between the publication of Adolph Murie’s classic monograph on coyotes (1940) and the modern era of coyote research. The latter began in 1989, six years before wolves were reintroduced to the ecosystem (Crabtree and Sheldon 1999a). Reintroduction of wolves was expected to have a negative effect on Yellowstone’s coyote populations, as larger species of canids tend to compete for prey with, kill, and physically displace their smaller competitors. Crabtree and Sheldon (1999a, 1999b) recently summarized data collected during these new studies of the coyote on Yellowstone’s northern range. Coyotes live in all GYE habitats below about 2,400m except for very steep, rocky areas and areas with deep snow. Crabtree and Sheldon (1999b) estimated an average pre-wolf density of 0.45 adult coyote per km2 on the northern range and densities from 0.1 to 0.4 coyote per km2 over much of the forest habitat in the GYE. Higher densities, sometimes exceeding 1 coyote per km2, occur in some of the more open grassland and shrub habitats. Craighead et al. (1999) estimated that there are fewer than 3,000 adult coyotes in the GYE. Yellowstone’s coyotes are protected from hunting and trapping and inhabit an environment with good food resources. Crabtree and Sheldon (1999a, 1999b) argued that these factors explain many of the differences between Yellowstone coyotes and those studied in other areas, including larger pack size, greater social stability, higher adult survival rates (91% per year), higher mean age of adults, and lower dispersal rates of juveniles. Coyotes on the northern range have a social system similar to that of wolves, living in territorial packs with an average size of six adults. However, unlike wolves, members of coyote packs often travel alone. Coyote territories are contiguous (no intervening spaces between them), nonoverlapping, and average 10.1 km2. Territory boundaries in the Lamar Valley and the Blacktail Plateau were very stable from 1990 through 1995, changing little from year to year.
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Ecological Dynamics on Yellowstone’s Northern Range Even though female coyotes become sexually mature around 10 months of age, Yellowstone females do not breed until they are 2 to 5 years old. An average of 5.4 pups are born per territory but an average of only 1.5 pups survive to 1 year of age. Principal causes of pup mortality are disease and starvation. Coyotes feed primarily on voles and elk carcasses and the diet varies seasonally (Murie 1940, Crabtree and Sheldon 1999b). Ungulates provide about 45% of the coyotes’ annual biomass consumption, most of which is consumed as carrion during the five winter months. The reintroduction of wolves in 1995 had dramatic effects on the coyote population (Crabtree and Sheldon 1999b). Wolves killed from 25% to 33% of the coyote population during each of the 1996–1997 and 1997–1998 winters, especially in the core areas used by wolves. Almost all these coyotes were killed near the carcasses of elk killed by wolves. The coyote population in the Lamar Valley dropped from 80 coyotes in 12 packs in 1995 to 36 coyotes in 9 packs in 1998 (Crabtree and Sheldon 1999b), and coyotes failed to recolonize their traditional territories in core wolf areas. However, coyotes are flexible and adaptable animals and have already begun to travel in larger groups (Crabtree and Sheldon 1999b). Some of the surviving coyote packs are smaller and are producing larger, healthier pups with higher survival rates. Coyote packs on the fringes of wolf territories have experienced little mortality and are able to benefit from elk carcasses killed by neighboring wolves.
Representative terms from entire chapter: