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Revisiting Brucellosis in the Greater Yellowstone Area (2020)

Chapter: 2 Geographic Scope of Populations and Disease and Change in Land Use

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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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2
Geographic Scope of Populations and Disease and Change in Land Use

1. INTRODUCTION

There have been significant changes in the population sizes and distributions of bison and elk in the Greater Yellowstone Area (GYA) since the 1998 National Research Council report. Both bison and elk numbers have increased overall. Bison ranges have expanded and there is increased intermixing among herds. Elk numbers have increased in many areas, while the population size of the well-known Yellowstone northern range herd has declined. Elk are now recognized as a large reservoir of B. abortus. In addition, wolves were reintroduced to the GYA and grizzly bear numbers have increased. As a result of all these changes, brucellosis transmission dynamics are considerably different than in 1998.

The changes in B. abortus reservoir and transmission dynamics in the GYA are also outcomes of processes associated with the ecologies, population dynamics, and spatial distributions of bison and elk. Bison and elk also play critical roles in the functioning of the Greater Yellowstone Ecosystem as a whole. They affect and also respond to vegetation, soils, other wildlife populations, and human activities.

This chapter examines the ecological context of brucellosis in the GYA as affected by the abundances and spatial distributions of its host species: elk and bison. It draws on best available data to provide a quantitative basis for understanding the abundances and spatial distributions of the two main host species, and it explores factors (such as climate, predators, land use, hunting, changes in management activities) that cause host abundances and distributions to change.

2. ELK POPULATIONS AND DISTRIBUTIONS

One of the most significant changes since 1998 is an increased recognition of the central role that elk play in B. abortus transmission. An increase in elk numbers across the GYA is one factor contributing to a change in the role of elk, with more than 125,000 counted elk distributed among 11 major herds. The ranges and migration pathways of 9 of these major herds are shown in Figure 2-1.

Dynamics of the northern Yellowstone elk population have been intensively studied for decades (Houston, 1982; Coughenour and Singer, 1996; Singer et al., 1997; Taper and Gogan, 2002; Barmore, 2003; White and Garrott, 2005a,b; Varley and Boyce, 2006; Eberhart et al., 2007). In 1969, a policy of natural regulation ended artificial reductions that enabled the elk population to swell to its highest levels in the 1980s with more than 18,000 counted elk (Coughenour and Singer, 1996) (see Figure 2-2). In the late 1990s, the population steadily declined following wolf reintroduction, with the 2016 population count at less than 5,000 elk. In modeling and predicting elk population equilibrium levels in the GYA pre-wolves, there was general agreement that food limitation would result in an equilibrium number of approximately 15,000-18,000 counted elk on the northern range, which corresponds to approximately 20,000-24,000 actual elk in the absence of wolves (Coughenour and Singer, 1996; Taper and Gogan, 2002; Coughenour, 2005; Varley and Boyce, 2006).

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Image
FIGURE 2-1 Map of migration corridors, winter ranges (blue polygons) and summer ranges (tan polygons) of 9 of 11 major elk herds in the GYA. This map excludes approximately one-third of the southern GYA that includes elk in the Afton-Pine, the Pinedale-South Wind River areas, and eastern Idaho, west of Grand Teton National Park (see Figure 2-4 and Tables 2-3 and 2-4). SOURCE: National Geographic, 2016.

2.1 Wolves and Hunting

The beginning of the decline in northern range elk numbers coincided with the reintroduction of wolves in 1995 and 1996, suggesting that wolves were, at least in part, responsible. However, other factors are also probably playing a role, including a high hunting removal of elk migrating north of Yellowstone National Park (YNP), and the fact that hunting harvests have been mostly of prime-aged female elk with high reproductive value (Eberhart et al., 2007). Wolf predation now exceeds hunter harvest, but it has a smaller effect on elk population dynamics because wolves concentrate on calves and older females with less reproductive value (White et al., 2003; Smith et al., 2004; White and Garrott, 2005a; Evans et al., 2006; Wright et al., 2006; Eberhart et al., 2007). Early empirical models of the effects of climate, harvest, and wolves on this elk population indicated that population responses to wolf predation were compensatory, meaning that predators mainly removed animals that would die of other causes anyway (Vucetich et al., 2005). Additionally, there were several consecutive years of drought during 2000-2006, which could have reduced forage and consequently affected elk (MacNulty, 2015). Elk starvation was documented in late

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

winter 2003-2004, which was mild but preceded by several years of low annual precipitation (Vucetich et al., 2005). Also, grizzly bears—which are major predators on elk, particularly elk calves—doubled to tripled in number between the mid-1980s and mid-2000s (Singer et al., 1997; Harris et al., 2007; Barber-Meyer et al., 2008; Haroldson, 2008; Schwartz et al., 2009; USFWS, 2016).

2.2 Causes of Changes in Elk Spatial Distributions

Wolves also shift elk distributions, as wolves reduce the availability of habitat and total forage. As a result, a greater number of elk are now found at lower elevations outside of YNP where wolves are less abundant (White et al., 2012). Elk are less likely to occupy areas with deeper snow or other conditions that increase predation risk in the presence of wolves (Mao et al., 2005; White et al., 2009, 2013). Thus, wolves may also have contributed to the decline in the northern Yellowstone elk herd indirectly, through a contraction of the elk range and associated forage. The numbers (see Figure 2-3 [top]) and proportions (see Figure 2-3 [bottom]) of elk herds using habitats north of YNP increased markedly during the mid- and late 1970s in response to increased population size, changes in the timing of elk hunts, and protection of winter ranges outside of YNP (Coughenour and Singer, 1996). The proportion of Yellowstone elk north of YNP in winter increased steadily through 2011 and has remained high during 2011-2015 (see Figure 2-3). The increased percentage is due to the decrease in total population size rather than an increase in numbers outside YNP.

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FIGURE 2-2 Northern Yellowstone elk numbers. SOURCES: Coughenour and Singer, 1996; Taper and Gogan, 2002; White and Garrott, 2005a; Cross, 2013.
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
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FIGURE 2-3 Elk numbers and percentages north of YNP and on Dome Mountain. SOURCES: Coughenour and Singer, 1996; Taper and Gogan, 2002; White and Garrott, 2005a; Cross, 2013.

Elk grouping behavior has also changed, as northern Yellowstone elk have been found in larger groups following wolf reintroduction (Mao et al., 2005). Although there was an increase in large groups found outside YNP, there was a decrease in large groups found inside YNP where the elk population has declined (White et al., 2012).

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

The decline in elk numbers on the northern winter range and the increased proportions of the herd wintering outside YNP could also be due in part to increased competition for forage from increasing numbers of bison. Unlike elk, which migrate to higher elevations in summer, bison remain on the low elevation elk winter range during summer, thereby depleting forage for wintering elk. Ecosystem modeling experiments indicate greater numbers of bison could reduce elk numbers due to dietary and habitat overlaps, but not to the extent to which elk numbers have actually declined (Coughenour, 1994).

2.3 Madison Headwaters Herd

The small herd of elk that resides in the Madison River headwaters within YNP is nonmigratory. This herd spends both winters and summers within YNP and is not subjected to human hunting (Garrott et al., 2003, 2009). The population has declined markedly from approximately 600 in 2001 to about 100 in 2009, likely due to wolves and grizzly bears (Hamlin and Cunningham, 2009). Multiple wolf packs became established by 2002 with a total of 30-40 animals. High wolf densities and moderate elk densities resulted in 20% of elk being taken by wolves (Garrott et al., 2005). Grizzly bear numbers increased from about 10 in the mid-1980s to more than 20 by 2004 (Haroldson, 2006, 2007; Hamlin and Cunningham, 2009). The ratio of grizzly bears to elk was higher in the Madison-Firehole and Gallatin Canyon areas than any other areas across the GYA (Hamlin and Cunningham, 2009).

2.4 Elk Herds Wintering North and West of YNP

Approximately 30,000 elk in elk management units (EMUs) are located north and west of YNP within the brucellosis designated surveillance area (DSA) (see Table 2-1 and Figure 2-4). Approximately 56% of 22 core winter ranges are privately owned, and 10 of the winter ranges in this area have higher than 80% private ownership (personal communication, Quentin Kujala, Montana Department of Fish, Wildlife & Parks [MDFWP], October 30, 2015). Elk numbers in EMUs north and west of YNP in Montana have been increasing since the late 1970s (Cross et al., 2010a), as they have been in many parts of Montana (MDFWP, 2004). In 2008, there were five to nine times more elk in EMUs in the western Paradise and eastern Madison River Valleys of Montana than in 1975. Elk numbers have also been increasing in EMUs just to the north and northwest of the DSA (see Figure 2-5). These include the Bridger, Crazy Mountain, Pioneer, Tendy, and Tobacco Root EMUs.

Elk group sizes have also been increasing, and group size distributions have been increasing in the eastern Madison River and western Paradise Valleys (Cross et al., 2010b). From 2003-2009, there were more large groups and larger group sizes in the northern YNP herd wintering outside YNP than from 1987-1992 (White et al., 2012). In the Madison drainage, elk aggregated in somewhat larger groups in response to wolf predation risk (White et al., 2009). Recently, a pattern was observed of increasing group sizes with increasing density across 27 herd units in this region, and there was no evidence that wolf predation risk affected elk aggregation patterns (Proffitt et al., 2015).

Changes in land ownership have also affected elk migration and aggregation patterns in this region, which have in turn affected hunter access. In the Madison-Gallatin EMU, MDFWP reported that “There is limited access to public land and adjacent private land in some portions of the EMU due to changes in land ownership. This has resulted from a change in land ownership toward landowners who do not make their primary living from ranching” (MDFWP, 2004). Elk migrating from YNP to winter in portions of this EMU, in combination with non-YNP elk, results in high numbers of elk which makes it difficult to control numbers through late-season hunting (MDFWP, 2004). Likewise in the Absaroka EMU, MDFWP further reports, “There has been an increasing number of landowners who do not make their primary living from ranching, and these landowners have less interest than traditional landowners in allowing elk hunting [on their property]” (MDFWP, 2004). As a result, this has created elk refugia, reduced elk harvest, and increased elk numbers. Counts in this EMU are far above management objectives (see Table 2-1).

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

TABLE 2-1 Elk Numbers in EMUs (Hunting Districts) North and Northwest of YNP, But Within the Brucellosis DSA, in 2015

Hunting District Count 2015 Objective
NorthernYellowstone
313 3,714 4,000
Absaroka
317 934 900
520 1,922 1,050
560 2,018 700
Total 4,874 2,650
Gallatin/Madison
301, 309 587 500
310 428 1,500
311 2,069 2,500
314 3,381 3,000
360 South 773 1,000
360 North 865 1,200
361 150
362 2,500 2,952
Total 10,603 12,802
Gravelly
322-327, 330 10,543 8,000
All Elk Management Units
Total 29,734 27,452

SOURCE: MDFWP, 2016.

Elk avoidance of hunted areas has resulted in elk groups being unavailable for harvest in the Madison River Valley, which is winter range for approximately 5,000 elk (Proffitt et al., 2010a). During the hunting season, elk shifted to areas that were closed to hunting, including privately owned lands and a state Wildlife Management Area (Wall Creek). The probability of finding elk on such designated refuge areas has more than doubled, with Global Positioning System (GPS)-collared elk in the Madison River Valley showing preference for areas that were privately owned because they faced south and had steeper slopes, lower road densities, and more green forage (Proffitt et al., 2010b). Elk selection for private lands with green forage increases the probability of overlap with cattle and increases the risk of disease transmission.

2.5 Elk Herds Wintering East and South of YNP Boundaries

The Clarks Fork Herd East of YNP

The Clarks Fork elk herd consists of about 4,500 migratory and nonmigratory elk that inhabit the Absaroka Mountains northeast of YNP (Middleton et al., 2013a,b). The winter range of the migratory herd segment includes areas east of YNP, extending to the foothills northwest of Cody. In the spring, the migrants move 40-60 km to high elevation summer ranges inside YNP (Middleton et al., 2013a). The resident herd segment spends winters and summers northwest of Cody, overlapping a portion of the migratory winter range. The wintering ranges of both herd segments and the summer range of the resident segment are included in the Clarks Fork Hunt Unit.

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Image
FIGURE 2-4 Elk management units in Montana, elk hunt units in Wyoming, and game management units in Idaho in relation to the designated surveillance area and the YNP boundary, as of 2014 (red lines). Elevations above 2,500 m are shown in gray. SOURCE: MDFWP and WGFD data provided to committee.

The productivity of the migrant herd has declined markedly, with calf recruitment decreasing 70% over 21 years and pregnancy decreasing 19% in 4 years. The decline may be partly due to increased dryness in the region, particularly on summer ranges, with shorter durations of green-up occurring (perhaps 16 days) since 2002. Also, the migrant Clarks Fork elk are exposed to four times as many grizzly bears and wolves as resident elk (Middleton et al., 2013a). Along with the Cody herd and the Jackson herd, the Clarks Fork elk have experienced reduced calf recruitment (4%-16%) and population growth rate (2%-11%) from 1987 to 2010 (Middleton et al., 2013c). During this time, some grizzly bears shifted their diets to less predation on trout and likely more predation on elk calves. This diet shift may have been a result of the decline in cutthroat trout in and around Yellowstone Lake, which in turn is due to the invasion of lake

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

trout (Middleton et al., 2013c). There is debate on whether the grizzly bear population has increased inside YNP (Schwartz et al., 2006; Hamlin and Cunningham, 2009); however, the population has increased in areas outside of YNP (Schwartz et al., 2006). The combination of more bears and shifts in bear diets could have acted synergistically to reduce calf recruitment of migratory elk in this herd (Middleton et al., 2013c).

The largest elk groups in this area tend to be in open range land systems. Wolf numbers are positively correlated with larger groups in open areas (Brennan et al., 2015), while in forested areas elk group sizes tend to be smaller in the presence of wolves (Creel and Winnie, 2005). As a result of wolf presence in large open areas in this region, there are a few very large elk groups (see Figure 2-6).

Elk Herds East of YNP

Herds east of YNP (see Figure 2-4) totaled 17,425 elk in 2015, with an objective of 17,065 (see Table 2-2). Numbers in eastern herd units could be markedly higher than numbers counted based on model estimates that correct for sightability (personal communication, B. Scurlock, Wyoming Game & Fish Department [WGFD]).

Herds south and southeast of YNP (see Figure 2-4) totaled 37,410, with an objective of 35,577 (see Table 2-3). Elk population trends in Wyoming herds east, southeast, and south of YNP are shown in Figures 2-7, 2-8, and 2-9.

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FIGURE 2-5 Trends in elk numbers in Montana EMUs. These EMUs are located just beyond the DSA, except for Gravelly. The gray bands represent the 95% confidence interval on a locally weighted scatterplot smoothing. SOURCE: MDFWP data provided to committee.
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Image
FIGURE 2-6 Histogram of the elk group size distribution from the eastern portion of the GYA in Wyoming. Arrows highlight thefew, but very large elkgroups. SOURCES: Cross et al., 2013; Brennan et al., 2015.

TABLE 2-2 Numbers of Elk in Herds East of YNP in Wyoming in 2015

Elk Hunt Area Total Counted Population Objective Posthunt Estimate
Clarks Fork 2,390 3,300 4,600
Cody 4,205 4,400 6,000
Gooseberry 2,090 2,015 2,000
Medicine Lodge 2,130 3,000 8,216
North Bighorn 6,610 4,350 6,610
Total 17,425 17,065 27,426

NOTES: “Total Counted” is the total number counted from the ground or air during classifications. “Population Objective” is set by the WGFD and “Posthunt Estimate” is statistically modeled and takes into account sightability and survey effort.

SOURCE: Persona lcommunication, B. Scurlock, WGFD.

NOTES: “Total Counted” is the total number counted from the ground or air during classifications. “Population Objective” is set by the WGFD and “Posthunt Estimate” is statistically modeled and takes into account sightability and survey effort.

SOURCE: Personal communication, B. Scurlock, WGFD.

Elk in Idaho, Southwest of YNP

There are five elk management zones in eastern Idaho that provide habitats for GYA elk (see Figure 2-4 and Table 2-4). These units contain herds that seasonally migrate over relatively short distances from low elevation winter ranges to higher elevation summer ranges. There is some movement between Idaho and YNP, Grand Teton National Park (GTNP), and the Rockefeller Parkway areas in Wyoming. In certain circumstances, Idaho permits emergency winter feeding of elk to prevent excessive mortality in drainages that would affect herd recovery. There is one elk emergency winter feeding area with four feeding sites near the border with Wyoming (personal communication, D. Cureton, Idaho Department Fish and Game).

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

TABLE 2-3 Numbers of Elk in Herds South and Southeast of YNP in Wyoming in 2015

Elk Hunt Area Total Counted Population Objective Posthunt Estimate
Fall Creek 3,813 4,400 4,500
Afton 1,837 2,200 1,837
Upper Green River 2,713 2,500 2,713
Piney 1,736 2,400 3,100
Jackson 11,051 11,000 11,200
Pinedale 2,081 1,900 2,081
West Green River 4,791 3,100 3,225
Hoback 1,104 1,100 1,104
Wiggins Fork 5,663 5,500 5,817
South Wind River 2,621 2,600
Targhee 0 200 200
Total 37,410 36,900 35,777

NOTES: “Total Counted” is the total number counted from the ground or air during classifications. “Population Objective” is set by the WGFD and “Posthunt Estimate” is statistically modeled and takes into account sightability and survey effort.

SOURCE: Personal communication, B. Scurlock, WGFD.

NOTES: “Total Counted” is the total number counted from the ground or air during classifications. “Population Objective” is set by the WGFD and “Posthunt Estimate” is statistically modeled and takes into account sightability and survey effort.

SOURCE: Personal communication, B. Scurlock, WGFD.

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FIGURE 2-7 Elk population trends in herds east of YNP. The gray bands represent the 95% confidence interval on a locally weighted scatterplot smoothing. SOURCE: WGFD data provided to committee.
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Image
FIGURE 2-8 Elk population trends in herds south and southeast of YNP. This area also has theelk feedgrounds. The gray bands represent the 95% confidence interval on a locally weighted scatterplot smoothing. SOURCE: WGFD data provided to committee.
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FIGURE 2-9 Elk populationtrends in herds thefurthestsouthof YNP. The gray bands represent the 95% confidence intervalon a locally weighted scatterplot smoothing. SOURCE: WGFD data providedto committee.
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

TABLE 2-4 Elk Herds in Idaho in the GYA

Idaho Elk Management Zone Count
Island Park 2,512
Teton 220
Palisades 797
Tex Creek 3,885
Diamond Creek 2,352
Total 9,766

NOTE: Idaho elk management zones are listed fromnorth to south.

SOURCE: Cureton and Drew, 2015.

2.6 The Jackson Elk Herds

The Jackson Herd Unit comprises most of the areas south and east of GTNP and the National Elk Refuge (NER), with three elk feedgrounds located in this area (see Figure 2-4). The Jackson elk herds winter on low elevation winter ranges, including the NER, the Gros Ventre drainage, and areas near Moran in GTNP. Many of these elk move to higher elevation summer ranges across a broad area to the north, including southern YNP (Boyce, 1989; Smith and Anderson, 2001; Cole et al., 2015).

The management objective determined for this area is 11,000 elk, which is based on judgment, past experience, and balance of conflicting objectives of different stakeholders. On average, there were 11,690 elk counted from 2009-2013 and 11,051 were counted in 2015 (WGFD, 2014; see Table 2-4). As many as 19,000 elk were estimated in the mid-1990s, but annual harvests have reduced the population (USFWS and NPS, 2007). Herd management goals were to feed 5,000 elk on the NER, 3,500 elk in the upper Gros Ventre feedgrounds and native winter ranges east of Crystal Creek, and 2,500 elk on other native winter ranges (WGFD, 2014). The number of elk on native winter ranges has decreased dramatically over the past decade (USFWS and NPS, 2007). On average, 1,307 elk were harvested by 3,082 hunters per year from 2009-2013. The NPS and the Wyoming Game and Fish Commission also carry out elk reductions based on yearly recommendations in two areas just outside the east boundary of GTNP.

An average of 536 Jackson herd elk have wintered in GTNP from 1989-2003. The objective is to support an average of about 356 elk in GTNP, with numbers ranging between 137 and 857 (USFWS and NPS, 2007). The NPS philosophy for national parks is to contribute to the conservation of species at larger landscape scales. However, there are no allowances for permitting elk or bison populations to exceed natural densities within GTNP, even when this would contribute to natural population levels for the larger landscape (USFWS and NPS, 2007).

An elk reduction program in GTNP was authorized by Congress in 1950. Removals occur in the Fall in two hunt areas east of the Snake River but within the boundaries of GTNP and are coordinated between the NPS and WGFD (personal communication, Sue Consolo-Murphy, National Park Service, February 23, 2016). In the mid-1990s, densities of elk were 2.5-fold higher in GTNP than densities outside of GTNP, likely as a result of the relative lack of hunting inside GTNP compared to outside of GTNP (Smith and Anderson, 1996). Smith and Anderson (1996) also concluded that elk numbers inside GTNP were not being regulated through food limitation and density dependence because they spend winter and are fed on the NER, and thus they argued that hunting removals are warranted.

In 2014, there were 129 wolves in 17 packs in the Jackson herd area (packs south and southwest of YNP excluding Wind River Reservation and Prospect packs) (Jimenez and Becker, 2015). As of the winter of 2004, the total number of elk killed by wolves each winter in the Gros Ventre portion of the Jackson herd area was estimated to represent less than 1% of the herd (USFWS and NPS, 2007; WGFC, 2007). Wolves incidentally preyed on the NER until 2004/2005; 18 elk were killed in 2004/2005 and 63 were killed in

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

2005/2006 (USFWS and NPS, 2007). Grizzly bear numbers were positively correlated with calf:cow ratios in this area, more so in unfed than in fed elk (Foley et al., 2015).

2.7 Elk Feedgrounds

Currently, 22 elk feedgrounds are maintained in Wyoming by the WGFD, independently of the NER. Feedgrounds are mostly located adjacent to active cattle allotments within the Bridger-Teton National Forest (BTNF) and along boundaries between U.S. Forest Service (USFS) lands and private lands. Bienen and Tabor note two reasons for feeding, including “keeping brucellosis infected elk from foraging on cattle ranches, and maintaining consistently higher numbers of elk than the available range could support, which satisfies hunters and outfitters and brings revenue to the state” (Bienen and Tabor, 2006). WGFD states three reasons for maintaining the feedgrounds: (1) to prevent depredation on stored crops, (2) to prevent elk-livestock comingling, and (3) to reduce winter elk mortality (WGFD, 2011). However, there is some disagreement about the benefits of feedgrounds (Bienen and Tabor, 2006). In the absence of feeding, elk disperse and give birth alone, which limits disease transmission. Additionally, there is concern that the mortality from an epidemic of chronic wasting disease that could be facilitated by high elk densities on feedgrounds would exceed any losses resulting from reducing or eliminating feeding.

From 1982-1987, the number of elk counted on the feedgrounds, including the NER, increased from 17,770 to 20,145, but since then the number has been relatively stable in the range of 20,000 to 26,000 (Cross et al., 2010a). From 1998-2013, the NER has fed 5,000-8,000 elk (USFWS and NPS, 2007), which means that approximately 15,000-18,000 elk have been fed on the other feedgrounds since 1998. Since 1998, WGFD tried to reduce large elk aggregations on several feedgrounds by distributing food across a broader area and by stopping feeding earlier in the year (Cross et al., 2013). Targeted elk hunts have been implemented in some areas of southwestern Montana and western Wyoming to disperse large groups, move them away from cattle, and reduce population sizes.

In theory, supplemental feeding would reduce negative effects of dry growing seasons and severe winters on forage availability, which would then result in increased elk reproduction and survival (Foley et al., 2015). However, one study found no evidence that feedgrounds affected midwinter calf:cow ratios (Foley et al., 2015). Calf:cow ratios of fed elk were more strongly correlated with environmental factors (snow and summer rainfall), while calf ratios of unfed elk were more strongly correlated with predator densities, particularly bear density. In contrast, an earlier study found that survival of calves supplementally fed in winter exceeded survival of calves not fed (Smith and Anderson, 1998).

Population growth is also affected by juvenile and adult survival (Lubow and Smith, 2004). However, variation in juvenile survival is primarily affected by environmentalconditions—particularly snowpack and duration of winter—and is little affected by feeding (Smith and Anderson, 1998). Also, female elk that fed on feedgrounds had negligibly higher survival rates than unfed elk, and any differences in survival due to feeding were due to effects on older animals, which have lower reproductive values (Foley et al., 2015).

Supplemental feeding can alter the seasonal migrations of elk (Jones et al., 2014). Fed elk migrate shorter distances, arrive on summer ranges later, and depart from summer ranges earlier than unfed elk. Feeding disrupts the migration of fed elk from the timing of spring green-up, and it decreases the time elk spend on summer range by 26 days, thereby reducing access to quality forage (Jones et al., 2014). If supplemental feeding were phased out, elk might make greater use of summer ranges to at least partially compensate for the loss of feed. Supplemental feeding of elk also increases dense aggregations which in turn increases stress levels; this has been detected through increases in glucocorticoid, a metabolite associated with stress that has been hypothesized to reduce immune function and increase disease susceptibility (Forristal et al., 2012). Relocating, reducing, or eliminating feedgrounds are options that have previously been considered but not pursued by WGFD in its Brucellosis Area Management Plans (WGFD, 2011). Reasons cited by WGFD for not pursuing these options include land availability constraints with relocating feedgrounds (including permitted grazing allotments) and lack of support from various constituencies (agriculture, land management agencies, sportsmen) for reducing or eliminating feedgrounds. However, as part of

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

a Target Feedground Project, major reductions in the length of the feeding season have already occurred at certain feedgrounds since 2008, and minor reductions have occurred at other feedgrounds since 2009.

2.8 The National Elk Refuge

In 1912, the NER was created as a place for supplemental elk feeding that would mitigate the loss of natural winter range and minimize impacts to livestock operations. Historically, elk moved longer distances to areas, including the upper Gros Ventre Basin, Idaho, the Green River area, and in severe winters, the Red Desert (Murie, 1951; Cole, 1969; Boyce, 1989; Cromley, 2000; USFWS and NPS, 2007). Recently, human settlement and conversion of winter range to livestock grazing areas has shortened migration routes and caused elk to remain in Jackson Hole (USFWS and NPS, 2007). Supplemental feeding increases the nutritional status1 of 68% to 91% of the Jackson elk herd and reduces winter weight loss, particularly in severe winters (Wisdom and Cook, 2000; USFWS and NPS, 2007).

The number of elk fed on the NER has varied from about 5,000-11,000 between 1971-1997, and from 5,000-8,000 between 1998-2013. The Bison and Elk Management Plan calls for a reduction to 5,000 elk on the NER (USFWS and NPS, 2007). The population objective of 5,000 elk for the NER is distinct from the population objective of 11,000 for the entire Jackson herd unit set by WGFD. The herd objective for the NER is set at a level that is in line with U.S. Fish & Wildlife Service (USFWS) policy for refuges to contribute to natural population densities and natural levels of variation at larger landscape scales, especially when habitat has been lost in the surrounding landscape or ecosystem (USFWS and NPS, 2007).

Given concerns over the negative impacts of supplemental feeding on brucellosis transmission, it is pertinent to determine how many elk could be supported on native ranges if feedgrounds were to be phased out. In modeling the number of elk that could be supported across an area corresponding to the Jackson herd unit, Hobbs and colleagues (2003) used a “Forage Accounting Model” to estimate forage production, snow cover, and resulting forage availability for different habitat types. The model indicated that supplemental feeding is necessary to support current numbers of elk in winters with above average snowpack, but supplemental feeding far over compensates for the loss of winter range in winters that have average or below average snowpack (Hobbs et al., 2003). Without supplemental feeding, about half as many elk could be supported compared to current numbers in winters with average snowpack. However, reductions in forage availability in severe winters are natural occurrences. The model indicated that habitat removals due to human settlements and livestock grazing have had negligible effects on forage availability (Hobbs et al., 2003).

According to the 2007 elk and bison management plan, a long-term goal is to implement a variety of actions to transition from intensive supplemental winter feeding on the NER to a greater reliance on freestanding forage (USFWS and NPS, 2007). This would need to be carried out with objective criteria and adaptive management actions that would be developed in collaboration with WGFD.

3. CHANGES IN LAND USE AND CONSEQUENCES FOR ELK

Changes in land ownership in areas outside of YNP have affected elk distributions and the ability of state wildlife authorities to manage elk populations. In three elk hunting districts (HDs) just north of YNP, there has been a shift in property ownership to more owners who are interested in natural amenities and who exclude hunters in order to support elk for their own enjoyment, which consequently has created refugia for elk (Haggerty and Travis, 2006). In those three HDs numbers—313, 314, and 317—18% of the winter range is privately owned in one district (HD 313) while 71% and 46% of winter range is privately owned in the other two districts (HD 314 and 317), respectively (Haggerty and Travis, 2006). MDFWP has been able to utilize a combination of general and late-season hunts to achieve population targets in HD 313,

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1 Nutritional status indicates the degree to which an animal’s nutritional requirements are being met through forage intake. Nutritional status will decrease when requirements are not being met, and it will increase when intake exceeds requirements.

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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but this has proven to be more difficult in HDs 314 and 317, because the lands are “out of administrative control” (Haggerty and Travis, 2006). Similar and even more pervasive land ownership changes have taken place in the Paradise Valley (north of the Northern Yellowstone Management Unit) and in the Madison River Valley (west of YNP). These land use and land ownership conversions could have contributed to a much larger fraction of the northern elk herd now being found outside of YNP in the winter and in larger and denser groups than previously found.

More people are also settling across the GYA. From 1990-2010, the population of census blocks in and near the GYA increased nearly 50%, with much of that growth occurring in rural home development (McIntyre and Ellis, 2011). From 1970-1999, the population in the GYA increased by 58%, with rural areas increasing by 350% due to exurban housing densities, demonstrating that developed land in the GYA is increasing faster than the rate of population growth (Gude et al., 2006). The GYA consists of the 145,635 km2 of land, with 32% privately owned, 32% managed by the USFS, 19% by the Bureau of Land Management (BLM), and 7% managed by the NPS (Gude et al., 2007). Using the current rates of population growth to predict future land use scenarios and their potential impact on biodiversity, Gude and colleagues (2007) predicted that 10% of elk winter range and 24% of wildlife migration corridors would be affected in 2020.

With many core winter ranges north and northwest of YNP in private ownership, MDFWP has identified a number of management challenges. These challenges arise from a large fraction of the elk population not being available to hunters due to reduced access to public land and adjacent private land; increases in landowners who have less interest in allowing elk hunting; and elk that have shifted onto privately owned lands during the hunting season (Proffitt et al., 2010b). However, as of the writing of this report, a proposed option in the Montana Fish & Wildlife Commission’s Brucellosis 2017 Annual Work Plan would allow for landowners in the Red Lodge area, which is outside the DSA, to request that a very limited number of potentially infected elk (no more than 10) be culled to prevent contact with livestock (French, 2016).

Changes in land use can also potentially increase the risk of elk-cattle contact. Across the GYA, scrub/shrub and grasslands are the predominant land cover types on private lands (35% and 26%, respectively), and they are the types most likely to be used for livestock grazing (McIntyre and Ellis, 2011). Winter ranges for large mammals (elk, mule deer, pronghorn antelope) also occur primarily on scrub/shrub (9,804 km2) and grassland/herbaceous (7,001 km2) land cover types. Consequently, increased development of private lands on land cover types that are used by both livestock and large mammalian wildlife (including elk) could result in an increasing number of wildlife finding refugia from hunting on exurban land holdings (Haggerty and Travis, 2006; Gude et al., 2007; McIntyre and Ellis, 2011). Increased elk-livestock interaction could occur in some areas where elk prefer private lands with livestock over lands where public hunting occurs (Proffitt et al., 2010b). Although exurban development could reduce elk-livestock interactions by reducing livestock numbers, this could be offset by increased elk-elk transmission due to denser concentrations of elk on exurban refugia.

4. BISON POPULATIONS AND DISTRIBUTIONS

4.1 The Yellowstone Bison Herds

The Yellowstone bison population consists of two herds:a centralherd and a northern herd, with some intermixing between them (Gates et al., 2005; Olexa and Gogan, 2007). The range for the central herd includes the Hayden and Pelican Valleys in the east, across to the Firehole Valley and the Madison River Valley in the west (see Figure 2-10). The range for the northern herd is at lower elevations and includes the Lamar River Valley in the east and the Gardiner Basin in the west. Under the Interagency Bison Management Plan (IBMP), bison are allowed to use habitats outside the northern and western boundaries of YNP (Zone 2 and Eagle Creek, see Figure 2-10).

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Image
FIGURE 2-10 Bison range distribution conservation areas and Zone 2 bison toleranceareas. SOURCE: Wallen et al., 2015.

In 1966, the total bison population was 366 and had been managed through periodic herd reductions. In 1968, a natural regulation policy was adopted for bison and elk, with the hypothesis being that populations would naturally achieve a dynamic equilibrium with forage production without human intervention. The bison population grew steadily from 1968-1995 (see Figure 2-11). The first removals outside YNP boundaries occurred in 1992, with considerable numbers of bison removed in winters of 1994-1998. In 2006, the population grew in size to 5,015 animals. Bison hunting was first allowed outside the YNP boundaries in 2005-2006, and a substantial number of bison were hunted in the following years. Due primarily to management removals from 2005-2008, the total population was reduced to less than 3,000 in 2009. Since that time, the total bison population has increased to nearly 5,000 in 2014, which increases the population average to about 4,000 over the longer term. Notably, most of this increase occurred in the northern herd, which has more than doubled in size since 2008; meanwhile, the central herd has remained nearly constant in size. In 2015, YNP managers recommended removing or hunting approximately 900 bison per year in the two following winters to achieve a population target of 3,500, as recommended in the IBMP (Geremia et al., 2014a). The number of bison that can actually be removed depends on the number that cross the YNP boundary; however, it is realistic to assume sufficient numbers would emigrate given the current size of the population.

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Image
FIGURE 2-11 Bison counts and annual removals, northern and central herds. SOURCE: Geremia et al., 2014b.

Bison migrate seasonally along elevational gradients: moving from higher-elevation summer ranges to lower elevations during autumn through winter, and returning to summer ranges in June (Meagher, 1989; Bjornlie and Garrott, 2001; Bruggeman et al., 2009; Plumb et al., 2009). Migration to lower elevations is primarily driven by earlier snowfall and greater snow depths at higher elevations in autumn and early winter. As the bison population increased, more bison began migrating earlier to lower elevation winter ranges for better access to food resources (Meagher, 1989; Bruggeman et al., 2009; Plumb et al., 2009). In the spring, bison progressively migrate to higher elevations, following the progressive snow melt and green-up with increasing elevation.

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Bison movements out of YNP are driven by a combination of density and snow conditions that reduce forage availability. Density dependent dispersal has been observed in many animal populations (Owen-Smith, 1983; Pulliam, 1988). Since 1998, more than 6,000 bison have been removed from the two dispersal areas outside the northern and western boundaries during their annual winter migrations to lower elevations, and the total population has been kept in the 3,000-5,000 range since 2002. However, the northern range has now become a dispersal area for the centralherd; numbers in the northern herd have more than doubled; and animals crossing the northern boundary are coming from the central herd.

An analysis of bison removals versus population size showed that there were generally few removals when the population was below 3,000. Above the threshold of 3,000, bison removals markedly increased and removals were highly correlated with population size when snow water equivalent was above 17 inches. It was also suggested that exceptionally large numbers of bison would leave YNP when the snow pack melts and refreezes to create an ice layer, as occurred in the winter of 1996-1997. A more recent analysis using data through 2008 indicated that in average winters, most movements outside YNP would be minimal if population sizes are kept <3,500 in the central herd and <1,200 in the northern herd (Geremia et al., 2009). Migration beyond the northern boundary is affected by herd size, snow water equivalent, and forage biomass whereas migration beyond the western boundary is less influenced by these variables (Geremia et al., 2011). Kilpatrick and colleagues (2009) predicted that with 7,000 bison and average snowfall, more than 1,000 bison would leave YNP in 74% of the winters; with 3,000 bison and average snow, more than 1,000 bison would emigrate in 9% of the winters; and with severe snow, more than 1,000 bison would leave in 25% of the winters (see Figure 2-12).

In the past three decades, the increases in bison populations and bison movements outside YNP have been partly attributed to more favorable conditions for bison movement and resultant range expansions. Although road grooming for snowmobiles and coaches could increase population growth and facilitate movements and range expansion (Meagher, 1993), more recent analyses have concluded that road grooming has not affected range expansion or population growth (Gates et al., 2005; Bruggeman et al., 2007). Bison have increasingly used road corridors to travel through certain landscape bottlenecks, such as canyons that connect the centraland the northern herd ranges (Gates et al., 2005; Bruggeman et al., 2006, 2007, 2009). The resultant increased connectivity between the central and northern herds has likely contributed to increased numbers of animals exiting the northern boundary.

Bison population growth rate decreases at higher population densities (Fuller et al., 2007). This is because bison become increasingly resource limited at higher population densities, despite the added resources resulting from range expansion as was seen in the 1980s and 1990s. Despite the declining population growth rate at higher densities and removals at the boundaries, population growth rates have remained positive, even at high densities (see Figure 2-12).

The use of an ecosystem model to estimate food limited carrying capacity can be useful for predicting population dynamics (Coughenour, 2005; Plumb et al., 2009). Coughenour (2005) predicted that a mean bison population size of 6,000 could be sustained at food limited carrying capacity with no removals at the boundaries. This is in comparison to the actual population of 5,000 where more bison could theoretically be supported by the forage base. However, bison are intolerant of increased levels of competition and nutritional stress at higher densities and prefer to migrate beyond the designated dispersal areas to maintain adequate nutritional status. Also, elk compete with bison for forage due to overlapping diets and habitat, and bison numbers are affected by elk abundance (Coughenour, 2005). The decrease in elk on the northern elk winter range could therefore have contributed to increased numbers of bison.

4.2 The Jackson Bison Herd

The Jackson bison herd is jointly managed by the NER, GTNP, WGFD, and BTNF. Bison were first introduced into GTNP near Moran in 1964 and were allowed to free range in 1969, enabling bison to establish well-defined seasonal movement patterns in GTNP. However, since the winter of 1975-1976, most of the herd has wintered on the NER. In 1980, bison discovered the NER feedlines and subsequently the herd greatly increased in size. Bison were initially culled or hunted, but no reductions have taken place

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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since 1990. WGFD reinitiated hunting in 1998 outside the NER and GTNP; however, few have been killed because most habitats are inside the NER or GTNP. In 1990, the Jackson bison population was approximately 110; in 1998, it increased to about 430; and in 2006, it had increased further to about 950 (USFWS and NPS, 2007). The Bison and Elk Management Plan (USFWS and NPS, 2007) calls for a reduction to 500 bison and 5,000 elk. During feeding operations for elk on the NER, bison are fed in order to minimize disruptions with elk feeding operations. After feeding is discontinued in late winter or early spring, the bison herd moves north of the NER to spring ranges, then moves further north to summer ranges on the east side of GTNP. Calving occurs on both the spring and summer ranges.

Image
FIGURE 2-12 Bison population growth rates versus population sizes in the previous year, northern and central herds. SOURCE: Geremia et al., 2014b.
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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5. LIVESTOCK

There are approximately 450,000 cattle and calves in the GYA comprising those in Bonneville, Caribou, Franklin, Fremont, and Teton Counties in Idaho; Gallatin, Madison, and Park Counties in Montana; and Lincoln, Park, Sublette, and Teton Counties in Wyoming (NASS, 2011; Schumaker et al., 2012). Approximately 85% of the operations are cow-calf producers with open-range grazing in summer and pasture supplemented with hay during the winter. Within the DSAs, there are 296 herds in Montana, 242 in Wyoming, and 191 in Idaho (USDA-APHIS, 2014).

Since 1998, cattle operations immediately adjacent to YNP have been reduced as part of the IBMP. Private lands just north of YNP have been acquired by USFS for inclusion into the northern bison management area. In 2006, there were 266 cattle in four herds in winter and 677 in nine herds in spring in the northern bison management area (Kilpatrick et al., 2009). More recently, in the northern bison management area, there were just two small (25 each) cattle operations (USDA-APHIS, 2014). In the IBMP western bison management area, there were no cattle in winter and 686 cattle in nine herds in spring (Kilpatrick et al., 2009). Recently, it was reported that there were four seasonal operators (no year-around operations), with approximately 600 cow-calf pairs utilizing the western management area during summer after June 15 (USDA-APHIS, 2014). Due to the reduced numbers of cattle and management operations that maintain temporal and spatial separation from bison, few cattle have any exposure to infected YNP bison (USDA-APHIS, 2014).

Originally, GTNP had 29 permittees grazing approximately 4,320 animals on 67,640 acres inside GTNP. The number of permittees has decreased to two as a result of permits expiring and ranches ceasing to operate (USFWS and NPS, 2007). The two remaining permittees graze on three grazing allotments that are inholdings: one with 525 cattle animal unit months (AUMs) permitted (264 cattle present) and the others with only horses. Just outside the GTNP are three ranches, with 5,514 cow-calf AUMs on two ranches that seasonally move animals from one area to another and one ranch with 60 breeding stock permitted (55 cattle present) (personal communication, S. Consolo-Murphy, NPS, 2015).

In the three counties in Wyoming at the southern end and just beyond the GYA (Lincoln, Sublett, and Sweetwater Counties), there are approximately 105,000 cattle and 500 producers (personal communication, B. Schumaker, University of Wyoming, 2015). These counties contain portions of 17 WGFD elk herd units and 15 of the 22 feedgrounds not including the NER. In conducting a cost-benefit analysis of various brucellosis management options, a risk map was produced of elk-cattle interactions for this tri-county area (Kauffman et al., 2013), which is a somewhat preliminary approach, but one that has considerable potential for use in the future.

While there are many legally designated grazing allotments throughout the GYA (see Figure 2-13), many of them are not active. The committee was unable to locate maps of all active versus non-active allotments throughout the GYA. However, the BTNF provided the committee with information showing permitted livestock numbers, turn-on and turn-off dates, head-months, and AUMs for each allotment (personal communication, T. O’Conner, USFS, 2016). In the BTNF, there are 63 active grazing allotments with approximately 99,000 permitted cattle (see Figure 2-14). A total of 34,337 livestock, 110,892 head-months, and 135,603 AUMs are permitted. The permits included mature cows with a nursing calf, yearlings, and bulls. Turn-on dates varied from June 1 to July 15 and turn-off dates generally varied from September 15 to October 15. In Idaho, there are currently 163 resident cattle herds within the DSA with approximately 15,000 head; there are also 80 seasonal herds that use USFS, BLM, and private lands with approximately 16,000 head (personal communication, B. Barton and D. Lawrence, Idaho Department of Agriculture, 2015).

Although livestock numbers have been reduced or closely managed immediately adjacent to the national parks, there are large areas of private lands and grazing allotments that have considerable overlap with elk throughout the GYA.

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Image
FIGURE 2-13 Grazing allotments throughout the GYA. Each of the drawn polygons is an allotment and the current use of many allotments across theentire region were not easily accessible. SOURCES: BLM (2014) and USFS (2008, 2009, 2015).

6. IMPLICATIONS OF CHANGING CLIMATE FOR ELK AND BISON

Climate change in the GYA has implications for elk and bison numbers and distributions and, thus, brucellosis in the GYA. A recent analysis of historic climate data concluded that over the past 100 years minimum temperatures have increased 2.9°F and maximum temperatures have increased 1.2°F (Northern Rockies Adaptation Partnership, 2014). Using climate model outputs from the Coupled Model Intercomparison Project, maximum temperature in the GYA is expected to rise 5-10°F and minimum temperature is projected to rise 7-12°F. Winter maximum temperature is predicted to rise above 32°F in mid-century,

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

and summer temperatures predicted to rise by nearly 5°F by mid-century and nearly 10°F by the end of the century (Littell et al., 2011). Predictions for precipitation are uncertain, but there could be a slight increase. Warming temperatures in the northern Rocky Mountains is associated with earlier spring snowmelt, warmer summers, and longer growing seasons (Romme and Turner, 2015). Spring and summer temperatures will rise 8-10°F by mid-century, with increased frequency of hot, dry summers (Westerling et al., 2011). Snowpacks in the GYA have consistently declined due to increased temperatures, and a long-term forecast in the GYA calls for less snow (Tercek et al., 2015). This conclusion is based on analyses showing that temperature increases are the primary cause of decreased snowpack and that temperatures are continuing to trend upward. Another projection calls for a 32% reduction in snowpack in areas typical of elk habitats at mid-elevations and a 56% reduction at higher elevations (Lapp et al., 2005; Creel and Creel, 2009).

Image
FIGURE 2-14 ActiveU.S. Forest Service grazing allotments in Bridger-Teton National Forest. SOURCE: Data provided by USFS and BTNF.
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×

Reduced winter snowpack will affect elk and bison by increasing winter forage intake and by causing spring snowmelt and green-up to occur earlier. Reduced snowpack may also reduce the energetic costs of foraging and traveling in winter. As a result of greater forage intake and reduced stress, population growth rates are likely to increase. An empirical elk population model predicted that warmer winters could raise equilibrium population size in Rocky Mountain National Park elk by 50%-100% depending on whether summers are drier or wetter (Wang et al., 2002). Using an empirical population model driven by climate model outputs, it is predicted that Montana elk populations will increase substantially due to reduced snowpack (Creel and Creel, 2009).

Similarly, the annual population growth rate of bison in the central YNP herd was negatively correlated with snowpack, but not so for the northern herd; this was likely due to deeper snow at higher elevations in central YNP than in northern YNP (Fuller et al., 2007). As previously discussed in this chapter, bison movements in winter to lower elevations (including areas outside YNP) are driven by a combination of increased animal density and increased snowpack. Thus, it is likely that reduced snowpack will reduce bison outmigration from YNP, but this could be offset by increased population size unless the population is managed. Increased population growth rate could also lead to increased numbers being removed outside the YNP boundary by management actions. Conversely, reduced snowpack will likely cause earlier migrations upslope in the spring due to earlier green-up, resulting in a shorter duration of bison at low elevations outside YNP.

Temperature and precipitation interactively affect plant growth and thus forage availability. Increased spring temperatures will result in earlier green-up and growth, but increased summer temperatures in water-limited environments can lead to increased evapotranspiration, reduced soil moisture, reduced growth, and earlier curing of forage, which thus results in reduced forage quality. During the dry period of 1989-2009 in the GYA, spring-summer temperatures were warmer and there was reduced spring precipitation, leading to an increased rate and shorter duration of green-up (Westerling et al., 2006; Middleton et al., 2013a). The dry conditions resulted in less green forage and lower pregnancy rate, and a mismatch between time of green forage availability and the period of lactation could also lower recruitment rate, presumably through reduced calf survival (Post and Forchhammer, 2008; Middleton et al., 2013a).

Overall, the positive effects of reduced snowpack and the negative effects of warmer temperatures and increased dryness could counteract one another. The net outcome can most likely be predicted with process-based models. Models of plant growth and snowpack that represent the effects of water, temperature, and snowpack on plant productivity, time of green-up, and time of senescence could be employed to predict future patterns of forage availability seasonally and across the landscape. Such models can be linked to models of animal distributions in response to changing distributions of snow, forage, and land use as well as process-based population dynamics models that consider the effects of forage availability on animal nutritional status and consequent rates of reproduction and survival (e.g., Coughenour, 2005). The implications for brucellosis arise from changes in predicted elk numbers and distributions in relationship to livestock numbers and potential elk management actions.

7. SUMMARY

With elk now known to be a primary source of B. abortus transmission in the GYA, the scope and dynamic complexity of brucellosis in the GYA has expanded. Whereas bison are primarily confined to YNP or just outside its immediate borders, many tens of thousands of elk are spread across a very large and heterogeneous area. Elk are likely a reservoir of brucellosis independent of bison. Elk populations in the GYA have increased for the most part, and elk now occur in larger aggregations than in the past. The change in elk numbers and distributions is in part due to land use changes, including land acquisitions by owners who discourage or prohibit access by hunters, which then creates refugia from hunting offtake and leads to elk aggregations. Also, large numbers of elk continue to be artificially fed in winter in the southern GYA. Despite recognition by management agencies that feeding contributes to B. abortus transmission and that it would be desirable to phase out feeding, this goal remains elusive due to extensive habitat overlap with

Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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livestock operations and questionable assertions that feeding is necessary to maintain abundant elk populations. These major factors have contributed to the sustainability of elk populations as a reservoir for brucellosis.

The bison population has also increased, which has also shifted the distribution of bison across the landscape. Bison are moving from central YNP to northern YNP, and consequently, the northern herd segment has increased in size. Most of the bison exiting YNP do so at the northern boundary, with more exiting when snow is deeper and the population is larger, and there are efforts to manage the bison population through opportunistic removals at the YNP boundary. Northern YNP has shifted from being elk-dominated to bison-dominated. Under the IBMP bison have been successfully contained in designated dispersal areas just outside the YNP boundary. The dispersal area has been enlarged, and bison have also been kept separated from livestock.

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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Page 39
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Page 40
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Page 41
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Page 42
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Page 43
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Page 44
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Page 45
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
×
Page 46
Suggested Citation:"2 Geographic Scope of Populations and Disease and Change in Land Use." National Academies of Sciences, Engineering, and Medicine. 2020. Revisiting Brucellosis in the Greater Yellowstone Area. Washington, DC: The National Academies Press. doi: 10.17226/24750.
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Page 47
Next: 3 Ecology and Epidemiology of Brucella abortusin the Greater Yellowstone Ecosystem »
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Brucellosis is a nationally and internationally regulated disease of livestock with significant consequences for animal health, public health, and international trade. In cattle, the primary cause of brucellosis is Brucella abortus, a zoonotic bacterial pathogen that also affects wildlife, including bison and elk. As a result of the Brucellosis Eradication Program that began in 1934, most of the country is now free of bovine brucellosis. The Greater Yellowstone Area (GYA), where brucellosis is endemic in bison and elk, is the last known B. abortus reservoir in the United States. The GYA is home to more than 5,500 bison that are the genetic descendants of the original free-ranging bison herds that survived in the early 1900s, and home to more than 125,000 elk whose habitats are managed through interagency efforts, including the National Elk Refuge and 22 supplemental winter feedgrounds maintained in Wyoming.

In 1998 the National Research Council (NRC) issued a report, Brucellosis in the Greater Yellowstone Area, that reviewed the scientific knowledge regarding B. abortus transmission among wildlife—particularly bison and elk—and cattle in the GYA. Since the release of the 1998 report, brucellosis has re-emerged in domestic cattle and bison herds in that area. Given the scientific and technological advances in two decades since that first report, Revisiting Brucellosis in the Greater Yellowstone Area explores the factors associated with the increased transmission of brucellosis from wildlife to livestock, the recent apparent expansion of brucellosis in non-feedground elk, and the desire to have science inform the course of any future actions in addressing brucellosis in the GYA.

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