Spring Migration

The spring migration of ungulates from a low elevation winter range to a higher elevation summer range is a relatively well-studied phenomenon. By the end of winter, ungulates tend to be in poor condition nutritionally relative to other seasons (Parker and others, 2009). As snow melts and vegetation green-up begins in late spring and early summer, migratory ungulates in temperate climates move to summer-range habitats to access higher quality forage and areas with vegetative cover that provides protection for them during fawning (Wasserman and Friggens, 2017). Spring migration typically begins in late March or early April, depending on winter severity and elevation (Messmer and Klimack, 1999; Kolar and others, 2011; Monteith and others, 2011; Collins, 2016), and is initiated by decreasing snow depth and increasing plant growth. Hoskinson and Tester (1980) found that the start of spring migration among pronghorn in southeastern Idaho coincided with the breakup of snow cover, and Monteith and others (2011) found that later snowfall events delayed the mean date of mule deer migration by several weeks. Similarly, Garrott and others (1987) found that mule deer migrated to the summer range earlier after relatively mild winters and later after more severe winters. By moving to summer ranges as soon as the snow cover disappears, animals can take advantage of improved forage and space resources.

Plant phenology is a critical factor that shapes the resource landscape for ungulates. Because plant growth is delayed at high elevations and latitudes in the spring, the onset of spring green-up occurs as a resource wave known as “the green wave.” Some populations track, or “surf,” the green wave during spring migration, timing their movements to maximize access to optimal forage conditions (Monteith and others, 2011; Merkle and others, 201675). Forage quality is highest in new plant growth due to the high cell-soluble content (for example, crude protein, sugars, and starches) at that time. Cell solubles are easily digestible but decline as plants mature and accumulate fiber; therefore, although forage quantity increases as plants mature and biomass increases, the quality decreases. According to the forage maturation hypothesis, ungulates balance forage quantity and quality by selecting for intermediate forage biomass (Fryxell, 1991).

By timing spring migration with the emergence of vegetation, animals can take advantage of highly digestible forage for a prolonged period, leading to improved nutritional condition, survival, and reproductive success (Fryxell and Sinclair, 1988). For example, Hebblewhite and others (2008) found that migratory elk achieved an average of 6.5 percent higher forage-quality exposure compared with resident elk. Other studies found that some ungulates “jump” rather than “surf” the green wave, moving quicker along their migration routes and arrive on the summer range before the peak productivity of forage occurs (Lendrum and others, 2014). Even in these cases, migrating ungulates could access more favorable forage conditions than if they remained on the winter range.

Summer Range

Female ungulates require adequate summer forage to meet the nutritional demands of lactation and build fat reserves to survive and maintain pregnancy during the winter months (Cook and others, 2004). Fawning occurs on the summer range, typically from May to August (Messmer and Klimack, 1999), and access to high-quality forage during lactation improves the condition of female mule deer and has a positive effect on fawn survival (Monteith and others, 2014). The lactation period is energetically intensive for pronghorn, which have the highest proportion of neonate to adult female mass of North America’s ungulate species (Byers and Moodie, 1990).

Summer nutrition conditions can limit elk pregnancy rates and calf production. Proffitt and others (2016) found that elk exposed to lower summer-range nutrition entered winter in a poor nutritional condition, which resulted in lower pregnancy rates (72 percent) compared with elk that experienced higher summer-range quality (89 percent). Similarly, the summer and autumn nutrition of calves influences a female’s probability of becoming pregnant as a yearling. The probability of pregnancy among yearling elk can approach 100 percent for those that have access to high summer through autumn nutrition in their first year (Cook and others, 2004). These results highlight the important role of summer nutrition in maintaining healthy ungulate populations, as ungulate population dynamics are driven by a combination of adult female survival and juvenile recruitment (Gaillard and others, 2000).

Autumn Migration

In October, migratory ungulates depart their summer ranges for lower elevation winter ranges. Autumn migration among ungulates is less well-studied compared with spring migration. Autumn migration can be less synchronous among individuals than spring migration (Monteith and others, 2011), suggesting that autumn migration triggers may be more complex than those driving spring migration. One of the known drivers of autumn migration is the combination of increasing snowfall and decreasing temperatures. Increased snow depth and colder temperatures result in increased energy expenditures associated with movement and thermoregulation. Deep snow also directly reduces the availability of forage, which affects the nutritional condition of individuals and winter survival rates. Kucera (1992) found that major fall snowstorms caused a migration pulse in which mule deer rapidly and simultaneously left their Sierra Nevada summer ranges.

Similarly, Monteith and others (2011) found that during a year with early snowfall and cold temperatures, mule deer completed their migration 1 month earlier than during a year with mild autumn conditions. However, in the absence of cold temperatures, the authors only found a modest increase in the daily probability of mule deer migration after an early snowfall. Other mule deer herds have migrated in advance of the first snow (Garrott and others, 1987; Monteith and others, 2011), suggesting that local conditions may influence autumn migration.

As with the timing of migration, the distances traveled by ungulates between summer and winter ranges can vary among populations and between years. For example, during autumn migration, more severe winters are associated with longer pronghorn migrations, and milder winters are associated with shorter migrations (Bruns, 1977; Hoskinson and Tester, 1980; Collins, 2016). Winter severity may also determine whether individuals choose to migrate to their summer range or remain on their winter range. Collins (2016) found that 65 percent of pronghorn monitored in the northern Great Basin migrated during a year with milder winter conditions, while 100 percent migrated during a year with harsher winter conditions.

Another potential driver of autumn migration is the senescence of plants on the summer range, as autumn nutrition is a known driver of overwinter survival and fecundity (Cook and others, 2004; Tollefson and others, 2010; Proffitt and others, 2016). As plants age, their cell walls increase in thickness and fiber, making them less digestible. However, several studies found that ungulates migrate before the senescence of plants on their summer range (Monteith and others, 2011; Rivrud and others, 2016; Mikle and others, 201979). These findings suggest that animals make a tradeoff by leaving their summer range before peak rates of forage senescence to avoid becoming trapped at higher elevations by severe weather.

Land management activities, such as hunting, can also drive autumn migration. Ungulates perceive human disturbance as a predation risk and can respond to hunting by altering resource selection or moving into areas where hunting is prohibited (White and others, 2010). Changes in behavior during migration, such as reduced foraging or displacement from high-quality habitats, can affect individual fitness (Paton and others, 2017). For example, Mikle and others (2019) compared the timing of autumn migration among two subpopulations of elk in southwestern Wyoming—one that migrates to winter range located on a protected area where hunting is prohibited, and one that migrates between lands accessible to hunters. They found that 67 percent of elk that used the protected winter range left their unprotected summer range before the start of archery season (September 1), prior to frost or snow, while no elk from the second subpopulation migrated before September 1. Departing the summer range earlier to avoid hunting had tradeoffs, with early migrants leaving two months before vegetation senescence and potentially decreasing foraging efficiency (Mikle and others, 2019).

Winter Range

Autumn migration is complete when animals arrive on their winter range. Winter range habitat is often composed of grasses, forbs, and shrubs such as sagebrush (Wasserman and Friggens, 2017) and is typically much smaller in area and lower in elevation than the summer range. By moving to lower elevations, ungulates can escape deeper snow and colder temperatures. Snow conditions directly affect the ability of ungulates to access forage (Fancy and White, 1985), which in turn has consequences for their nutritional condition and winter survival. In fact, in northern climates, most annual mortality for ungulates occurs during winter (White and others, 1996).

The quality of winter-range habitat can be a limiting factor, affecting ungulate population dynamics (Bishop and others, 2009). Nutrition during winter is vital for minimizing fat loss. Winter severity thus interacts with body condition to affect the overwinter survival of juveniles and adults (Cook and others, 2004). For example, Singer and others (1997) found that winter severity had an additive effect on elk calf mortality by contributing to malnutrition. Winter nutrition can also affect calf birth weight, an essential predictor of neonate calf survival (Griffin and others, 2011). However, it is important to note that the traditional view of the winter range as the most critical and limiting factor for ungulates has begun to shift, as the importance of summer and spring or fall nutrition has been documented (Copeland and others, 2014). For example, in Nevada, summer drought and extreme temperatures can affect forage quality during those months, leading to a delayed mortality among mule deer during the winter months (Wasley, 2004).

Stopover Sites

While migrating between seasonal ranges, animals require access to adequate forage to maintain and renew energy reserves. As a result, many long-distance migrants spend part of their migration period in habitat patches along movement routes known as “stopovers,” where animals can rest and forage (Dingle and Drake, 2007). This phenomenon is well-studied among flying avian migrants but not among land migrants (Sawyer and Kauffman, 2011).

The emerging literature on the use of stopovers by ungulates demonstrates that these sites play a crucial role in their migration strategies. Most notably, Sawyer and Kauffman (2011) found that mule deer in Wyoming spent 95 percent of spring and autumn migration in stopover sites. Seidler and others (2015) found that pronghorn in Wyoming spent a significant portion of the spring migration period (78 percent) in stopovers. These sites had higher forage quality than movement corridors, and deer used the same stopover sites for years. During their spring migration, deer used stopover sites 44 days before peak green-up (Seidler and others, 2015), enabling them to exploit vegetation during the period of growth when plants are more easily digestible. Had deer not used these sites, they would have arrived on their summer ranges several weeks before optimal forage conditions. These results suggest that ungulates can reduce the amount of time they need to spend on the winter range by using stopover sites, allowing them to recover their body condition earlier in the spring and maintain a good condition longer into the autumn. More recently, Jachowski and others (2018) found that that the availability of stopover sites dampened the physiological stress response induced by human disturbances on the landscape among migratory mule deer in western Wyoming. The results of these and other studies led to a growing awareness about the importance of conservation strategies that prioritize stopover locations and the maintenance of connectivity between those locations.

Migratory Bottlenecks

Bottlenecks are features of migration routes that can influence ungulate movements. Bottlenecks are narrow areas along migration routes where landscape features such as vegetation, topography, or human development restrict the width of the corridor and, therefore, animal movements (Sawyer and others, 2005). Bottlenecks are often less than 1 km wide and can be as narrow as 100 meters (m) (Berger and others, 2006). Because of their small area, minor changes to the habitat in these corridors can completely sever established migration routes. One of the best-known bottlenecks is Trapper’s Point, Wyoming, which is used by an estimated 2,500–3,500 mule deer and 1,500–2,000 pronghorn as they migrate between the Upper Green River Basin and Grand Teton National Park. This narrow sagebrush ridge was once 2 km wide, restricted by the Green River to the west and the New Fork River’s riparian zone to the east; today, it has been reduced to 0.8 km by housing development and transportation infrastructure (Sawyer and others, 2005; Berger and others, 2006).

Residential Development

The effects of residential-scale development on ungulates have received less attention. Exurban development, which is characterized by low-density, vehicle-dependent communities outside of cities and towns, can be particularly detrimental to wildlife. This type of dispersed development can fragment habitat, alter animal movement patterns and behavior, cause species’ home ranges to overlap, and reduce fitness (Riley, 2006). In the Rocky Mountain West, areas targeted for residential development, such as valley bottoms and low-elevation foothills, frequently intersect with ungulate winter ranges (Polfus and Krausman, 2012).

Research shows that ungulates can exhibit short-term behavioral responses to human disturbance, although responses vary by species. For example, Goad and others (2014) found that elk demonstrated decreased occupancy of high-density exurban areas, while mule deer showed relatively less avoidance behavior. In areas of human activity within protected areas, there is documentation of elk and pronghorn exhibiting behavior consistent with reduced predatory threat, such as higher levels of feeding, suggesting that they use more-developed spaces as refugia from predators (Shannon and others, 2014). Few studies tested whether population-level consequences result from these behavioral shifts, which would likely require long-term (>5 years) studies to detect (Polfus and Krausman, 2012). While some research exists on how energy development can alter migration behavior, similar studies have not explored the effects of residential development on ungulate migration.

Transportation Infrastructure

Roads and highways represent a direct source of mortality for ungulates due to the risk of wildlife-vehicle collisions (Bolger and others, 2008) and are a top concern among western big-game managers. These collisions can result in human injury and fatalities and lead to costly property damage. For example, in 2007, the average cost associated with deer-vehicle collisions was $6,617, while the average cost associated with elk-vehicle collisions was $17,483 (Huijser and others, 2009). Coe and others (2015) found that 10 percent of mule deer mortalities were due to vehicle collisions, which was roughly equivalent to the mortality caused by legal (11 percent) and illegal (13 percent) hunting.

In addition to the threat of collisions, transportation infrastructure can affect ungulates by reducing habitat connectivity and impeding migratory movements. Dodd and others (2010) found that a paved highway with fenced right-of-ways represented a near-total barrier to pronghorn passage. Similarly, Seidler and others (2015) found that a highway with high traffic volume and non-wildlife-friendly fencing represented a complete barrier to pronghorn movement: no animals were able to cross the highway. However, a highway with lower traffic levels and wildlife-friendly fencing remained permeable, and all pronghorn were able to successfully cross. Gavin and Komers (2006) identified behavioral changes in response to transportation infrastructure, with pronghorn demonstrating increased vigilance and decreased time foraging along high-traffic roads. Meanwhile, mule deer are able to search for less busy portions of highways to cross during migration (Coe and others, 2015). Lendrum and others (2012) found that in areas with high road-density, mule deer did not substantially deviate from their migration route but increased their movement rate. This finding suggests that deer avoid roads if they can do so without significantly altering their migration route. In highly developed areas, deer might not have this option, and fidelity to their migration route wins out (Lendrum and others, 2012). A lack of avoidance behavior could put migrating mule deer populations at increased risk if their movements crossing roads increased mortality.

Some States have installed—or are exploring the feasibility of installing—wildlife crossings over or under highways to reduce wildlife-vehicle collisions and support safe passage across transportation infrastructure. In Wyoming, the construction of seven underpasses, plus game-proof fencing on Wyoming Highway 30, which intersects a migration route used by thousands of mule deer, reduced deer-vehicle collisions by 81 percent (Sawyer and others, 2012). Detailed information on the location of migration corridors and where animals tend to cross roads is critical to ensuring the effective placement of crossing structures (Sawyer and others, 2012; Coe and others, 2015).

Railroads are another type of transportation infrastructure that can be a direct source of mortality for ungulates. In Canada’s Yoho National Park in British Columbia and Banff National Park in Alberta, train strikes are the second-largest source of mortality for deer, elk, and moose (Dorsey and others, 2017). Railroads are travel corridors for some ungulates, such as elk, and deep snow can make an escape from oncoming trains difficult (Popp and others, 2018). Despite substantial documentation of ungulate-train collisions, our understanding of the effects of this type of transportation infrastructure on ungulates is limited compared with the effects of roads (Popp and others, 2018).

Fences

Fencing can prevent ungulates from crossing roads and being involved in collisions. Game-proof fencing may be necessary for guiding ungulates to underpasses or overpasses, improving the effectiveness of these structures (Sawyer and others, 2012). In northern Arizona, the installation of fencing along roadways designed to guide animals to existing crossing structures resulted in a 97 percent reduction in elk-vehicle collisions along a 9.17-km stretch of road (Gagnon and others, 2015).

Fencing not friendly to wildlife, including that associated with roads, livestock, or property, can be a source of direct mortality and a complete barrier to movement. Ungulates can snare their legs in fences if they attempt to jump over them. For example, wire fences in Colorado and Utah killed an annual average of one ungulate per 4 km of wire fence, with juveniles being eight times more likely to die in fences than adults (Harrington and Conover, 2006). Pronghorn more often attempt to crawl under fences when they cross, whereas deer jump (Jones, 2014). There are several solutions for making fences wildlife friendly. These solutions include limiting fence heights to 42 inches (in.), which adult deer and elk can jump, and leaving an 18-in. clearance under the bottom wire or rail for animals such as pronghorn that prefer to crawl under, rather than jump over, fences. Gates, dropdowns, and other mechanisms that allow wildlife passage can also be installed in areas where wildlife concentrate or where livestock is not present (Paige, 2012).

Energy Development

Over the last decade, researchers started to explore how the expanding scale and intensity of oil and gas development in the Western United States could affect migratory ungulates (Beckmann and others, 2012; Sawyer and others, 2013; Christie and others, 2015; Seidler and others, 2015). Two of the largest natural gas fields in the contiguous United States partially occupy the southern reaches of the Greater Yellowstone Ecosystem, an area of Wyoming that is home to more than 100,000 wintering ungulates (Beckmann and others, 2012).

Studies suggest that ungulates alter their migratory behavior in response to energy development. For example, Sawyer and others (2013) found that mule deer can migrate through moderate levels of energy development, but in areas of intense development, behavioral changes were identified, including detouring from traditional migration routes, increasing the rate of movement, reducing the use of stopover sites, and reducing the use of migration routes. Mule deer have also increased their rate of movement during spring migration in areas high in natural gas development, resulting in earlier arrivals on their summer range (Lendrum and others, 2013). Among pronghorn, Seidler and others (2015) found that animals reduced their use of stopover sites in the most intensively developed areas of two natural gas fields. Beckmann and others (2012) found that pronghorn avoided habitat patches with the highest level of disturbance from energy activities. In addition to the density of energy development, the level of human activity at these sites may also influence ungulate habitat selection. For example, Sawyer and others (2009a) found that mule deer avoided all natural-gas infrastructure and selected areas farther from well pads with higher levels of vehicular traffic.

These changes in migratory behavior and habitat use can affect the fitness of individuals, as when animals migrate more quickly and reduce the time spent at stopover sites with high-quality forage (Christie and others, 2015). Behavioral changes may be precursors to demographic responses, such as lower reproduction and survival rates in subsequent years (Beckmann and others, 2012). Christie and others (2015) identified a demographic cost to pronghorn caused by energy development, determining that 8 percent of an observed 73 percent decline in abundance was likely attributed to oil and gas development. One potential reason is that this type of infrastructure reduces the net primary productivity of forage plants because of the removal of vegetation to build oil pads, roads, and other infrastructure (Allred and others, 2015). Oil and gas development are also associated with increased road-density and vehicular traffic, representing an additional source of mortality and habitat fragmentation (Christie and others, 2015).

Warming Temperatures and Plant Phenology

Plant phenology, a central determinant of ungulate habitat use (Fryxell, 1991), is sensitive to climatic variation because of the role of temperature in plant development. Large-scale climate variability plays an important role in shifting spring onset on interannual and multidecadal time scales (Ault and others, 2015), and shifts have been observed at both the individual plant and landscape levels (Cleland and others, 2007). The observed changes in plant phenology appear to vary with altitude and are affected by changes in both temperature and precipitation phases. While there is increasing evidence of longer growing seasons at low altitudes, there is less evidence of shifts at higher altitudes (Inouye and others, 2000). This difference is likely due to variations in snow accumulation across altitudinal zones. For example, in the Colorado Rocky Mountains, at an elevation of 2,945 m, snowfall increased over a 25-year period; therefore, spring green-up did not shift significantly despite warming temperatures (Inouye and others, 2000).

In a study of phenological shifts across the elk range in Wyoming (1,315–3,066 m), Christianson and others (2013) found that snow cover influenced the timing and rate of green-up. The authors found that years with extended periods of snow cover tended to result in later but more rapid spring green-up, while winters with snow cover that receded early resulted in an earlier spring onset but slower green-up. Therefore, shifts in plant phenology can vary both latitudinally and altitudinally. The extent to which ungulates are affected depends on the location and altitude of their winter and summer ranges.

Winter Precipitation and Severity

Warming winter and spring temperatures across the Western United States has contributed to more winter precipitation falling as rain than snow and earlier spring snowmelt (Knowles and others, 2006). As a result of warming temperatures, snowpack has declined across much of western North American since the 1950s and is projected to continue to decrease throughout the 21^st century (Pederson and others, 2011). Precipitation is projected to become more variable, with more precipitation generally occurring from October to April and less expected during the summer months (DeVos and McKinney, 2007). In higher elevation areas, the projected increase in winter precipitation could result in deeper snowpack, unless warming temperatures push the rain-snow line farther upslope and precipitation falls instead as rain (Pettorelli and others, 2007).

Across many Western State mountain ranges, climate-model projections suggest the length of consistent snowfall-conducive temperatures will decrease from approximately five months (November–March) to three months (December–February) by the mid-21^st century (2036–2065) (Klos and others, 2014). The Western United States could see a 30-percent average monthly reduction in the amount of land area that can remain within a wintertime snowfall regime by the mid-21^st century (Klos and others, 2014). However, changes to the rain-snow transition line are influenced by elevation, latitude, and topographic relief. Areas of the Western United States with lower relief, mid-elevation mountains, such as the Northern Rocky Mountains and Northern Cascade Mountains, are projected to shift more quickly into new precipitation-phase regimes. Areas with steeper elevational gradients, such as the Sierra Nevada, Middle Rocky Mountains, and Southern Rocky Mountains, are likely to transition less quickly (Klos and others, 2014). Despite these projected changes, colder and higher areas in the intermountain West are expected to continue receiving winter snow at the highest elevations (Gonzalez and others, 2018). Climate extremes, including winter storms, are also expected to increase in frequency and magnitude. Whether or not these storms produce heavy snowfall depends on winter temperatures (Kunkel and others, 2013). However, mild and wet conditions can lead to heavy rain-on-snow events, which can result in a layer of ice forming above forage that prevents wildlife from accessing vegetation (Hansen and others, 2011).

Overall, the projections of future winter conditions suggest that warming temperatures and shifting precipitation phase regimes could, on average, lead to reductions in snowpack and less severe winters in the Western United States. However, interannual variability in winter conditions is expected to continue or increase, and severe winters could still occur if intense winter precipitation events become more frequent and coincide with temperatures that are snow-conducive, particularly in high-elevation areas.

Drought, Wildfire, and Invasive Species

Warming temperatures and more variable precipitation patterns are resulting in more frequent and intense droughts in the Western United States. In the Southwest, increasing temperatures amplify the severity and effect of droughts and increase the likelihood of decadal and multidecadal megadroughts. Decreasing snowpack and early peak snowmelt in the region further exacerbate drought conditions (Gonzalez and others, 2018). In the northern Great Plains, fluctuations between heavy rainfall events and severe drought suggest that the region’s climate is becoming more variable as temperatures warm (Conant and others, 2018).

Fire activity also increased across the Western United States in recent decades. Warming spring and summer temperatures and earlier spring snowmelt resulted in increased fuel aridity and more favorable fire conditions in western forests (Abatzoglou and Williams, 2016). The occurrence of large wildfires increased dramatically in the mid-1980s, with a spike in large-wildfire frequency (nearly four times the average), longer wildfire durations (increased from 1 to 5 weeks), and longer wildfire seasons (increased by 78 days) (Westerling and others, 2006). The most notable increases occurred in the Northern Rocky Mountains, where spring and summer temperatures increased, and spring snowmelt occurs earlier in the season (Westerling and others, 2006).

Climate change and historical fire suppression also aid the spread of invasive plant species that degrade western rangelands and perpetuate the wildfire cycle. Cheatgrass and other invasive plants were brought into the region with crop seeds in the early 1900s and have since led to dramatic changes in rangeland vegetation structure and composition (WAFWA Mule Deer Working Group, 2003). Invasive species such as cheatgrass compete with native species for resources and can increase erosion and the frequency and intensity of wildfires (Watkins and others, 2007). More frequent and intense wildfires on mid- and low-elevation winter ranges destroy the native shrublands that ungulates use for forage and cover (Watkins and others, 2007).

Ungulate Adaptive Capacity

Ungulates could adapt to changing climate conditions through behavioral plasticity, such as altering the timing of migration to better track spring green-up, or through evolutionary adaptations, such as altering the timing of parturition. Behavioral plasticity by ungulates is documented in several studies and may prove important for the long-term persistence of migratory ungulates under rapidly changing climate conditions, particularly if the rate of change outpaces evolutionary adaptations (Loe and others, 2016). Although evolutionary adaptations to climate change are observed among other taxa, ungulates have long generation times, and the pace of evolutionary change might be too slow compared with the rate of climate change. For example, caribou in West Greenland were unable to advance their reproductive phenology to coincide with the fast rate of warming temperatures and advancing spring green-up, and therefore experienced significant reductions in reproductive success (Post and Forchhammer, 2008).

The ability of ungulates to advance or delay the timing of migration could directly affect the ability of populations to persist in the face of changing climate conditions (Monteith and others, 2011). Large herbivores can adjust migration timing in response to environmental conditions, suggesting that they can buffer the adverse effects of climate change by using flexible migration strategies (Monteith and others, 2011). For example, observed interannual variation in elk migration timing, particularly the winter range departure date and the summer range arrival date, demonstrates the ability of elk to shift migration timing based on changing environmental conditions. It is possible that elk could continue to access high-quality forage as climate change alters forage conditions, assuming quality forage is available in the region and elk can locate it (Rickbeil and others, 2019). Similarly, migratory caribou on the northern Quebec-Labrador Peninsula demonstrate interannual variation in habitat use based on the climatic and habitat conditions available each year (Sharma and others, 2009).

The use of stopover sites has served as a mechanism for helping migrating ungulates better track forage conditions (Sawyer and Kauffman, 2011). Ungulate migration is cued mainly by forage conditions. As they arrive at stopover sites along migration routes, ungulates can delay or advance their movements based on the cues received at these sites. In effect, ungulates use stopover sites to ensure they arrive on their winter or summer range at the appropriate time, avoiding trophic mismatch (Sawyer and Kauffman, 2011).

Alternatively, migratory ungulates could adapt to climate change by switching to nonmigratory behaviors. For example, the partially migratory Ya Ha Tinda elk population in Alberta, Canada, demonstrated that migratory behavior is an individually variable trait that can respond to external drivers, including environmental forces (Eggeman and others, 2016). If more individuals become nonmigratory, overcrowding on ranges could become an issue (Wang and others, 2002), particularly if warming temperatures and less severe winters lead to increased population growth. However, Eggeman and others (2016) found that high elk abundance caused individuals in a partially migratory population to switch to a migrant strategy. Therefore, if climate change results in more interannual variability for factors such as winter severity, snowpack, and drought occurrence, ungulate populations that migrate every year may become partially migratory, selecting their strategy each year based on current environmental conditions and population abundance.

Response Scenario One: Altered Migration Timing

Response Scenario Two: Altered Migration Route

Response Scenario Three: Behavioral Switching—From Migratory to Resident

Implementation

A national coordinator oversees all implementation activities related to SO3362. States were grouped into five “unified regions” for the implementation of SO3362, and each region was assigned a Federal liaison from either the BLM, FWS, or NPS. The primary role of the liaisons is to work with the States to implement SO3362. Within 60 days of SO3362 being signed, each State was asked to identify their top three to five migration corridor, winter range, and stopover habitat priorities and their top two or three research priorities. This information was summarized in a State action plan for each State. Each plan describes the State-defined priority areas and research needs, as well as relevant, ongoing State and Federal management and research activities. The State action plans, set to be updated on an annual basis, guide the allocation of Federal funds that can protect and enhance both winter range and corridor habitat. The DOI acted immediately after the completion of these plans and provided up to $300,000 in funding for each State to support priority research needs. Table 1 lists additional funding activities related to SO3362.

U.S. Geological Survey Roles and Activities

Concerning science and research activities, SO3362 directs the USGS to work closely with States to develop maps and mapping tools related to ungulate movement or land use, evaluate the effectiveness of habitat treatments in the sagebrush ecosystem, and develop a greater understanding of the locations used by ungulates as winter range and migration corridors.

State Action Plans

Across all State action plans, acquiring better data on ungulate movement patterns through GPS collaring was listed as a priority. Data that can help States understand the location of winter range, summer range, corridors, and stopover sites, as well as how ungulates use these habitats, was a common theme. In many of the plans, these needs were identified for specific herds. Table 2 lists the research priorities identified by each State.

In each State action plan (see table 2 for the State action plan citations for each State), States identified three to five priority corridors or ranges and listed the risks and threats to each priority area. Many States identified similar threats and risks. All States identified either wildlife-vehicle collisions or transportation infrastructure as a threat. Different barriers to movement were identified as threats by almost every State. These barriers include fencing, transportation infrastructure, energy development, and irrigation infrastructure.

The threats can be grouped into 23 categories. Of these, 14 threats were identified by 2 or more States. Table 3 lists the categories of threats identified by two or more States.

The additional threat categories, which were identified by only one State, include barriers to movement (irrigation), insufficient understory, lack of early seral conditions, low genetic diversity, predation, recreation (type not specified), vegetation changes (not specified), and weather (not specified).

Arizona, California, and Washington each mention climate change in their State action plans (State of Arizona and U.S. Department of the Interior, 2018; State of California and U.S. Department of the Interior, 2018; State of Washington and U.S. Department of the Interior, 2018). Arizona mentions that an “altering climate regime” is responsible for shifts in vegetation and available resources, while the California plan states that addressing several of their outlined research needs will help understand “effects from climate change.” The Washington plan identifies climate change as affecting the State’s wildlife and biodiversity more broadly. The plan also identifies the need to prioritize habitat work to “protect climate refugia and buffer migratory corridor changes driven by climate” and identifies the need for “explicit [F]ederal support for global reduction in greenhouse gas emissions” (State of Washington and U.S. Department of the Interior, 2018). Protecting climate refugia and reducing greenhouse gases are listed as actions that should be taken in response to the increasing incidence of extreme weather events, such as drought and low winter snowpack, that reduce the nutritional carrying capacity of the landscape and the body condition of mule deer during migration periods.

Although climate change is not mentioned explicitly in the other plans, several of the threats identified by States are affected by changing climate conditions. These threats include drought, wildfire, habitat conversion due to invasive species, and pinyon-juniper encroachment. Understanding how the distribution, frequency, or severity of these threats might change is important for long-term planning.

Stakeholder Meetings

For this report, three State wildlife agencies in the Western United States, three Federal land management agencies (BLM, NPS, FWS), and three scientists from the USGS involved in ungulate-related research activities were consulted to assess information needs related to ungulate migration. Of the climate-related information needs identified by stakeholders, a common need was information that can help agencies prioritize the application of habitat treatments and restoration efforts. State agencies want to ensure that they are strategic in the use of limited resources. If information is available to help managers identify how different parts of the landscape may be altered by changing climate conditions, managers could more effectively place habitat treatments by focusing efforts on those areas. Understanding the resistance and resilience of the landscape first requires an understanding of how the landscape responded to past climate changes. Projections of wildfire activity, drought frequency and severity, invasive species distributions, and other climate-related stressors can then be incorporated to provide forward-looking assessments of how the landscape might change.

Additionally, the vulnerability of winter and summer ranges was of more interest than the vulnerability of migration-corridor habitat because of the influence of summer- and winter-range foraging on ungulate condition and reproductive success. State stakeholders were most interested in short-term outlooks of approximately 2–3 years than in long-term outlooks.

Potential Projects to Support Ungulate Migration Research