How to Conserve Wildlife Migrations in the American West

New science examines challenges, reveals solutions to habitat fragmentation

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How to Conserve Wildlife Migrations in the American West
Pronghorn migration
Conservation of ancient pathways used by migratory ungulates—such as the pronghorn, pictured, traversing an aspen grove in Wyoming—can help protect these animals and the many benefits they provide to other wildlife and human communities.
Joe Riis

Overview

The natural spectacle of wildlife migration has beguiled people for millennia—a sense of wonder that continues today. Aristotle and his contemporaries marveled at the sudden disappearance of birds and animals in the fall and their reappearance in spring.1 Now, in the 21st century, science is uncovering fascinating new information about the amazing treks that mule deer, elk, and pronghorn make each year across the American West.

Every day, from some of the most remote and spectacular reaches of the United States, special GPS collars affixed to large ungulates (hoofed mammals), other mammals, and birds beam tens of thousands of electronic signals via satellite to researchers across the country. This information, which primarily indicates where a particular animal is located at a given moment, is revolutionizing the understanding of wildlife ecology by enabling scientists for the first time to pinpoint when and where wildlife moves at a grand scale.

The massive amount of data generated from these research endeavors is also transforming the work of wildlife managers, land stewards, transportation officials, and policymakers. In addition to answering simple questions about where animals go every spring or fall during their migration seasons, the data is providing critical new insights into why and how animals move between seasonal ranges. Although GPS has been around for several decades, it has only recently been scaled up for broad applications in wildlife research. And as the data pours in, that new knowledge has significant implications for how wildlife and their habitats can be managed to ensure that migrating species can persist and thrive in the face of old and new challenges.

This report is based on a synthesis, conducted by the Wyoming Migration Initiative on behalf of The Pew Charitable Trusts, of the growing body of science regarding the migration of western North America’s populations of mule deer, elk, pronghorn, and other ungulate species and identifies the most substantive threats to migrating wildlife. The key findings are:

  • Migration between seasonal habitats is a critical part of the life cycle of most ungulate populations in the western U.S. Moving from place to place, often across hundreds of miles, allows these animals to negotiate changes in weather, food availability, predator activity, and other ecological variables. Ungulates that migrate successfully tend to be fatter than those that do not, and fatter females are more likely to survive the winter and produce healthier young, which in turn supports the overall well-being of herds.
  • When migration pathways are blocked or disrupted, animal populations tend to decline. Wildlife depends on resources that are found in different places throughout the year, and when animals cannot access food or important habitat, individual animals and the herd can suffer. Further, because animals in some species learn how and where to migrate from older members of the herd, disturbance of a route for more than a generation can mean the long-term or permanent loss of that inherited knowledge and, with it, vital nutritional and fitness resources.
  • Human activity disrupts migratory behavior and jeopardizes the health of ungulate populations. Fencing, roadways, residential development, and energy and mineral production operations can block, alter, or fragment migration routes and limit access to adjacent habitat that ungulates rely on. And climate change is altering the growth patterns and abundance of forage at key locations and times of year. Additionally, wildlife-vehicle collisions on our nation’s roads take a costly and sometimes deadly toll on animals and people, while entanglement in fences claims hundreds of animals’ lives each year.

In light of these findings, The Pew Charitable Trusts has developed a set of broad recommendations to help land and wildlife managers, conservationists, industry, and other stakeholders better coordinate and direct their efforts to protect these majestic species:

  • Increase mapping and research efforts. Core research focused on better understanding where migrations occur, how widespread they are, and what challenges and threats migrating ungulates face is the first step to conserving wildlife. More such research is needed from academia; Tribal governments; and state, federal, and nongovernmental stakeholders.
  • Identify and coordinate with human stakeholders whose activities intersect with wildlife migration routes. Most migrating animals cross property boundaries at some point in their travels. Better alignment of land use rules and priorities among landowners and managers is essential to conserving the wildlife that transits public, Tribal, and private lands.
  • Remove or modify fences in key migration corridors. Fences are ubiquitous features in the U.S. West and present challenges to wildlife along migration routes. Fences that no longer serve a purpose can be removed, and fences that are still in use can be retrofitted to wildlife-friendly specifications. In addition, new “virtual fence” technology that uses GPS to contain livestock while allowing wildlife to pass unrestricted is gaining in popularity and functionality, offering the potential for a future of safer, more open landscapes.
  • Build wildlife-crossing structures along roads and highways. Millions of large animals die every year in the U.S. from collisions with vehicles. The cost to humans is also significant, with tens of thousands of motorists injured annually. Wildlife bridges and underpasses are highly effective at reducing these crashes, improving safety for animals and humans alike. They also facilitate essential wildlife movement, making herds more resilient in the face of drought and other changes in their habitat.
  • Provide conservation easements or other incentives to landowners. The development of private land that intersects migration habitat can have long-term detrimental effects on migrating wildlife. Incentives that encourage landowners to keep land free of structures, industrial or commercial activity, fencing, and other barriers are a key strategy for conserving migration corridors.
  • Practice smart energy and mineral development on federal lands. Oil and gas leasing, mining, and renewable energy development carried out on known migration routes can harm migrating wildlife. Siting extractive development operations away from migration corridors—or at least minimizing their density within those corridors—would support animals’ ability to use important routes and access key forage opportunities.
  • Engage in local and regional land use planning. Especially in exurban areas where residential neighborhoods are encroaching on wildlife habitat, county, city, Tribal, or federal land use plans can help identify important areas such as migration corridors and put protective zoning restrictions or other conservation overlays in place as the human footprint increases in these areas.

This report details the current state of knowledge about the many long migrations in western states—and how to conserve this compelling and vital process for the health of ungulate species, the diverse habitats they transit, and the character of the American West.

The science of ungulate migration

Migration is the annual back-and-forth movement of animals between seasonal ranges. (See Figure 1.) In the western U.S., mule deer, elk, pronghorn, moose, and bighorn sheep spend their summers at high elevations eating green vegetation, which fuels the creation of body fat necessary to survive the lean winter months.2

When the snow begins to fall in autumn, these animals slowly move down to their wintering grounds at lower, warmer altitudes. And as the first green buds emerge the following spring, they begin this roundabout journey again, following the spring green-up up the mountainsides.

Ungulates give birth in late spring and early summer, after they have had a chance to replenish some of their depleted winter fat reserves and can provide the best nutrition for their young.3

A cluster of arrowleaf balsamroot that has been nibbled on. Elk, pronghorn, and other ungulates will graze on these and other young plants in the spring.
Fresh sprouts of arrowleaf balsamroot have been nibbled by mule deer along the Red Desert-to-Hoback migration corridor in the foothills of the Wind River range in Wyoming.
Gregory Nickerson Wyoming Migration Initiative, University of Wyoming
Two mule deer stand in tall spring foliage among sagebrush-covered hills.
Mule deer in spring forage on the Red Desert-to-Hoback migration corridor near Willow Lake in Wyoming.
Gregory Nickerson Wyoming Migration Initiative, University of Wyoming

Why migrate?

Ungulates migrate for several reasons: seeking out abundant forage, avoiding inhospitable areas, and distancing themselves from predators and parasites.4 For ungulates in the western U.S., migration is the solution to the seasonal fluctuations in weather and food availability across the region’s many mountains and valleys.

For example, the peaks of the Rocky Mountains can receive up to 50 inches of precipitation a year, which in turn supports abundant grasses, forbs, and shrubs, but they also retain deep snow for many months, which makes them inhospitable in winter. And although the valley floors get only about a tenth of the precipitation that the peaks receive and so have a lower abundance of forage, they provide winter refuge from the deep snows and harsh temperatures of the mountains. Migrating between the mountains and valleys in concert with the seasons allows these animals to access the best food with the least effort throughout the year.

Yet the benefits of migration do not end with simply getting animals to the right range at the right time of year. Recent research indicates that the migration routes themselves, and the time at which animals move through them, also provide substantial benefits. For instance, as all gardeners know, lettuce and other greens taste best when harvested young. Ungulates, which are primarily herbivores, also know this, and it is critical to their diet and nutrition. Foraging ungulates seek out plants in their earliest growth stages, when nutrient-dense plant parts are at their peak relative to indigestible parts, such as stems.5 And although gardeners can control when their plants will be ready to harvest, ungulates must move around the landscape to find plants at peak forage quality.6

Map showing three paths—long, medium, and short—that mule deer take between their winter range in the Red Desert near Rock Springs, Wyoming, and three summer ranges. The long path, shown in purple, runs 150 miles to Hoback Rim, south of Jackson, Wyoming, with deer that follow this route dispersing to various areas around the high country in the Wind River and Wyoming Range mountains. The medium path, in orange, ends near South Pass City, and the short route, in green, leads to the southern end of the Wind River Range.

Because temperature is moderated by elevation, the start of the growing season arrives later as the altitude increases. This creates a “green wave” in which spring conditions slowly propagate upward and from south- to north-facing slopes throughout the spring months.7 For example, in the Rocky Mountains, the green-up date extends five days for every 300 feet of additional elevation.8

Ungulates capitalize on this phenomenon by timing the start of their migration with the onset of spring, when plants are at their earliest growth stages, and “surfing the green wave” uphill over the course of spring and early summer.9 (See Figure 2.) Although ungulates could complete their migrations in a matter of days, they take their time. Mule deer, for example, can spend up to three months migrating, maximizing their exposure to early spring conditions that provide high-quality forage along the way.10

Graph with seven horizontal bars, each bar representing a segment of an elk migration corridor from winter range (bottom bar) to summer range (top bar). The bars are shaded in graduated green and yellow tones denoting the progression of new plant growth. Each bar also features a vertical dark green line indicating the point of peak forage quality on that segment of the migration as well as a small purple box depicting when elk arrive at each segment. Between the bars are purple arrows, pointing upward and toward the right, indicating the elks’ weekly movement. Overall, the graph demonstrates that elk usually arrive at each spot along their migration near the moment of peak forage quality.

Animals that move in near-perfect concert with the green wave end up fatter at the end of the summer than their counterparts that surf the wave less well or stay in one place year-round.11 In North American female ungulates, autumn fat levels are clearly linked to the probability of getting pregnant, surviving the winter, and giving birth in the spring, which suggests that effective green-wave surfing plays a role in bolstering migratory populations.12 (See Figure 3.) The ability of migratory ungulates to move freely and exploit forage at specific times of the year is critical to their success and is one of the reasons for their abundance in the West and on the planet.13

Line graph showing the correlation between migration length and fat gain in migratory ungulates. The graph features three lines, long (in purple), medium (in orange), and short (in green), that start at a similar point around 5% body fat but diverge as they move higher and toward the right, indicating the progression of time from late winter in March, through summer, to early winter in December. The purple line shows that the greatest increase in body fat—roughly 4.6%—occurs in mule deer that undertake the longest migrations. Deer that migrate shorter distances gain much less fat, 2.5% for medium migrations and only 1.7% for short migrations.

When to migrate?

Although seeking out the best forage is important, ungulates must also minimize the costs of moving. (See Figure 4.) One substantial hazard they face in the western U.S. is snow, which not only buries forage, forcing animals to dig for their food, but also increases their costs of movement—the energy they must expend while traveling—and their vulnerability to predation.14 In most cases, animals incur significant costs only when they encounter snow at a depth that requires excessive exertion to traverse. These snow thresholds vary by species based on hoof size, chest height, and the mechanics of their legs.15 For example, mule deer are particularly sensitive to snow depths of more than 1.5 feet, but moose have a much higher tolerance because they have relatively long legs with special shoulder joints that allow them to lift their hooves up and out of the snow.16

But snow levels are just one of a suite of environmental characteristics that trigger ungulates’ spring migration. When an animal starts its end-of-winter journey and how fast it travels are key to its ability to successfully exploit the best forage and reduce the costs of moving in deep snow.17 For mule deer in the Sierra Nevada Range of California, the start of spring migration correlates with snow depth but is also associated with the onset of plant green-up on the winter range.18 And for elk in the Greater Yellowstone Ecosystem (GYE), it correlates with the timing of snow melt on winter range and green-up along migration routes, as well as winter range productivity and migration distance.19 

Similarly, in fall, ungulates must balance several factors, including the benefits of staying on summer range longer to continue foraging versus the risk of getting caught in deep snow on the way to lower elevations. Research shows that although capitalizing on bits of green-up in the autumn results in larger-bodied mule deer fawns, early winter snowfall decreases fawn survival rates.20 Animals typically begin heading to lower elevations when temperatures drop and snow begins to fly, with the species that are most sensitive to snow, such as pronghorn, leaving their summer ranges first.21

For mule deer in the Sierra Nevada, the start of fall migration correlates with snow depth, mean daily temperature, daily snowfall, and change in daily temperature, but less so with the timing of forage decline.22 By contrast, the start of the fall migration among elk in the GYE correlates with the first snowfall on the summer range and increased exposure to hunting pressure, as well as summer range elevation and migration distance.23 Changes in the availability of forage on agricultural fields or in supplemental feeding—when wildlife managers place hay near wintering herds—can also influence the timing of migration and whether individual animals migrate at all.24

A combination shaded-area and line chart showing temperature changes and precipitation totals in the Green River Basin from August through December and their effects on elk behavior. The precipitation is represented by peaks and valleys in light blue for rain and dark blue for snow. An orange line shows the temperatures. Small brown dots, which usually overlap the precipitation peaks, represent elk leaving their high summer range. Three specific periods, identified as “trigger events,” are highlighted. First, a five-day period in late October when 7 inches of snow and a 25-degree temperature drop occurred and 31 elk left the summer range. Second, in mid-November, 7 inches of snow fell over 10 days and the temperature dropped 28 degrees over three days; 16 elk left the summer range during that span. And third, a few days later, 3 ½ inches of snow fell and the temperature dropped 44 degrees over four days, pushing another seven elk off the summer range. Over the full span covered by the graph, very few elk left their summer range except during those three periods.

Yet, whether in spring or fall, once ungulates start migrating, the time they spend en route varies greatly. Some herds and species migrate in just a couple of days, while others travel for weeks or months.25 For instance, mule deer migrate on average approximately 0.5 to 1.5 miles a day in western Wyoming, but these average movements can be broken up into bouts of rapid movement, punctuated by days spent in a single area.26 These two modes can be classified as “fast directed movements within corridors” and “slow circular movements within stopovers.” (See Figure 5.)

A herd of pronghorn migrating uphill, with apparent difficulty, through snow up to their bellies.
Pronghorn struggle in deep snow in Wyoming’s Upper Green River Basin.
Joe Riis Yellowstone Migrations

Stopover behavior is well known in birds, which use stopovers to rest, refuel, and accelerate their migration to breeding grounds.27 Ungulates, on the other hand, use stopovers as foraging sites and to slow down and keep pace with the maturation of forage plants.28 Mule deer in Wyoming, for example, spend 95% of their migrations on stopovers where forage quality is higher compared with movement corridors.29

Map showing the paths taken by three mule deer moving from winter range near Aztec, New Mexico, to their summer ranges. The first, identified as Deer 31 and represented by a purple line, traveled for 63 days during 2012 and ended up well north of Pagosa Springs, Colorado, in the San Juan Mountains west of Route 160. The second, designated Deer 02, took 51 days in 2014 to reach summer range, also well north of Pagosa Springs but east of Route 160. And the third deer, Deer 01, migrated along the state line toward Chama over 15 days in 2014, mostly staying in New Mexico.  Each line also features several yellow sections representing spots where the deer “stopped over” to forage for periods of two to 25 days.

Whether to migrate?

Unsurprisingly, given the vast distances and variable topography that ungulates must navigate during their journeys, their migratory behavior is highly complex. Each year, individual animals must choose which route to take, which summer range to go to, which individuals to migrate with, and, in some cases, whether to migrate at all.30

Deciding whether to migrate is the first choice they must make. Ungulates fall along a spectrum in this regard. Some species and populations, notably pronghorn, switch frequently between migration and residency—that is, remaining in one area year-round.31 Others switch more occasionally. For instance, approximately 15% of one elk herd in Alberta, Canada, switches between migration and residency in any given year.32 And some populations— such as mule deer of the Rosa herd, which migrate between the Navajo Reservoir in New Mexico and Pagosa

Springs, Colorado—never seem to switch.33 Further, a synthesis of data from more than 300 mule deer in Colorado, New Mexico, and Wyoming that were monitored for two to six years found that animals that were resident at the start of monitoring remained resident and those that were initially migrants stayed migratory.34

Switching behavior has been linked to population density, predator abundance, winter severity, and the amount of precipitation on summer range, and to characteristics of individual animals, such as age.35 Similar predictors of the choice to migrate have also been found in other ungulates around the world, such as wildebeest, red deer, and moose.36 Notably, a recent meta-analysis of various populations and landscapes across the globe found that ungulates were more likely to exhibit a migratory strategy in settings where the green wave phenomenon is more pronounced, as is the case in many landscapes of the American West.37

Where to migrate?

If migration is chosen, the next decision is which route to take. Ungulates have strong spatial memory—they can remember places they have been to in the past and efficiently return to them—which is an essential ingredient for successful navigation during migration.38 For example, an analysis of mule deer found that the location of their summer range and the migration path they used the previous year were much stronger predictors of animals’ migratory movements than key habitat variables, such as forage quality, snow depth, and topography.39 (See Figure 6.) This means that following the green wave or seeking out specific habitats alone are not sufficient to drive migration route choices. Rather, an individual or herd’s memory of their migration routes largely informs their movements.

However, knowledge of a particular migration route requires learning it in the first place. Ungulate biologists have long speculated that migratory behavior is passed down from mother to young.40 And evidence increasingly indicates that ungulates not only can rapidly learn the location of movement paths but also rarely forget them once learned.41 A recent analysis of movement data from multiple populations of reintroduced and translocated bighorn sheep and moose strongly supports the idea that migratory behavior is developed through social learning, probably from mother to young: Very few of the reintroduced animals migrated after their release, but each population developed migratory behavior over the course of generations.42

Map showing the paths, one in blue and the other in red, that two mule deer traveled between their winter range near Aztec, New Mexico, and their summer range near Pagosa Springs, Colorado, annually from 2012 to 2017. The lines mostly overlap, with only a few deviations. Additionally, insets show that although the deer generally did not step in their exact footsteps year over year, they stayed within a fraction of a mile of their previous years’ paths throughout their routes.

Although learning a migration route as a juvenile is key, adult animals may still make small adjustments to their routes or learn entirely new ones based on whom they migrate with. Many ungulates are gregarious and migrate in groups. And for those groups to remain together during migration, the animals must agree on when and where to go.43 Further, interactions among individuals can provide an animal with new knowledge related to foraging sites and migration routes.44 For instance, among bison, joining a group with a home range different from one’s own increases an individual animal’s chance of visiting and learning about new areas.45 Yet because some ungulates, including moose, migrate alone and others, such as elk, migrate in groups of a hundred or more, the role of learning in migration behavior and success varies greatly among big game in the West.

In addition, the migratory behavior of some animals can be unpredictable and occur outside typical spring and fall seasons. For example, pronghorn in northern Montana, southern Alberta and Saskatchewan, and the Greater Hart-Sheldon area of Oregon and Nevada may migrate, or they may engage in nomadism: diffuse, irregular movements that rarely overlap.46 (See Figure 7.) Other examples include elk that repeatedly move back and forth between winter and summer ranges during fall and other ungulates whose migrations feature significant elevation changes over relatively short distances and that similarly move between ranges multiple times throughout the year.47 These variations illustrate the diversity of factors that influence ungulate migrations and the movement strategies they employ in response to those many variables.

Map showing more than a dozen paths taken by various pronghorn from south of the Sheldon National Antelope Refuge—a large area in northwest Nevada—to northwest of the Hart Mountain National Antelope Refuge, a smaller area in southeastern Oregon. Some of the paths are short, indicating that those pronghorn are considered resident in those areas, while others move back and forth in a regular pattern between the two refuges, converging in a concentrated zone between the two protected areas, which means that those pronghorn are migrating. Still other lines wander in various directions, seemingly at random. Those pronghorn are considered nomadic. 

Effects of migration on ecosystems and the economy

Seasonal movements of ungulates provide more than just a boon to these species’ own population numbers. Migration promotes abundance in herds, which in turn provides a range of critical ecosystem services: natural processes and products that benefit people. And the ability of migratory ungulates to move between seasonal ranges cascades through the food web and underpins other ecosystem processes.48 (See Figure 8.)

As prey animals, migratory ungulates provide food for carnivores, scavengers, and omnivores, while as consumers, they influence plant abundance, quality, and life cycles.49 Further, ungulates often forage in one place and defecate in another, facilitating plant dispersal and establishment as well as nutrient cycling.50 And ungulate movements enable the spread of disease in some cases but help snuff it out in others.51 Taken together, these observations make clear that migratory ungulates play a key role in the functioning of the broader ecosystem.

A hunter, carrying a backpack and with a rifle slung over his shoulder, stands in silhouette at dusk with mountains in the distance.
A hunter stands on the range at dusk. Migration supports healthy ungulate populations, which bring significant economic activity to rural communities from hunting, wildlife watching, and related activities.
Craig Moore Getty Images
A black and brown wolf from the Wapiti Lake pack and a crow stand atop a bison carcass in a snowy landscape at Yellowstone National Park.
A wolf from the Wapiti Lake pack and a raven prepare to dine on bison in Yellowstone National Park. Migratory ungulates provide food for predators and contribute to healthy ecosystems.
John Morrison Getty Images

Humans also capitalize on migratory ungulates. These animals have been a mainstay of North American Indigenous peoples’ food, clothing, and cultural practices for thousands of years.52 And today across the western U.S., abundant herds support hunting and ecotourism industries that contribute about $1 billion annually to the national economy.53

Detailed evaluations of the modern economic impact of ungulate migrations are scarce. But one analysis of 2015 data from Wyoming found that the state’s big game species, most of which are migratory, support $224 million in retail sales, have a total economic impact of more than $303 million, and support at least 3,100 jobs, mainly guides who host out-of-state hunters.54 And hunting and outfitting provide sizable economic benefits to other western states. For example, during 2014, elk hunters spent $138 million on food, lodging, transportation, and equipment in Montana.55

Further, in spring, elk give birth, and as the herds migrate from winter ranges into Yellowstone National Park, those calves feed the wolves and grizzly bears that are a main draw of tourists to Yellowstone and fuel a robust tourism economy in the surrounding communities. In one recent survey, over 70% of the more than 3 million visitors to the park indicated that they came to see wildlife, especially predators, noting that “viewing wildlife in their natural habitat” was an “extremely” or “very” important motivation for their trip.56 

Illustration of an annual elk migration with notations showing the benefits it presents for humans, other wildlife, the ecosystem, and the elk themselves. The fall and spring migrations are identified by two sets of black and gray lines, respectively. One simply shows the routes. The other shows the direction of travel, west (leftward) in spring and east (rightward) in fall, and the change in herd sizes, represented by varying line thicknesses, throughout the journey. The fall migration begins as a moderately thick black line leaving the summer range in Yellowstone National Park. This migration supports wildlife viewing and tourism, as well as hunting, which results in herd losses shown by the thinning of the line as it reaches the winter range at right. The spring migration begins as a very narrow gray line indicating winter herd losses to hunting, predation, and starvation. Moving leftward, the line widens significantly due to calf births, although some elk are lost to malnutrition and predation. The thickest line—and the largest herd size—of the year approaches Yellowstone, where bears take some calves, reducing the herd’s size slightly as it reaches its summer range.

Challenges and threats to migratory species

Migration ecology is a rapidly advancing field. Much more is known about migratory ungulates today than just a decade ago. For example, researchers now quickly and easily collect fine-scale data to reveal migratory patterns; understand how green-up influences migration; recognize that migratory behaviors can be flexible but may vary among individuals and species; are starting to grasp the role that disease can play in affected populations; and have sophisticated maps and infographics that allow migration to be considered in land use planning.57 Yet, despite these advances, the conservation of migratory ungulates remains a daunting challenge as landscapes and the climate change at unprecedented rates.58 Furthermore, ungulates require large and relatively undisturbed landscapes with long distances between seasonal habitat patches, which often means they must move across many jurisdictional boundaries. These extraordinary space and distance requirements make ungulates in the western U.S. and around the world especially susceptible to threats such as habitat fragmentation and loss, mainly related to roads, residential growth, and energy and mineral development; other human activities; disease; and environmental changes.59

Habitat fragmentation and loss

Nearly all the world’s wildlife now lives in environments that have been altered in some way by humans.60 In the American West, conversion of land to commercial, industrial, or residential uses is increasing rapidly as the human population in the region grows faster than the national average. The effects of these changes on wildlife habitat fall broadly into two categories, fragmentation and loss, each of which contributes to the other and strongly influences migratory species’ distribution and viability.61

Fragmentation, or breaking apart, of habitat may have positive or negative effects on wildlife.62 For example, fires may disrupt and fracture habitat in ways that benefit migratory ungulates by spurring new plant growth, and some agricultural development may provide new food sources in the form of tasty crops such as alfalfa that benefit ungulate populations.63 For seasonal migrations, however, fragmentation is generally detrimental, blocking movements, limiting access to critical habitats, and increasing the risk of injury and death.

Similarly, habitat loss, as its name suggests, is the destruction or removal of habitat. More so than habitat fragmentation, habitat loss has persistent negative effects. Although any single instance of habitat loss rarely causes meaningful harm, the cumulative loss from multiple human activities adds up to significant acreage and restricts ungulate movements.64

The physical footprint of human activities that eliminate or carve up valuable wildlife habitat is plain to see throughout the western U.S.65 Less obvious and more complex, however, are the behavioral and physiological responses of wildlife to the resulting changes in their environments, migration routes, and ranges.66

For example, mule deer tend to move more quickly through altered areas, which allows them to minimize their exposure to the disturbance but can also reduce the amount of stopover habitat that they use.67 In such cases, movement is maintained between seasonal ranges, but the functionality of the migration route is compromised because removal or reduced use of stopover habitat impedes an animal’s ability to surf the green wave and maximize nutritional intake.68

However, such behavioral effects during migration may not become evident until after a certain threshold of disruption is exceeded, at which point they become more difficult to address or reverse.69 And those thresholds are difficult to detect and may vary with the type of human activity, habitat, and species involved.70 Nevertheless, understanding the major human activities that affect wildlife in the American West and how key species respond can begin to reveal possible solutions.

Seven elk cross a curved, two-lane road in front of a white SUV.
Wildlife-vehicle collisions pose serious risks to motorists and ungulates, such as these elk crossing U.S. Highway 89 north of Jackson, Wyoming, during their spring migration.
Mark Gocke

Roads

As animals traverse roads during seasonal migrations and at other times of the year, the resulting wildlife-vehicle collisions present substantial hazards and costs for motorists, including roughly 200 human deaths, 29,000 animal injuries, and the deaths of an estimated 1 million to 2 million large mammals, mostly deer, in the U.S each year.71 That adds up to a cost to American drivers, businesses, and taxpayers of $6 billion to $12 billion annually.72 And the number of these collisions is only expected to increase as traffic volume rises and more roads are built.73

Roadways can also obstruct migratory routes across large regions.74 Interstate 80, for example, extends more than 400 miles across southern Wyoming, blocking the movements and migrations of mule deer, elk, pronghorn, and moose.75 (See Figure 9.)

Map demonstrating the obstructive effect of roadways on ungulate migrations. The area covered roughly spans the Great Divide Basin, which is bisected by Interstate 80, which runs east to west along the length of southern Wyoming. Along the highway are shown dozens of migration routes of pronghorn (in yellow), mule deer (in purple), elk (in green), and moose (in blue). Most of the migrations start as lines miles away from the interstate and end in a chaotic jumble along the edge of the highway; very few cross the highway.

Fences

Although they have been less studied than roads, fences often outnumber roads in linear feet and represent one of the most prevalent human-made features on the landscape.76 An estimated 620,000 miles of fencing crisscrosses the western U.S., much of it in wildlife habitat away from population centers.77

A dead deer lies on the ground, entangled in barbed wire, on a dry open prairie.
Fencing can be a direct source of mortality for ungulates, often as a result of entanglement in wires, and can restrict animals’ movements, such as when high fences intended to keep elk off of ranch land block the way.
Shane Moore

Although fences can provide some benefit to wildlife by keeping animals off roadways, assisting with sensitive species recovery, and maintaining working ranches, more often they are a cause of injury and death for ungulates and, like roads, act as barriers that restrict or limit animal movements.78 Wild ungulates may cross dozens of fences in a single migration. Fences higher than 4 feet are problematic for animals that jump over, and those with low bottom wires or woven wire, or fences partially buried in snow, are challenging for others such as pronghorn, which prefer to crawl under.79 Many animals have scars from barbed wire cutting them. Migrating ungulates also tangle in wire fences, become trapped, and slowly die.80

The magnitude of these effects may vary with fence type and species. In general, however, fences with high top wires and low bottom wires and those with woven wire tend to be more problematic for wildlife than fences that allow animals to move underneath or jump over.81

New research shows that ungulates’ behavioral responses to fences can be complex. These behaviors take three main forms: “tracing,” in which animals move long distances parallel to fence lines; “bouncing,” in which animals move close to fences and then retreat away; and “restricted movements,” in which animals become trapped.82 These responses, in turn, probably have other effects on ungulate movement and distribution, such as restricting access to key habitat at particular times of the year, but those secondary effects are more difficult to document.

Residential, commercial, and industrial development

Residential development is one of the most common sources of habitat loss.83 In places such as the Front Range of Colorado; the Reno, Nevada, area; the Boise, Idaho, region; and the Gallatin River Valley in Montana, rapid growth and development over the past decade have consumed large tracts of previously undeveloped wildlife habitat. And that growth is continuing: The latest U.S. census found that some of the fastest-growing counties are still in those Intermountain West areas.84

Because people tend to settle in valleys and foothills, residential development often conflicts with ungulates’ prime winter habitat.85 To the extent that ungulates avoid or rush through residential areas, they may be missing out on places with less snow, more warmth, and better forage, which can affect the quality and quantity of nutrition they consume in those crucial winter months.86

Energy and mineral development

Demand for and consumption of energy (e.g., natural gas, oil, wind, and solar) and minerals (e.g., gold, trona, and uranium) is increasing along with the growing human population and footprint, leading to rapid expansion of extraction facilities across the West. For instance, central North America adds more than 50,000 new oil and gas wells each year, and an estimated 77,000 square miles of additional land in the U.S. will be directly affected by energy development by 2040.87 

A close aerial view of a well pad, dusted with snow, with vehicles, drills, container boxes, and other equipment densely packed together.
Energy development—such as this natural gas facility within the Pinedale Anticline of western Wyoming, which converted native sagebrush into well pads, roads, and other infrastructure over more than two decades—is a common cause of habitat loss across western rangelands.
Joe Riis Yellowstone Migrations

From a conservation perspective, such development presents a serious challenge, because much of it overlaps with ungulate habitat.88 This is especially true for species such as pronghorn and mule deer that occupy open basins, grasslands, and shrublands where energy and mineral development tend to be more feasible.89

The two main consequences of energy and mineral development are direct habitat loss—such as the building of 6-to-8-foot-tall security fences around a utility-scale solar development project, which preclude ungulates from accessing or moving through the areas—and indirect habitat loss, in which animals avoid a location.90 Such avoidance behavior can be long term, can extend for years, and is especially problematic because it displaces animals away from habitat that would otherwise be usable, effectively reducing the amount of functional habitat.91 Such indirect habitat loss is much larger—in some cases up to 4.6 times—than the habitat lost directly and has been linked to population declines.92

Avoidance is often the primary form of habitat loss related to oil and gas development and can reduce foraging opportunities or cause other behavioral changes that harm animal health.93 For instance, one study observed that between moderate and intensive phases of a Wyoming natural gas field development project, deer use of stopover habitat decreased by 60%.94 (See Figure 10.)

Map of central south Wyoming bordered by Interstate 80 and state highways 789 and 70, showing gas wells, represented by red dots, within the Atlantic Rim energy development project area, outlined also in red. Thin arrows show the movements of two mule deer herds through the gas field. Thicker shaded lines in purple for the Dad herd and green for the Baggs herd overlay the arrows, with deeper shades indicating higher use by the deer. The paths of both herds cross the project area, but the lines steer almost entirely clear of the existing wells, illustrating the ungulates’ avoidance behavior.

Hardrock mining activity, such as the exploration and removal of earth in search of copper, gold, or rare earth minerals, also tends to result in direct loss. Habitat changes from these activities are usually concentrated and intensified with 100% removal of vegetation and topsoil, whether in the area being mined or where the debris is placed. These effects, combined with security fencing, make mines largely or entirely inaccessible to ungulates for any portion of their daily or seasonal needs, with large mining operations consuming tens of thousands of acres of former habitat.95 

Ultimately, when energy or mineral facilities are constructed in crucial habitat, most animals exhibit some behavior changes, but the nature and extent of those behavioral responses can vary by species, geography, and type of development.96 For example, the level of avoidance is lower in regions where rugged topography or vegetation provides refuge from human activity than in open landscapes, such as basins and grasslands, and behavioral responses to wind development are less clear than those related to oil and gas.97

Recreation

Another growing concern among land and wildlife managers is the booming outdoor recreation industry.98 Mountain biking; use of e-bikes, all-terrain vehicles, and motorcycles; backcountry skiing and snowmobiling; mountain climbing; dispersed camping; and other activities are growing in popularity. Existing trail systems are receiving more use, and new trail systems are quickly expanding into areas that historically had low levels of human use. And the potential impacts on migratory animals of these growing recreational demands on public lands, including possible displacement, increased energy expenditures, and lower rates of reproduction, are not yet well understood.99

Similarly, the potential impacts of hunting on animal movements are also a concern, though the science is unclear about their exact mechanisms or consequences. For example, hunting may trigger early autumn migrations of some elk and red deer populations or push them onto lands where there are no hunters, such as private property, but where they may also cause damage to crops or fences.100 However, other studies indicate that hunting does not trigger autumn migrations in mule deer, but they suggest that ungulate responses to hunting probably depend on access to secure cover or roadless areas.101

Disease

The role of disease and parasites in wildlife conservation has always been complex and challenging, but it has arguably become one of the most important factors for managers and biologists around the globe. For example, more than 30 years ago, researchers documented the first clinical description of chronic wasting disease (CWD) from a captive mule deer population in Colorado. Today, the disease, which is almost always lethal and can trigger population declines, has infected deer, elk, moose, and reindeer populations across North America and parts of Europe.102

In addition to CWD, ungulates face an assortment of other diseases and parasites. Elk and bison populations in the GYE must contend with brucellosis.103 Deer can be susceptible to outbreaks of epizootic hemorrhagic disease, which can kill 10% or more of affected populations.104 And moose are increasingly influenced by heavy winter tick infestations.105

Migration can complicate or even contribute to the spread of disease and parasites by moving infected animals across landscapes or uninfected animals closer to infection risk.106 But it can also help depress the spread of disease by allowing animals to escape infected areas.107 The role of disease management and forecasting in the conservation of migratory ungulates is becoming more important, especially in areas where habitat and movement options are limited.108

Climate change

Climate plays a key role in plant ecology and in limiting or expanding the range of various wildlife species.109 Understanding how plants, landscapes, disease, and various ungulate species will respond to a warming climate is critical to conservation efforts. Some regions of the western U.S. have been more affected by climate change than others, including several that have already warmed by 2 degrees Celsius, resulting in drought, altered plant life cycles, wildfires, and other changes to ungulate habitat. (See Figure 11.)

Nine elk descend a scree-covered high-altitude slope in single file with light snow on the ground and mountains in the distance.
Climate change is altering the way ungulates—such as these elk in the Absaroka Mountain Range near Yellowstone National Park— migrate. Warming temperatures and persistent drought are causing ungulates to travel to their summer ranges earlier, and they may not find as much forage along the way as they did in past decades.
Joe Riis Yellowstone Migrations

One of the primary ways in which climate change may affect migratory ungulates is by shortening growing seasons and increasing the frequency and intensity of droughts, which can reduce the forage that animals access along their migratory routes.110 These effects may diminish the foraging benefits of migration to the point that animals will either experience increased mortality and lower reproduction or choose year-round residency rather than migration.111 This will be especially serious for temperature-sensitive species, such as moose, and species on the edge of their geographic range that have nowhere else to go for food as the plant abundance in their range declines.

Ungulate populations also will be affected by large-scale habitat alterations linked to climate change,112 such as the mountain pine beetle epidemic that has killed more than 1.5 million hectares of pine trees in northern Colorado and southern Wyoming.113

Heat map of the 11 westernmost states in the continental U.S. with red shading indicating the extent of temperature rise across the region from 1895 to 2019, with the lightest shades in white representing zero degrees Celsius, and rising in half-degree increments to a maximum of 3 degrees Celsius, in a deep red. The darkest areas are along the Utah-Colorado border, on the southern coast of California and in southeastern Oregon near the borders with Nevada and California.

Relatedly, warming temperatures affect the timing of ungulate migrations, pushing them later into the fall, causing animals to spend more time on summer ranges, potentially altering prey-predator and other ecosystem dynamics, and putting hunting seasons out of sync with herd movements.114 

Property boundaries

The western U.S. is characterized by a mix of land ownership patterns that include private, state, federal, and Tribal entities, and when ungulates migrate long distances, they often cross various administrative boundaries that mark distinct land uses for humans but are invisible to them. (See Figure 12.) Coordinating management of land uses that affect migrating animals across these jurisdictional borders can be a difficult and time-consuming process but is needed to ensure long-term conservation of migratory populations and the continuity of their routes.

For example, the thousands of elk that summer in Yellowstone National Park spend the other half of the year on private lands that have other uses, including ranching and farming, where the elk may cause problems, such as by grazing in areas meant for livestock and damaging crops.115 Similarly, pronghorn that summer in Grand Teton National Park migrate 100 miles each year to winter on federal lands where they must negotiate extensive energy development.116

Map showing various land owners in the border region of Idaho, Montana, and Wyoming. The legend indicates ownership by the National Park Service in brown, specifically for Yellowstone and Grand Teton national parks; the U.S. Bureau of Land Management in yellow; the U.S. Fish and Wildlife Service in red; Tribal nations in tan; states in purple; the U.S. Forest Service in green; and private landowners in gray. Five thick lines overlay the area and show the routes of the Northern, Paradise Valley, Madison Valley, Blacktail and Sand Creek elk herds, which all summer in or near Yellowstone. The lines highlight the many ownership borders that the elk cross during their migrations using darker shades of the same seven colors.

Solutions and tools for conservation of migration corridors

Conserving migratory ungulates in the face of the many complex challenges that they face will require a diverse set of tools and programs aimed at retaining quality habitat and improving landscape connectivity. Fortunately, these efforts enjoy strong public support. For instance, in a 2022 survey, at least 80% of residents from each of several intermountain states and from across the political spectrum said they were strongly or somewhat in favor of wildlife migration conservation.117 This support represents an important opportunity for substantive action at the local, state, Tribal, and federal levels to implement evidence-based solutions to many of the threats facing migratory ungulates and their critical habitat.

Migration maps and assessment for planning

The widespread availability of movement data collected from GPS collars makes it possible to map migration corridors for entire herds, including stopover habitat, and to categorize routes as low, moderate, or high use. (See Figure 13.) Migration maps are easy to interpret and consider in land use planning.

Recent efforts in northeastern Montana and in areas of Wyoming around Yellowstone National Park provide examples of how transboundary ungulate migrations can be conserved through mapping and coordinated management across jurisdictions.118 In both cases, researchers or land managers began by mapping migration routes, noting pinch points and stopover habitat, across a patchwork of private working lands, public lands managed for various uses, Tribal lands, and a few protected areas.119 That data, in turn, gives land trusts and other stakeholders a tool for identifying areas where agreements with private landowners and administrative policies can be most effective in maintaining open ungulate migration routes.120

Two side-by-side maps showing the same mule deer migration corridors across and along the New Mexico-Colorado border, beginning at winter range in New Mexico, marked in blue. The map on the left shows in gray all the GPS points, collected from tagged deer along an array of migration routes to three summer ranges. The map on the right shows the use level for those corridors, represented as low use in yellow, medium use in orange, and high use in dark red.

Researchers can also use this type of migration assessment to overlay maps showing potential threats, such as fences, roadways, and development. Policymakers, Tribes, and other interested parties can then identify potential solutions to the hazards and conflicts identified by the assessment.121 (See Figure 14.)

Such research and mapping initiatives—ideally undertaken as collaborations between states, Tribes, federal agencies, academia, and nonprofit organizations—are critical for conserving migration, but they require significant funding and other resources. In recent years, the U.S. Department of the Interior has provided support for collaborative research and mapping efforts by the U.S. Geological Survey as well as grants (administered by the National Fish and Wildlife Foundation) to states and Tribes for investigations of migration routes. However, many migrations remain unmapped, and more federal and state funding is needed to fully document the travels of ungulates in the western U.S.

Map of the Green River Basin region, between the Wyoming and Wind River mountain ranges. The background map is shaded to indicate land ownership: the National Park Service in brown; the U.S. Bureau of Land Management (BLM) in yellow; the U.S. Fish and Wildlife Service in dark gray; the Bureau of Reclamation in lighter gray; Wyoming Game and Fish Department in orange; Tribal nations in tan; states in purple; the U.S. Forest Service in green; and private landowners in white. The ownership here is predominantly BLM, Forest Service, and Tribal. Running southeast to northwest is an outline in purple of a mule deer migration. Ten text boxes along the migration route identify the top threats to the migration. These include areas where deer have only narrow bottlenecks to travel through; residential development; road crossings; fences; and possible future oil and gas development.

Landowner identification and coordination

A first step in developing management or conservation priorities for a particular herd is to identify the full set of state and federal agencies, Tribes, private landowners, and conservation organizations—as well as their various administrative boundaries. However, for some migrations, especially those that span long distances and feature dozens of transboundary crossings, this can be a difficult task. One tool that can support this effort is “ribbon maps,” which clearly show all land ownership along a given migration route. (See Figure 15.)

Such holistic efforts that prioritize inclusiveness in conservation planning provide myriad benefits, including a better understanding of people’s intent or values, a sense of shared purpose, and a greater sense of how conservation efforts fit into a larger vision of protecting wildlife and other land uses. One model adopted in several western states involves ad hoc groups of landowners and other concerned people and organizations that meet regularly to discuss and act on conservation policy and funding needs. These collaborations have prompted widely supported policymaking efforts to identify and conserve ungulate migration corridors. Similarly, in Wyoming, the governor has created formal groups that assess known migration routes and consider whether the governor should designate them for conservation. Although these groups require significant time investments by members, they help ensure that proposed solutions are effectively designed to conserve migrating wildlife over the long term.

Bar graph with five color-coded bars depicting landownership patterns along the winter-to-summer-range migration routes of the Sand Creek, Blacktail, Madison Valley, Paradise Valley, and Northern elk herds. The length of each bar represents the distance traveled: from 25 miles for the Paradise Valley herd to almost 70 miles for the Blacktail herd. The colors indicate how much and what sections of each migration cross each type of land ownership. For instance, the Northern herd leaves its winter range on state land (in purple), crosses about 4 miles of private land (in gray), enters U.S. Forest Service land (in green) for roughly 15 miles, and then spends the remaining 40-plus miles of its route on U.S. National Park Service land (in brown). Other ownership types shown are U.S. Bureau of Land Management (yellow), U.S. Fish and Wildlife Service (red), and Tribal (tan).

Fence removal and modification

Removing fences or modifying their design can help reduce the risk of harm and improve the continuity of migration routes.122 Fence designs that accommodate ungulate movements are often referred to as “wildlife friendly” and have specific guidelines for the number, type, and height of wires. (See Figure 16.) Although wildlife-friendly fencing does make it easier for most ungulates to move about the landscape, it is not a panacea and can still be an impediment for animals in certain sex and age groups and during winter months when snowpack obstructs the space under a fence.123 Most western states have private or government-funded programs that help landowners modify fences to wildlife-friendly standards.

Line drawing of a wildlife-friendly fence design, with four strands of wire running between two posts.  The top wire is smooth and between 40 and 42 inches high; text suggests this top wire should have a PVC cover, flagging, or a top rail or be made with high-visibility wire to increase visibility. The middle two wires are made of traditional barbed wire and are placed 12 inches below the top wire to prevent entanglement. The bottom wire is smooth to protect animals that prefer to pass under fences and is placed 18 inches above the ground. 

Wyoming and other western states encourage landowners to use wildlife-friendly fencing, which can take several forms depending on the type of livestock being contained, the wildlife species using the area, and other factors. In general, however, the ideal fence design includes a smooth bottom wire 18 inches above the ground, a smooth top wire no higher than 42 inches, no less than 12 inches of spacing between the top two wires, and very taut wires, which don’t tangle as easily as loose wires.

Virtual fencing technology is also an increasingly viable option for maintaining migration corridors while effectively containing domestic animals, such as cattle, within predetermined boundaries. These systems use GPS-enabled collars that deliver vibrations or gentle electrical shocks to domestic animals that venture outside those boundaries, effectively eliminating the need for physical fences and creating more open landscapes for wildlife to move through.

Roadway crossing structures

Crossing structures that allow animals to move safely under or over roadways have evolved in recent decades from basic concrete or steel culverts to sophisticated underpasses and overpasses. When properly sited in or near existing migration corridors, crossing structures can effectively reduce wildlife-vehicle collisions by more than 80%, improve motorist safety, and reduce the effects of habitat fragmentation on wildlife and ecosystems.124 (See Figure 17.)

Two maps, side by side, show Colorado state Route 9 running northwest to southeast between Kremmling and the Green Mountain Reservoir. The map on the left shows purple dots arranged in clusters, some large, where dozens of animals, mostly mule deer, died in wildlife-vehicle collisions in winters from 2011 through 2015. The map on the right shows animal deaths from 2016 to 2020 after two overpasses and five underpasses, marked in black and red, respectively, were constructed to allow wildlife to safely cross the road. Protective fences, indicated with yellow lines on either side of the highway, run between the crossing structures to guide animals along the safe path. The purple dots on this map are far fewer and are concentrated at the southern end of the project where the fence ends, and just north of the first overpass.

These structures are most successful when gameproof fencing is installed adjacent to them and parallel to roadways to help guide animals toward the crossing.125 Their effectiveness can also be improved by minimizing nearby human activity and incorporating species-specific preferences.126 For example, pronghorn strongly prefer overpasses to underpasses.127

Although crossing structures have high construction costs, they often pay for themselves relatively quickly through savings associated with reduced collisions, which cost $9,086 per deer on average.128 For example, one study estimates that a $2 million overpass, completed in 2012 on U.S. Highway 191 in Wyoming, saves $570,000 annually in reduced collisions, surpassing its construction cost in only four years.129

And new federal and state funding opportunities for these structures are becoming available. The 2021 bipartisan infrastructure law includes a variety of new monies for wildlife crossings, including a dedicated $350 million competitive grants program.130 State legislatures are also increasingly active in funding wildlife crossings through a variety of mechanisms, including specialty license plates, mitigation programs, and general fund appropriations.

The Trappers Point wildlife crossing over U.S. Highway 191 in Wyoming. A red pickup truck emerges from the tunnel under the land bridge at sunset. The Wind River mountains are in the distance.
Each year, thousands of mule deer and pronghorn safely move across U.S. Highway 191 near Pinedale, Wyoming, via this overpass and adjacent underpasses.
Gregory Nickerson and Leon Schatz Wyoming Migration Initiative, University of Wyoming

Conservation easements and private land incentives

Government agencies and nonprofit organizations such as land trusts and hunting conservation groups can maintain open habitat on private lands and working ranches by agreeing to conservation easements— voluntary, contractual agreements with private landowners—to ensure that specified lands will be maintained as open space. Landowners may donate an easement without compensation or choose to receive a financial benefit, typically tax breaks for donated land, cash payments for purchased easements, or a combination, and the initiating organization or agency assumes the right to monitor and enforce restrictions in the easement agreement. Most such easements are in perpetuity, but some preclude development without a permanent time restriction.

Conservation easements have helped protect ungulate migrations throughout the Intermountain West.131 Other tools, including wildlife occupancy agreements or habitat leases, can also be used to compensate landowners for providing quality wildlife habitat with or without an associated easement. For instance, Montana is developing a program to lease priority wildlife habitat areas for 30- or 40-year terms from private landowners for conservation purposes and, in some cases, for uses such as hunting, recreation, and livestock grazing or other agricultural functions.132

Additionally, some conservation agreements are structured to be active only during certain times of the year, such as while animals are migrating between seasonal habitats. In these instances, landowners may allow for certain fences to be temporarily laid down or removed or agree to move domestic livestock to pastures away from migration routes during the migration period.

Federal, state, and local entities will need to develop tools that foster new ideas and opportunities for collaboration among landowners and managers. For example, a U.S. Department of Agriculture initiative, launched as a pilot program in Wyoming in 2022, draws on several existing USDA programs, including the Grassland Conservation Reserve Program, Environmental Quality Incentives Program, Regional Conservation Partnership Program, and Agricultural Conservation Easement Program, to provide financial and technical assistance for landowners who want to conserve wildlife and migration corridors on their working lands.133 USDA plans to use lessons learned from the pilot to eventually expand the program across the West.

Smart energy and mineral development on federal lands

Federal agencies with mandates to manage public lands for multiple uses, such as the Bureau of Land Management (BLM) and U.S. Forest Service, collectively oversee more than 400 million acres in the 11 western states and Alaska.134 These lands serve as critical seasonal and migratory habitat for much of the region’s ungulate populations while also serving economic development, recreation, and other conservation interests.

These agencies have discretion over the placement of fossil and renewable energy development on federal lands. BLM and Forest Service land use plans, which identify suitable areas for these types of development and serve as important repositories for migration information, can be revised or amended to incorporate the best available science regarding thresholds and tolerances of ungulates and to apply various land use restrictions on known migration corridors and other crucial habitat. For instance, limiting the density of oil and gas well pads, timing construction work around key migration periods, and keeping aboveground extraction activity and infrastructure out of critical migration areas can all help maintain functional migrations.

Two pronghorn stand on a grassy field, facing away from the camera. A tall blue-and-yellow drill rig stands toward the right, beyond the animals.
Ungulates, such as these pronghorn approaching the Pinedale Anticline Natural Gas Field in the Upper Green River Basin, Wyoming, increasingly encounter oil rigs and other energy development in their habitat.
Joe Riis Yellowstone Migrations

States also can use their authority over permitting for energy projects to support conservation efforts. Colorado, for instance, recently changed the mission statement of its oil and gas commission to include consideration of human health and environmental stewardship and implemented new rules requiring the commission to identify ungulate migration routes and then consult with the state wildlife agency to develop plans to minimize the impacts of development on migrating wildlife.135

Similarly, although the General Mining Act of 1872 limits federal agencies’ control over the location of mines, more recent legislation—the Federal Land Policy and Management Act of 1976—empowers the BLM, which manages mining on federal lands, to halt or restrict “unnecessary or undue degradation” of those lands by regulating the timing of operations, disposal of waste, or siting of aboveground facilities, for example.136 However, the Mining Act also allows miners to “patent” or take ownership of their claim, in which case the land becomes private, and conflicts must be resolved as with any other private landowner.

Local and regional land use planning

The four fastest-growing states in the U.S. are also home to large populations of migrating wildlife. Utah and Idaho, for example, grew by 18.4% and 17.3%, respectively, between 2010 and 2020.137 Much of this growth has occurred in urban areas such as Salt Lake City and Boise, but smaller “gateway” cities and towns adjacent to scenic areas and public lands have experienced significant booms in housing demand in recent years, especially since the start of the COVID-19 pandemic.138

The location of new residential developments, associated road networks, and other intensive land uses, such as shopping malls and server farms, in previously rural areas often overlaps with important winter habitat for deer, elk, pronghorn, and other migratory species.139 Western counties and municipalities are grappling with how to accommodate this growth while maintaining the wildlife resources that are part of what attracts new residents and visitors. Comprehensive land use plans, often at the county level, offer community leaders a tool for managing future growth in ways that can reduce the impacts of urban and exurban development on critical corridors and other habitat. Several components of county or city planning processes, including zoning restrictions, open space acquisitions or easements, and transportation planning, can focus growth away from migration routes and other crucial habitat.

Conclusion

Thanks to evolving technology and emerging research, scientists, conservationists, policymakers, and other stakeholders are developing a fuller understanding of how migratory behavior plays a vital role in maintaining abundant and healthy ungulate populations, which in turn provide significant cultural, economic, and dietary benefits to humans. The science of wildlife migration has advanced at a rapid pace over the past decade, although much remains to be learned about the breadth of this phenomenon across the western U.S. Tens of thousands of data points now accumulate daily, continually adding to the body of knowledge about how and when deer, elk, pronghorn, and other large mammals move across landscapes.

This data provides the foundation for policy and management decisions at all levels of government that can best conserve migrating wildlife. By identifying the routes that animals use and the timing of their journeys, states, managers, Tribes, landowners, and other concerned parties can tailor management strategies to maintain ancient wildlife pathways.

Migrating wildlife faces diverse challenges. Land use trends in the West, particularly in rural areas, almost invariably result in habitat loss and fragmentation, and because migration routes tend to span many miles across multiple jurisdictions, stakeholders will need to employ a range of solutions rather than a single tool. Fortunately, a full toolbox already exists. Conserving a given migration might involve a bridge across a highway, an exclusion zone for new energy development, a county land use plan that incorporates wildlife movement areas, or one of the other approaches highlighted in this report.

The science is clear, and solutions exist. All that remains is for the various stakeholders to embrace those solutions. When deployed in tandem, science and action can provide a bright future for the migrating wildlife that are emblems—and economic drivers—of the American West.

Acknowledgments

How to Conserve Wildlife Migration in the West is in part based on research by the Wyoming Migration Initiative at the University of Wyoming and cartography and data visualizations by the InfoGraphics Lab in the Department of Geography at the University of Oregon.

Endnotes

  1. E.J. Milner-Gulland, J.M. Fryxell, and A.R.E. Sinclair, eds., Animal Migration: A Synthesis (Oxford University Press, 2011), https://oxford.universitypressscholarship.com/view/10.1093/acprof:oso/9780199568994.001.0001/acprof-9780199568994.
  2. M.J. Kauffman et al., Wild Migrations: Atlas of Wyoming’s Ungulates (Corvallis, OR: Oregon State University Press, 2018), https://migrationinitiative.org/wild-migrations-atlas.
  3. Ibid.
  4. Milner-Gulland, Fryxell, and Sinclair, Animal Migration; T. Avgar, G. Street, and J.M. Fryxell, “On the Adaptive Benefits of Mammal Migration,” Canadian Journal of Zoology 92, no. 6 (2013): 481-90, https://cdnsciencepub.com/doi/10.1139/cjz-2013-0076.
  5. J.M. Fryxell, “Forage Quality and Aggregation by Large Herbivores,” The American Naturalist 138, no. 2 (1991): 478-98, https://www.journals.uchicago.edu/doi/abs/10.1086/285227; M. Hebblewhite, E. Merrill, and G. McDermid, “A Multi-Scale Test of the Forage Maturation Hypothesis in a Partially Migratory Ungulate Population,” Ecological Monographs 78, no. 2 (2008): 141-66, https://esajournals.onlinelibrary.wiley.com/doi/10.1890/06-1708.1.
  6. Hebblewhite, Merrill, and McDermid, “A Multi-Scale Test of the Forage Maturation Hypothesis”; B. van Moorter et al., “Understanding Scales of Movement: Animals Ride Waves and Ripples of Environmental Change,” Journal of Animal Ecology 82, no. 4 (2013): 770-80, http://dx.doi.org/10.1111/1365-2656.12045.
  7. S. Van der Graaf et al., “Surfing on a Green Wave: How Plant Growth Drives Spring Migration in the Barnacle Goose Branta Leucopsis,” Ardea 94, no. 3 (2006): 567-77, https://www.researchgate.net/publication/235007576_Surfing_on_a_green_wave_-_How_plant_growth_drives_spring_migration_in_the_Barnacle_Goose_Branta_leucopsis; R. Bischof et al., “A Migratory Northern Ungulate in the Pursuit of Spring: Jumping or Surfing the Green Wave?” The American Naturalist 180, no. 4 (2012): 407-24, https://doi. org/10.1086/667590.
  8. Hebblewhite, Merrill, and McDermid, “A Multi-Scale Test of the Forage Maturation Hypothesis.”
  9. J.A. Merkle et al., “Large Herbivores Surf Waves of Green-Up in Spring,” Proceedings of the Royal Society B: Biological Sciences 283, no. 1833 (2016): 20160456, http://dx.doi.org/10.1098/rspb.2016.0456; E.O. Aikens et al., “The Greenscape Shapes Surfing of Resource Waves in a Large Migratory Herbivore,” Ecology Letters 20, no. 6 (2017): 741-50, https://doi.org/10.1111/ele.12772; Van der Graaf et al., “Surfing on a Green Wave”; Bischof et al., “A Migratory Northern Ungulate.”
  10. H. Sawyer and M.J. Kauffman, “Stopover Ecology of a Migratory Ungulate,” Journal of Animal Ecology 80, no. 5 (2011): 1078-87, https://besjournals.onlinelibrary.wiley.com/doi/10.1111/j.1365-2656.2011.01845.x; H. Sawyer et al., “The Extra Mile: Ungulate Migration Distance Alters the Use of Seasonal Range and Exposure to Anthropogenic Risk,” Ecosphere 7, no. 10 (2016): e01534, https://doi.org/10.1002/ecs2.1534.
  11. A.D. Middleton et al., “Green-Wave Surfing Increases Fat Gain in a Migratory Ungulate,” Oikos 127, no. 7 (2018): 1060-68, https://onlinelibrary.wiley.com/doi/abs/10.1111/oik.05227.
  12. J.G. Cook et al., “Effects of Summer-Autumn Nutrition and Parturition Date on Reproduction and Survival of Elk,” Wildlife Monographs 155, no. 1 (2004): 1-61, https://wildlife.onlinelibrary.wiley.com/doi/10.2193/0084-0173(2004)155[1:EOSNAP]2.0.CO;2; K.L. Parker, P.S. Barboza, and M.P. Gillingham, “Nutrition Integrates Environmental Responses of Ungulates,” Functional Ecology 23, no. 1 (2009): 57-69, https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2009.01528.x; Middleton et al., “Green-Wave Surfing.”
  13. M. Fryxell, J. Greever, and A.R.E. Sinclair, “Why Are Migratory Ungulates So Abundant?” The American Naturalist 131, no. 6 (1988): 78198, https://www.journals.uchicago.edu/doi/10.1086/284822.
  14. E.S. Telfer, “Winter Habitat Selection by Moose and White-Tailed Deer,” The Journal of Wildlife Management 34, no. 3 (1970): 553-59, http://www.jstor.org/stable/3798862; E.S. Telfer and J.P. Kelsall, “Adaptation of Some Large North American Mammals for Survival in Snow,” Ecology 65, no. 6 (1984): 1828-34, https://www.jstor.org/stable/1937779; D.J. Huggard, “Effect of Snow Depth on Predation and Scavenging by Gray Wolves,” The Journal of Wildlife Management 57, no. 2 (1993): 382-88, https://doi.org/10.2307/3809437.
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  87. B.W. Allred et al., “Ecosystem Services Lost to Oil and Gas in North America,” Science 348, no. 6233 (2015): 401-2, https://www.science.org/doi/10.1126/science.aaa4785; Trainor, McDonald, and Fargione, “Energy Sprawl.”
  88. H. Sawyer et al., “Winter Habitat Selection of Mule Deer Before and During Development of a Natural Gas Field,” The Journal of Wildlife Management 70, no. 2 (2006): 396-403, https://doi.org/10.2193/0022-541X(2006)70%5b396:WHSOMD%5d2.0.CO;2.
  89. Sawyer et al., “Migratory Plasticity Is Not Ubiquitous.”
  90. H. Sawyer et al., “Trade-Offs Between Utility-Scale Solar Development and Ungulates on Western Rangelands,” Frontiers in Ecology and the Environment (2022), https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/fee.2498.
  91. H.E. Johnson et al., “Caribou Use of Habitat Near Energy Development in Arctic Alaska,” The Journal of Wildlife Management 84, no. 3 (2020): 401-12, https://wildlife.onlinelibrary.wiley.com/doi/full/10.1002/jwmg.21809; J.M. Northrup, C.R. Anderson, and G. Wittemyer, “Quantifying Spatial Habitat Loss From Hydrocarbon Development Through Assessing Habitat Selection Patterns of Mule Deer,” Global Change Biology 21, no. 11 (2015): 3961-70, https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.13037; H. Sawyer et al., “Mule Deer and Energy Development—Long-Term Trends of Habituation and Abundance,” Global Change Biology 23, no. 11 (2017): 4521-29, https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.13711.
  92. S.P.H. Dwinnell et al., “Where to Forage When Afraid: Does Perceived Risk Impair Use of the Foodscape?” Ecological Applications 29, no. 7 (2019): e01972, https://doi.org/10.1002/eap.1972; Johnson et al., “Increases in Residential and Energy Development”; Sawyer et al., “Mule Deer and Energy Development.”
  93. Dwinnell et al., “Where to Forage When Afraid”; Northrup, Anderson, and Wittemyer, “Quantifying Spatial Habitat Loss”; Sawyer et al., “Mule Deer and Energy Development”; Lambert, Sawyer, and Merkle, “Responses to Natural Gas Development.”
  94. Sawyer et al., “A Framework for Understanding Semi-Permeable Barrier Effects.”
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  97. Northrup, Anderson, and Wittemyer, “Quantifying Spatial Habitat Loss”; K.T. Smith et al., “Pronghorn Winter Resource Selection Before and After Wind Energy Development in South-Central Wyoming,” Rangeland Ecology & Management 73, no. 2 (2020): 227-33, https://www.sciencedirect.com/science/article/abs/pii/S1550742419301150; K.L. Taylor, J.L. Beck, and S.V. Huzurbazar, “Factors Influencing Winter Mortality Risk for Pronghorn Exposed to Wind Energy Development,” Rangeland Ecology & Management 69, no. 2 (2016): 108-16, https://www.sciencedirect.com/science/article/abs/pii/S1550742415001542.
  98. C.L. Larson et al., “A Meta-Analysis of Recreation Effects on Vertebrate Species Richness and Abundance,” Conservation Science and Practice 1, no. 10 (2019): e93, https://conbio.onlinelibrary.wiley.com/doi/full/10.1111/csp2.93?af=R; S. Marion et al., “A Systematic Review of Methods for Studying the Impacts of Outdoor Recreation on Terrestrial Wildlife,” Global Ecology and Conservation 22 (2020): e00917, https://www.sciencedirect.com/science/article/pii/S2351989419308923.
  99. R. Kays et al., “Does Hunting or Hiking Affect Wildlife Communities in Protected Areas?” Journal of Applied Ecology 54, no. 1 (2017): 242-52, https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2664.12700; A.C. Keyel et al., “Modeling Anthropogenic Noise Impacts on Animals in Natural Areas,” Landscape and Urban Planning 180 (2018): 76-84, https://www.sciencedirect.com/science/article/abs/pii/S0169204618308065; A.R. Taylor and R.L. Knight, “Wildlife Responses to Recreation and Associated Visitor Perceptions,” Ecological Applications 13, no. 4 (2003): 951-63, https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/1051-0761(2003)13%5B951:WRTRAA%5D2.0.CO%3B2; M.J. Wisdom et al., “Elk Responses to Trail-Based Recreation on Public Forests,” Forest Ecology and Management 411 (2018): 223-33, https://www.sciencedirect.com/science/article/abs/pii/S0378112717319928.
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  101. Rodgers et al., “Sex-Specific Behaviors of Hunted Mule Deer.”
  102. DeVivo et al., “Endemic Chronic Wasting Disease”; D.R. Edmunds et al., “Chronic Wasting Disease Drives Population Decline of White-Tailed Deer,” PLoS ONE 11, no. 8 (2016): 1-19, https://doi.org/10.1371/journal.pone.0161127; I.H. Plummer et al., “Mineral Licks as Environmental Reservoirs of Chronic Wasting Disease Prions,” PLoS ONE 13, no. 5 (2018): e0196745, https://doi.org/10.1371/journal.pone.0196745; E.S. Williams and S. Young, “Chronic Wasting Disease of Captive Mule Deer: A Spongiform Encephalopathy,” Journal of Wildlife Diseases 16, no. 1 (1980): 89-98, https://pubmed.ncbi.nlm.nih.gov/7373730/.
  103. P.C. Cross et al., “Effects of Management and Climate on Elk Brucellosis in the Greater Yellowstone Ecosystem,” Ecological Applications 17, no. 4 (2007): 957-64, https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/06-1603.
  104. S.A. Christensen et al., “Spatial Variation of White-Tailed Deer (Odocoileus Virginianus) Population Impacts and Recovery From Epizootic Hemorrhagic Disease,” Journal of Wildlife Diseases 57, no. 1 (2021): 82-93, https://doi.org/10.7589/JWD-D-20-00030.
  105. Jones et al., “Mortality Assessment of Moose Calves.”
  106. Rayl et al., “Elk Migration Influences the Risk of Disease Spillover.”
  107. Altizer, Bartel, and Han, “Animal Migration and Infectious Disease Risk.”
  108. N.L. Galloway et al., “Supporting Adaptive Management With Ecological Forecasting: Chronic Wasting Disease in the Jackson Elk Herd,” Ecosphere 12, no. 10 (2021): e03776, https://doi.org/10.1002/ecs2.3776; A. Mysterud et al., “Spatial Clustering by Red Deer and Its Relevance for Management of Chronic Wasting Disease,” Animals 11, no. 5 (2021): 1-13, https://doi.org/10.3390/ani11051272.
  109. Davidson et al., “Ecological Insights From Three Decades of Animal Movement”; K.L. Monteith et al., “Effects of Climate and Plant Phenology on Recruitment of Moose at the Southern Extent of Their Range,” Oecologia 178 (2015): 1137-48, https://link.springer.com/article/10.1007/s00442-015-3296-4.
  110. E.O. Aikens et al., “Drought Reshuffles Plant Phenology and Reduces the Foraging Benefit of Green-Wave Surfing for a Migratory Ungulate,” Global Change Biology 26, no. 8 (2020): 4215-25, https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.15169; A.D. Middleton et al., “Animal Migration Amid Shifting Patterns of Phenology and Predation: Lessons From a Yellowstone Elk Herd,” Ecology 94, no. 6 (2013): 1245-56, https://esajournals.onlinelibrary.wiley.com/doi/10.1890/11-2298.1; Middleton et al., “Green-Wave Surfing.”
  111. Aikens et al., “Wave-Like Patterns of Plant Phenology”; Middleton et al., “Green-Wave Surfing.”
  112. See, for example, B.G. Lamont et al., “Multi-Scale Habitat Selection of Elk in Response to Beetle-Killed Forest,” The Journal of Wildlife Management 83, no. 3 (2019), https://doi.org/10.1002/jwmg.21631.
  113. L.J. Kayes and D.B. Tinker, “Forest Structure and Regeneration Following a Mountain Pine Beetle Epidemic in Southeastern Wyoming,” Forest Ecology and Management 263 (2012): 57-66, https://www.sciencedirect.com/science/article/abs/pii/S0378112711005949.
  114. Rickbeil et al., “Plasticity in Elk Migration Timing”; Middleton et al., “Conserving Transboundary Wildlife Migrations”; J.D. Tack et al., “Beyond Protected Areas: Private Lands and Public Policy Anchor Intact Pathways for Multi-Species Wildlife Migration,” Biological Conservation 234 (2019): 18-27, https://www.sciencedirect.com/science/article/abs/pii/S0006320718313892.
  115. Middleton et al., “Conserving Transboundary Wildlife Migrations.”
  116. Berger, “The Last Mile”; Sawyer et al., “Migratory Plasticity Is Not Ubiquitous.”
  117. Colorado College Conservation in the West Poll, “Loss of Wildlife Habitat Is Seen as a Problem in Every Western State” (2019), https://www.coloradocollege.edu/other/stateoftherockies/documents/Wildlife%202019%20d1b.pdf.
  118. Middleton et al., “Conserving Transboundary Wildlife Migrations”; Tack et al., “Beyond Protected Areas.”
  119. Tack et al., “Beyond Protected Areas.”
  120. Middleton et al., “Conserving Transboundary Wildlife Migrations.”
  121. Ibid.
  122. A.F. Jakes et al., “A Fence Runs Through It: A Call for Greater Attention to the Influence of Fences on Wildlife and Ecosystems,” Biological Conservation 227 (2018): 310-18, https://doi.org/10.1016/j.biocon.2018.09.026.
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  125. Clevenger, Chruszcz, and Gunson, “Highway Mitigation Fencing.”
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Wildlife Migration Corridors Saves Lives

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Wildlife migration—the act of traveling between seasonal habitats—is widespread among terrestrial species, including many culturally and economically important animals such as mule deer, elk, and pronghorn antelope

A line of brown female elk walk from left rear to right front along a deep path in snow with snow-flecked fir trees, blue sky, and puffy clouds in the background.
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Wildlife Migrations in the U.S. Sustain Ecosystems

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Each year, millions of mule deer, elk, pronghorn, and other large ungulates (hoofed mammals) traverse the vast landscape of the American West. In recent years, GPS technology, in the form of collars affixed to individual animals, has revolutionized scientists’ understanding of these migrations by enabling researchers to pinpoint when, where, and how wildlife moves at a grand scale.

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Wildlife Crossings Can Protect Migrating Animals

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Designated bridges and tunnels offer passage for herds, preserve habitats, and keep people safe.

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Mule Deer
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Western States Use Science to Safeguard Migrating Wildlife

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By the early 20th century across the American West, a host of far-ranging native wildlife had been reduced to a small fraction of their former numbers or geographical extent, with some teetering on the brink of extinction because of overexploitation by early non-Indigenous settlers.

Graduate students in Wyoming tag a mule deer with a GPS tracker to monitor its migration path across hundreds of miles.
Graduate students in Wyoming tag a mule deer with a GPS tracker to monitor its migration path across hundreds of miles.
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Innovative Wildlife Migration Maps Can Help States

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Using the latest technologies involving GPS-enabled collars, wildlife biologists can track big game migrations in real time as animals move—in some cases—hundreds of miles. With this data, researchers are then able to analyze the effects of development, roads, and climatic conditions on animals’ historic migration routes, which can help state and federal agencies make informed wildlife management and development decisions.