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The forest ranger had a troubled look on his face. It was the summer of 2022 and my kids and I were trudging up a steep trail in the Indian Peaks Wilderness, near Denver, when we encountered him. He stood amid a small grove of subalpine fir, clutching a walkie-talkie tightly in his hand. As we came closer, he brought one index finger to his lips and pointed with the other into the distance. “Moose,” he whispered. Below us, perhaps 100 yards away in a flower-strewn meadow, a cow and her calf munched grass without concern. “Cute!” exclaimed my teenage daughter. “Go that way,” the ranger said gruffly, pointing up a steep slope covered in boulders. We walked on, weaving through a crowd of curious onlookers. Some inched closer to the moose for a better look. Others held cellphones, swiping fingers across screens to bring the animals into better view. Few creatures evoke American wilderness like Alces americanus, the American moose. It is the largest member of the deer family and the second largest land animal in North America behind the American bison (Bison bison). Its imposing size is undercut by its goofy countenance—the wide fan of horns, the thin legs that suspend a hefty body, the face like a hand-puppet fashioned from a worn-out sock. Despite their ungainly appearance, moose are formidable and, at times, graceful, reaching speeds of 35 miles per hour at full gallop. Growing up in Colorado in the late 1980s and early ’90s, I took trips with my father into designated wildernesses in the northern part of the state—the Flat Tops, Mount Zirkel, the Rawah—hoping to glimpse a moose. We never did. These days I often encounter them when out hiking. For a while, I thought my luck had changed. But I’ve since learned that these experiences are nothing particularly special. Though moose are notoriously hard to count, the Colorado Parks and Wildlife Department estimates that there are now around 3,000 scattered through the state’s major mountain ranges. That figure, however, does not adequately describe their growing presence here. The comment sections for dozens of hikes in Colorado’s Front Range and the San Juan, Sawatch and Elk Mountains on the popular AllTrails app are a litany of moose sightings. Several moose have even made their way into the suburban sprawl of metro Denver, the state’s capital and largest city, browsing in greenbelts, sauntering across golf courses, loitering in mall parking lots. As Colorado’s human and moose populations have grown in tandem, so have the number of conflicts. Over a two-week span in spring of 2022, moose attacked people in three separate incidents. One of those occurred near the mountain town of Nederland, where a mother moose trampled and severely injured a hiker and a dog; a police officer shot her, and wildlife officials took her calf into custody. In September 2022, a moose gored and nearly killed a bowhunter in northern Colorado after the hunter’s arrow whistled wide of its mark. More often than not, however, moose come out on the losing end of these clashes. According to the Colorado Department of Transportation, cars struck and killed 59 moose in 2022. In 2012, the number was just four. As human and moose populations grow in Colorado, so too have their interactions: Both moose attacks on humans and car strikes on moose have increased dramatically in recent years. David Dietrich Despite the increase in dangerous encounters, the moose has emerged as a potent symbol and ambassador of the wild in a state enamored of its outdoor places—depicted in murals and statues in many mountain towns. A large painting of a moose even graces Coors Field, the home of the Colorado Rockies baseball team. There’s just one problem. As much as Alces americanus seem to belong in Colorado, the species’ native range is in more northerly latitudes and doesn’t extend into the state. Colorado’s wildlife department introduced moose from Wyoming and Utah beginning in the 1970s to put money into its own coffers through the sale of hunting licenses. In that bygone era of wildlife management, the will of a few high-ranking state officials was enough to set a great ecological experiment into motion. To be sure, human values have always helped shape wildlife policy. In Colorado and elsewhere in the American West, game animals, including mountain goats, elk and bison, have been introduced to places where they never lived or have been sustained in unnaturally high numbers to satisfy hunters and wildlife watchers. Those efforts have frequently caused dramatic environmental changes. Indeed, now that moose are flourishing in Colorado, they are behaving in unexpected ways, challenging management paradigms and emerging in new environments. As moose occupy an ever larger part of Colorado’s natural present, biologists are working to understand their effects on native plants and animals. All of which leads to an all-consuming question: In an environment increasingly altered by agriculture, urbanization and the ever-expanding footprint of human infrastructure, do moose have a place in the state’s ecological future? A light dusting of snow covers a moose. David Dietrich In the winter of 1978, a handful of state wildlife staff huddled together one morning in the Uinta Mountains in northern Utah. Led by chief of big game, Dick Denney, the team had traveled there to search for moose, a smallish subspecies known as Shiras (pronounced SHY-rass) found in the Rocky Mountains. Deep snows coated the peaks and filled the valleys. To fight off the chill, the officials wore government-issue olive drab winter gear—all save one, an older gentleman with a pompadour of white hair in a bright red snowsuit. This was the signature attire of Marlin Perkins, zoologist and co-host of “Mutual of Omaha’s Wild Kingdom,” who had traveled to Utah to capture the event for an episode called “Moose Airlift.” As the capture got underway, a pair of helicopters cruised over the landscape. A man with a rifle under his arm sat perched in the smaller of the two aircraft, which descended toward a cow moose and her yearling calf in a snowy meadow. There was a sharp report, not from a bullet but a tranquilizer dart, and the cow took off at a run. Within minutes, her legs went wobbly, and the crew landed and set to work. They placed a blindfold over the animal’s eyes and drew her blood, testing to ensure she was not infected with brucellosis or leptospirosis, two diseases that can pass to (and from) domestic cattle. The team then fitted the cow moose with a telemetry collar and an ear tag, and carefully slid a specially designed sling under her belly, attached by a rope to one of the helicopters. For a moment, as the pilot eased into the air, the moose lurched, drawing her legs upward as her feet left the ground—“a common reflex,” as Perkins described it in his folksy narration. At last, the animal appeared to relax as she soared over the rugged valley, bound for her new home—a vast expanse of sagebrush and willow between two major mountain ranges in northern Colorado, known as North Park. Moose were rarely seen south of Yellowstone National Park before the early 1900s. Their populations grew in Colorado following their airlifted transport to the region in the 1970s.  Courtesy of Denver Public Library She would not, technically, be the first moose to set foot in the state: The animals appear in a few scattered accounts from settlers in the mid-1800s. One of the best-known comes from Milton Estes, a member of the family that founded the northern mountain town of Estes Park, who killed a bull moose in that area in the 1860s as it mingled with a herd of elk. Biologists today believe moose like the one Estes killed were transient, perhaps dispersing juveniles entering the state from Wyoming, and officials generally agree that Colorado never supported a breeding population. To make their case for introducing moose to the state’s mountains, Denney and his colleagues had argued that moose would have eventually migrated to and thrived in Colorado on their own, had people not blocked the way. Settlers and Indigenous hunters were “undoubtedly the primary limiting factor in Colorado moose establishment,” Denney wrote in an article for Colorado Outdoors in 1977. “Practically every moose that has come into Colorado has ended up by being eaten or shot and abandoned.” That’s a plausible explanation, according to noted Colorado State University wildlife conservation expert Joel Berger. Moose were rarely sighted south of the lands that would become Yellowstone National Park, in northwestern Wyoming, before the early 1900s, he said. Then, after settlers extirpated predators from the Yellowstone area, a member of the Shoshone tribe encountered a moose on the east side of the Wind River Mountains, in central Wyoming. “He didn’t know what it was, because they hadn’t occurred there before,” said Berger. The Red Desert, a vast expanse of arid land in southwestern Wyoming, was also likely a formidable obstacle. In total, between 1978 and 1979, Colorado’s wildlife department airlifted a dozen moose out of the Uintas—along with a dozen more from Wyoming’s Tetons—and hauled them to North Park. There, they remained in a small enclosure for several days before being released into the rolling high plains along the Illinois River. A young biologist named Gene Schoonveld was among the officials with the Colorado Division of Wildlife who orchestrated the process. An avid moose hunter, Schoonveld had moved from Canada to Colorado in the late ’60s to attend graduate school at Colorado State University. When he wasn’t in class, he spent days exploring the mountain valleys and basins of the Rockies, marveling over the copious stands of willow and aspen, favorite food sources for moose. After landing a job at the state wildlife department, he immediately pestered Denney, his supervisor, to pursue moose introduction. “I knew that moose could live down here and I let Dick know how I felt,” he told me when I reached him by phone in the fall of 2022, shortly before his death from a long illness. Dick Denney, former Colorado chief of big game, displays the antlers of an adult moose. Courtesy of Denver Public Library The idea of introducing moose to Colorado had been kicked around for decades, but ranchers in rural communities who feared moose would compete with their cattle for forage resisted those plans, and they never materialized. Denney’s 1976 “proposal” to introduce the half-ton animals is a mere 54 pages and includes no comprehensive studies of their potential ecological impacts. And although Schoonveld and Denney interviewed residents of northern Colorado about the releases, they dismissed the opposition as unfounded. After all, moose wouldn’t be feeding on hay bales or grass, Schoonveld said; they’re browsers that subsist almost entirely on willow, aspen and other woody material. “We brought them to Colorado because we could,” he said, “because we had the space and the habitat for them.” Amid North Park’s rich willow stands, the two dozen transplanted moose kicked into reproductive overdrive. In 1980, nearly one in five gave birth to two offspring at once—a phenomenon called “twinning” that often occurs among ungulates when food is especially plentiful. By the winter of 1988, a decade after introduction, the moose population had grown to around 250. The animals proved so successful and so popular with residents and visitors that, between 1987 and 2010, wildlife officials transplanted more moose to other parts of Colorado, where they thrived in a variety of habitats. On the semi-arid slopes of Grand Mesa near the state’s western border, for example, where moose were introduced in 2005, moose subsist mainly on Gambel oak rather than willow. They’ve also adjusted to high-elevation valleys of the San Juan Mountains near Colorado’s southern border, where they were introduced in the early 1990s. That makes them the southernmost moose herd in the world, according to Eric Bergman, a research scientist and moose specialist with Colorado Parks and Wildlife. The species may be pushing still farther southward. Last fall, a moose was spotted in the mountains of northern New Mexico, near Taos, presumably after crossing the Colorado border. “Biologists generally expected them to do well,” Bergman said of the introduction, “and they certainly did.” Dust drifts up between two moose. David Dietrich Rocky Mountain National Park, just east of North Park, is among the places that have witnessed that rapid growth. Park biologists estimate that 40 to 60 moose now wander the western side of the park. On the more touristed east side, moose now inhabit every drainage and are likely increasing. And little wonder: The 415-square-mile preserve has some of best moose habitat in the state, with deep glacially carved valleys and willow-thick stream bottoms. Last April, I sat down with landscape ecologist Will Deacy in his office at Rocky Mountain National Park headquarters as he called up a satellite map on his computer. The park service has fitted 23 moose with telemetry collars, and Deacy showed me one of their routes. The path, transmitted over the course of a season, looked like a child’s scribble, moving to and fro with little regard for the ragged topography. Animals have been known to traverse the entire park in just a few days, hinting at the expansive size of their overlapping ranges, which have been shown elsewhere to cover areas as large as 50 square miles. Deacy next pulled up an infrared image of a mountainside covered in dark trees, gathered by an aircraft mounted with an infrared camera. A closer look revealed several white silhouettes, like small Bullwinkles, scattered amid the pines: moose going about their mysterious business. “They are a new species in a new context,” Deacy said. These supremely adaptable animals could behave very differently in Rocky Mountain than they do in, say, Yellowstone or Glacier National Parks, he explains. “There is so much we just don’t know.” One of those unknowns is just how moose will affect a landscape already heavily browsed by native elk. Settlers once hunted elk nearly to extinction in this part of the state, but in 1913, officials reintroduced them within the protective boundaries of the national park, where hunting was banned. By the latter half of the 20th century, elk here also no longer faced predation by wolves or grizzlies, both of which were extirpated from the state by hunters and trappers. The local herd ballooned to as many as 3,500 animals by the early 2000s—far more than the maximum of 2,100 that the park service deemed sustainable. The elk rapidly chewed through willow stands, particularly along streams, and the park’s mature willow plants declined by 96 percent between 1999 and 2019. Under the auspices of the park’s Elk and Vegetation Management Plan, officials called in sharpshooters to cull some elk and constructed tall fences called “exclosures” around more than 200 acres of sensitive aspen and willows along creeks, wetlands and rivers, to keep large ungulates out. They also set in motion surveys of hundreds of scattered plots to monitor browsing and the health of the park’s willows, foundational plant species along its streams. The fragrant shrubs stabilize soil and prevent erosion, while providing food and sanctuary for hundreds of species of mammals, insects, fish and birds. On a brisk morning during my April visit to the park, I followed Deacy and biological technicians Nick Bartusch and Kim Sutton to one of those plots, in a meadow near the headwaters of the Fall River. Our feet crunched through a thick layer of frost, and deep snow still blanketed the 12,000- to 13,000-foot peaks of the Mummy Range towering above. Sutton swiped a metal detector across the matted grass until she found four markers. Then, Bartusch strung orange thread between them, forming a crude square, and began to evaluate the plants within. Though the spring bloom was approaching, the limbs remained leafless, making evidence of herbivory easier to see. Bartusch looked for signs, gently caressing the plants. The largest in the plot had clearly been browsed, with buds missing and limbs chewed to ribbons. As the team recorded their findings, I wandered around the plot’s perimeter. Impressed into a semi-frozen patch of mud was a single, six-inch-long hoofprint. I showed Deacy. “Looks like moose,” he said. Currently the park has no equivalent of the elk plan for its moose. Though moose arrived here in 1980, just two years after the North Park releases, visitors and researchers rarely encountered them prior to 2015, said Bartusch. “Now it’s almost daily.” That sudden prevalence complicates existing efforts to recover park vegetation. A single adult moose can eat up to 60 pounds of willow per day, far more than an adult elk, which consumes roughly a third of that amount of forage, only a fraction of which is willow. In other words, too many moose could create new problems for the host of other creatures that depend on this critical plant. For example, Berger, the Colorado State University wildlife biologist, conducted research in riparian zones in the Grand Teton and Yellowstone National Parks and found that neotropical migratory songbirds, such as warblers and flycatchers, occur at much lower densities where there are large populations of moose, particularly where moose don’t face pressure from predators. Four bird species that he expected to see during that study didn’t occur at all, Berger said, “because moose browsing had been so intense.” And because national parks ban hunting, moose tend to congregate within their borders, achieving densities almost five times higher than outside of them, Berger added, meaning Rocky Mountain National Park may see magnified effects over time. Scientists conduct willow surveys to assess the impact of moose populations on park vegetation. Moose can eat up to 60 pounds of willow per day, significantly impacting local plants and other wildlife that rely on them.  Jeremy Miller Meanwhile, the moose here are exhibiting new and surprising behaviors that could affect the park’s ailing vegetation. Moose tend to be solitary animals, said Bartusch. In 2019, however, he had an encounter in the park that challenged that notion. He and a crew member were working on the park’s west side when they spotted a couple of moose in a large meadow. “We weren’t worried about it because they were a long way off,” said Bartusch. “So we went about our business and suddenly we realized we’d somehow managed to get surrounded. My partner and I counted 33 individual moose.” According to Deacy, groups of moose sometimes “yard up” in the winter to stomp out a comfortable spot in deep snow. But such congregations are rare in summer. In this case, said Bartusch, the animals seemed to be moving in a herd. If the behavior became commonplace among Rocky Mountain’s moose, it could concentrate their impacts. “People love their moose,” said Elaine Leslie, former chief of the National Park Service’s Biological Resource Management Division. But too many animals could very well threaten “the primary purpose of the park, which is the preservation of resources.” What might a Rocky Mountain National Park moose management plan look like? First of all it requires sound scientific data on moose populations. If they determine there are too many moose, Leslie said, options include working with the state to increase moose hunting on Rocky Mountain’s periphery. She also mentioned dosing animals with contraceptives delivered via darts. The worst-case scenario, she said, would be having to conduct a moose cull, as other parks have done periodically to bring down their elk populations. Further complicating management is the degree to which Rocky Mountain’s ecosystems have already been modified by people. Before the park was established, ranchers and farmers plowed willows under to provide forage for horses and cows; others dewatered and altered stream channels and meadows to make way for roads, parking lots, visitor centers and other bits of infrastructure. Directly restoring the park’s beleaguered willow stands and wetlands, therefore, would go a long way toward making the environment more resilient against future moose damage. To that end, the park is attempting to coax beaver back within its boundaries from surrounding waterways to build ponds and raise the water table. That, in turn, would help willows regenerate and grow. Park staff are counting on the exclosures to do double duty, protecting beavers and their handiwork from any boost in elk or moose numbers that willow regrowth might bring. Leslie sees another potential solution in Colorado’s wolf reintroduction, which brought ten animals to Grand County, in the Central Rockies, in December 2023. Wolves are the main predator of elk and moose, and they could help ease pressure on the park’s willow and aspen if they recolonize the area and reduce populations or induce herds to keep moving. That’s what happened in Yellowstone after the federal government restored wolves, and as grizzly bear and other struggling predator populations rebounded. A moose wades through the water. David Dietrich On a bright late-July morning last year, I visited State Forest State Park, in the same region where officials originally released moose in 1978. Today, as many as 700 roam the area, comprising nearly one-fourth of the state population. “It’s the last frontier,” said Tony Johnson, a State Forest law enforcement ranger, “where there are no chain stores, but moose on every corner.” I headed to a campground and trail that Johnson identified as a “moose hotspot.” “There is a moose there that goes from being a very neat encounter to a potentially dangerous situation pretty quickly,” he had told me. At the trailhead, as if on cue, a large juvenile male emerged from a stand of pines. It stood mere feet from the dirt path, munching on willows as a procession of ultra-marathoners plodded by. Some stopped to gawk. Others glanced at the animal as if it were a hallucination—understandable, perhaps, given that the runners were about 15 miles into a punishing 65-mile race. Even though moose pose potential threats to native ecosystems and people, local communities are learning to co-exist with the animals. In Walden, 25 minutes north, moose have become such frequent visitors that a sign on the way into town proudly proclaims it “The Moose Viewing Capital of Colorado.” “We have them in town quite often,” said Josh Dilley, State Forest’s park manager, who met me on the trail. They especially like to congregate around the elementary school, Dilley explained, “so we’ll go sit strategically between the moose and the kids while they’re going to school.” When moose loiter too long in front yards and public parks, wildlife officials haze them away with firecrackers or non-lethal rubber buckshot. On rare occasions, they sedate an unruly moose with a dart and transplant it elsewhere by truck. Along the trail, Dilley and I encountered dozens of hikers and several bags of dog poop, which Dilley dutifully retrieved. Dogs, Dilley explained, present one of the greatest sources of conflict with moose. Moose do not distinguish a Pomeranian from a gray wolf. And rather than run away, an adult moose will stand its ground or chase an unleashed dog back to its owner, often attempting to gore a dog with its antlers or crush it with its hooves. A week later, at State Forest’s annual “Moose Fest,” I spoke with Trina Romero, a wildlife viewing coordinator with Colorado Parks and Wildlife, who said that moose attacks in the state now outnumber bear and mountain lion attacks combined, even though moose numbers are significantly lower. Despite growing pains as Coloradans figure out how to co-exist with this large, non-native ungulate, the state has become something of a de facto refuge for the species. Moose populations in much of their native range across the northern U.S. are plummeting. In New Hampshire, they declined by nearly half between the mid-1990s and late-2010s, owing to habitat loss from clear-cutting and warming temperatures, which have triggered a sharp rise in ticks. Wyoming also used to be a moose stronghold, but today Colorado has more moose than its neighbor to the north. And there are signs that Colorado’s moose numbers may be naturally stabilizing. “We have some evidence that our moose population is expressing characteristics of being at or near carrying capacity, such as lower pregnancy rates and animals skipping breeding,” Bergman said. Because biologists don’t have great information on the long-term trajectory of state moose populations, Bergman said, his agency is conservative when it comes to apportioning moose tags to hunters each year. “We could probably use [hunting] as a tool to bring down density … but we also face social pressure to maintain high densities of animals. People love seeing moose, so it really is about finding trade-offs and middle ground.” Others are not so optimistic. Moose “are one of my favorites,” said Elaine Leslie. “But I’m worried about what is happening at the ecosystem level, especially in Rocky Mountain National Park. That is a very biodiverse area right now.” Moose gather together. David Dietrich For the sake of Colorado’s moose and the ecosystems they inhabit, Leslie said, the state’s ardor must turn to more research, rigorous population counts and science-based management. “You have to look at the big picture, at what happens 20 and 30 years down the road.” Otherwise, Colorado residents may find sorrow after sorrow: increasingly denuded streambanks, more frequent attacks and car collisions, and greater numbers of moose in the crosshairs. “It’s partly everybody’s fault, the state and the feds, because we don’t think into the future very well,” Leslie said. “And we don’t learn from history. Unless everybody gets on the same page, it’s going to get ugly.”This story originally appeared in bioGraphic, an independent magazine about nature and regeneration powered by the California Academy of Sciences. Get the latest Science stories in your inbox.

Roughly 3,000 animals now roam the state's mountain ranges