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The North American plains bison has a long history here, albeit one that suffered . For a century and a half, their distinctive humped shoulders and bearded faces were missing from America's grasslands.

Tribes and conservation organisations have recently started bringing the bison back. And scientists studying the returns are discovering that getting the 2,000lb (900kg) grazer back in its native ecosystem could be ?key to the future of the prairie. When I visit one of these projects in summer 2023, wilting in the searing summer heat of the scorched prairie, I wonder what a giant, fur-covered herbivore is doing to help.

The bison's biggest service, I soon find out, is to assist the prairie in holding onto its water. Bison on the shortgrass prairie send a strong message about the importance of native grazers for landscape resilience. They may offer the prairie a lifeline .

Hila Shamon, a research ecologist with the Smithsonian's National Zoo and Conservation Biology Institute in Washington DC, has spent five years tracking the ecology of the remote grasslands.

She partners with , a conservation organisation with parcels of land on either side of the million acre , to study how bison are changing the ecosystem.

Shamon spends her field season crawling across American Prairie property measuring everything from vegetation density and species diversity to the habits of grassland birds and prairie dogs.

Shamon recently co-authored which suggests bison do something remarkable in this hostile environment. "There is no doubt, year-round grazing of bison is good for the landscape," she tells me. Bison, it turns out, have an ingrained capacity to boost biodiversity.

Plains bison co-evolved with the short-grass prairie. In the 12,000 years since the end of the Pleistocene, they have proven themselves to be potent .

An adult bison eats about 25lb (11kg) of grass a day. The grasses adapted to their foraging. Vegetation across the plains uses the nutrients in their dung. Birds pluck their fur from bushes to insulate their nests.

Bison also . They roll in the dust and create indentations known as "wallows" that hold water after rainstorms. After the bison move on, ?insects and become a feast for birds and small mammals. Pronghorn antelope survive by following their tracks through deep winter snows.

Plains bison spent thousands of years engineering a distinctive grassland ecology from Northern Canada through Montana to Mexico. But more than a century ago, this influence abruptly stopped. A few decades of slaughter led bison numbers to plummet from 60 million to barely 800 living wild in the US and Canada by 1889. replaced bison and native people with ranches, white settlement and cattle. For a hundred years, cattle claimed the prairie as their own. But while they were easier to turn into steaks, they were not as finely tuned for prairie life.

Today, however, bison are getting a second chance. Tribal reservations are at the forefront of their recovery, taking and restoring them to treaty lands. American Prairie is also playing a part, buying ranches and returning bison to places where cattle were once king. About 30,000 bison in various parks and protected areas across the country. More .

Scientists like Shamon are watching it all closely to see how the return of bison affects the land.

"Cows don't move as much," Shamon says. When the weather gets hot, they . When it's cold, cows hunker down behind creekside shrubs to shelter from the wind.

This preference for spending time in the starts a vicious cycle. By browsing the trees and shrubs they reduce the amount of available shade, Shamon tells me. Their chewing means less leaf litter to cover the ground and, when the shrubs die, less roots to stabilise the stream banks. Over time, the soil around the creeks dries out, the boxelder, cottonwoods, wild rose and snowberry disappear, and the streambanks get taken over by non-native grasses. What was once a cooling refuge for wildlife becomes almost as forbidding as the uplands.

Bison, by contrast, are quite happy to tough it out on the prairie. It's what evolution finetuned them to do. They visit streams and ponds periodically to drink, but bison were built to keep moving. , they generally don't bother spending time near creeks. They don't need as much shade. They prefer the to woody shrubs in the river bottoms. As a result, they do less damage to creeks than cows.

Although rivers and streams cover less than 2% of the prairie, they are crucial refuges for its wildlife. Shamon's study . She also found more deer and elk. Other studies show that year-round, low-density grazing by bison is associated with in riparian areas than seasonal grazing by cattle. Well-vegetated creeks are the prairie's lifeblood. They keep the soil moist, the vegetation green, and they provide dispersal corridors for large mammals like mountain lions and black bears. One day, they may provide safe passage for recovering grizzly bears.

***

American Prairie increasingly thinks of its bison as a highly mobile, low-tech restoration tool. During my stay on Montana's shortgrass prairie, Danny Kinka, the non-profit's wildlife restoration manager, drives me to see an example of a creek in full-blown, bison-assisted recovery.

Bumping along the rough gravel roads in his truck, the 37-year-old wildlife ecologist talks enthusiastically about bison's role in creating the wildlife havens that exist near water. As we emerge into the heat alongside Beaver Creek, two night herons, a flock of white pelicans and a great blue heron all take flight.

The stretch of creek was changed by American Prairie from cattle grazing to bison in 2017. "When we bought this property, Beaver Creek looked like a single channel," says Kinka. The area around Beaver Creek was the second piece of American Prairie property to hold bison after Sun Prairie 20 miles to the south-east. One hundred and fifty bison had 12,000 acres (4,856 hectares) on which to graze.

Replacing cattle with bison has greened up a floodplain nearly 180m (594ft) wide. Standing by one of the braids of the creek, the tops of the cattails wave in the breeze above our heads.

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Kinka explains how the bison had set the stage for beavers ¨C ¨C to help bring the landscape back to health. The abundance of new growth made possible by the bison gave beavers the building material they needed.

We walk across a beaver dam made entirely of mud and cattails. Over the years to come, the flooding created by the dams will cause cottonwood seeds buried in the streambanks to sprout, leading to the regrowth of trees, says Kinka. As more shrubs and trees return, more beavers will move in and build more dams, he adds. The virtuous cycle will continue.

Kinka warns that if a creek channel has degraded too far it might need assistance getting to the point where it can spill water onto the floodplain again. Near where I camp at Telegraph Creek, I see workers from the Montana Conservation Corps, an organisation putting Montana youth to work rehabilitating public and private lands, pounding barriers made of wooden posts, brush and mud in the creek to partially block water during spring runoff. The structures, known as Beaver Dam Analogues (or BDAs), slow the water, trapping sediment and raising the creek bed. It's a technique known as .

"We don't do it with backhoes [diggers]," Kinka says. "We do it with people in waders." Sometimes a few BDAs can raise the water table enough to get a real beaver to move in and take over.

Here on Beaver Creek, this hard work wasn't necessary. What excites Kinka about the recovery here is its simplicity. "We didn't go down there, we didn't reintroduce beavers, we didn't build BDAs. We didn't do anything," he tells me. "We just put bison here. This is resilience that is inherent in the system. It just needed a little bit of a break from grazing to be able to do this all on its own."

What's more, the creek is still improving, he says. "Every year the creek gets a little wider, a little more meandery, a little more like a pool and less like a trench."

***

While the evidence for the benefits of bison is compelling, many cattle-owners think the story needs more nuance. Malou Anderson-Ramirez and her husband run cattle on a ranch bordering Yellowstone National Park, 300 miles (480km) to the south-west of American Prairie. When we talk about grazing and stream health, she inserts an important qualifier.

"It's not the cow, it's the how," she says.

If you graze cattle carefully, you can avoid the damage that occurs when they are left to their own devices. Anderson-Ramirez reinforces the places she wants cattle to cross a stream with rocks and fences. She pipes creek water into tanks hundreds of feet away to ensure animals aren't wading into the stream to drink. She lets vegetation rest for several weeks between grazing rotations to give plants time to rebuild their roots. This type of treats livestock as a tool that can improve the health of the land.

Shamon agrees that cattle can provide benefits in systems that evolved with large herbivores. In a study of grassland birds, she found that, outside of riparian areas, birds benefit as they do from bison. "If you manage the cattle in a proper way and you continue to rotate them, you could get an effect similar to bison," Shamon tells me.

Kinka shares the view that grasslands need a big grazer, whether that be a cow or a bison. The key is to thwart cattle's natural inclination to loiter in the water.

Martin Townsend of the ?in Malta, Montana is helping ranchers implement these better grazing strategies. "Ranchers are very aware of what cows can do to riparian areas if left unattended," he says. "They know it's not in their interest."

Townsend gets grants from state agencies and environmental organisations to defray the costs of wildlife-friendly ranching. "Ranchers have always known that when they have successful wildlife populations they are in a better place to have a successful ranch," he says. His organisation encourages fences and strategically placed mineral licks to keep cattle away from creeks. They hire the same Montana Conservation Corps workers as American Prairie to build BDAs. The BDAs slow the streamflow down and allow more water to penetrate the soil. This creates greener grass, a benefit to ranchers and wildlife alike, he tells me with a grin.

Conservationists often to restore balance to ecosystems. The importance of bringing back large, native grazers is lesser known but equally important. In Kansas, 30 years of bison grazing in has been shown to . In Europe, it is hoped the in Kent in the UK will do something similar for ancient woodlands. Across the world, to resume their lost roles.

In places where native species reintroduction may be difficult, careful management of livestock can . On landscapes as big as Montana's prairie, it seems likely there is room for both. People still want to eat Montana beef and ranching has an important place in the region's culture and economy. But as climate change intensifies, it is worth considering which animals are best adapted to the emerging conditions. Plains bison, frugal around water and constantly on the move, are built for this land. They are also a healthy source of protein. Outside Yellowstone National Park, they are already from the tribes. And, on its land, American Prairie is offering this year.

***

On my last night at American Prairie, I wake in my tent with a start. The bison are moving noisily through the campground.

All around me, I hear soil shifting under bison hooves, rocks getting scuffed and big lungs expelling the nighttime air. The bison are keeping in touch with each other in the dark with a constant stream of grunts, gurgles and huffs. I feel like a stranger in the middle of a big family gathering, with mothers coaxing their young to keep moving, teenagers acting boisterously in the crowd, and big males asserting their independence around the edges. The cacophony keeps me awake until the sound starts to fade and the bison wander off to higher ground.

When I rise the next morning, I look around and see the herd on a hill about a mile (1.6km) to the east of camp. I quickly finish my breakfast and pack the car. I want one last look at the bison before I head home. I grab my binoculars, drive to a small knoll and scan the hill.

It is empty. As quickly as they had arrived, the 200 bison had gone. The bison are on the move, in the heat. Doing their part to keep the prairie healthy.

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