On an island in the Arctic Ocean, in a region that is warming as much as seven times as fast as the rest of the planet, the food chain is being turned upside down.

Underwater kelp forests are surging into once-frozen waters, replacing other native species. Reindeer, cut off from traditional foraging routes over vanishing sea ice, now graze on seaweed when they cannot reach inland grasses and lichen.

And polar bears, deprived of the ice platforms they once used to hunt seals, have turned inland, raiding bird nests, hunting reindeer and, increasingly, clashing with humans.

Scientists are watching this ecological upheaval unfold in real time from an international research station in Svalbard, a cluster of islands near the North Pole. And it’s making their work more dangerous.

The scientists must carry rifles. A new brochure warns that if anyone comes face-to-face with a polar bear, “Stay calm. DO NOT RUN.” If it charges: “Be prepared to use any possible deterrence (shovels, ski poles, rocks, blocks of ice, water in a thermos, etc.).”

Svalbard is controlled by Norway but governed by an international treaty that allows foreigners to live and work. At the Ny-Alesund international research station, the world’s northernmost human settlement, scientists study every level of the Arctic ecosystem.

As the region continues to warm, driven by the burning of fossil fuels, the full consequences of this rapid transformation are still unfolding and it is unclear if plants and animals will be able to adapt.

“The question is can they find a winning strategy that will let them survive given those changes?” said Gil Bohrer, an environmental engineer at The Ohio State University. He helped create an archive of data from sensors that track animal movements across the Arctic to understand how wildlife responds to rapid environmental change.

Vanishing ice

On a cold, sunny morning this spring, two scientists — one German, the other originally Russian — loaded their gear into a small boat: drills, snow testing kits, more than 20 pounds of protective suits, and, of course, two rifles and two flare guns.

“We are always watching for polar bears,” said Sebastian Gerland, as he suited up in layers. “They can swim and they can dive.”

For nearly 29 years, Dr. Gerland has returned to Ny-Alesund, which was a coal mining town in the early 20th century. After a series of fatal accidents in the 1960s, the Norwegian government shut down the mines and the town was reborn as an international research hub.

Each spring, scientists drill into the sea ice in the same fjord, extracting cylindrical samples to measure thickness, temperature and salinity.

Their data tells a clear story. The ice is forming later, melting earlier and getting thinner each year, and the snow cover is lighter. What was once a glacier 17 years ago has melted into the sea. As the ice disappears, more heat gets absorbed by the dark fjord water, contributing to warming and shrinking the next year’s ice even more.

Over decades, Dr. Gerland and other scientists have watched this feedback loop play out, a frozen world slipping into retreat. Collapsing ice affects everything, Dr. Gerland said.

Seals can’t dig breeding dens without snow cover, which means less food for polar bears and foxes. Indigenous communities in other areas of the Arctic lose the frozen highways they’ve used for hunting and travel for ages.

After spending an hour collecting samples under a bright blue sky, the two scientists rode in their boat back through iceberg-filled waters to the research base in time for lunch.

The canteen felt like a school lunchroom: boots piled by the door (beside a stuffed polar bear), snow coats hung by the lobby, scientists lined up with trays in hand, steaming food. The weekly cargo ship had just come in, bringing a haul of grapefruits, bananas, tomatoes, lettuce and other fresh food in a place otherwise cut off from basic goods.

There was a cheerful mix of languages. The Germans and French usually sat together. The Norwegians, largest in number, often formed their own group. At another table, four Indian scientists ate quietly.

India has sent scientists to Svalbard since 2008, with some on the current team returning almost every year. Pradeep Kumar, director of the Geological Survey of India, was on his seventh visit. He mentioned a fresh paw print from a polar bear spotted just the day before, not far from the canteen. “Maybe we just missed the bear by half an hour,” he said. A viral video had been circulating of a polar bear chasing a man in one of Svalbard’s Russian settlements. In a dramatic escape, he runs, jumps onto his snow scooter, and rides off — just in time.

The ripple effect

Dr. Kumar’s roommate, Vipindas Kavumbai, an Indian microbiologist, spends his days in a marine lab near Ny-Alesund’s small port. He analyzes bacteria from fjord water. He filters, samples and freezes them, then ships his work to India for DNA sequencing.

As Arctic waters warm, cold-adapted bacteria are decreasing, replaced by faster-growing species better suited to rising temperatures.

“When the sea ice melts and more heat comes, these organisms cannot survive,” he said. “Other organisms replace them.”

The result, at the microscopic level, is what scientists call a “community shift.”

Just offshore, underwater brown kelp forests are thriving in newly ice-free waters. Carlos Smerdou, an ecologist from Spain who has studied seaweed in the Arctic for 23 years, said these forests are “reorganizing everything.”

Some of these vast underwater forests have attracted a surprising new grazer: hungry reindeer. Ashild Onvik Pedersen, a Norwegian ecologist, has seen it firsthand.

A dog musher from a small village in southern Norway, she splits her time between Svalbard’s capital, Longyearbyen, and Ny-Alesund, where she studies reindeer along the coast. For decades, they were climate winners. Not anymore.

“The sea ice is the glue here,” she said. “They don’t have it anymore. So they are stuck.”

Once, these reindeer roamed widely across Svalbard’s coast, using frozen fjords to reach better grazing grounds. Now, with that route gone, they’re hemmed in by mountains and glaciers.

Dr. Pedersen travels by snowmobile across the frozen terrain and tracks reindeer movement, body condition and survival rates. The temperatures can dip to minus 20 degrees Celsius, or minus 4 Fahrenheit, which turns her cheeks red.

As she explained her work, a small group of reindeer emerged in the distance, trotting along the ridge. A young one even made its way toward her.

“They’re very curious,” she said, looking through binoculars.

And they are finding new sources of food. Reindeer dig through snow to reach the lichens, an important part of their diet. But as the climate changes in Svalbard, there are more freeze-thaw cycles that leave thick crusts of ice over the snowy ground. That makes it difficult for reindeer to dig. Landlocked and starving, the reindeer have turned to kelp, Dr. Pedersen said. Seaweed is less nutritious for reindeer than lichens.

“Survival food,” she calls it.

But the story is not the same everywhere. In Svalbard’s inland valleys, reindeer populations have nearly quadrupled, reaching record highs in 2018. Warmer summers mean longer growing seasons and more abundant vegetation, and those herds are thriving.

It’s created what Dr. Pedersen called “diverging population trends” across an ecosystem that once functioned as a connected whole.

Adapting a hunt

As sea ice vanishes, polar bears are showing up in unusual places and turning to new prey.

For centuries, they relied almost exclusively on seals, whose thick blubber sustained them through the Arctic winter. But as the ice retreats, many of Svalbard’s roughly 300 bears have shifted to land-based prey and an array of new hunting tactics.

There’s the cliff strategy, where bears climb underneath reindeer on steep terrain, driving the reindeer higher up the slopes until they fall. Or they ambush the reindeer from above, using their considerable bulk to power down the hills faster than the reindeer expect. Others chase reindeer into water and outswim them.

“I have been surprised with how they’ve found ways to catch reindeer,” said Jon Aars, a Norwegian ecologist, who has studied polar bears for more than 20 years.

Polar bears now come ashore nearly a month earlier than they did in the 1990s, Dr. Aars said. If they arrive before seabird eggs hatch, they can wipe out up to 90 percent of nests, he added.

Human encounters are also rising, with more sightings near Ny-Alesund than five to 10 years ago. The town now employs armed guards to patrol its perimeter. This July, a 4-year-old male bear was shot and killed near the settlement after it was determined that it posed a threat to humans.

Since polar bear hunting was banned in 1973, the bear population in Svalbard has recovered and stabilized. Some bears are even recolonizing areas they had abandoned a century ago and Svalbard is considered one of the best places to see polar bears anywhere in the Arctic.

During the three-month seal hunting season, the bears obtain about 70 percent of their annual energy needs. Right now, they supplement their diet with reindeer meat, bird eggs, grass and seaweed.

Dr. Aars isn’t sure whether these alternative foods will provide enough nutrition if the bears eat fewer seals over time.

But on the other side of the Arctic, in Hudson Bay, polar bear populations are crashing. Longer ice-free seasons have left them too thin, too soon, pushing many toward starvation.

“I think a lot of what we have today will be lost,” Dr. Aars said.

Still, he believes some ecosystems will survive in a few places, or give way to new ones, with different species and behaviors emerging in response to a warmer world.

“I’m less pessimistic than others,” Dr. Aars said. “I think we will have polar bears in parts of the Arctic quite far into the future.”