Caribou Herds in Arctic Alaska Tundra Areas are on Opposite Trends

The Western Arctic Caribou Herd, once the biggest in Alaska, is faltering, having fallen from a high of 490,000 animals in 2003 to only 152,000 as of 2023. But to the east, the Porcupine Caribou Herd appears to be thriving, with an all-time high of 218,00 animals recorded at the last census¹. That makes it, rather than the Western Arctic herd, the state’s largest.

This Article was originally published by the Alaska Beacon on Jan. 20, 2025 and has been edited only for length and style. It is published here under Creative Commons license CC BY-NC-ND 4.0. Notes from our editor are marked as superscripts and can be found at the end of the article.

Why are the herds following opposite trends? An answer, Alaska scientists say, is found in what is growing on the ground – and the way the warming climate has changed those plants.

Woody shrubs and even trees are spreading rapidly over Arctic regions of Northwest Alaska, the area where the Western Arctic herd ranges, said Roman Dial, a professor at Alaska Pacific University. But that plant transformation, which scientists refer to as “shrubification,” has been much slower on the eastern side of Arctic Alaska, the range for the Porcupine Caribou Herd, he said.

For caribou, growth of woody plants like alders and willows means problems. Caribou depend on tundra plants like lichen and mosses; the shrubs and trees taking over the terrain are reducing the availability of that food favored by the animals.

Dial has studied changes in Alaska plant growth for several years and has been traveling in the Brooks Range since he was a teenager in the 1970s.

Even though his recent years’ work in Arctic Alaska has been focused on plants, he said encountering willows and other woody plants covering what used to be open tundra west of the Dalton Highway made him think right away of animals.

“A lot of caribou trails were getting overgrown and disappeared, and you’d find really old antlers that were in skulls that were kind of buried in the tundra, so caribou hadn’t been there for a long time,” he said. “Right away it was, like: ‘Wow, caribou are changing their routes.’ And you could see it.”

The overgrown state of caribou trails that had been etched into tundra terrain over multiple years of migration was instructive, Dial said.

When he presented his studies during the December annual meeting of the Western Arctic Caribou Herd Working Group, the advisory panel representing villagers and others dependent on the herd, Dial described what might be a caribou’s view of the plant takeover, and he used a brief video from the field to illustrate his point.

He does not like walking through willows that can be eight feet tall, “and I don’t think caribou like going through willows either,” Dial told the working group. “If my antlers were all tender and velvet, I wouldn’t want to go through a bunch of tall willows. And also, when you go through willows, there’s bears in there.”

Changes in caribou habitat are linked to reduced Arctic sea ice, which itself is a direct result of accelerated climate warming in the Northern Hemisphere, Dial said.

Open water leads to more snowfall, he said, as there is more moisture sent into the atmosphere to fall as precipitation, More snowfall insulates the ground, keeping soil temperatures warmer through the winter, he explained. Warmer soil temperatures encourage plant growth and the spread of woody shrubs and trees. More woody plants on the ground make life harder for caribou, both by displacing their usual tundra food sources and by creating new obstacles to movement.

Open water does not affect Alaska’s western and eastern Arctic tundra regions equally, and the results are seen on the ground, Dial said.

Sea ice retreat usually forms later and melts earlier in the Chukchi Sea, which lies off the northwestern coast, than in the Beaufort Sea, which lies off Alaska’s northeastern coast. Utqiagvik – the nation’s northernmost community – is the point where the two Arctic seas meet. Relatively warm Pacific Ocean water flows into the Chukchi through the Bering Strait, making ice there more seasonal, meaning it forms and melts earlier each year. In contrast, an ocean circulation system called the Beaufort Gyre sends old multiyear ice from north of Canada into the Beaufort, making the freeze there a little more resilient. While ice retreat has been significant over the past decades in both seas, the characteristics of the Chukchi make it particularly vulnerable, and it has lost both the thickness and extent of ice at a faster rate than almost any marginal sea in the Arctic, according to climate scientists.

Both the Western Arctic herd range and the Porcupine herd range have become warmer in summer and snowier in winter, according to records kept by the Alaska Center for Climate Assessment and Policy² at the University of Alaska Fairbanks. But the summer change has been more intense in the western range, particularly in coastal areas off Nome and Kotzebue, according to the data.

The work by Dial and his colleagues to track the changes involved an old-fashioned method: walking the ground.

The idea to do that was inspired by studies of shrub growth that is spreading up to higher elevations around Anchorage – and made necessary by the COVID-19 pandemic, which shut down National Science Foundation-funded air travel in 2020, Dial said. Some of the Alaska Pacific University students who were working with him on studying vegetation were enlisted to go north to be part of the Brooks Range expeditions.

In 2020 and 2021, they walked hundreds of miles of the terrain, smartphones in hand, looking down and recording changes in the plants growing on either side of the Dalton Highway. “It kind of added a new dimension to hiking,” he said. A study published in June details the findings from their treks in 2020.

A separate but related study, published about a year ago, examined tree rings to show a correlation between growth and proximity to open Arctic water. The study, which Dial did with Patrick Sullivan of the University of Alaska Anchorage and other scientists, focuses on white spruce trees from 19 different sites along the Brooks Range. Those trees were small, ranging from ankle to chest height, indicating that they were recent arrivals, Dial said.

The on-the-ground work by Dial, Sullivan and their colleagues adds to past research that tracked the northward spread of woody plants by more distant methods. A 2018 study by scientists from the U.S. Fish and Wildlife Service, UAF and other organizations, for example, used 50 years’ worth of aerial photographs to identify shrub and tree expansion into the Arctic National Wildlife Refuge in northeastern Alaska, where the Porcupine herd ranges. The study found that shrubs had spread into the refuge’s tundra regions over the half- century period, but they had done so at a slower pace than in other Arctic Alaska tundra regions.

The growth of woody plants in Arctic tundra regions affects more than caribou.

In northwestern Alaska, where the growth has been most dramatic, it has attracted a proliferation of beavers, for example. And as beavers colonize the landscape, they are transforming it with thousands of new dams that pool water that, in turn, speed thaw of permafrost and feed into the cycle of shrub expansion.

Broader climate change impacts

Climate change impacts on the Western Arctic Caribou Herd go beyond the spread of shrubs displacing tundra plants.

Warm winter conditions in 2005 produced two days of rain in the herd’s winter range, creating a thick layer of ice that encased the tundra plants that the animals eat. A large die-off followed, according to the Alaska Department of Fish and Game.

Wildland fires that are becoming more common as conditions warm can also affect caribou by destroying the slow-growing lichen and other tundra plants the animals eat. That has long been known to be an issue for caribou in more southern and boreal regions, such as Interior Alaska’s Nelchina herd. Now wildfire has emerged as a threat to the Western Arctic Caribou Herd’s habitat.

The Western Arctic herd’s declines are part of a circumpolar trend.

Tundra caribou populations across the Arctic have declined by 65 percent over the last two to three decades, according to the 2024 Arctic Report Card released in December by the National Oceanic and Atmospheric Administration. Alaska’s Western Arctic Caribou Herd was one of those identified as having the most dramatic declines.

“Warmer summer and fall temperatures, changes in winter snowfall, and an increasing human footprint collectively stress Arctic caribou, altering their distribution, movements, survival and productivity,” the report card said.

For the Western Arctic herd, changes go beyond its sliding population numbers. A key metric measured by federal and state biologists who study the herd is the date when southward-moving caribou cross the Kobuk River, a waterway that flows west from the Brooks Range into Kotzebue Sound. Over the decades, most collared caribou spend summers north of the river and winters south of it, though in five years since 2016, fewer than half of the collared animals went that far south in their fall migration, according to the data.

At the December meeting of the Western Arctic Caribou Herd, National Park Service biologist Kyle Joly said the average river-crossing date in 2023 was notably late: Nov. 8.

“It was the latest-ever average time that they crossed. I actually had to extend my graph here because the number didn’t fit,” Joly told working group members.

Caribou that do cross have also shifted the location of where they do so, and where they spend the winter. There has been a notable lack of caribou on the Seward Peninsula in the western part of the traditional range, according to the data from collared animals. None were tracked into Bering Land Bridge National Preserve, Joly said.

On-the-ground observations match the data, he said. “Twenty-five years ago, Unalakeet was a great place to see caribou,” he said. Residents haven’t seen caribou there for several years, he said.

Editor’s Note

1. Since the time of this article’s first publication, the Alaska Department of Fish and Game, working with the Government of Yukon, conducted another photocensus which estimated the Porcupine Caribou Herd at 143,000 caribou. The census shows a marked decrease since the herd’s population peaked in 2017 but is still the largest herd in the state and stable enough to support normal hunting guidelines.
2. Now the Alaska Center for Climate Assessment and Preparedness