Hungrier summers mean harder winters

Caribou—iconic symbols of the northern wilderness—are facing steep population declines as climate change reshapes their habitats. LIBBY EHLERS studied Alaska’s and Canada’s Fortymile caribou herd using GPS video camera collars to capture “bite-sized” views of their foraging behaviour. She explains what the research revealed about how competition, insects and shifting vegetation are affecting their survival and reproduction. 

Across the sweeping alpine tundra landscapes of Alaska and Canada, the Fortymile caribou herd moves continuously, heads down, constantly searching for food beneath endless skies. During their two-month summer, swarms of mosquitoes and flies swirl in the heat, driving the animals to windswept ridges that provide needed relief from harassment but offer little to eat.  

Traditionally, shrubs and lichens have carpeted these lands, but their availability to caribou is shifting as wildfires, warming and competition for high-quality summer foods change the landscape. For Indigenous Peoples, scientists and wildlife managers alike, caribou are more than animals—they are a vital thread in northern ecosystems. Understanding what, where and how these animals search for and find quality foods in summer has become a pressing question in a rapidly changing Arctic. 

For many northern Indigenous and rural communities, caribou remain the most important land animal for food, clothing and cultural traditions. But many herds continue to decline. By 2024, the Fortymile herd, which lives in east-central Alaska and west-central Yukon, had dropped to about 30,000 animals, down from 80,000 in 2017.

Changing food sources and more insects

One of the biggest reasons for this recent decline is climate change. Warmer temperatures, changing precipitation patterns, and stronger and more frequent wildfires are modifying the plant communities that make up the tundra. Shrubs and trees are spreading into open areas where lichens—the main winter food source for caribou—once thrived.  

Unlike shrubs, lichens grow very slowly, sometimes taking decades to recover if they are damaged or eaten too heavily. This means that even if caribou find enough shrubs to eat in summer, they may struggle to survive in winter, when lichens are scarce. 

Insects are also making life harder for caribou. Scientists estimate that in Alaska alone, there are trillions of mosquitoes in summer. These and other flies harass caribou so severely that they spend less time eating and more time trying to escape.  

Warmer temperatures and changing wind patterns caused by climate change may exacerbate this problem, extending the duration and intensity of the insect season and leaving caribou in a weaker nutritional state and potentially challenged to rebuild their bodies, give birth, and feed and raise their young during the short, demanding summer season. 

A new way to study caribou

To better understand how these pressures are affecting the Fortymile caribou herd, researchers from the University of Montana and a team of partners—including the Bureau of Land Management, Alaska Department of Game and Fish, Yukon government, and U.S. National Park Service—used a new technology: GPS video camera collars. These collars were fitted on caribou and recorded short video clips showing exactly what the animals were eating throughout summer and where they were feeding.  

With help from community scientists and botanists, the team analyzed more than 18,000 videos and identified more than 7,000 food items. I led the team to build mathematical models to predict how caribou decided where to feed, considering food availability, weather, insect activity, wildfires and competition with other caribou. 

The results showed clear patterns. When many caribou were packed into the same area, they ate fewer lichens—likely because competition between animals had already reduced the amount available. As a result, they turned more often to shrubs, especially willows, which are a higher-quality food in summer. Willow leaves provide proteins in summer and can regrow quickly after being eaten. This shift makes sense for short-term survival, but there are long-term risks if lichens are depleted in areas used year-round. Because the Fortymile herd’s summer and winter ranges largely overlap, heavy use in summer can damage lichens needed for winter, compromising survival and reproduction over time. 

Hope for the future

Despite these challenges, our study points to constructive steps that can be taken. We used our data to build detailed seasonal maps of the herd’s range, showing areas where caribou were most likely to find high-quality foods in summer. These maps are now being used by wildlife managers to guide conservation and land-use decisions. This project also relied on extensive collaborations between scientists, government agencies and volunteers who classified thousands of video clips. By combining advanced technology with community involvement, we are finding new ways to understand and protect caribou. 

The story of the Fortymile caribou herd is both a warning and an opportunity. It shows how climate change and shifting food communities can push an iconic species toward decline. But it also shows how creative tools, teamwork and diverse knowledge systems can help us find solutions. Protecting the spaces and foods that caribou need to survive in a rapidly changing Arctic is not only vital for the animals themselves, but for the northern people and ecosystems that have always depended on them. 

Caribou are truly adapted for the north. They are lifelines for people, indicators of ecosystem health, and symbols of resilience in a rapidly changing world. The story of the Fortymile caribou herd shows us that what caribou choose to eat today shapes whether they, and the communities that depend on them, can thrive tomorrow. Protecting their habitats and food sources now is the key to keeping this species—and the systems that thrive with them—alive for generations to come.