Shift in caribou movements may be tied to human activity

Each year caribou take on one of nature's longest land migrations, trekking hundreds of miles across Alaska and Canada to find food and give birth in their preferred calving grounds.

A UC study published today in the journal Frontiers in Ecology and Evolution identified a shift in one herd's movements after the 1970s that coincided with changes in herd size and climate, and the construction of new roads and other energy infrastructure.

Researchers used isotope analysis of antlers shed by female caribou to track their historical patterns of movement over the landscape. Female caribou are unique among deer for growing and shedding antlers each year like males.

The study is timely given the auction this year of oil and gas leases in the Arctic National Wildlife Refuge. Indigenous Gwich'in opposed the leases, arguing development could disrupt the migration of caribou they depend on for sustenance.

An international team of researchers led by UC geologist Joshua Miller focused on the antlers of female caribou, which are shed within a few days of giving birth each spring. The location where antlers drop marks their spring calving grounds.

Caribou then spend the summer growing a new pair of antlers.

Miller and his collaborators found that analyses of isotopes from the antlers could not only identify one caribou herd from another but also identify changes in their summer range over time.

Miller, an assistant professor in UC's College of Arts and Sciences, traveled extensively across the Arctic during five expeditions with collaborators from the U.S. Fish and Wildlife Service. Using inflatable boats, the team navigated rivers, avoiding bears and enduring mosquitoes to collect caribou antlers across the Arctic National Wildlife Refuge in northeast Alaska.

"It's one of the most remote places on the planet," Miller said. "So it poses all sorts of logistical challenges. It is a real adventure."

The refuge is home to grizzly and polar bears, musk ox and hundreds of thousands of caribou found in different herds. Caribou are an important staple food for Indigenous Alaskans who seasonally hunt them.

Two populations of caribou are found in the Arctic Refuge: the Central Arctic herd and the Porcupine caribou herd, which is named for the Porcupine River that flows in the heart of its range. While caribou numbers can fluctuate year to year, the Porcupine herd is home to about 200,000 caribou. The Central Arctic herd has approximately 60,000 more, though its numbers may be declining.

The collected antlers were shipped back to Miller's UC geology lab, where researchers, including UC graduate student Abigail Kelly, prepared them for isotopic analysis.

Strontium, which is found virtually everywhere on Earth, is absorbed up the food chain through plants that caribou and other herbivores eat. Strontium exists as different isotopes, which vary with the geology like an isotopic footprint. By comparing the ratios of strontium-87 and strontium-86, researchers could track where the antlers were grown.

Since new female antlers grow in just a few months each year, they make an ideal time capsule to identify where a caribou has been feeding.

Miller said finding the antlers is straightforward on the flatter terraces of the Arctic Refuge away from the thick grass tussocks.

"On these flat areas, they can be everywhere -- more than 1,000 antlers per square kilometer. In some places, you can find one every several steps," he said.

Some of the antlers had been lying on the tundra for hundreds of years. One was dated to the 1300s.

Researchers observed a shift in summer movements among caribou in the Central Arctic herd before and after the 1970s. This coincides with three factors known to alter caribou migration: population growth, climate change and increased human disturbances to their summer and calving ranges.

Human development in the 1970s included oil field expansion and construction of the Trans-Alaska Pipeline. Previous studies have found that pregnant caribou avoid pipelines and roads while calves born in the vicinity of roads and other development are underweight compared to those living farther from human development.

"An important future area of research will be to test this shift in preferred summer landscapes using an expanded sampling of antlers shed across each herd's calving grounds," researchers said.

Scientists only began studying caribou migration using radio telemetry in the 1970s and '80s. Miller said. With antlers, it is possible to track historical caribou landscape use long before that.

"The question is, how can we evaluate the effects of human impacts given that we only recently started paying close attention? Antlers provide opportunities to look at the past and fill in some of these gaps in our knowledge," he said.

"One thing we know about caribou is they often avoid human-modified landscapes: pipelines, roads, tourism lodges," Miller said. "They are surprisingly sensitive to these changes."

UC associate professor Brooke Crowley, a study co-author, has employed similar methods to identify critical hunting areas for endangered goshawks in Madagascar, track endangered jaguars in Belize and even follow the migrations of long-extinct animals like mammoths and mastodons.

"Strontium isotopes allow researchers to understand mobility of animals on temporal and spatial scales that complement other conservation tools," Crowley said. "It is particularly valuable to be able to reconstruct what a species or population did in the past because then we have some baseline data that we can compare to modern trends."

The other co-authors included Clément Bataille from the University of Ottawa, Eric Wald from the U.S. Fish and Wildlife Service, Volker Bahn from Wright State University and Patrick Druckenmiller from the University of Alaska Fairbanks.

UC doctoral student and paper co-author Madison Gaetano said tools developed by paleontologists to study long-extinct animals are helping researchers answer pressing questions about wildlife conservation.

"Bones lying on modern landscapes accumulate over many generations and record data that are applicable to a myriad of questions about the evolution and ecology of animals and their ecosystems," Gaetano said. "Our role is to develop methods to access, interpret and apply this information, which I think is nicely demonstrated by this research."