Every spring, tens of thousands of elk follow a wave of green growth
up onto the high plateaus in and around Yellowstone and Grand Teton
national parks, where they spend the summer calving and fattening on
fresh grass. And every fall, the massive herds migrate back down into
the surrounding valleys and plains, where lower elevations provide
respite from harsh winters.
These migratory elk rely primarily on environmental cues, including a
retreating snowline and the greening grasses of spring, to decide when
to make these yearly journeys, shows a new study led by University of
California, Berkeley, researchers. The study combined GPS tracking data
from more than 400 animals in nine major Yellowstone elk populations
with satellite imagery to create a comprehensive model of what drives
these animals to move.
“We found that the immediate environment is a very effective
predictor of when migration occurs,” said Gregory Rickbeil, who
conducted the analysis as a postdoctoral researcher in Arthur
Middleton’s lab at UC Berkeley. This is in contrast with some other
species, such as migratory birds, which rely on changing day length to
decide when to move, Rickbeil pointed out.
The results, published in the current issue of the journal Global Change Biology, suggest that, as climate change reshapes the weather and environment of the park, elk should have the means to adjust their migratory patterns to match the new conditions.
While this adaptability may benefit the survival of the elk, it may
also have unknown ripple effects in local economies and throughout the
Greater Yellowstone Ecosystem — one of the last remaining large, nearly
intact ecosystems in Earth’s northern temperate zone, which encompasses
about 18 million acres of land managed by more than 25 public entities
and hundreds of private landowners. Another recent UC Berkeley-led study
suggests that climate change is likely to hit National Parks harder
than other areas of the country.
“The decisions that these animals make about when to migrate are
absolutely dependent on changes in the landscape, changes that are
ultimately governed by the climate,” said Middleton, an assistant
professor of environmental science, policy and management at UC Berkeley
and senior author on the study. “And in the future, with climate
change, we should expect the timing of these mass movements to be
altered, which will affect the other wildlife and the people who depend
on them, including predators, scavengers and hunters across the
Though the migration study’s period was too short to say whether or
not climate change is already affecting migratory timing, the tracking
data did reveal a surprising trend: Elk on average arrived on their
winter ranges 50 days later in 2015 than in 2001. This change had been
noted by wildlife managers in the area, but had yet to be quantified on
the ecosystem scale until now.
“This [study] provides great insight into the adaptation strategies
of elk to climate change in the Greater Yellowstone Ecosystem,” said
Jonathan Jarvis, former director of the National Park Service, who now
serves as executive director of the Institute for Parks, People, and
Biodiversity at UC Berkeley.
Jarvis noted that more broadly, the new picture of the Yellowstone
elk migrations provided by the study’s comprehensive mapping effort
“clearly demonstrate the need to think and operate at the landscape
scale.” He added, “For the park managers, this kind of research gives
them options and incentives, such as protection of migration corridors
and seasonal habitats, for ensuring elk and other keystone species in
the Greater Yellowstone Ecosystem will persist.”
Eating and being eaten
Yellowstone’s approximately 20,000 migratory elk are among the most
important large mammals in the ecosystem, comprising about 10 million or
so pounds of animal biomass pulsing in and out of the parks and
adjacent wilderness areas each year — so where they can be found at any
given time matters to both animals and humans alike.
“These elk eat a lot of things, and they are eaten by a lot of
things, so wherever these masses of hundreds or thousands of elk are on
the landscape determines who gets to eat and who doesn’t,” Middleton
said. “In some cases, this could be sensitive populations of carnivores,
like grizzly bears or wolves, and on the human side, it could be
hunters, some of whom are making their income as outfitters and guides.”
Recent studies have shown that threatened grizzly bears depend
heavily on newborn elk calves as a food source in spring — right when
the migration is happening — and that a Yellowstone wolf kills, on
average, 16 elk per year. Meanwhile, each fall, thousands of hunters
from around the country pay guides for the chance to harvest an elk in
the wilderness near Yellowstone.
While a smattering of studies has investigated the migration of
individual herds in the park, none before this study had investigated
the phenomenon on an ecosystem scale. To get a more complete picture of
migration, Middleton partnered with state and federal wildlife managers
in the Yellowstone region to pool information on 414 elk across nine
herds that had been fitted with GPS collars between 2001 and 2017.
Rickbeil then analyzed the data to pinpoint when each elk made its
trek from winter range to summer range and back again and used satellite
images to infer the conditions on the ground during journeys.
He found that elk tended to leave their winter ranges and set out to
their summer ranges as soon as the snow had melted and during the
“green-up,” when fresh, nutritious plant growth began to sprout.
Likewise, encroaching snowfall and hunting pressure cued them to make
the return journey.
The team was surprised by the extent of the elks’ flexibility: One
year, a female elk might migrate in early May, but the next year in late
July, depending on the timing of snowmelt and green-up.
“They’ve got a big brain and big eyes, and they can look around and,
to a large degree, see changes on the landscape and react to them,”
However, Rickbeil notes, the snow cover and vegetation couldn’t
fully explain why the elk are now arriving so much later at their winter
ranges. Variations in snow depth, which cannot be inferred from
satellite data, might explain part of the dramatic change, Rickbeil
Alyson Courtemanch, who manages the Greater Yellowstone Ecosystem’s
Jackson elk herd as part of her job as a wildlife biologist with the
Wyoming Game and Fish Department, says knowing the whereabouts of the
elk is critical to her job setting hunting seasons and managing the
spread of diseases among wild elk and domestic cattle.
“We’ve been observing a lot of really interesting changes over the
past decade about the way that elk are moving across the landscape,
specifically of the timing of the migrations,” said Courtemanch, who
supplied GPS data on the Jackson herd for the study. “This analysis
helped confirm a lot of things that people on the ground had suspected
were happening, but that weren’t really quantified.”
“It seems like these animals can adapt to changing climates, which
is likely a good thing,” Rickbeil said. “But there will be a lot of
consequences to these changes.”
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