Moose, iconic Alaskan residents, became the state's official land mammal in 1998. But these animals haven't always lived everywhere in the largest state. Before the 20th century, moose did not live in the northern Arctic tundra region of Alaska.
What changed? It got warmer, scientists say.
A new study suggests that decades of rising temperatures made the region more welcoming to moose. The warmth led to larger shrubs, which both feed and protect the bulky mammals, according to a new paper published Wednesday in the journal PLOS ONE.
"When you come to the Arctic, it seems like a very timeless place," says study lead author Ken Tape, an ecologist at the University of Alaska Fairbanks, in a phone interview with The Christian Science Monitor. "But, at least on the scale of the last century, it's changing quite a bit."
Since the turn of the 20th century, Dr. Tape estimates that moose territory expanded between 52 and 60 million acres – an area the size of Minnesota.
Historical records showed Tape that moose only lived in boreal forests in Alaska before the 1900s. One explanation put forth was that reduced hunting in the early 20th century allowed for the moose to expand into other regions, a theory Tape calls "pretty compelling."
But that explanation leaves out the region's climatic and environmental changes, which the team hypothesized might have had an even larger impact.
"There are existing relationships between summer warmth and the height of vegetation," Tape says: the shrubbery that grows along little creeks and floodplains in this region, for example, is particularly affected by warmer summers.
Tape and his colleagues calculated how tall those shrubs would have grown in the summer temperatures of the 19th century and did the same calculation for more recent times. They found that in 1860, those shrubs would have grown to about 3.6 feet tall on average, whereas in 2009, they would have reached 6.6 feet.
Why does height matter?
Although caribou, which migrate through the region, dig around in the snow for food, moose tend to nibble on what is freely available. So if the snowpack is more than 3 feet deep, as it likely was, short shrubs would be less accessible to moose.
Furthermore, moose use plants for protection from predators. The average moose stands at about 6.3 feet at the shoulder, Tape says, so a 3.6-foot shrub wouldn't provide much cover.
"The idea of a moose hanging out in a shrub patch where it's actually sticking above the shrubs is kind of hilarious actually," Tape says. "I don't think I've ever seen that. If a moose is sticking out, it's usually on route to a place where it will be hidden in very short order."
Tape admits that there's no way to know exactly how deep the snow used to be. But, he says, even if a shallower snowpack revealed more shrubbery to snack on, the height of the shrubbery itself would have still been necessary for cover.
The hunting hypothesis might figure into the story as well, says Peter Pekins, a wildlife ecologist at the University of New Hampshire who was not part of this study. "I think you can't discount that," he says in a phone interview with the Monitor. "It's probably a combination."
But Tape and his team's work makes sense, he says.
"Habitat dictates animal presence," Dr. Pekins says. "So it's not surprising that we see animals respond to that."
He points to other examples of animals shifting their range because of a changing climate. In Alaska, the snowshoe hare is moving into the tundra along with the moose. The red fox is also moving north, encroaching on arctic fox territory.
Overall, animals are practically sprinting northward, according to a study published in 2011 that found some 2,000 species shifting their ranges north at a rate of more than 10.5 miles each decade.
As animals move north, they aren't just moving into a new neighborhood. They're also encountering new neighbors.
"You may see a reduction in diversity of vegetation due to the browsing pressures of these animals," Pekins says. As the new residents snack on the shrubs, they'll be competing with the other browsing species that were already there.
And, Pekins adds, "Moose have parasites. They bring these parasites into new areas."
So while "this may be viewed as a positive outcome of climate change for moose," he says, "the flip-side is that that community is now changed. As the moose become winners, there's probably going to be some losers in there, too."