beaver. The dams they build and the ponds they
create are hard to miss; these newly formed bodies
of water even show up on satellite images.
Beavers have infiltrated three watersheds in
northwestern Alaska in the last couple of decades.
Together these drainages cover more than
18,000 square kilometers — an area larger than
On images of the region collected by Landsat
satellites in summer months from 1999 through
2014, Tape and colleagues looked for new areas
of wetness that covered at least half a hectare
( 1. 24 acres), or about four times the area covered
by an Olympic swimming pool.
The researchers then used newer, high-res-olution satellite images to verify the presence
of beaver ponds. Available aerial photographs
taken before 1999 didn’t pick up any signs of beaver activity in the area, Tape says. Kirk notes that
beavers were present in the Lower Noatak River
watershed before 1999, but in vastly smaller numbers than they are today.
Based on the images at hand, the researchers
found 56 new complexes of beaver ponds in the
area over the 16-year study period. On average,
beavers expanded their range about 8 kilometers
per year, Tape and colleagues reported in the
October Global Change Biology.
“This is remarkable, but it shouldn’t come as a
surprise,” Tape says. “Beavers are engineers that
work every day, all summer long.”
The animals have also made their way into
western Alaska’s Seward Peninsula and the north-
ern foothills of the Brooks Range, mountains
that stretch east to west across northern Alaska,
the researchers found. If the animals’ recent rate
of expansion continues, beavers could spread
throughout Alaska’s North Slope in the next 20 to
40 years, the researchers say.
The Lower Noatak River watershed, one of
the areas that Tape and colleagues studied, is
mostly tundra. By definition that means treeless
plain. But the area also is about 3. 5 percent forest, mainly concentrated along the river and its
tributaries. The watersheds just to the north are
completely tundra. So how do the beavers there
build dams without trees? In those areas, Tape
says, the animals construct smaller dams than
they might at lower latitudes, using the branches,
twigs and foliage of willows and other shrubs.
“I never expected to see beavers on the tundra,”
Roth says, intrigued by Tape’s team’s findings.
The beavers are not only persisting on the tundra,
they’re thriving. The moderately sized streams
and flat terrain provide ideal habitat. And once
they gain a foothold, these industrious creatures
set about making improvements that are probably
an overall plus for myriad other species, Tape says.
For instance, frigid conditions in the region
cause shallow streams to freeze solid in winter.
But when a beaver builds a dam, the water that
gathers upstream of the structure becomes deep
enough to remain liquid below a sheet of ice that
provides insulation from the chilly winter air.
That persistent liquid lets the beavers move
about under the ice even in the depths of winter.
The water gives them a place to stockpile food, too,
Tape notes. That constant supply of liquid water
also provides year-round habitat for fish, amphibians and even some insects in their larval stages.
None of these species are part of the beaver’s diet,
but they could serve as food for other creatures.
“All that diversity would add whole new layers to
food webs,” Roth says.
Ecological changes could extend well beyond
the beaver pond. The water impounded by beaver dams sometimes finds its way past the dam,
Tape says. The satellite photos that he and his
colleagues analyzed revealed that some stretches
of river just downstream of beaver dams now
remain unfrozen even in winter. That flowing
water probably spills over the dam or around its
edges, but some may seep through or under the
That liquid water also helps thaw the underlying permafrost. Previous studies have shown
that even a shallow pond less than a meter deep
can boost sediment temperature by as much as
10 degrees C above the locale’s average air temperature. That kind of warming causes permafrost to thaw decades earlier than it would
Moving on up
Beavers have begun
moving (yellow arrows)
beyond the tree line
(orange) in Alaska and
Yukon in northwestern
Canada. In the next few
decades, the rodents
could spread farther into
Alaska’s North Slope