across, plastered with mud and moss and sit-
ting in a waist-high lake surrounded by marsh.
The water had been diverted from the river by
a series of dams. “That entire swamp around
Lodge Mahal is new,” Tape said. “If you went
back 50 years, there’d be zero beavers here.”
Tape and Wald had wanted to explore the
Alatna in part because a guide who works for
Wald had earlier found beaver-chewed wood
along the Nigu River. The Nigu starts near Gae-
deke Lake, the Alatna headwaters, but on the
other side of the Continental Divide—and so it
flows north into the Colville River and the Arctic
Ocean. Along the Alatna, above Lodge Mahal, we
found other ponds and abandoned dams. Tape
now thinks that beavers are on their way to the
North Slope, and that they’re using the Alatna as
a route through the Brooks Range. “We’re seeing
this expansion in real time,” he said.
He can’t prove that climate change is driving
it; the beaver population also has been rebound-
ing since the end of the fur trade, a century and a
half ago. But in any case, the bucktoothed engi-
neers could significantly remake permafrost
landscapes. “Imagine if you were a developer
and you said, I’d like permission to put three
dams on every other stream in the Arctic tun-
dra,” Tape said. “That’s what this could be like.”
Tape has seen a preview. Southeast of Shish-
maref, on Alaska’s Seward Peninsula, photos
of a tributary of the Serpentine River show no
change at all between 1950 and 1985. By 2002
beavers had moved in and flooded the land-
scape. By 2012 some ground had collapsed and
become wetlands. Permafrost was on its way out.
A few hundred beavers won’t reengineer the
Arctic. But the animals may be heading north
in Canada and Siberia too, and they reproduce
quickly. Argentina’s experience is instructive:
Twenty beavers were deliberately introduced in
the south in 1946 in order to foster a fur trade.
Today that population hovers around 100,000.
N THE ZIMOVS’ VISION OF
the past and future of
Arctic permafrost, wild
animals also play a cen-
tral role—but the beasts
are bigger than beavers,
and their effect on per-
mafrost more benevolent.
The herds of bison, mammoths, horses, and
reindeer that lumbered across the Pleistocene
steppes, Sergey Zimov has long argued, did
more than just eat the grass. They maintained
it. They fertilized it with their waste and packed
it down, trampling mosses and shrubs and rip-
ping out tree saplings.
Since the last ice age, those dry, rich grass-
lands have been replaced in eastern Siberia by
damp tundra, dominated by mosses in the north
and forests farther south. One key driver of that
change, according to Zimov, was human hunters
who decimated the herds of large grazers, by
about 10,000 years ago. Without grazers to fer-
tilize the soil, grasses withered; without grasses
to soak up water, the soil got wetter. Mosses and
trees took over. But if humans hadn’t pushed the
ecosystem beyond a tipping point thousands
of years ago, there would still be mammoths
grazing in Siberia.
Almost 25 years ago, on lowlands near Cher-
skiy, Zimov created a 56-square-mile demon-
stration project called Pleistocene Park. His
idea was to bring large grazers back and see
whether they would bring back the grasslands.96 NATIONAL GEOGRAPHIC