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GLOBAL VOICES OF SCIENCE:
Pleistocene Park: Return of the Mammoth's Ecosystem
Sergey A. Zimov
Sergey A. Zimov
Russia
Sergey A. Zimov, director of the Northeast Science Station in Cherskii
in the Republic of Sakha (Yakutia), received his academic training in
geophysics at the Far East State University in Vladivostok, Russia. He
subsequently did fieldwork in northern Siberia for the Pacific
Institute for Geography, part of the Far East Branch of the Russian
Academy of Sciences. In 1980, he organized the science station that he
now directs. Research at the center includes studies of global carbon
and methane budgets and animal extinctions that occurred in Siberia
when the Pleistocene epoch gave way to the ongoing Holocene about
10,000 years ago. In 1989, Zimov initiated a long-term project known as
"Pleistocene Park," which he now is pursuing with a number of partners.
The goal of the project is to reconstitute the long-gone ecosystem of
the Pleistocene epoch that supported vast populations of large animals
including mammoths, horses, reindeer, bison, wolves, and other large
predators. If the effort succeeds in the park, Zimov and his co-workers
would like to see the ecosystem restored over much larger areas in an
effort to stave off what otherwise could be a massive release of carbon
that now is sequestered in the permafrost but that could be released
into the atmosphere as global temperatures rise. His hunting of mammoth
remains in the tundra and his bold vision of controlling and restoring
ecosystems have earned him coverage in books, documentaries, and other
media.
CREDIT: DENISE BERJAK |
All essays appearing in this series can be found online at www.sciencemag.org/sciext/globalvoices/
During the last ice age, the world's most extensive ecosystem stretched
from France across the Bering Strait to Canada and from the arctic
islands to northern China. It was at the very end of a more than
million-year epoch, the Pleistocene, during which colossal ice sheets
repeatedly advanced and retreated, plowing up much of northern Europe
and America. At the same time, from a geological perspective,
northeastern Siberia remained relatively unscathed. There, vast
dust-covered plains and valleys dominated the landscape. Mammoths,
woolly rhinoceroses, bison, horses, reindeer, musk oxen, elk, moose,
saiga, and yaks grazed on grasslands under the predatory gaze of cave
lions and wolves.
The ground, as in Siberia today, froze, contracted, and cracked each
winter. In spring, water penetrated and froze in deep, narrow cracks,
creating networks of ice wedges. Over time, because of the slow
accumulation of dust, river silt, and ice, the northern lowlands of
Siberia became covered with a thick sedimentary mantle of frozen loess.
These frozen sediments are filled with rootlets of grasses, microbes,
and animal bones, all of which have enabled scientists to chronicle the
rise and fall of the region's Pleistocene ecosystem.
About 10,000 years ago, at the beginning of the Holocene epoch, this
vast system, which I refer to as the mammoth tundra-steppe, disappeared
completely. In northern Siberia, mossy tundra and forest tundra
replaced the mammoth ecosystem. The only herbivores to survive were
reindeer that grazed on lichens and moose that fed on willows. The
mammoths and their large animal companions, which had survived even the
worst conditions the ice age could muster, disappeared during the
Holocene warming.
It actually might not have been the climatic changes that killed off
these great animals and their ecosystem, however. More consequential,
perhaps, were shifts in ecological dynamics wrought by people who
relied on increasingly efficient hunting practices, which decimated the
very populations of grazing animals that maintained the tundra steppe.
To test this possibility, my colleagues and I for the past decade have
been working to reconstitute the mammoth ecosystem in one modest parcel
of the northern Siberian region of Yakutia. We call our project
Pleistocene Park. The primary scientific goal is to determine more
precisely the role that Pleistocene animals played in maintaining their
own ecosystem. However, we also suspect that by learning how to
preserve and extend Pleistocene-like grasslands in the northern
latitudes, we could subsequently develop means for mitigating both the
progress and effects of global warming. The amount of carbon now
sequestered in soils of the former mammoth ecosystem, and that could
end up as greenhouse gases if released into the atmosphere by rising
global temperatures, surpasses the total carbon content of all of the
planet's rain forests.
The Vanishing of the Herbivores
Grassland ecosystems are evolutionarily the youngest of ecosystems.
These ecosystems have the highest rates of biogeochemical cycling.
Grasses use water resources more rapidly than their less productive
competitors, such as cactuses and trees, rather than spending energy
for making thorns and toxins to ward off enemies. When their numbers
reach a level that can be sustained by the landscape, herbivores eat
and trample all the grassland vegetation produced during the rainy
season and return nutrients to the soil through their manure. On
different continents, at different latitudes, grassland ecosystems have
been, and are now, composed of different species, but they share a
similar set of functional types or guilds. These include grasses,
elephants, horses, rodents, dung-beetles, large cats, vultures, and so
on. The greater the diversity within and among these functional types,
the more active the biological cycles and the more successful and
extensive the ecosystem can become.
Horse sense. Grazing on a snow-covered tundra meadow
in northern Siberia, rugged Yakutian horses like these could help
reduce the effects of global warming by stabilizing vast expanses of
grassland.
CREDIT: S. ZIMOV |
In the Pleistocene, grassland ecosystems occupied about half of the world's land mass.
Homo species
emerged in these pasture ecosystems, where they left tools, weapons,
cave paintings, and other signs of their presence. Starting with
unpretentious ambitions to survive in a hostile environment,
Homo
ended up assuming the powerful role of ecosystem terminator. The
mammoth ecosystem was the first large-scale victim, but the global
destruction of grasslands only accelerated in the Holocene when people
invented agriculture and began raising cattle.
Twenty years ago, scientists explained the disappearance of numerous
animals in the northern grasslands very simply--the arid steppe climate
changed into a humid one, and when the steppe vanished so did the
steppe's animals. In short, the moist Holocene climate was a
catastrophe for them. In the last few years, however, a growing
accumulation of radiocarbon dates of animal remains has been suggesting
a different story. It appears now that mammoths survived the
Pleistocene-Holocene shift. For the first 7000 years of the Holocene,
they persisted on Wrangell Island in the Arctic Ocean. Bison, horses,
and musk oxen also lived in the north of Siberia in the Holocene.
Horses and musk oxen lived there even up to historical times.
In Alaska, bison survived throughout the entire Holocene. They
disappeared only in the historical period at the hands of human
hunters. Alaskan native elders still tell stories that chronicle the
taste of bison meat. Another indication that climate change has had
little to do with the survival of bison is that in the past century,
bison were brought back to Alaska, and they have been breeding there
successfully. What's more, when musk oxen were reintroduced from the
coldest, driest islands of the Canadian Arctic to Alaska in the 20th
century, they immediately began to breed actively, even though the
climate in Alaska was warmer and wetter. The same thing happened
wherever musk oxen were reintroduced in Siberia. Even in the west
Norwegian climate, musk oxen have prospered.
The recent history of horses bolsters the case against climate change
as the factor that destroyed the mammoth ecosystem and its diversity of
large animals. In the Republic of Yakutia in northern Siberia, the
biomass of horses is greater than that of reindeer. Although horses are
classified as domesticated animals, in practice most of them are wild,
living without any aid from people. Evidently, they are suited to the
present climate.
Yet, these great herbivores disappeared by the millions from northern
Siberia and elsewhere. As has happened elsewhere and at other times,
their vanishing coincides with the introduction by humans of new
hunting technology. In Australia, 46,000 years ago, when people first
arrived, 23 animal species vanished, all but one heavier than 45 kg
(about 100 pounds). In America, 12,000 years ago, hunters began using
small, sharp lances and arrowheads. After that, 70% of the large animal
species vanished. By the time people started recording their own
history, bison, aurochs, dziggetai (koulan), wild horses, saiga, and
many other herbivores had already been exterminated from the steppes
and prairies.
Out to Pasture
Just as the great herbivore herds disappeared at the end of the
Pleistocene, so did the northern grasslands that nurtured them. One
possible explanation for this is simply that the cold, arid climate of
the steppes changed into a humid one, turning the steppes into mossy
tundra. However, the Holocene climate shift was not unique. Similar
shifts occurred in previous interglacial periods, yet these did not
cause catastrophic landscape reconstructions.
During the last glacial, when mammoths still roamed on the steppes that
covered Europe, the annual precipitation there was 200 to 250 mm, and
January temperatures were in the range of -25º to 35ºC. Such climate
conditions are similar to those of present-day northeastern Siberia. By
many criteria, the present climate there is not humid, but rather is
characteristic of an arid steppe. According to all weather stations of
northeast Siberia, the annual radiation input is about twice what is
necessary to evaporate the annual precipitation. This only adds to the
mystery of why Siberia is no longer dominated by a grassy, steppe
landscape.
The physiological traits associated with Holocene vegetation partially
explain the vegetation changes that coincided with loss of the
Pleistocene megafauna. Plant transpiration accounts for most of the
water loss from landscapes, and high transpiration rates are associated
with more productive plants. Rates of water loss must therefore have
been high in the north when productive Pleistocene meadow and steppe
vegetation prevailed. As a result, vast amounts of water were sucked up
from the ground, resulting in dry conditions, while the plants
themselves sequestered nutrients to drive their own productivity.
Holocene vegetation, in contrast, is dominated by unproductive moss and
shrubs. This type of vegetation does not transpire enough moisture to
dry out the soil. Moss does not even have roots. This leads to wet
conditions conducive to the growth of mosses, which account for a
substantial proportion of the northern Siberian biomass.
Water-saturated soils inhibit decomposition of biomass and therefore
the availability of nutrients to support plant growth. What's more,
mosses insulate the ground efficiently--a 20-cm layer of moss prevents
the underlying frozen soil from thawing. This also has the effect of
sequestering nutrients and preventing their cycling through the
ecosystem. All of these factors indicate that moss communities, once
they are in place, create and sustain their own environment and do not
depend so much on particular climate conditions.
They are quite vulnerable to physical disturbance, however, and this is
where their ecological connection to herbivores comes in.
The Future of the Past
When mosses are destroyed on loess soils, the site becomes overgrown
with grasses within 1 to 2 years. The grasses then dry out the soil
through their high transpiration rates, creating a steppe-like
ecosystem. But when herbivore populations are low, grass productivity
begins to decrease within a few years, because grass litter accumulates
on the soil surface, shading and insulating the soil. In turn, soil
fertility declines. As a result, shrubs and mosses, which have lower
nutrient requirements than grasses, ultimately become dominant.
Pleistocene Park. This territory in the Republic of
Yakutia is roughly an even split of meadow, larch forest, and willow
shrubland. This Siberian region could become the venue for a
reconstituted ecosystem that vanished 10,000 years ago.
CREDIT: S. ZIMOV |
In the mammoth ecosystem, the collective behavior of millions of
competitive herbivores maintained the grasslands. In the winter, the
animals ate the grasses that grew the previous summer. All the while
they fueled plant productivity by fertilizing the soil with their
manure, and they trampled down moss and shrubs, preventing these plants
from gaining a foothold. It is my contention that the northern
grasslands would have remained viable in the Holocene had the great
herds of Pleistocene animals remained in place to maintain the
landscape.
In the southern steppes, the situation is different. There, the warmer
soil allows for more rapid decomposition of plant litter even in the
absence of herbivores. In the north today, the soil is too cold to
foster such decomposition, which means that the steppe ecosystem can be
stable there only with the help of herbivores that decompose organic
matter in their stomachs and that disturb mosses. Today's African
savannas, in which trees and shrubs have supplanted grasses in much the
same way that mossy tundra has supplanted grasses in Siberia,
demonstrate this principle. These savannas would disappear without
large herbivores, which are present there in large numbers. The large
numbers of animals on African savannas amaze many people. However,
similar animal densities exist in northern and middle latitudes. For
example, at Elk Island National Park in Canada, about 60 bison browse
on each square kilometer of grassland. The animal is much bigger than
the gnus and zebra of Africa. Forests in the park are preserved only by
strongly controlling the number of animals.
This is why I believe that the changing climate of the Holocene would
have had little bearing on the survival of the mammoth ecosystem. In
some places, such as sandy and stony ground, trees and shrubs would
have appeared. And that might have caused changes in the relative
proportions of horses and moose. But overall, if climate were the only
controlling factor, the total pasture productivity and the number of
herbivores should have increased in the Holocene. Support for this view
comes from the climate history that is chronicled in the Greenland ice
sheet. It shows a sharp warming and dramatic increase of precipitation
~14,700 years ago, leading to conditions that resemble the present
climate. Even so, in the north of Siberia, mammoth populations soared
at this time.
This view means that the present Holocene climate of northern Siberia,
particularly near the present tree line, is likely just now to be
optimal for the mammoth ecosystem. If we accept the argument that the
pasture landscapes were destroyed because herbivore populations were
decimated by human hunting, then it stands to reason that those
landscapes can be reconstituted by the judicious return of appropriate
herbivore communities.
In northern Siberia, mainly in the Republic of Yakutia, plains that
once were covered by tens of meters of mammoth steppe soils now occupy
a million square kilometers. The climate of the territory is near
optimal for northern grassland ecosystems. Thus, in principle, the
ancient mammoth ecosystem could be restored there.
In Yakutia, we are trying to do just that. The government has adopted a
program to restore the republic's former biodiversity. One thrust of
this effort has been through the nonprofit organization of Pleistocene
Park--of which I am a founding member--on 160 km2
of Kolyma lowland. One-third of the territory is meadow, one-third is
forest, and one-third is willow shrubland. Today, many of the animals
of the mammoth ecosystem and grasses remain in northern Yakutia.
Reindeer, moose, Yakutian horses, recently reintroduced musk oxen,
hares, marmots, and ground squirrels forage for vegetation, and
predators, including wolves, bears, lynxes, wolverines, foxes, polar
foxes, and sables, prey on the herbivores. However, strong hunting
pressure has kept the overall number of animals low. Therefore, their
influence on vegetation is small. The first step for Pleistocene Park,
which we are just now initiating, is to gather the surviving megafauna
of the mammoth ecosystem (initially without predators) within the part
of the parkland that is rich in grassland. The second step will be to
increase the herbivore density sufficiently to influence the vegetation
and soil. As animal densities increase, the fenced boundary will be
expanded.
The most important phase of the program will be the reintroduction of
bison from Canada and subsequently, when the herbivores are
sufficiently abundant, the acclimatization of Siberian tigers. In many
regions of the Amur River basin, where this formidable predator
survives, January temperature is as low as -25º to -30ºC. The tigers'
survival there is limited more by poaching and herbivore density than
by climate. Scientifically, Pleistocene Park is important because it
directly tests the role of large herbivores in creating and maintaining
grassland ecosystems, something that can only be surmised but not
proven from the paleorecord.
There is more than just scientific discovery at stake here. Northern
Siberia will influence the character of global climate change. If
greenhouse gas-induced warming continues, the permafrost will melt. At
present, the frozen soils lock up a vast store of organic carbon. With
an average carbon content of 2.5%, the soil of the mammoth ecosystem
harbors about 500 gigatons of carbon, 2.5 times that of all rainforests
combined. Moreover, this carbon is the relatively labile product of
plant roots that were incorporated from productive steppe vegetation
during the Pleistocene. As soon as the ice melts and the soil thaws,
microbes will begin converting this long-sequestered soil carbon into
carbon dioxide under aerobic conditions or into methane under anaerobic
conditions. The release of these gases will only exacerbate and
accelerate the greenhouse effect.
Preventing this scenario from happening could be facilitated by
restoring Pleistocene-like conditions in which grasses and their root
systems stabilize the soil. The albedo--or ability to reflect incoming
sunlight skyward--of such ecosystems is high, so warming from solar
radiation also is reduced. And with lots of herbivores present, much of
the wintertime snow would be trampled, exposing the ground to colder
temperatures that prevent ice from melting. All of this suggests that
reconstructed grassland ecosystems, such as the ones we are working on
in Pleistocene Park, could prevent permafrost from thawing and thereby
mitigate some negative consequences of climate warming.
10.1126/science.1113442
The author is at Northeast Scientific Station, Pacific Institute for
Geography (Far East Branch), Russian Academy of Sciences, Post Office
Box 18, Cherskii, Republic of Sakha 678830, Russia. We thank G. Zimova,
F. S. Chapin, and M. Chapin for their helpful comments.
