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Focus: Endgame
MEGAFAUNA
EXTINCTION
Blitzkrieg overkill by humans or a
gradual, climate-induced die-out?
By Madeline Ross
“A few thousand
years after humans
first crossed the Bering Strait, 72% of the
megafauna genera in
North America were
gone.”
36 Harvard Science Review • spring 2007
B
efore the North American continent was populated with humans,
it was populated with super-sized versions of today’s familiar mammals, a
group of gigantic animals known as
megafauna. The mastodons, sabertooths, and ground sloths of the Pleistocene period existed on this continent
for hundreds of thousands of years.
However, a few thousand years after
humans first crossed the Bering Strait,
72% of the megafauna genera in North
America were gone (1). This sort of
massive extinction occurred not just
in North America, but in every continent except Antarctica. The age of
the mammoths came to an abrupt end,
and scientists today are still involved in
a heated debate over what caused the
death of all the giants. Much of the
discourse centers upon one question:
what role, if any, did humans play in
the extinction of megafauna?
There are two primary theories in
the debate: climate change and overkill. Both of these premises are relatively straightforward: either humans
killed off the animals or the changing
weather and environment did. But as
more information is uncovered, the
situation becomes less clear. Instead of
resolving the debate, the new evidence
is contradictory and difficult to interpret. Controversial sites in Australia
have caused many scientists to argue
for some combination or variation
upon the two main hypotheses. Some
scientists even support completely
separate theories, such as the hyperdisease hypothesis, which posits that
there was a super-virus that decimated
the large mammals (2). Several currently
existing species still fulfill the definition
of megafauna. Even the average adult
human weighs more than the cut-off
point of 44 kilograms, making us, along
with the lions and hippos, survivors
of the Pleistocene megafauna. But
many of the animals now extinct were
much, much larger. The ground sloth,
a cousin of the tree-hugging sloths living today, could weigh as much as 5000
kilograms (3).
Followers of the overkill hypothesis
can be divided into two main camps
– those who believe that the main cause
of extinction was a blitzkrieg killing of
the naïve, slow-moving megafauna
Focus: Endgame
Figure 1. Many of the megafauna surviving today live in areas of Africa and
Eurasia where humans and megafauna
co-evolved, supporting the overkill
theory.
Credit: www.nature.com
and those who argue for second-order
predation theory. Supporters of both
theories point to the fact that slowbreeding survivors (most megafauna
were slow-breeding) tended to exist
in places that were hard for humans
to reach – trees, mountaintops, or the
deep forest (1). They also highlight
comparisons between island-based
and continental extinctions. Animals in
isolated, island communities frequently
survived after those on the continent,
until humans arrived on the island. For
example, there is a distinct difference in
the dates of sloth extinctions between
North America, South America and the
West Indies. In the West Indies, humans
did not arrive until the middle of the
Holocene era, well after the hypothesized climate change. Sloths in the West
Indies are thought to have survived well
past their continental counterparts, until the Holocene era (3). The blitzkrieg
hypothesis is further supported by the
higher prevalence of surviving megafauna in Africa and Eurasia, areas where
humans and megafauna co-evolved.
Rhinoceroses, elephants and tigers are
all living remnants of the Pleistocene
world. However, in spite of co-evolution, 36% of the Eurasian megafauna
still disappeared (1).
The other variation on the overkill
hypothesis, second-order predation,
argues that the human policy of killing large carnivores ultimately (and
paradoxically) caused the extinction
of the megaherbivores. Once humans
had killed the predator species, the
herbivore populations began to soar
and then crash. The environment was
unable to support the population boom
and many of the prey populations
starved. The slow reproductive rate of
the larger mammals would have made
it harder for them to recover from the
population crashes (4). This theory
directly conflicts with another human
impact hypothesis arguing that a large
factor in the extinction of the North
American large carnivores, including
the giant short-faced bear, the dire wolf,
and the saber-tooth, was the demise of
the megaherbivore (5).
Herein lies the dilemma of the extinction debate: for every theory, there is
another theory diametrically opposed
to the first, as well as a third theory that
argues for some combination of the
two, such as the human impact hypotheses. These theories, more subtle than
that of the blitzkrieg overkill, argue that
human arrival disrupted the balance of
the existing environments.
How might humans have changed
the environment? One of the ways
that climate change is known to affect
animals is by altering plant life in the
environment (1). Some scientists argue
that humans may have changed the lo-
“Some scientists
argue that
humans may
have changed the
local vegetation,
permanently
altering the
ecosystem around
them as they
settled.”
cal vegetation, permanently altering the
ecosystem around them as they settled
in. Early humans are thought to have
frequently built fires, eventually leading
to the change of the landscape from
one of trees and grasslands to one of
desert scrub in locales like Australia
(6). This eventual desertification of
the continent could have had dire
consequences for the resident animals.
The large, slow-breeding, plant-eating
mammals would have been especially
vulnerable to such change.
However, new evidence in Northeast
Australia and other locales has led some
scientists to other conclusions. In these
areas, extinction seems to have occurred simultaneously with a sudden,
spring 2007 • Harvard Science Review 37
Focus: Endgame
climate-caused change of environment. This has caused some scientists
to completely dismiss the possibility
of a blitzkrieg, sudden human overkill
hypothesis, at least in Australia (1).
The Australian continent remains a key
piece in the megafauna puzzle. A 2001
study concluded that the late Pleistocene disappearance of large animals in
Australia occurred quickly and suddenly
throughout the continent. Moreover, it
claimed that this abrupt disappearance
closely coincided with the arrival of
humans in Australia, thereby seemingly
supporting the overkill hypothesis (7).
However, multiple scientists soon chal-
lenged the very nature of the study,
as it had neglected to consider several
sites as evidence, specifically the site of
Cuddie Springs (8).
Cuddie Springs is located in semiarid northern Australia. Several thousand years ago, Cuddie Springs was
an ephemeral lake, a source of water
for all of the nearby thirsty mammals.
Megafauna fossils were first discovered
at Cuddie Springs in the 1870s, when
a well was built in the claypen. What
makes Cuddie Springs so dangerous
to the hypothesis of the 2001 study
is that evidence found there suggests
that humans and megafauna co-existed
in Australia for over 10,000 years (9).
Similar evidence in Southeast Asia has
previously been used to argue against
“Evidence found
[in Cuddie Springs]
suggests that
humans and
megafauna coexisted in Australia
for over 10,000
years.”
Human arrival on
Australian
continent
46
56
.5
ka
ka
HABITAT DISRUPTION TIMELINE:
Australian
megafauna
extinct
Human burning disrupts ecosystem
ka
ka
30
36
41
ka
OVERKILL TIMELINE:
Australian
megafauna
extinct
Human arrival on
Australian continent
Climate change and resulting ecological
change cause species extinction
Human
arrival on
Australian
continent
ka
Virtually all
Most
megafauna megafauna
extinct
extinct
Figure 2. Timelines of Australian megafauna extinction based on various theories. (12)
38 Harvard Science Review • spring 2007
16
ka
30
ka
45
13
0
ka
CLIMATE AND ECOLOGICAL CHANGE TIMELINE:
the overkill hypothesis and in
favor of climate change (10).
Some scientists have completely discarded the overkill
versus climate change debate,
developing interesting new
theories that ignore both. One
of these is the hyperdisease
hypothesis, which argues that
humans brought a virulent
disease to North America
when they crossed the Bering
Strait. This disease could have
been dormant either in their
own bodies or in those of the
domesticated dogs that traveled with them (11). However,
many claim that a virus sufficiently strong and infectious
to decimate entire populations
is incredibly rare. In addition, while earlier computer
simulations supported the
hyperdisease theory, a recent
study questions the ability of
such a disease ability to cause
a size-biased extinction. An
examination of the spread of
West Nile Virus (also labeled a
“hyperdisease”) among North
American birds argues that it is
highly unlikely that a disease so
deadly would not also affect
smaller mammals (2). However, in the early 1990s domestic
dogs living on the outskirts of
Focus: Endgame
the Serengeti National
Park transmitted canine distemper to the
lions within the park,
killing 1,000 lions. The
disease was then transmitted to a reserve in
Kenya where it infected not just lions, but
also leopards, hyenas,
and foxes (11). While
in this instance the
virus did not become
pandemic to any of the
species infected, it did
have a sizable impact
on the populations.
The true cause of
the giant mammal extinctions remains
a mystery to all, buried under all of the
varied and contradictory theories. Scientists are pushing ahead the only way
they can: with more research. Many of
the field’s leading experts bemoan the
lack of data that causes gaping holes
in what we know about the disappearance of the megafauna. Certain areas
of Southeast Asia and South America
are largely unexplored (2). Methods
and definitions vary across continents
and disciplines; for example, Africa
and Eurasia have a definition of what
constitutes evidence of human hunting
of megafauna that differs vastly from
that of North America (1).
Scientists are not only attempting to
find more data in the same ways (i.e.
locate more “kill sites” to investigate
archaeologically), but are also using new
technology to help clarify the situation.
Many scientists look to computer simulations to solve the puzzle. Some, like
Geobiologist Elin Whitney-Smith, have
developed dynamic computer models
that combine the various theories and
test them with simulations (4).
The advent of these new tools has
also benefited the study of the interaction between climate change and
extinctions. The study of fungal spores
in dung has helped to date the hitherto
unknown survival of many species,
particularly the sloths. In addition, the
Figure 2.
Left:
Artist’s
rendering of
Procoptodon,
a 3 meter high
short-faced kangaroo native to
Australia.
Right:
Model of the
mammoth, one
of the most
well-known
megafauna.
Credit: www.nature.com
study of pollen records can help track
the change in floral vegetation, which
can then be compared with changes in
large animal populations (1). However,
so far such studies have been inconclusive. While changes in floral vegetation
and megafauna extinction do coincide
in some areas, in others they do not.
In recent years, the question of what
killed the megafauna has taken on extra
significance as global warming has entered the public consciousness. Could
what happened to the giants happen to
us? The fact that many of the theories
attribute some part of the extinctions
to climate change seems ominous. As
long as the cause of the mammoths’
deaths is unknown, it is impossible to
tell whether humans are threatened
by similar forces. After all we, too, are
megafauna. Perhaps, buried somewhere
in the lake basins of Australia or the
unexplored caves of South America lies
the secret to our own survival.
—Madeline Ross, ’08, is an English
concentrator in Kirkland House.
References
1.Barnosky et al. “Assessing the Causes of Late
Pleistocene Extinctions on the Continents.” Science
306 (1 October 2004): 70-75.
2. Lyons et al. “Was a ‘hyperdisease’ responsible for
the late Pleistocene megafaunal extinction?” Ecology
Letters 7 (2004): 859-868.
3. Steadman et al. “Asynchronous extinction of
late Quaternary sloths on continents and islands.”
Proceedings of the National Academy of Sciences
of the United States of America 102:33 (August 16,
2005): 11763-11768.
4. Whitney-Smith, Elin. “Late Pleistocene extinctions
through second-order predation.” Settlement of the
American Continents: A multidisciplinary approach
to human biogeography. Tuscon: University Arizona
Press, 2004. 177-189.
5. Koch and Barnosky. “Late Quaternary Extinctions:
State of the Debate.” Annual Review of Ecology,
Evolution, and Systematics. 37 (2006): 215-250.
6. Miller et al. “Sensitivity of the Australian Monsoon
to insolation and and vegetation: Implications for
human impact on continental moisture balance.”
Geology 33:1 (January 2005): 65-68.
7. Roberts et al. “New ages for the last Australian
megafauna: Continent-wide extinction about 46,000
years ago.” Science 292 (June 8 2001): 1888-1892.
8. Johnson, Christopher N. “What can the data on
late survival of Australian megafauna tell us about
the cause of their extinction?” Quaternary Science
Review 24 (2005): 2167-2172.
9. Wroe and Field. “A review of the evidence for a
human role in the extinction of Australian megafauna
and an alternative interpretation.” Quaternary Science Review 25 (2006) 2692-2703.
10. Louys et al. “Characteristics of Pleistocene
megafauna extinctions in Southeast Asia.” Palaeogeography, Palaeoclimatology Palaeoecology 243
(2007) 152-173.
11. Fiedel, Stuart J. “Man’s best friend – mammoth’s
worst enemy? A speculative essay on the role of dogs
in Paleoindian colonization and megafaunal extinction.” World Archaeology 37(1): 11-25.
spring 2007 • Harvard Science Review 39