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Ecology a nd Evolution . 201 8 ;1–1 4 .
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1
www.ecolevol.org
1 | INTRODUCTION
Research on human impacts on the environment, whether study-
ing greenhouse gas emissions, overharvesting fisheries, or
deforestation of rain forests, has grown significantly over the last
40 years. The growing number of new journals focusing on the
Anthropocene (e.g., Anthropocene, Anthropocene Review, Elementa)
reflects this increase in interest. An important area of discussion
Received:4December2017
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Revised:10May2018
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Accepted:20June2018
DOI: 10.100 2/ece3.43 93
ORIGINAL RESEARCH
The overkill model and its impact on environmental research
Lisa Nagaoka1 | Torben Rick2 | Steve Wolverton1
This is an op en access article under t he terms of t he Creat ive Commons Attr ibutio n License , which pe rmits u se, dist ributi on and rep roduc tion in any m edium,
provide d the orig inal work is proper ly cited.
© 2018 The Aut hors. Ecology an d Evolutionpu blishedbyJohnWiley&SonsLtd.
1Depar tment of G eography and the
Environment, University of North Texas,
De nton , Texas
2Depar tmentofA nthrop ology,Smithsonian
Institution,NationalMuseumofNatural
Histor y,Washington,Dist rictofColumbia
Correspondence
Lisa Nag aoka, D epartment of Ge ography
and the Env ironment, Unive rsity of North
Tex as , Dent on , Texa s.
Email: lisa.nagaoka@unt.edu
Abstract
Research on human- environment interactions that informs ecological practices and
guides conservation and restoration has become increasingly interdisciplinary over
the last few decades. Fueled in part by the debate over defining a start date for the
Anthropocene, historical disciplines like archeology, paleontology, geology, and his-
tory are playing an important role in understanding long- term anthropogenic impacts
on the planet. Pleistocene overkill, the notion that humans overhunted megafauna
near the end of the Pleistocene in the Americas, Australia, and beyond, is used as
prime example of the impact that humans can have on the planet. However, the im-
portance of the overkill model for explaining human–environment interactions and
anthropogenic impacts appears to differ across disciplines. There is still considerable
debate, particularly within archeology, about the extent to which people may have
been the cause of these extinctions. To evaluate how different disciplines interpret
and use the overkill model, we conducted a citation analysis of selected works of the
mainproponentoftheoverkillmodel,PaulMartin.Weexaminedtheideasandargu-
mentsforwhichMartin’soverkillpublicationswerecitedandhowtheydifferedbe-
tween archeologist s and ecologists. A rcheologists cite over kill as one in a combination
of causal mechanisms for the extinctions. In contrast, ecologists are more likely to
accept that humans caused the extinctions. Aspects of the overkill argument are also
treated as established ecological processes. For some ecologists, overkill provides an
analog for modern- day human impacts and supports the argument that humans have
“always” been somewhat selfish overconsumers. The Pleistocene rewilding and de-
extinction movements are built upon these perspectives. The use of overkill in eco-
logical publications suggests that despite increasing interdisciplinarity, communication
with disciplines outside of ecology is not always reciprocal or even.
KEYWORDS
citation analysis, communication, conservation, human impacts, interdisciplinarity, megafauna
extinctions, Pleistocene overkill
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NAGAOKA et Al .
revolves ar ound how far back hu mans have been having a si gnificant
impactontheenvironment(Boivinetal.,2016;Braje&Erlandson,
2013a;Smith& Zeder,2013). Itisthe subjectof notonlydefining
the boundaries of the Anthropocene and other concepts such as
theSix thExtinction,butunderstandingthenatureoftherelation-
ship between humans and the environment . It is into this discussion
of how long people have been having a significant impact on the
environment that the overkill explanation for Pleistocene megafau-
nal extinctions plays a role.
Inthe1960s,PaulMartin(1958,1967a),ageoscientistandpa-
leobiologist, developed the overkill hypothesis, in which human
hunting was proposed to have caused the extinction of the mega-
fauna that roamed North America during the Pleistocene. During
thelast50years,thehypothesishasbeenextended toincludeall
anthropogenic factors and has been applied to human colonization
virtuallyanywhereintheworldatanyperiod(Burney&Flannery,
2005; Martin,1984).Recently,overkill (Pleistocene or other wise)
has been used as a prime example in ecological or conser vation
studies stating that humans have profound impact s on the en-
vironment and have been doing so for millennia (Donlan, 2007;
Donlan etal., 2006; Sherkow & Greely, 2013; Svenning etal.,
2016). However, among researchers studying the extinction of
Pleistocene megafauna (many archeologists and paleobiologists),
the cause of the extinctions and the validity of overkill as an expla-
nation are still being debated. Thus, what is a subject of debate (the
cause of the terminal Pleistocene megafauna extinctions) to some
is being used as a prime example of anthropogenic environmental
destructionbyothers.So,howisitthatthesedifferentcommuni-
ties of researchers view the role of overkill in megafaunal extinc-
tions so differently?
In this paper, we summarize the overkill hypothesis and the
debate about the cause of the late Pleistocene megafaunal extinc-
tions. Wethendelveinto problemsof cross-disciplinary commu-
nication by conducting a citation analysis of cited works of Paul
Martin,theauthoroftheoverkillhypothesis.Wedocumenthow
overkill is interpreted and used differently by archeologist s and
ecologists. For many ecologists, overkill holds significant meaning
for the relationship between humans and the environment that
has consequencesfor conservation. While a number of import-
ant studies have been conducted by archeologists, ecologists,
and Quaternary scientists since Martin’s research, including re-
cent studies by ecologists that support climate/multidisciplinary
models(e.g.,DiFebbraroetal.,2017;Lima-Ribeiro&Diniz-Filho,
2013, 2017; Nogues-Bravo, Rodiguez, Hortal, Batra, & Araujo,
2008) and others that support human impact s (Bartlett et al.,
2016;Sandom,Faurby,Sandel,&Svenning,2014),ourfocushere
is not to review that extensive literature, but instead is to focus
on interdisciplinary communication, particularly through citation
oftheseminalworksonoverkillbyMar tin. Ifenvironmentaland
anthropological scientists are to study the Anthropocene to-
gether, researchers face a challenge to improve interdisciplinary
communication.
2 | THE OVERKILL HYPOTHESIS
DISSECTED
By the end of the Pleistocene, a suite of 37 genera of large- bodied
mammalsbecameextinc tinNorthAmerica(Grayson,2015;Meltzer,
2015). There a re two main competin g hypotheses to explain the
extinction of these megafauna that are based on the timing of the
extinctions and either the arrival of people to the Americas or cli-
mate change at the end of the Pleistocene. Research evaluating
these hypotheses has involved investigators from archeology to the
geosciencestoevolutionarybiologyandecology(Koch&Barnosky,
2006;Meltzer,2015).Untilthe1960s,theextinctionswereprimar-
ily a paleontological subject of research, believed to be caused by
warming climate that occurred during deglaciation in the transi-
tion from the late Pleistocene to early Holocene. There was little
evidence that people interacted with megafauna let alone that they
lived in the same places at the same time. However, with the advent
of radiocarbon dating, the arrival of people in North America was
documented back to the Late Pleistocene (Haynes, 1964). Thus, a
temporal association was est ablished between people and mega-
fauna. M artin arg ued that if pe ople were pre sent in the Am ericas
alongside the megafauna, then they could have been a factor in their
extinction. Asan alternative explanation to climatechange,Martin
(1958,1967a,1973) proposed the overkill hypothesis in which hu-
mans hunted the megafauna to extinction.
While the t iming of both clim ate change and hum an coloniza-
tion overlaps with megafaunal extinction, the mechanisms for how
climate change was able to cause extinction in this context were un-
clear.In particular, Martin (1967a)questioned why megafaunahad
survived multiple interglacial periods during the Pleistocene only to
go extinct at the end of the last glacial period. On the other hand,
with the rise of environmentalism in the 1960s, the mechanism for
Martin’s overkill model was intuitive and self-evident (Grayson,
2001:41;Grayson& Meltzer,2003:590).Itwaseasytoconceptual-
ize how people could have caused an extinction event because the
impact s of (and protests against) human- caused environmental deg-
radationwereonthenightlynews.Bythe1980s,Martin(1984)had
expanded the overkill model beyond North American Pleistocene
extinctions to explain mass extinc tions globally as a function of
human colonization: wherever peoplego,speciesgo ex tinct.Since
then, the model has grown considerably outside of archeology and
paleontology and is often used as evidence for the harm that people
can perpetrate on the environment.
The mechanism for how people were able to cause the extinc-
tions thr ough hunting makes s everal key assumpt ions. When the
argument is teased apart, it is easier to evaluate whether or not
overkill adequately explains the extinctions. The first two assump-
tions use the “island analogy” (Nagaoka, 2012). First, the mechanisms
for extinction of continental megafauna are similar to those that impact
island fauna. In his explanation for overkill, Martin (1967a, 1984,
1990) described many prehistoric and historic examples of extinc-
tion of island species following human colonization in places such
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NAGAOK A et Al.
as Madagascar, New Zealand, Hawaii, and other Pacificislands as
supportfortheideaofoverkill(seealsoSteadman&Martin,2003).
Island fauna often evolves in the context of low predation pressure
resulting in traits such as flightlessness and ground- nesting in birds,
and naïve behavior in general. These traits along with high ende-
mism and small populations make island species more vulnerable
to predation and environmental perturbations, and thus extinction.
While it is widelyrecognized thatthecircumstances for islandex-
tinctions can dif fer from those on continents, these island examples
demonstrated that people could and did cause extinctions, which
planted the seeds for the process of anthropogenic extinctions in
other contexts.
To bolster the analogy between naïve island fauna and continen-
tal Pleis tocene mega fauna, Mar tin develop ed the secon d assump-
tion: continental megafauna were vulnerable to extinction like island
fauna because humans are superpredators. Continental megafauna
coexisted with a large predator guild and thus had evolved a suite of
predator defenses. However, if humans were hyper- efficient preda-
tors, then megafauna could be naïve to their specific type of preda-
tion. People were so ef ficient at hunting that the megafauna went
extinct before they could develop an appropriate predator response
(Martin,1973).Indeed,theBlitzkriegversion ofoverkill haspeople
hunting me gafauna in a w ave across Nor th Ameri ca (Mosiman n &
Martin,1975).Thisassumptionisoftenacceptedasfact.Neitherthe
degree of human hunting efficiency nor the absence of predator re-
sponse has yet to be evaluated or demonstrated.
A third assumption relates to the empirical requirements of
the model. Archeological data are particularly important for eval-
uating the overkill hypothesis because the test implications are
not just that people coexisted with the megafauna, but that they
directly interacted with the megafauna in such a way as to cause
extinction. Thus, stone tools embedded in megafauna bones reflect
hunting, cut marks reflect butchering, and (potentially) burnt bone
suggests cooking. Empirically, however, there is little archeological
evidence for these types of direct association between people and
megafauna, let alone that human predation had a significant im-
pact on megafaunal populations. The megafauna that humans are
directly associated with are limited to five (mammoths, mastodons,
gomphotheres, camels, horses) rather than all 37 genera, with mam-
mothasthemostcommontaxon(Grayson&Meltzer,2002;Melt zer,
2015).Andonlya small numberofsites, 15–26(theveracityofthe
association is debated among archeologists), have been identified as
showing evidence for a direct association between stone artifacts
and remai ns of extinc t megafauna (e.g ., Meltzer, 2015; Surove ll &
Waguespack,2008).
The paucity of archeological evidence for interaction between
people and megafauna has been called the “associational critique”
(Grayson,1984a;Meltzer,1986). But ithasbeen deftlyhandled by
Martin(1973,1984)andothers(Fiedel&Haynes, 2004;Surovell&
Grund, 2012), who assume that there is a small sample of sites with
evidence of association only because the extinction process was so
rapid that the remains were not buried and thus did not preserve. The
absence of evidence, specifically the absence of association, is used
as evidence for overkill. Requiring evidence of association is consid-
eredtoostringentacriteriontoexpectforancientdeposits(Surovell
&Grund,2012).
Critics have countered this explanation in several ways. Arguing
that the “absence of evidence is evidence” is not a scientific means
to evaluate a hypothesis. If there is a paucity of data, then other
alternative means should be found to test the hypothesis. Even if
the absence of association between people and megafauna was a
valid measure, it could be used to both suppor t both the overkill and
climate hypotheses. If climate change was the major cause of mega-
faunal ex tinction, then a paucity of sites with association would not
contradict expectations. However, it is more important for overkill
to demonstrate that the lack of sites is a result of poor preservation
of sites and remains. Interestingly, there are many paleontological
sites from the Late Pleistocenewith mammoth (Agenbroad, 2005;
Widga etal ., 2017) and other ext inct megafaun a (Meltzer, 2015).
The higher proportion of remains in paleontological contexts com-
pared to archeological ones suggests that megafaunal mortality may
be better explained by natural rather than anthropogenic causes.
The alternative argument is that preservation in archeological con-
texts is less likely than in paleontologic al ones. But this has not been
demonstrated.
Whatispart icularl ys tar tlingabou ta dv oc atingthatasso ci at io n
should not be a requirement for evaluating overkill is that this is
a foundational concept for historical disciplines, such as archeol-
ogy, geology, and paleontology. Association is used to argue that
spatial relationships between fossils and artifacts within depos-
its reflect past events and behaviors. Thus, to argue that demon-
strating association is not necessary or that it is too onerous of a
requirement is to argue that these disciplines are not necessary
for understanding these extinc tions. This is unfortunate given that
archeology is the only one of these three historical disciplines that
can provide evidence of direct interaction between humans and
megafauna.
When overk ill was introduced , the model appea red to have
a clear mechanism for how megafaunal extinction occurred.
However, the reality is that the argument uses a series of untested
assertions about human–environment interactions. Thus, the best
evidence for overkill is the temporal association between mega-
faunal extinctions and human colonization. Unfortunately, the
extinctions also co- occur with climate change at the end of the
Pleistocene. Further compounding the problem is that archeology
over the last few decades has continued to demonstrate that many
of the earliest peoples in the Americas had broad spectrum diets
focused on small game, aquatic resources, and a variety of foods
thatwerefarmoreabundantthanmegafauna(Cannon&Meltzer,
2004,2008;Dillehayetal.,2017;Erlandsonetal.,2011).Similarly,
other studies demonstrate that many megafauna species were
extin ct prior to huma n arrival (Bo ulanger & Lyman, 2014; Lima -
Ribeiro&Diniz-Filho,2013).Givent hatcausesforthePleistoce ne
extinctions are unresolved, it is interesting to see that the overkill
model features prominently in the ecological and conservation
literature.
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NAGAOKA et Al .
3 | DIFFERENT DISCIPLINES, DIFFERENT
INTERPRETATIONS
The use and relevance of ove rkill as the cause of t he Pleistocene ex-
tinctionsvarieswithinandbetweendisciplines.Withinarcheology,
the literature on overkill has become polarized between perspec-
tives of proponents and critics of overkill (e.g.,Fiedel & Haynes,
2004;Grayson & Melt zer,20 03, 2004). Thus,itmayappear that
there is a debate for or against overkill. However, the average ar-
cheologist is not representedin such black and whiteterms. We
surveyed archeologists about what killed the megafauna during a
poster sessionat the annualmeetingofthe SocietyforAmerican
Archaeology in 2012 (n = 91). Eighty- two percent believed that the
extinctions were caused by multiple variables with climate change
as the only single cause identified (Figure 1). In a separate but simi-
lar survey of 112 archeologists, 63% of archeologists identified a
combinat ion of factor s caused t he extinc tions (W heat, 2012). In
our survey, respondents who believed there were multiple causes
for the extinctions were asked to identif y which causes were in-
volved in the extinctions. Most respondents identified climate
change more often as one of the causes, with human impacts
either directly through hunting or indirectly through landscape
change as the other factor (Figure 2). To archeologists, overkill is
not the dominant explanation for the extinctions.
Yet, outside of archeology, overkill as the prime mover for the
extinctions seems to have taken on a different trajectory. As arche-
ologist s, it is surprising to encounter publications in which overkill
explains megafaunal extinctions and is used as an example of human
impact s in general. For example, a recent popular science book and
New York Times 10 Best Books of 2014 use the overkill model as
follows:
If…people were to blame [for the extinctions] – and
it seems increasingly likely that they were – then the
import is almost disturbing. It would mean that the
current extinction event began all the way back in the
middle of the last ice age. It would mean that man was
a killer – to use the term of ar t an “overkiller” – pretty
much right from the st art
Kolbert (2014: 229–230)
FIGURE1 Archeologists’responses
to the prompt, “The main cause of
megafaunal extinctions in North America
is” (n = 91)
FIGURE2 The causes for megafaunal
extinction identified as playing a role
by archeologists who believe that the
extinctions were multicausal
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NAGAOK A et Al.
Whilethisexcerptwaswrittenbyasciencewriter,thesectionafter
this quote d escribes Pa ul Martin’s overk ill model. Th e lack of direc t
archeological evidence is never discussed.
To evaluate how the overkill literature is being used dif ferently
by different communities of researchers, we conducted a citation
analysis of j ournal arti cles that cite four of P aul Martin’s pub lica-
tions on overkill. His 1967 book chapter, “Prehistoric overkill” and
his 1973 Science article “The discovery of America,” are the first full
descriptions of the overkill model. In 1984, he coedited the book,
Quaternary Extinctions, with Richard Klein, which reviews extinc-
tionsacrossdifferentregions.Martinalsoauthoredachapterinthe
book entitled “Pleistocene overkill: The global model” that brings
together his perspective on overkill as the cause for megafaunal ex-
tinctions worldwide.
Weused thecitedreference searchinThompsonReuters’Web
of Science d atabase to fin d articles t hat cited these f our publica-
tions. The articles spanned from 2015 whentheanalysis was orig-
inally done and 1995, the earliest extent of the Web of Science
database at the time. We then categorized the publications into
groups—archeology, Quaternary, ecology, and other. The other cat-
egory consisted of publications in fields such as philosophy, law, or
sociology. For this study, we focus on archeological, Quaternary, and
ecological publications. They differ in the subject matter and time
depth. Archeological publications were those written by archeolo-
gists on human prehistory or paleoecology. Quaternary publications
represent paleontological, historical biogeography, or paleoecologi-
cal publications that generally focus on evolutionary processes re-
lated to a specific taxon. Ecological publications are neoecological
studies that study taxa in contemporary contexts or that presented
research related to conservation.
WefoundadifferenceinhowMartin’spublicationswerecitedin
these three areas of research. Authors in each research area tended
to cite different publications when referring to overkill (Table 1). Of
the three groups, Mar tin’spublications tendtobecited the mostin
Quaternary publications. Archeologist s cite Mar tin’s earlier publica-
tions, particularly his 1973 article, probably because it discusses the
relationship between human colonization and overkill. In contrast,
withinecologicalpublications,Mar tin’slaterworksarecited.
TounderstandhowMartin’spublicationswerebeingcitedinthe
different types of publications, we analyzed the text associated with
each cit ation. We evaluated t he citations on ly for the 1984 publi-
cations because they represent more recent thought and also have
more even coverage across all three categories of research. Of the
388 references, copies of 378 articles were obtained. Of those, 363
were categorized as archeological, Quaternary, or ecological. The re-
maining fifteen articles were in social sciences and humanities pub-
licatio ns. For each pu blication , the text ass ociated with t he Marti n
citation was recorded. Each use of the citation was then categorized
based on the claim it was used to support. Citation examples are pre-
sented below.
All three research areas cite Martin’s work as evidence that
either a large number of species went extinct at the end of the
Pleistocene or that the cause of the extinction is being debated
(Table 2). However, about one- third of the ecological publications,
or five or six times the archeological or Quaternar y publications, use
Martin’sworkasevidencetosupporttheclaimthathumansare di-
rectly responsible for the extinctions (i.e., human predation) or that
humans are capable of causing great damage to the environment,
including extinctions. For example:
“There may be a variet y of situations in nature, of
course, in which consumers or consumer popula-
tions are not controlled by predation. For instance,
before the late Pleistocene overkill of large mammals
in Australia and North and South America (Martin
&Klein, 1984),most of theearth’secosystemscon-
tained megaherbivore species whose adult members,
liketoday’s elephants, weretoolargetobekilled by
the largest predators.
SouléandTerborgh(1999:811–812)
TABLE1 ThepercentageofcitationsforfourofPaulMartin’spublicationsbypublicationtype
Reference No. citations % Archeology % Quaternary % Ecolog y % Other
Mar tin(1967a) 87 34 45 17 4
Martin(1973) 117 43 31 12 14
Martin(1984) 213 26 46 26 2
MartinandKlein(1984) 175 14 47 33 6
TABLE2 ThepercentageoftimesMartin(1984)andMartin&
Klein (1984) were cited within the three types of publications, and
the claim for which the publications were cited
Publication type
Archeological
(n = 74)
Quaternary
(n = 143)
Ecological
(n = 148)
Cause of
extinctions
debated
64.9 58.7 20.9
Large- scale
extinctions
occurred
10.8 21.0 23.0
Humans killed off
the megafauna
6.8 4.9 32.4
Extinctions linked
to human
colonization
8.1 7. 0 16.9
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NAGAOKA et Al .
In addition, a number of articles claim that there is “growing con-
sensus” or increasing or mounting evidence that humans caused
the ext inctions (e.g., B londel, 20 08; Kodric-B rown & Brown, 2007 ).
Interestingly,manyofthese articles citeMartin&Klein’s(1984)book
(Table 3), which is a general compendium about extinctions during
the Quaternary, including papers that suggest alternatives to overkill
(Grayson, 1984b; Kiltie, 1984). Authors are thus incorrectly citing the
book, p ossibly confus ing it with Mar tin’s chapter in th e book. Thus ,
while many of the publications in the archeological and Quaternary
categories suggest overkill as one potential explanation for megafau-
nal extinctions, a greater proportion of the ecological sample promotes
it as a likely cause and/or a well- founded process and cites either of
Martin’s(1984)publicationsasjustification.
Because overkill is more commonly used within the neoeco-
logical literature as an example of anthropogenic impacts, the as-
sumptions of the model are also being used as if they are confirmed
ecological processes. For example, a small percentage (7%) of the
ecological articles cite the publications to support the idea that hu-
mans are hyper-efficient predators rendering continental fauna in ef-
fect, naïve. But only one of the archeological or Quaternary articles
uses the publications for the same purpose. The following quotes
illustrate how the assumption about humans as superpredators has
been used:
The late Pleistocene invasion of the Americas by hu-
mans might be the most recent case of an introduced
predator exerting large impacts on continental prey
(Barnosky, Koch, Feranec, Wing, & Shabel, 2004);
once again, however, it is likely that human impact
was magnif ied by the naivete′ of New World p rey
toward this novel predator archet ype (Wilson 1992;
Martin,1984).
Cox and Lima (2006)
Rather than having a continent of fearless ani-
mals waiting to be killed by an advancing wave of
hunters (e.g., Flannery 2001), it is more likely that
human hunters posed unique threats, and that
while not entirely predator naïve, the hunted ani-
mals did not have a sufficient antipredator behavior
to cope with these unique threats.
Blumstein (2006)
Interestingly, in both of these cases, the authors have brought up
thenaivete’ of Pleistocene megafauna becauseitisanexception not
seen elsewhere that they need to explain. Alternatively, the authors
couldhavearguedthattheroleofnaivete’inmegafaunalex tinctionis
an untested assumption.
The assumption that fauna were naïve to human hunting at initial
contact has also had an impact on studies on the historical biogeog-
rap hyofAfr ica.Afewoftheecologic alarticlesciteMartin’sp ublica-
tions to argue that Africa maintained a high diversit y of large- bodied
mammals following the Pleistocene because the fauna had evolved
withhumansandthuswerenotnaïvetothem.Martinhimselfmade
this same claim in his 1967 article, Africa and Pleistocene overkill.
WebelievethatAfrica’suniquenessinrangecontrac-
tion is the r esult of a fundame ntal differe nce in the spa-
tial dynamics of the extinction forces in Africa. Human
evolution in Africa allowed species there to adapt to
coexist with humans(Martin,1984).However,ashu-
mans expanded their range out of Africa and into the
other regions of the world they encountered animals
that were naive to their abilities and suffered extinc-
tions(Diamond,1984;Mar tin,1984).
Channell&Lomolino(2000)
These authors are arguing that the difference in biodiversity
across continents is partly a result of the distribution of humans.
Specifically,itis argued thatpost-Pleistocenespecies diversity
in Africa is greater because the fauna coevolved with humans
and thus was adapted to their superpredatory skills (Faith, 2014;
Wroe,Field, Fullagar,&Jermin, 2004).Thus, the biodiversityof
Africa is due to the fact that hunter–gatherers have not been
able to hunt species to ex tinction as they have elsewhere. As dis-
cussed above, the superpredatory skills of humans are linked to
the analogy between islands and continents. Continent al extinc-
tions are similar to island extinctions if people are superpreda-
tors and megafauna were naïve to their predatory skills. But if the
extinctions involved any other predator–prey relationship, would
extinctions of small, endemic, island populations be studied to-
gether with extinctions of widespread continental populations?
Indeed, if we exclude all island examples, the causes for conti-
nental megafaunal extinctions are diverse, often multicausal, and
have limited evidence for overkill (Barnosky et al., 2004).
Another ecological process that overkill seems to demonstrate
is that human colonization of new lands leads to faunal extinction.
Remember that island extinctions following human colonization were
usedtoprovideamechanismforextinction.Martinextendedhuman
colonization as a causal factor from islands to virgin lands in general
in this 1984 book chapter, such that the coincident timing of people
and extinction is proof that humans had a negative impact on fauna.
Colonization and hunting by aboriginal humans played
a major role in the extinction of the Pleistocene mega-
fauna in North America and other par ts of the world
(Martin&Klein,1984;Owen-Smith1987).
BrownandMcDonald(1997)
TABLE3 Reference cited when supporting overkill in the
ecological literature
Citation %
Martin(1984) 52.1
MartinandKlein(1984) 47.9
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NAGAOK A et Al.
The idea that human colonization had detrimental impacts on
fauna even shows up as an important fact in guidelines proposed to
promote conservation literacy.
Impacts of human colonization in ancient times:
Human societies have a long histor y of causing ex-
tinctions and major changes in ecosystems. (1) In the
prehistoric(Martin&Klein,1984)andhistoric(Crosby
1993) past, arrival of humans to new areas led to ex-
tinctions of other species and large- scale changes in
natural communities.
Trombulaketal.(2004:1185)
Unfortunately, the underlying idea behind this process is not
just that human colonization causes extinctions but that humans as
a species are inherently destructive. In the conservation and neo-
ecology literature, this idea has been used two ways. First is to use
the relationship to set up the argument that humans have been det-
rimental to the environment; thus, ecological reparations are required.
This thinking has led to proposals such as Pleistocene rewilding and
de-extinction(Donlan,2007;Donlan etal., 2006;Sherkow&Greely,
2013;Svenningetal.,2016).Someproponentsofbothproposalsuse
overkill to argue that North American fauna is depauperate because
humans caused the mass extinctions at the end of the Pleistocene.
Thus, it is argued that it is our moral and ethical responsibilit y to re-
populate the landsc ape with descendants, close relatives, or clones of
themegafauna.Wenotethatmanyecologistshavebeencriticalofthe
overkill model and particularly of Pleistocene Rewilding (Fernández,
Navarro, & Pereira, 2017; Lima-Ribeiro & Diniz-Filho, 2013, 2014,
2017;McCauley,Hardesty-Moore, Halpern,Young, &Seddon,2017;
Nogués-Bravo, Simberloff, Rahbek, & Sanders, 2016; Richmond,
McEntee, Hijmans, & Brashares, 2010; Rubenstein & Rubenstein,
2016). However, the overall trend in the citation analysis and liter-
ature we reviewed is that overkill is more likely to be treated as the
explanation for the extinctions such that support for overkill can be
found even in the argument of some critics of rewilding (Oliveira-
Santos&Fernandez,2010;Perringetal.,2015).
4 | COMMUNICATION BREAKDOWN
Sowhydothese researchcommunitiesdifferintheirperspectives
on overkill and megafaunal extinctions? One explanation is that the
archeological literature, which discusses the empirical research on
the role of people in the extinctions, is less likely to be accessed by
researchers publishing in the ecological literature. Recent bibliomet-
ric and citation analyses appear to support limited interaction be-
tween these two groups. For example, Rosvall and Bergstrom (2011)
analyzed citations from over 9 million articles across nearly 8,000
journals to understand connectivity and information networks
among academics. They identified four major clusters of research.
The physical sciences and the life sciences form the two largest
clusters of research. The third cluster of ecology and earth sciences
includes ecology, conservation biology, and Quaternary research.
Social sc iences, into whic h archeolog y falls, form th e fourth clu s-
ter. Thus, ecologists and Quaternary scientists may be more likely to
readandciteoneanother’sresearchthantheyare toreadandcite
archeological journal articles.
If archeologists are publishing about megafaunal extinctions
only in archeologic al journals, then ecologists may be less likely to
encounter these articles. Donald Graysonand David Meltzer are
prominent critics of overkill whose work is published predominantly
in archeological journals (Grayson, 1984a,b, 2001, 2007; Grayson
&Meltzer,2002, 2003, 200 4,2015;Meltzer,1986,2015;seealso
Wroe, Fiel d, & Grayson , 2006). Whil e they are comm only cited in
publications about the Pleistocene ex tinctions in the archeologi-
cal and Quaternar y literature, they are rarely cited in the neoeco-
logical literature (Table 4). Thus, researchers outside of the social
sciences may have been less likely to encounter information on the
importance of association and the paucity of evidence for associa-
tion between megafauna and humans. However, none of the pub-
lications by archeologists that support overkill that are published
in broaderscientific journals (e.g., Faith & Surovell, 2009;Haynes,
2007,2013; Surovell&Waguespack, 2008;Surovell,Waguespack,
& Branti ngham, 2005) te nd to be cited either. This su ggests that
cross- disciplinary communication, particularly from archeology to
ecology, is limited. Researchers publishing in the neoecologic al liter-
aturemaynotrecognizearcheology’sroleinevaluatingoverkilland
may not look to the archeological literature as an impor tant source
of information.
One solution would be for archeologists to publish their findings
in ecological journals. Currently, the peer- reviewed ecological litera-
ture in which archeologists assert that there are issues with the over-
killhypothesisislimited(e.g.,Wolverton,2010).Butthisisnotfroma
lack of effort. In our experience, claims made by archeologists about
the data and underlying assumptions of overkill are downplayed by
some ecologists. Unfortunately, these issues are not being debated
in the ecological literature, but occur in discussions at conferences
and in reviews of grant proposals and publications (see Grayson and
Alroy, 2001 for a rare exception). Colleagues who are ecologists typ-
ically will cite papers by ecologists to question our claims. During the
peer review process for past research articles on this topic, we have
been often pointed toward ecological research that examines the
streng th of the correlation between the extinctions, climate change,
and human colonization at the regional or global scale (e.g., Bartlett
TABLE4 PercentageofpublicationscitingMartin(1984)that
also cite publications by Grayson
Archeology Quaternary Ecology
# of Grayson
citations
39 40 6
#ofMartin,
1984 citations
56 98 56
% Grayson
citations
69. 6 40.8 10.7
8
|
NAGAOKA et Al .
etal., 2016; Pre scott, Wi lliams, Balm ford, Gree n, & Manica , 2012;
Sandometal.,2014).Thesepublicationsarecitedduringpeerreview
as evidence that ecologists do not suppor t overkill or the idea that
humans alone were responsible for the extinctions.
Our rebuttal to these assertions is twofold. First, the nature and
source of the human impact data in these studies is rarely questioned.
Reviewers may be surprised to lear n that the data on the magnitude of
human impact used in many of these models is directly derived from
overkill. They often rely on the assumption that human colonization
causes extinctions to model human impact rather than on empiric al
data. Thus, the evidence for climate change is contrasted against as-
sertions about human impacts to evaluate the strength of the cor-
relation between each factor and extinctions. These models further
strengthen rather than detract from our argument that the tenets of
overkill are deeply embedded in ecology and conservation biology.
Second , we find it curiou s that some rese archers app ear reti-
cent to accept arguments and data from archaeologists, particularly
given that our field of expertise is studying the interaction and im-
pact of human actions on species and ecosystems across time and
space.Summarystudiesbypaleo-orneoecologistsarenotequiva-
lent to archeological studies that assess the quality of evidence for
association between megafauna and humans during the terminal
Pleistocene. As archeologists, we take for granted that archeologi-
cal data are necessary for evaluating the role that humans may have
played in the extinction of the megafauna. The presence of mega-
faunal remains in archeological sites is required to demonstrate that
people interacted with megafauna, not just that they coexisted on
the continent at the same time. Archeological data are also needed
to demonstrate the nature of those interac tions that people hunted,
butchered, and ate megafauna. These data are necessary to under-
stand the magnitude of impac t that humans had on megafaunal pop-
ulations. As was mentioned earlier, these types of data are in short
supply. Only five of the 37 genera of extinct megafauna in North
America have direct evidence of association with ver y few archeo-
logical sites that exhibit convincing evidence that people commonly
andeffectivelyhuntedthefivegenera.Incontrast,over95%ofthe
sites dating to the early period of human occupation and contain-
ingremains ofthose five extinct taxaare paleontological(Meltzer,
2015;Table1).Thus,thereisanextensiverecordofnonassociation
between the extinct megafauna and people. In addition, the record
on the timing of the extinctions is variable indicating that some
megafauna went extinct before human arrival in North America
andsome persisted for a periodaf ter human colonization(Faith&
Surovell, 2009; Gr ayson, 2007; Grayson & Meltzer, 2002, 2003 ,
2004;Meltzer,2015). Weassume that ifthese data were incorpo-
rated into the models comparing the impact of climate change ver-
sus human colonization, the results would be substantially different.
Because of the variability in the timing of extinc tions across taxa
and in the evidence for interactions between humans and mega-
fauna species, archeologists have argued that unraveling the mech-
anisms for the extinctions will require a “Gleasonian” approach, in
which the extinction process is studied species by species (Grayson,
2007;Meltzer,2015;forspecies examples, seeHill,Hill, &Widga,
2008;Widgaetal.,2017).
Thus, in terms of interdisciplinary communication, researchers
publishing in the ecological and archeological literature seem to
have knowin gly or unwittingl y settled into a s tatus quo. Mart in’s
work serves different purposes for those publishing in the different
research areas. Those using it to support claims about human–en-
vironment interactions and ecological processes that the overkill
model promotes may not recognize the shortcomings of the model
because information flow bet ween the various groups is limited.
Archeologists have the necessary datasets to evaluate the human
role in the extinctions and bring to the table a different but relevant
perspective. But bringing this perspective into the neoecological lit-
erature has been limited and challenging.
5 | CHARACTERIZING HUMAN–
ENVIRONMENT INTERACTIONS WITH
OVERKILL
While communication about overkill between archeological and
neoecological research areas is limited, there is greater informa-
tion flow between Quaternary and neoecological publications.
However, like archeological publications, the Quaternary literature
does not promote overkill as the dominant explanation for the ex-
tinctions, but generally suggest s that more research is still needed
(Barnosky etal., 2004; Koch & Barnosky, 2006). Thus, favoring
the Quaternary literature over the archeological still does not ex-
plain why the use of overkill to characterize human–environment
interactions is still more prevalent in the ecological literature. In
addition, our study only focuses on publications citing Martin’s
publications and overkill specifically. But the ideas promoted
by overkill c an also be found i n articles t hat do not cite Mar tin
(e.g., Smith,Elliott Smith, Lyons, &Payne, 2018). Overtheyears,
Grayson andMeltzer (2003, 2004)haveargued that overkillper-
sists because it supports a particular philosophical perspective on
anthropogenic environmental impacts. Over several articles, they
have evaluated the history of the overkill model, particularly the
logic of the argumentation, as well as the empirical evidence for
themodel.Withinthelast15years,theyhavearguedmorevehe-
mently that,
The overkill position has also, despite a clear lack of
empirical archaeological support, been adopted on
faith by an influential subset of ecologists and used to
support what are essentially political arguments.
(Grayson&Meltzer,2004:135)
…the overkill argument captured the popular imagi-
nation during a time of intense concern over our spe-
cies’destructivebehaviortowardlifeonearth…[it]is
|
9
NAGAOK A et Al.
inextricably linked to modern times and to the homily
of ecological ruin.
(Grayson&Meltzer,2003:590)
Thus, they assert that overkill is used as evidence of the damage
that humans can do to the environment. If humans have been causing
mass extinctions for thousands of years, then they are and will always
be a destructive force and a significant threat to biodiversity.
While it is clearthat people are havingasignificant impacton
the environment today, it is another thing to extend this behavior
back into deep time, especially when there is considerable debate
on the topic (see Bartlett et al., 2016; Di Febbraro et al., 2017;
Lima-Ribeiro&Diniz-Filho,2013,2017;Nogues-Bravoetal.,2008;
Sandom etal.,2014).However,thismonolithicviewof human–en-
vironment interactions is not uncommon in neoecological publica-
tions. It is linked to a viewpoint of humans as outside of nature, in
which dominion over nature is a pan- human trait. The recent de-
bate about the old versus new conservation has highlighted these
philosophical differences in how we view human’s placein nature
(Doak,Bakker,Goldstein,&Hale,2014a,b;Kareiva,2014;Kareiva&
Marvier,2012;Marvier& Kareiva,2014;Miller,Soulé,&Terborgh,
2014;Soulé,2013).Whenhumansareinherentlyseparatefromna-
ture, then the relationship between humans and the environment is
fixed, and the outcome is inevitable (ecological ruin). As such, nature
must be preserved and kept separate from humans if biodiversity
is to be maintained and the extinction threat minimized. Overkill
provides justification for this preservationist perspective. However,
overkill can be found even in public ations advocating for a more plu-
ral isticvie wofthehuman–naturedynam ics(e.g.,Ka reiva&Marv ier,
2011), suggesting that the notion of humans as a destructive force is
deeply embedded. If the use of overkill is motivated by a humans- as-
separate- from- nature worldview, what impact does it have on how
the public and the scientific community conceptualize human–envi-
ronmentinteractionsingeneral?Weusearecentdiscussionabout
the transition to the Anthropocene as an example of how overkill
influences views about anthropogenic impacts on the environment.
Specifically,overkillisusedtosuppor ttheperspectivethathuman
actions are monolithic in impact. This perspective leaves little room
for research that examines variability, sustainability or resilience.
The Anthropocene is both a potential new geologic period of
time and aperception abouthumans’ role within the environment
(see Crutzen, 2002).While geoscientistsareempirically evaluating
the Anthropocene as a potential new geologic epoch, a broader defi-
nition of the Anthropocene used beyond the geosciences has be-
come synonymous with the age when anthropogenic activities came
todominatetheEarth’secosystems(Autin,2016;Braje&Erlandson,
2013a). The concept has so widely captured the imagination and
interest of scholars that it has led to a plethora of recent ar ticles
and several new journals focusing on the Anthropocene as a period
of anthropogenic environmental impacts. It has become a powerful
interdisciplinary rallying point around which scholars from diverse
disciplines weigh in on human–environment issues like never before
(Ellis, 2018).
For both the geosciences’ and the broader version of the
Anthropocene, the start date is very important. But each approach
uses different criteria. For the geologic epoch, defining the lower
boundary has focused on empirically identif ying the markers that
can be used to differentiate the Anthropocene from the Holocene
ingeologicdeposits(Lewis&Maslin,2015;Zalasiewiczetal.,2008).
Whendidglobalhumanimpactsbecomesignificantlydifferentfrom
whatisseenintheHolocene?Theconsensusseemstobethat1950
willlikelybethestartdate(Zalasiewiczetal.,2015).Thus,the geo-
logic Anthropocene is recent and represents modern anthropogenic
impacts.
In contrast, with the broader usage of the term Anthropocene,
the start date varies widely. But each is linked to historic turning
points s uch as industrial ization and Western e xploration and e x-
pansion, or major cultural developments such as the rise of civiliza-
tionsorthebeginningsofagriculture(Braje&Erlandson, 2013a,b;
Glikson, 2013; Ruddiman, 2013; Smith & Zeder, 2013; Steffen,
Grineva ld, Crutzen, & M cNeill, 2011). Unlike the geo logic epoch,
however, the broader use of the Anthropocene tends to focus on
similarities between the past and present rather than when the
impact s become markedly different. Thus, the farther back in time
the period extends, the more the issues of the present may be pro-
jected onto the past. While it may appear that the Anthropocene
represents the history of processes that led to modern- day environ-
mental impacts, it is often treated as a monolithic period of human
behavior and environmental impacts by humans before which ex-
isted a potentially pristine nature. The conceptual implications of
an Anthropocene with significant time depth are clearly illustrated
when overkill and megafaunal ex tinctions are used to define the be-
ginning of the period.
Human- c aused megafaunal extinc tions are used by some to argue
that initial human occupation of a place marks the beginning for the
Anth ro pocen e( Do ughty, Wolf,&F ield,2010).I ti sb ui ltof fofMartin’s
idea that human arrival has had a significant, impact on biodiversity
everywhere people migrate (Boivin et al., 2016). For example, given
overkill in North America, the impact of humans has been significant
and severe since the late Pleistocene when people arrived to the con-
tinent and overkilled the megafauna. Unfortunately, the logical ex-
tension of this argument is that humans are inherently destructive as
a species. Thus, it could also be argued that the Anthropocene should
extend back to the beginning of Homo sapiens as a species. This may
seem like an extreme or marginal view, but it is a relatively common,
implicit perception of humans when discussing environmental issues.
Forexample,E. O. Wilsonpresented justsucha scenario when dis-
cussing threats to biodiversity and human- caused extinctions.
‘Human hu nters help no species.’ That is a general
truth and the key to the whole melancholy situation.
As the human wave rolled over the last of the virgin
lands like a smothering blanket…., they were con-
strained by neither knowledge of endemicity nor any
ethic of conservation.
Wilson,E.O.(1992)The Diversity of Life,p.253
10
|
NAGAOKA et Al .
When overk ill is used as a cau tionary t ale and a means to r ally
support for environmentalism, it portrays humans as a destructive
species.
There are several important contradictory consequences for
this line of thought. Using overkill to establish an older benchmark
implies that prehistoric communities significantly altered the en-
vironment such that they should be classified as similar to that of
modern s ocieties. W hile the popul ar literature may h ighlight pre-
historic examples of societal collapse due to environmental degra-
dation (e.g., Diamond, 2011), much of the archeological record is
characterized by persistence rather than extirpation, with the mag-
nitude of anthropogenic impacts varying significantly across time
and context.
When theAnthropocene is extended back tothe evolutionary
beginning of humans, then humans are a destructive or invasive
species. Indeed, the blitzkrieg version of the overkill model portrays
people as locusts killing megafauna and eating their way across
North Americ a (Mosimann & Martin, 1975). The invasive species
analogy suggests that humans do not belong in any environment al
context and that they are separate from nature. By extension, at
no time in the evolutionary history of humans are they under the
purview of ecological or evolutionary processes. Unfor tunately, the
extreme, yet logical solution for healing the environment would be
to rid the planet of humans.
Extending the Anthropocene into deep time also ignores the
real factors that make modern anthropogenic impacts particularly
damaging—the combination of an exponentially increasing pop-
ulation, efficient and destructive extraction techniques, massive
consumption, and rapid technological innovation and knowledge
transmission. If humans have always been destructive, then study-
ing the historic or prehistoric past also provides no understanding of
how certain cultures were able to mitigate their impacts or in which
contexts the impacts were exacerbated or what tipping points might
have looked like.
In addition to unchanging human impacts across time, cross-
cultural diversit y in human interactions with the environment is
also ignored. Diversity in local ecological knowledge of people
in all areas of the world and across all times must be considered
uniform. Anthropogenic impacts are often structured as a choice
between being inherently destructive and “noble savages” who are
completelyintunewithnature(e.g.,Penn&Mysterud,2017:3).
The well- documented occurrence of prehistorical
overkill in the Americas, Australia, New Zealand,
Madagascar,O ceania, and elsewhere shouldput us
on notice that premodern indigenous people have not
always been exemplary stewards of biotic resources.
Terborgh (2000)
When hum an–environment i nteractio ns are viewed as f ixed and
unchanging, studying resilience and sustainable practices of modern
peoples can offer no solutions. However, like any other organism,
humans can destroy, modify, enhance, or preserve depending on
context. And there is an extensive continuum of human–environment
interactions that range from extinctions to sustainable coexistence
(Anderson,2010;Turner&Berkes,2006;Ricketal.,2016;Wolverton,
Nolan and Ahmed 2014). Archeological research on long- term rela-
tionships between humans and the environment and modern studies
of local ecological knowledge (LEK) examine how cultural practices
and institutions can mitigate environmental impact and result in sus-
tainability and resilience. In essence, human (phenotypic) diversity is
devalued when overkill is used to support a human–nature dichtomy,
resulting in the view that the past is a clone of the present. As such, it
is easy to deny a role in environmental and conser vation discussions to
any research areas that study human–environment interactions across
time and space.
Thus, there are many reasons why overkill is problematic as a
source for ecological explanations. Overkill remains hotly contested,
and its use highlight s two problems for conservation and management.
First, conservation research is not maximizing its interdisciplinary po-
tential even though it has been touted as multidisciplinary from its in-
ception (Soulé,1985).Thecitationpatterns inthisstudy suggestthat
communication between researchers publishing on environmental re-
search in the ecological and social sciences literature may be limited.
However, archeology and other social science disciplines provide the
source data to the human side of human–environment relationships
(Briggsetal.,20 06;Erlandson& Braje,2013; Lane,2015; Rick,Kirch,
Erlandson, & Fit zpatrick, 2013). In addition, archeology also contrib-
utes to paleoecology in similar ways as paleontology (e.g., Grayson,
1993,2015; Lyman, 2012), but this areaof researchmaybelesswell
known simply because archeology is classified as a social science or
because of methodological differences between the disciplines.
Second,whenoverkill is used to extendlarge-scale anthropo-
genic impacts back into the deep past, it homogenizes these im-
pacts across time and space. Human impacts become monolithic
and always catastrophic. However, even if overkill is demonstrated
to have been the cause of Pleistocene megafauna extinctions,
there are alternative ways of using this information. These extinc-
tions could be used as one data point in millennia of different “ex-
periments” of humans interacting with the environment. Thus, the
focus would be on documenting the variability of anthropogenic
impacts to understand when human actions are more sustainable
versus more destructive. That some researchers default to treating
human actions as inherently destructive indicates a core belief that
humans are beyond nature and that nature, thus, needs to be pro-
tec ted(Callicott,C rowder,&Mumford ,1999).Thisisaninteresting
conundrum for environmental researchers. The logical extension
is that if human–environment interactions are uniform, then not
only were human impac ts similar in the past, but future restoration
and management are futile. If this belief is deeply embedded and
if overkill as an explanation for extinctions is disproven, then the
likelihood is that these researchers may look for another similar ex-
ampletobolstertheperceptionofhumans’roleintheenvironment
rather than shift the focus to understanding how people in their
diverse cultural, social, political, and historical contexts impact
biodiversity.
|
11
NAGAOK A et Al.
Understanding Late Quaternary extinctions has long been an im-
portant, but often polarizing area of study and considerable debate
remains.Whileour focushas been onissues with theoverkillmodel,
particularly as they relate to interdisciplinary scientific communication,
there have also been important critiques levied against the climate
change model (see Bartlet t etal., 2016).Some researchers inarche-
ology, Quaternary sciences, and ecology are focused on multicausal
explanations for Late Quaternary extinctions, with humans often
seen as the final tipping point on already dwindling megafauna pop-
ulations (seeBarnosky etal., 20 04; Boulanger & Lyman, 2014;Braje
&Erlandson,2013b;Lima-Ribeiro&Diniz-Filho,2013).Animportant
step for future research on Late Quaternary extinctions, particularly
as applied to conservation, as well as for researchers working on other
highly interdisciplinary topics will be for scholars to read, critically
evaluate, and cite material on the topic across the varied fields that are
investigating this important area of interdisciplinary study.
CONFLICT OF INTEREST
None declared.
DATA ACCESSIBILITY
CitationandsurveydatawillbeuploadedtoFigShare.com.
AUTHORS’ CONTRIBUTIONS
L.N. collected and analyzed the data. All authors contributed to the
manuscript.
ORCID
Lisa Nagaoka http://orcid.org/0000-0001-9562-8842
Torben Rick http://orcid.org/0000-0002-8254-5885
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