The overkill model and its impact on environmental research

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 history 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 importance 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 main proponent of the overkill model, Paul Martin. We examined the ideas and arguments for which Martin's overkill publications were cited and how they differed between archeologists and ecologists. Archeologists cite overkill 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 ecological publications suggests that despite increasing interdisciplinarity, communication with disciplines outside of ecology is not always reciprocal or even.

revolves around how far back humans have been having a significant impact on the environment (Boivin et al., 2016;Braje & Erlandson, 2013a;Smith & Zeder, 2013). It is the subject of not only defining the boundaries of the Anthropocene and other concepts such as the Sixth Extinction, but understanding the nature of the relationship 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 megafaunal extinctions plays a role.
In the 1960s, Paul Martin (1958Martin ( , 1967a, a geoscientist and paleobiologist, developed the overkill hypothesis, in which human hunting was proposed to have caused the extinction of the megafauna that roamed North America during the Pleistocene. During the last 50 years, the hypothesis has been extended to include all anthropogenic factors and has been applied to human colonization virtually anywhere in the world at any period (Burney & Flannery, 2005;Martin, 1984). Recently, overkill (Pleistocene or otherwise) has been used as a prime example in ecological or conservation studies stating that humans have profound impacts on the environment and have been doing so for millennia (Donlan, 2007;Donlan et al., 2006;Sherkow & Greely, 2013;Svenning et al., 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 explanation 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 destruction by others. So, how is it that these different communities of researchers view the role of overkill in megafaunal extinctions so differently?
In this paper, we summarize the overkill hypothesis and the debate about the cause of the late Pleistocene megafaunal extinctions. We then delve into problems of cross-disciplinary communication by conducting a citation analysis of cited works of Paul Martin, the author of the overkill hypothesis. We document how overkill is interpreted and used differently by archeologists and ecologists. For many ecologists, overkill holds significant meaning for the relationship between humans and the environment that has consequences for conservation. While a number of important studies have been conducted by archeologists, ecologists, and Quaternary scientists since Martin's research, including recent studies by ecologists that support climate/multidisciplinary models (e.g., Di Febbraro et al., 2017;Lima-Ribeiro & Diniz-Filho, 2013Nogues-Bravo, Rodiguez, Hortal, Batra, & Araujo, 2008) and others that support human impacts (Bartlett et al., 2016;Sandom, Faurby, Sandel, & Svenning, 2014), our focus here is not to review that extensive literature, but instead is to focus on interdisciplinary communication, particularly through citation of the seminal works on overkill by Martin. If environmental and anthropological scientists are to study the Anthropocene together, researchers face a challenge to improve interdisciplinary communication.

| THE OVERK ILL HYP OTHE S IS D I SS EC TED
By the end of the Pleistocene, a suite of 37 genera of large-bodied mammals became extinct in North America (Grayson, 2015;Meltzer, 2015). There are two main competing 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 climate change at the end of the Pleistocene. Research evaluating these hypotheses has involved investigators from archeology to the geosciences to evolutionary biology and ecology (Koch & Barnosky, 2006;Meltzer, 2015). Until the 1960s, the extinctions were primarily a paleontological subject of research, believed to be caused by warming climate that occurred during deglaciation in the transition 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 established between people and megafauna. Martin argued that if people were present in the Americas alongside the megafauna, then they could have been a factor in their extinction. As an alternative explanation to climate change, Martin (1958, 1967a, 1973) proposed the overkill hypothesis in which humans hunted the megafauna to extinction.
While the timing of both climate change and human colonization overlaps with megafaunal extinction, the mechanisms for how climate change was able to cause extinction in this context were unclear. In particular, Martin (1967a) questioned why megafauna had 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). It was easy to conceptualize how people could have caused an extinction event because the impacts of (and protests against) human-caused environmental degradation were on the nightly news. By the 1980s, Martin (1984) had expanded the overkill model beyond North American Pleistocene extinctions to explain mass extinctions globally as a function of human colonization: wherever people go, species go extinct. 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 extinctions through hunting makes several key assumptions. When the argument is teased apart, it is easier to evaluate whether or not overkill adequately explains the extinctions. The first two assumptions 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 (1967aMartin ( , 1984Martin ( , 1990) described many prehistoric and historic examples of extinction of island species following human colonization in places such as Madagascar, New Zealand, Hawaii, and other Pacific islands as support for the idea of overkill (see also Steadman & 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 endemism and small populations make island species more vulnerable to predation and environmental perturbations, and thus extinction.
While it is widely recognized that the circumstances for island extinctions can differ 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 continental Pleistocene megafauna, Martin developed the second assumption: 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 predators, then megafauna could be naïve to their specific type of predation. People were so efficient at hunting that the megafauna went extinct before they could develop an appropriate predator response (Martin, 1973). Indeed, the Blitzkrieg version of overkill has people hunting megafauna in a wave across North America (Mosimann & Martin, 1975). This assumption is often accepted as fact. Neither the degree of human hunting efficiency nor the absence of predator response has yet to be evaluated or demonstrated.
A third assumption relates to the empirical requirements of the model. Archeological data are particularly important for evaluating 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 impact 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 mammoth as the most common taxon (Grayson & Meltzer, 2002;Meltzer, 2015). And only a small number of sites, 15-26 (the veracity of the association is debated among archeologists), have been identified as showing evidence for a direct association between stone artifacts and remains of extinct megafauna (e.g., Meltzer, 2015;Surovell & Waguespack, 2008).
The paucity of archeological evidence for interaction between people and megafauna has been called the "associational critique" (Grayson, 1984a;Meltzer, 1986). But it has been deftly handled by Martin (1973Martin ( , 1984 and others (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 considered too stringent a criterion to expect for ancient deposits (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 support both the overkill and climate hypotheses. If climate change was the major cause of megafaunal extinction, 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 Pleistocene with mammoth (Agenbroad, 2005;Widga et al., 2017) and other extinct megafauna (Meltzer, 2015).
The higher proportion of remains in paleontological contexts compared 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 contexts is less likely than in paleontological ones. But this has not been demonstrated.
What is particularly startling about advocating that association should not be a requirement for evaluating overkill is that this is a foundational concept for historical disciplines, such as archeology, geology, and paleontology. Association is used to argue that spatial relationships between fossils and artifacts within deposits reflect past events and behaviors. Thus, to argue that demonstrating 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 extinctions. 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 overkill was introduced, the model appeared 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 megafaunal 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 that were far more abundant than megafauna (Cannon & Meltzer, 2004, 2008Dillehay et al., 2017;Erlandson et al., 2011). Similarly, other studies demonstrate that many megafauna species were extinct prior to human arrival (Boulanger & Lyman, 2014;Lima-Ribeiro & Diniz-Filho, 2013). Given that causes for the Pleistocene extinctions are unresolved, it is interesting to see that the overkill model features prominently in the ecological and conservation literature.

| DIFFERENT DISCIPLINE S , DIFFERENT INTERPRE TATI ON S
The use and relevance of overkill as the cause of the Pleistocene extinctions varies within and between disciplines. Within archeology, the literature on overkill has become polarized between perspectives of proponents and critics of overkill (e.g., Fiedel & Haynes, 2004;Grayson & Meltzer, 2003. Thus, it may appear that there is a debate for or against overkill. However, the average archeologist is not represented in such black and white terms. We 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 art an "overkiller" -pretty We used the cited reference search in Thompson Reuters' Web of Science database to find articles that cited these four publications. The articles spanned from 2015 when the analysis was originally 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 category 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 archeologists on human prehistory or paleoecology. Quaternary publications represent paleontological, historical biogeography, or paleoecological publications that generally focus on evolutionary processes related to a specific taxon. Ecological publications are neoecological studies that study taxa in contemporary contexts or that presented research related to conservation. We found a difference in how Martin's publications were cited in these three areas of research. Authors in each research area tended to cite different publications when referring to overkill (Table 1) 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 Quaternary publications, use Martin's work as evidence to support the claim that humans are directly 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 variety of situations in nature, of course, in which consumers or consumer populations 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 the earth's ecosystems contained megaherbivore species whose adult members, like today's elephants, were too large to be killed by the largest predators. Soulé and Terborgh (1999: 811-812)  TA B L E 2 The percentage of times Martin (1984) and Martin & Klein (1984) were cited within the three types of publications, and the claim for which the publications were cited In addition, a number of articles claim that there is "growing consensus" or increasing or mounting evidence that humans caused the extinctions (e.g., Blondel, 2008;Kodric-Brown & Brown, 2007).
Interestingly, many of these articles cite Martin & 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, possibly confusing it with Martin's chapter in the book. Thus, while many of the publications in the archeological and Quaternary categories suggest overkill as one potential explanation for megafaunal 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) publications as justification.
Because overkill is more commonly used within the neoeco- once again, however, it is likely that human impact was magnified by the naivete′ of New World prey toward this novel predator archetype (Wilson 1992;Martin, 1984). (2006) Rather than having a continent of fearless animals 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 animals 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 the naivete' of Pleistocene megafauna because it is an exception not seen elsewhere that they need to explain. Alternatively, the authors could have argued that the role of naivete' in megafaunal extinction is an untested assumption.

Cox and Lima
The assumption that fauna were naïve to human hunting at initial contact has also had an impact on studies on the historical biogeog- We believe that Africa's uniqueness in range contraction is the result of a fundamental difference in the spatial dynamics of the extinction forces in Africa. Human evolution in Africa allowed species there to adapt to coexist with humans (Martin, 1984). However, as humans 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 extinctions (Diamond, 1984;Martin, 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, it is argued that post-Pleistocene species 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 (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 used to provide a mechanism for extinction. Martin extended human 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 megafauna in North America and other parts of the world (Martin & Klein, 1984;Owen-Smith 1987 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 history of causing extinctions and major changes in ecosystems. (1)

| COMMUNIC ATION B RE AKDOWN
So why do these research communities differ in their perspectives 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 bibliometric and citation analyses appear to support limited interaction between 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 sciences, into which archeology falls, form the fourth cluster. Thus, ecologists and Quaternary scientists may be more likely to read and cite one another's research than they are to read and cite archeological journal articles.
If archeologists are publishing about megafaunal extinctions only in archeological journals, then ecologists may be less likely to encounter these articles. Donald Grayson and David Meltzer are prominent critics of overkill whose work is published predominantly in archeological journals (Grayson, 1984a(Grayson, ,b, 2001(Grayson, , 2007Grayson & Meltzer, 2002, 2003Meltzer, 1986Meltzer, , 2015; see also Wroe, Field, & Grayson, 2006 al., 2016;Prescott, Williams, Balmford, Green, & Manica, 2012;Sandom et al., 2014). These publications are cited during peer review as evidence that ecologists do not support 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 learn 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 empirical data. Thus, the evidence for climate change is contrasted against assertions about human impacts to evaluate the strength of the correlation 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 curious that some researchers appear reticent to accept arguments and data from archaeologists, particularly given that our field of expertise is studying the interaction and im- and some persisted for a period after human colonization (Faith & Surovell, 2009;Grayson, 2007;Grayson & Meltzer, 2002, 2003Meltzer, 2015). We assume that if these data were incorporated into the models comparing the impact of climate change versus human colonization, the results would be substantially different.
Because of the variability in the timing of extinctions across taxa and in the evidence for interactions between humans and megafauna species, archeologists have argued that unraveling the mechanisms for the extinctions will require a "Gleasonian" approach, in which the extinction process is studied species by species (Grayson, 2007;Meltzer, 2015; for species examples, see Hill, Hill, & Widga, 2008;Widga et al., 2017).

Thus, in terms of interdisciplinary communication, researchers
publishing in the ecological and archeological literature seem to have knowingly or unwittingly settled into a status quo. Martin's work serves different purposes for those publishing in the different research areas. Those using it to support claims about human-environment interactions and ecological processes that the overkill model promotes may not recognize the shortcomings of the model because information flow between 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 literature has been limited and challenging.

| CHAR AC TERIZING HUMAN-ENVIRONMENT INTER AC TI ON S WITH OVERKILL
While communication about overkill between archeological and neoecological research areas is limited, there is greater information flow between Quaternary and neoecological publications.
However, like archeological publications, the Quaternary literature does not promote overkill as the dominant explanation for the extinctions, but generally suggests that more research is still needed (Barnosky et al., 2004;Koch & Barnosky, 2006). Thus, favoring the Quaternary literature over the archeological still does not explain 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 can also be found in articles that do not cite Martin (e.g., Smith, Elliott Smith, Lyons, & Payne, 2018). Over the years, Meltzer (2003, 2004) have argued that overkill persists 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 the model. Within the last 15 years, they have argued more vehemently 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 imagination during a time of intense concern over our spe-

cies' destructive behavior toward life on earth… [it] is
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 clear that people are having a significant impact on 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, 2013Nogues-Bravo et al., 2008;Sandom et al., 2014). However, this monolithic view of human-environment interactions is not uncommon in neoecological publications. It is linked to a viewpoint of humans as outside of nature, in which dominion over nature is a pan-human trait. The recent debate about the old versus new conservation has highlighted these philosophical differences in how we view human's place in nature (Doak, Bakker, Goldstein, & Hale, 2014a,b;Kareiva, 2014;Kareiva & Marvier, 2012;Marvier & Kareiva, 2014;Miller, Soulé, & Terborgh, 2014;Soulé, 2013). When humans are inherently separate from nature, 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 publications advocating for a more pluralistic view of the human-nature dynamics (e.g., Kareiva & Marvier, 2011), suggesting that the notion of humans as a destructive force is to dominate the Earth's ecosystems (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 articles 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  (Lewis & Maslin, 2015;Zalasiewicz et al., 2008).

When did global human impacts become significantly different from
what is seen in the Holocene? The consensus seems to be that 1950 will likely be the start date (Zalasiewicz et al., 2015). Thus, the geologic 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 such as industrialization and Western exploration and expansion, or major cultural developments such as the rise of civilizations or the beginnings of agriculture (Braje & Erlandson, 2013a,b;Glikson, 2013;Ruddiman, 2013;Smith & Zeder, 2013;Steffen, Grinevald, Crutzen, & McNeill, 2011). Unlike the geologic epoch, Human-caused megafaunal extinctions are used by some to argue that initial human occupation of a place marks the beginning for the Anthropocene (Doughty, Wolf, & Field, 2010). It is built off of Martin'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 continent and overkilled the megafauna. Unfortunately, the logical extension 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.
For example, E. O. Wilson presented just such a scenario when discussing threats to biodiversity and human-caused extinctions.
'Human hunters 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 constrained by neither knowledge of endemicity nor any ethic of conservation. 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 conservation 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 highlights two problems for conservation and management.
First, conservation research is not maximizing its interdisciplinary potential even though it has been touted as multidisciplinary from its inception (Soulé, 1985). The citation patterns in this study suggest that communication between researchers publishing on environmental research 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 (Briggs et al., 2006;Erlandson & Braje, 2013;Lane, 2015;Rick, Kirch, Erlandson, & Fitzpatrick, 2013). In addition, archeology also contributes to paleoecology in similar ways as paleontology (e.g., Grayson, 1993Grayson, , 2015Lyman, 2012), but this area of research may be less well known simply because archeology is classified as a social science or because of methodological differences between the disciplines.
Second, when overkill is used to extend large-scale anthropogenic impacts back into the deep past, it homogenizes these impacts 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 extinctions could be used as one data point in millennia of different "experiments" 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 protected (Callicott, Crowder, & Mumford, 1999). This is an interesting conundrum for environmental researchers. The logical extension is that if human-environment interactions are uniform, then not only were human impacts 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 example to bolster the perception of humans' role in the environment rather than shift the focus to understanding how people in their diverse cultural, social, political, and historical contexts impact biodiversity.
Understanding Late Quaternary extinctions has long been an important, but often polarizing area of study and considerable debate remains. While our focus has been on issues with the overkill model, particularly as they relate to interdisciplinary scientific communication, there have also been important critiques levied against the climate change model (see Bartlett et al., 2016). Some researchers in archeology, 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 populations (see Barnosky et al., 2004;Boulanger & Lyman, 2014;Braje & Erlandson, 2013b;Lima-Ribeiro & Diniz-Filho, 2013). An important 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.

CO N FLI C T O F I NTE R E S T
None declared.

DATA ACCE SS I B I LIT Y
Citation and survey data will be uploaded to FigShare.com.

AUTH O R S' CO NTR I B UTI O N S
L.N. collected and analyzed the data. All authors contributed to the manuscript.