BIODIVERSITY LETTER: The North American Pleistocene overkill hypothesis and the re-wilding debate
Correspondence: Steve Wolverton, Department of Geography, University of North Texas, 1155 Union Circle #305279, Denton, TX 76203-5017, USA.
The conservation agenda to re-wild North America may or may not be realistic in terms of political ecology. However, it represents a real conservation recommendation to re-wild North America with the extant megafauna most closely related to those that became extinct at the end of the Pleistocene. The recommendation is based on the presumption that society bears an ethical responsibility to re-wild because humans caused the extinctions. However, the extent to which Pleistocene megafaunal extinctions were the result of overkill is hotly debated. As a result, the ethical imperative for North American re-wilding should be questioned. It will not be questioned unless members of the conservation community read the extensive archaeological and geological literature concerning the North American Pleistocene extinctions. Overlooking the assumptions underlying this particular recommendation is costly to conservation science and to archaeology because it represents an over-simplified, unwieldy and troubling fusion of the two.
The re-wilding gambit
The recent controversial conservation agenda to re-wild North America with Old World analogue species that are the closest living relatives of now-extinct Pleistocene mammals has garnered much attention. The merits of and flaws in this perspective have been debated and summarized in some detail (Donlan et al., 2005; Rubenstein et al., 2006; Caro, 2007; Oliveira-Santos & Fernandez, 2010). An aspect of the debate that has been overlooked in the ecological literature is the validity of the assumption that humans were the most important cause of the Pleistocene extinctions. The notion of overkill has gained popularity amongst a broad cadre of scientists from a number of disciplines, including those within ecology, geology and archaeology. However, there are a number of scientists who view the debate on overkill as inconclusive and open (e.g. Grayson, 2007; Hill et al., 2008; see a balanced summary of the debate by Koch & Barnosky, 2006).
Is there an overkill debate?
Palaeobiologists and ecologists increasingly accept that the Pleistocene extinctions in North America were caused by human overkill. However, this topic is still debated. For example, for the argument that Pleistocene fauna were ecophenotypically vulnerable to predation (Brook & Bowman, 2005), there is a counter-argument that they may have been vulnerable to environmental disturbance (Guthrie, 1984; Kiltie, 1984; Wolverton et al., 2009). The position that some African and Asian megafauna species escaped extinction because they adaptively avoided human predation (Johnson, 2002; Kelt & Meyer, 2009) can be countered by the argument that shifts in seasonality driven by late Pleistocene climate change at temperate latitudes did not occur in the same parts of Africa and Asia (Guthrie, 1984; Kiltie, 1984; Wolverton et al., 2009). For each assessment that humans hunted megafauna in North America and elsewhere (Surovell et al., 2005; Surovell & Waguespack, 2008), there are positions that humans were not big-game specialists at the end of the Pleistocene (e.g. Cannon & Meltzer, 2004). For those who argue that the timing of extinctions correlate with human arrival to North America (Lyons et al., 2004; Faith & Surovell, 2009), others have argued that understanding the precise timing of extinctions (Grayson, 2007) and human arrival (Meltzer, 2004) requires more study. Ecological arguments that hunting occurred (e.g. Lyons et al., 2004; Brook & Bowman, 2005) can be countered with case studies that suggest harvest pressure on big game may not have occurred (e.g. Hill et al., 2008; Campos et al., 2010). Regardless of the side chosen or of how forcefully arguments are made, there is debate as to the timing of the extinctions and the role that humans played in them.
Archaeological implications of overkill
The debate on overkill is still open for a number of reasons. Overkill is an archaeological hypothesis at its heart (Surovell, 2008: 1372–1373; Wolverton et al., 2009); that is, the most provocative empirical tests of whether or not humans hunted large game at the end of the Pleistocene must rely on archaeological data (e.g. Hill et al., 2008; Surovell & Waguespack, 2008). The archaeological test implications that must be met to affirm hunting of late Pleistocene big game are rarely framed in a clear, organized manner (cf. Hill et al., 2008). First among these implications is that multiple kill sites must indicate ubiquitous hunting of Pleistocene mammals. Determining whether or not sites represent hunting events is a related, important archaeological question. To date, the North American archaeological record of interactions between humans and Pleistocene mammals (of any kind, hunting or scavenging) is sparse with evidence of hunting for only two taxa of proboscideans (Grayson & Meltzer, 2003; but see also Surovell & Waguespack, 2008). Equally important is evidence that environmental change at or near the end of the Pleistocene may have contributed to the extinctions (summaries in Koch & Barnosky, 2006; Wolverton et al., 2009). Add to the debate that understanding the timing, nature and extent of human entrance into the New World relies on a multivariate argument that tends to change as new evidence is encountered (see recent summary by Goebel et al., 2008), and the assumption that human arrival and spread correlate precisely with mass extinctions (Grayson, 2007) can be challenged, although it cannot be rejected (Surovell, 2008; Faith & Surovell, 2009).
Within the literature related to this debate, there is a long record of modelling human demography, agency, harvesting of Pleistocene mammals, and spatial patterning and preservation of sites (e.g. Alroy, 2001; Surovell & Waguespack, 2008). These studies bolster the archaeological record, paint scenarios that may have happened, and represent important contributions to the debate. Another approach has been to assess the Pleistocene extinctions as a macrogeographic (multicontinent) phenomenon (e.g. Lyons et al., 2004) emphasizing correlations between human arrival in an area and extinctions. Again, the timing of human arrival in an area is an archaeological hypothesis; Europe and North America represent two of the better known case studies (Surovell & Waguespack, 2008). Assessment of the extinctions at the multicontinent scale has the disadvantage of muting environmental and cultural variability, which undoubtedly relate to finer geographic scales (continents and smaller).
What is at stake?
The importance of debating the late Pleistocene extinctions has never been greater because overkill has been used to justify the ethics of re-wilding (Donlan et al., 2005). The ethical charge is that because humans caused the extinctions, today there is an obligation to restore North American ecosystems to approximate the ecology of the late Pleistocene by introducing species closely related to those that became extinct (e.g. extant proboscideans, large cats, et cetera). Thus, re-wilding would represent a sweeping change in conservation practice (Oliveira-Santos & Fernandez, 2010), which is of general public and scholarly interest. The actual role that overkill may have played, which is contentiously debated, is an issue that should not be overlooked; otherwise, re-wilding may simply represent exotic wilding (biological invasion) of North America based on poorly founded environmental ethics.
When debates about conservation policy come into play, testing the overkill hypothesis becomes much more than a scholarly exercise but instead represents fuel for conservation agendas. At stake is the validity of the role that palaeozoological research can play in conservation biology and restoration ecology (sensu Lyman, 2006; Frazier, 2007). Indeed, conservation scientists debate the pragmatic and theoretical validity of re-wilding, which involves a variety of potential risks and benefits. In the case of Pleistocene re-wilding, however, it is important to recognize that archaeology and ecology are very public enterprises with county, state, regional and national organizations that engage society. This debate and the ethical charge upon which it is based influence societal values. Treatment of North American Pleistocene overkill by scientists as factual information upon which to base environmental management instead of hypothetical deceives the general public and scholars who may not be familiar with the archaeological and palaeontological literature on the extinctions (e.g. Koch & Barnosky, 2006; Grayson, 2007; Hill et al., 2008; Surovell & Waguespack, 2008; Faith & Surovell, 2009). This represents an unwieldy and troubling fusion of palaeozoology with conservation biology.
Five anonymous reviewers provided helpful criticisms and helped to correct errors.
Steve Wolverton is an ecologist and archaeologist specializing in the palaeozoology of North America during the Holocene. His research focuses on vertebrate and invertebrate biology and the use of data sets from palaeozoology to address issues in conservation science.