Climate Change Frames Debate Over the Extinction of Megafauna in Sahul (Pleistocene Australia-New Guinea) Stephen Wroea,B, Judith H
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PERSPECTIVE PERSPECTIVE Climate change frames debate over the extinction of megafauna in Sahul (Pleistocene Australia-New Guinea) Stephen Wroea,b, Judith H. Fielda,1, Michael Archera, Donald K. Graysonc, Gilbert J. Priced, Julien Louysd, J. Tyler Faithe, Gregory E. Webbd, Iain Davidsonf, and Scott D. Mooneya aSchool of Biological, Earth, and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia; bSchool of Engineering, University of Newcastle, NSW 2308, Australia; cDepartment of Anthropology and Quaternary Research Center, University of Washington, Seattle, WA 98195; dSchool of Earth Sciences, The University of Queensland, St. Lucia, QLD 4072, Australia; eSchool of Social Science, The University of Queensland, St. Lucia, QLD 4072, Australia; and fSchool of Humanities, University of New England, Armidale, NSW 2351, Australia Edited by James O’Connell, University of Utah, Salt Lake City, UT, and approved April 9, 2013 (received for review February 12, 2013) Around 88 large vertebrate taxa disappeared from Sahul sometime during the Pleistocene, with the majority of losses (54 taxa) clearly taking place within the last 400,000 years. The largest was the 2.8-ton browsing Diprotodon optatum, whereas the ∼100- to 130-kg marsupial lion, Thylacoleo carnifex, the world’s most specialized mammalian carnivore, and Varanus priscus, the largest lizard known, were formidable predators. Explanations for these extinctions have centered on climatic change or human activities. Here, we review the evidence and argu- ments for both. Human involvement in the disappearance of some species remains possible but unproven. Mounting evidence points to the loss of most species before the peopling of Sahul (circa 50–45 ka) and a significant role for climate change in the disappearance of the continent’s megafauna. megafauna extinction | Pleistocene extinctions | archaeology | human colonization | faunal turnover Explaining Pleistocene faunal extinctions 2], ∼28–19 ka, that the first people crossed bandicoots, dasyurids, and rodents (27), also remains one of the most challenging prob- the biogeographic divide (Wallacea) to enter occurred. Furthermore, a range of medium- lems in the prehistory of Sahul (1–3). The Sahul at ∼50–45 ka (15, 16). It seems likely and small-bodied species disappeared or vast majority of extinctions across geological that the size, distribution, and density of hu- underwent major geographic range shifts on time are wholly attributable to climate- man populations in pre-LGM times have time frames similar to those of larger taxa related factors (4), but claims that some, or been overestimated (17), although these (28–33). Also notable is the fact that not all even all, Pleistocene extinctions of large- were evidently behaviorally modern people megafauna went extinct: Some underwent gigantic vertebrates (Fig. 1) in Sahul were the (18, 19). dwarfing, whereas others appear to have consequence of human activity have gen- survived relatively unchanged, such as emus, Late Pleistocene Fauna and Extinction erated particularly robust debate. Polarized cassowaries, wombats, salt-water crocodiles, views have emerged to account for the mode Chronologies and many species of large kangaroos, in- As commonly used in the context of extinc- and timing of these events (2, 5–11). A pau- cluding reds (Macropus rufus), grays (Mac- tions in Sahul, the term megafauna refers to city of empirical data; shortfalls in radiomet- ropus giganteus and Macropus fuliginosus), an arbitrary compilation of relatively large ric dating; and, until recently, a limited and wallaroos (Macropus robustus, Macropus mammalian, reptilian, and avian taxa, rang- appreciation of the paleoenvironmental re- antilopinus,andMacropus bernardus). Taxa ing in size from ∼10 kg or less up to >2,000 kg fromthewholegamutofbodysizedis- cord (7, 12) have placed considerable con- (20–23). In addition to Diprotodon opta- tributions, not just the largest, were affected straints on the ability to resolve “who or tum and Thylacoleo carnifex (Fig. 1), other by extrinsic factors during the Pleistocene. what” was responsible for these extinctions. well-known marsupial “giants” included the Only some of these factors led to extinctions. Given these limitations, assertions such as “... 230-kg kangaroo Procoptodon goliah (24), There is no reason to assume that all species the question is no longer if, but rather how, the tapir-like Palorchestes azael,andthe lumped together as extinct megafauna were humans induced this prehistoric extinction bull-sized Zygomaturus trilobus.Among ” event (ref. 13, p. 563) are premature. nonmammalian megafaunal species were — Author contributions: S.W. and J.H.F. designed research; S.W., J.H.F., Sahul mainland Australia, Tasmania, the massively built flightless bird Genyornis — ∼ G.J.P., and J.L. performed research; G.J.P., J.L., J.T.F., and S.D.M. and New Guinea comprised up to 11 mil- newtoni (25), the anaconda-like madtsoiid 2 analyzed data; and S.W., J.H.F., M.A., D.K.G., J.T.F., G.E.W., I.D., lion km at glacial maxima. Although domi- snake Wonambi naracoortensis,andthe and S.D.M. wrote the paper. nated by an expansive desert core (14), 5-m-long mekosuchine crocodile Pallim- The authors declare no conflict of interest. environments ranged from periglacial in Tas- narchus pollens (26). This article is a PNAS Direct Submission. mania to tropical in New Guinea (Fig. 2). It It is important to note that extinctions in 1To whom correspondence should be addressed. E-mail: judith. was against a backdrop of deteriorating con- Pleistocene Sahul were not restricted to the fi[email protected]. ditions leading up to the Last Glacial Maxi- large-bodied species described above. Extinc- This article contains supporting information online at www.pnas.org/ mum [LGM; in Marine Isotope Stage (MIS) tions of small-bodied species, including frogs, lookup/suppl/doi:10.1073/pnas.1302698110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1302698110 PNAS | May 28, 2013 | vol. 110 | no. 22 | 8777–8781 Downloaded by guest on September 25, 2021 explanation of the data are that the extinc- megafauna disappeared (6). Modeling studies tions were indeed staggered over a period based on the same dataset suggested that that began long before human arrival. A human activities could have accounted for staggered extinction event is further sup- the decline and disappearance of all mega- ported by recent rigorous statistical tests of fauna within 600 y (39). Various publications stratigraphically intact prehuman fossil se- before and since have offered proxy data to quences (Fig. 4) that show progressive support the notion of a primary human role losses of diversity over time (12), strongly (8, 40). suggesting that temporally progressive mid- Human activities as a primary extinction dle to late Pleistocene declines in diversity driver are not, however, supported by the are not sampling artifacts. paleontology or the archeology. The com- Extinctions of a comparable magnitude to plete lack of evidence for predation on, or Fig. 1. Thylacoleo carnifex, the 100- to 130-kg mar- those of megafauna have been observed for even consumption of, megafauna by people supial lion with massive “bolt cutter-like” cheek teeth and small-bodied species in a range of specific aside, the extinction window noted above is the most powerful bite for its size of any mammalian localized studies. For example, small-sized based on a statistical analysis of just seven carnivore, was a formidable predator of large animals. < (Drawing by Peter Schouten.) species ( 3 kg) from Mount Etna in north- sites(7).Fourofthesehavenopublisheddata eastern Australia (Fig. 2) experienced a 50% and cannot be scrutinized. A fifth site, reduction in species richness (from 28 to 35 Menindee Lake (Fig. 2), is 10,000 y younger sufficiently similar ecologically or entangled species to 16 to 12 species) over the course than originally proposed but still 10,000 y (or contemporaneous, see below) to enable of the middle to late Pleistocene (35). These older than the archeological materials from one simultaneous event to account for all losses track progressive environmental change thesamesite(7).Theremainingtwolocali- > these extinctions. from rainforest to less mesic habitats. Like- ties are 100 ka in age (7), and thus earlier ThelastappearancedatesinSahulforthe wise, the eastern Darling Downs records than the arrival of people by at least 50,000 y. suite of taxa traditionally referred to as mega- middle to late Pleistocene extinctions among Only two sites in Sahul have secure exca- fauna are consistent with a staggered extinc- both small- and large-bodied species con- vated contexts with co-occurrence of extinct tion process that was in train well before comitant with progressively drying envi- megafauna and people: Cuddie Springs in the arrival of humans (3, 10, 12) (Fig. 3). In- ronments (30–33). southeastern Australia and Nombe Rock- cluding new middle Pleistocene species re- shelter in the New Guinea highlands (7, 34) corded from south-central Australia (2), as Human-Mediated Extinction Processes (Fig. 2). Two other sites have yielded evi- many as 50 of the 88 known extinct mega- The argument for a primary human role in dence for single instances of megafauna faunal taxa are absent from fossil records the extinction of megafauna has been based persisting after the LGM. These fauna were postdating the Penultimate Glacial Maxi- on the presumption that the arrival of identified in archeological sites but were re- mum (PGM; MIS6) (3) at ∼130ka(Fig.3and humans on Sahul was synchronous with the covered from noncultural (prehuman) hori- Table S1). Additional taxa disappeared at disappearance of all now-extinct megafauna zons (Cloggs Cave and Seton Rockshelter) ∼85–80ka(3,6,12).Thereisfirm evidence (36). A link between the two seemed obvious (7). No kill-sites are known (3, 7). Although for only ∼8–14 now-extinct megafaunal spe- to advocates of a human-driven process (37), megafauna are known from Tasmania, most cies overlapping with human presence on the and a role for climate change was subse- extinct species had vanished before humans continent.