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Explaining the Pleistocene Megafaunal Extinctions: Models, Chronologies, and Assumptions

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Explaining the Pleistocene Megafaunal Extinctions: Models, Chronologies, and Assumptions Perspective Explaining the Pleistocene megafaunal extinctions: Models, chronologies, and assumptions Barry W. Brook* and David M. J. S. Bowman Key Centre for Tropical Wildlife Management, Northern Territory University, Darwin 0909, Northern Territory, Australia Understanding of the Pleistocene megafaunal extinctions has been advanced recently by the application of simulation models and new developments in geochronological dating. Together these have been used to posit a rapid demise of megafauna due to over-hunting by invading humans. However, we demonstrate that the results of these extinction models are highly sensitive to implicit assumptions concerning the degree of prey naivety to human hunters. In addition, we show that in Greater Australia, where the extinctions occurred well before the end of the last Ice Age (unlike the North American situation), estimates of the duration of coexistence between humans and megafauna remain imprecise. Contrary to recent claims, the existing data do not prove the ‘‘blitzkrieg’’ model of overkill. he cause of the extinction of giant birds, extra-Australian counterparts) apparently due solely to over-hunting by humans (16, Treptiles, and mammals in the late Pleis- occurred independent of climate change. 17). However, in both these models the tocene is, for palaeobiology, what Fermat’s For this reason, the situation in Greater unstated assumption was complete and un- last theorem was for mathematics (1): a Australia and New Zealand are seen as changing prey naivety. Recent field research long-standing scientific puzzle that has important ‘‘independent tests’’ of the impact has shown that both contemporary marsu- captured the imagination of specialists of human colonization on megafauna (3). pial and eutherian prey populations can and nonspecialists alike (2). Debate about Although a residual extant megafauna develop vigilance toward novel predators the possible cause of the extinction has did survive the Pleistocene extinction event quite rapidly (21, 22), which raises the ques- continued for over 150 yr (3–5), stimulated (e.g., red kangaroo, bison, Asian elephant, tion: What would happen to the predictions by new fossil finds, dating techniques, and llama, etc.), the only continent on Earth of these models if this assumption was modes of analysis. The debate is not strictly where a diverse assemblage of megafauna relaxed? scientific, however, because it impacts on remains is Africa, which is also where mod- the broader understanding of the evolution- ern humans arose. The African ‘‘anomaly’’ Sensitivity to the Assumption of Prey Naivety. ary theater of early human cultures, the fate is typically explained by long-term coevolu- We ran a simplified (single-species) version of contemporary global biodiversity, and the tion of megafauna with humans such that of Alroy’s (16) overkill model to evaluate rights of indigenous hunters (4, 6, 7). In- the prey and predator are matched evenly, the sensitivity of its results to axiomatic prey deed, the demise of the megafauna is often thereby creating trophic equilibrium. By naivety. This was done by systematically packaged as an environmental morality tale contrast, the extra-African megafauna are varying the term z in the equation that (8, 9). No matter why the debate interests characterized as completely naive to the describes predation efficiency (see Support- people, there is no doubt that it is of great human predator and therefore vulnerable ing Text, which is published as supporting significance. to overkill and the disintegration of food information on the PNAS web site, www. A great variety of competing scenarios webs (8, 15). pnas.org, and Fig. 1 for details). To date, all have been proposed to explain the extinc- A recent series of articles, widely reported overkill models assume prey off-take is de- tion of the megafauna such as climate in the international media, have been her- scribed by a ‘‘type II functional response,’’ change, disease, altered habitat condition alded as a major breakthrough in this de- where z ϭ 1, implying complete naivety of (particularly due to the effects of landscape bate. First, a mathematical simulation prey (e.g., refs. 10 and 16); or else take the burning by humans), and the breakdown of model (16) was interpreted as showing the extremely unrealistic view that off-take is food webs (3, 10, 11), but the presently North American megafauna most likely suc- entirely independent of prey density (e.g., ascendant idea is the so-called overkill hy- cumbed to over-hunting after human colo- ref. 17). pothesis. This theory posits that extinctions nization at the end of the last Ice Age. This When a functional response consistent were exclusively a result of human hunting article built on an earlier modeling exercise with ‘‘predator-hardened’’ prey is used, the (12), with a popular variant being the most concerning New Zealand extinctions (17). probability of overkill rapidly decays (Fig. restrictive case (termed ‘‘blitzkrieg’’), where Second, a metaanalysis of megafaunal re- 1). Yet when we set z ϭ 1 (Alroy’s assump- the extinction phase occurred within a few mains (18) found a coincidence between tion), our prediction of the probability of thousand years of human colonization (9, megafaunal extinction and human coloniza- overkill (93%) actually exceeded Alroy’s Ϸ 13). In North America, human colonization tion of Greater Australia (19, 20) 40,000– results (81%, based on the 37 scenarios in coincided with the end of the last Ice Age, 50,000 calendar years before present (yr which he treated megafauna as an undiffer- stymieing the disentanglement of human B.P.). Given the significance and impact of entiated ecological unit, as we have done). from environmental causes (12, 14). None- these articles, we chose to examine their Our higher predicted probability of overkill theless, proponents of the overkill hypoth- assumptions by undertaking some addi- when z ϭ 1 reflects our conservative as- esis point out that the megafauna had sur- tional simulations and statistical analyses sumption that human population dynamics vived previous glacial cycles where there was and reflect on possible future avenues that were independent of prey density, in con- no human predation. In Greater Australia may advance this complicated problem. trast to Alroy’s coupling of these terms. (Australia and the islands of Tasmania and New Guinea) and New Zealand, by con- Modeling Overkill trast, extinction of the megafauna (which Recent computer modeling has been used *To whom correspondence should be addressed. E-mail: were generally more diminutive than their to ‘‘prove’’ that megafaunal extinction was [email protected]. 14624–14627 ͉ PNAS ͉ November 12, 2002 ͉ vol. 99 ͉ no. 23 www.pnas.org͞cgi͞doi͞10.1073͞pnas.232126899 Downloaded by guest on October 2, 2021 critical importance of considering whether rect consequence of human activities such as all of the extinct megafauna were naive and habitat modifications caused by landscape incapable of adapting to a new predator. burning. For instance, the destruction of woody vegetation by burning has been pos- ‘‘Calibration’’ of Model Assumptions. Con- tulated to explain the extinction of the Pleis- temporary studies provide the only realistic tocene Australian giant flightless bird, opportunity to test assumptions about how Genyornis newtoni, and all other megafauna megafauna adapt to human predation (10). Ϸ48,000 yr B.P. (11). This theory is consis- For instance, Berger et al. (21) studied how tent with the absence of evidence of human the relationship between moose (megafau- predation of the Australian megafauna nal prey) and wolves (predator) changed in such as kill sites or specialized tools (ref. 7 response to the time period over which they and S. Wroe, J. Field, R. Fullagar, and L. S. had coexisted. Compared with populations Jermiin, unpublished data) with the carbon- in which wolves and moose have always isotope signature of Genyornis fossil egg coexisted, moose exposed to reintroduced shells that indicates that it ate woody (C3 wolves showed an initially elevated level of photosynthetic pathway) plant species, in Fig. 1. Declining probability of simulated overkill predation and lack of response to threaten- contrast to fossil egg shells of the surviving as the cause of the megafaunal extinctions in relation ing stimuli but also rapidly lost this naive ratite, the Emu (Dromaius noveahol- to the departure from maximal predation efficiency vulnerability (although the z parameter can- ϭ landiae), which also eats tropical grass spe- (when z 1 in the equation of functional response, not be calculated from their data). Berger et cies (C4 photosynthetic pathway), and the where predation rate ϭ FP z͞[G ϩ P z]; see ref. 47). al. (21) concluded that the megafauna that apparent destruction of tropical rainforest The parameter z can be interpreted as a proxy survived the end-Pleistocene extinctions in Ϸ of relative megafauna naivety and has been fixed at in the Australian humid tropics 47,000 yr 1 in all previous megafaunal extinction models, im- North America may have been better able to B.P. (30). plying complete prey naivety. adapt to novel predation than the species We conclude that models alone cannot that went extinct. resolve the question of whether the In northern Australia, experience gained megafauna were hunted to extinction be- Alroy (16) conceded that his model failed from management programs designed to cause of the critical importance of numer- to explain the differential survival of ‘‘geo-
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