Anthony D Barnosky

Earth's most recent major extinction episode, the Quaternary Megafauna Extinction, claimed two-thirds of mammal genera and one-half of species that weighed >44 kg between «50,000 and 3,000 years ago. Estimates of megafauna biomass (including humans as a megafauna species) for before, during, and after the extinction episode suggest that growth of human biomass largely matched the loss of non-human megafauna biomass until «12,000 years ago. Then, total megafauna biomass crashed, because many non-human megafauna species suddenly disappeared, whereas human biomass continued to rise. After the crash, the global ecosystem gradually recovered into a new state where megafauna biomass was concentrated around one species, humans, instead of being distributed across many species. Precrash biomass levels were finally reached just before the Industrial Revolution began, then skyrocketed above the precrash baseline as humans augmented the energy available to the global ecosystem by mining fossil fuels. Implications include (i ) an increase in human biomass (with attendant hunting and other impacts) intersected with climate change to cause the Quaternary Megafauna Extinction and an ecological threshold event, after which humans became dominant in the global ecosystem; (ii) with continued growth of human biomass and today's unprecedented global warming, only extraordinary and stepped-up conservation

Department of Integrative Biology and Museums of Paleontology and Vertebrate Zoology, University of California, Berkeley, CA 94720.

228 / Anthony D. Barnosky efforts will prevent a new round of extinctions in most body-size and taxonomic spectra; and (/"/"/") a near-future biomass crash that will unfavorably impact humans and their domesticates and other species is unavoidable unless alternative energy sources are developed to replace dwindling supplies of fossil fuels.

The Quaternary Megafauna Extinction (QME) killed >178 species of the world's largest mammals, those weighing at least 44 kg (roughly the size of sheep to elephants). More than 101 genera perished. Beginning «50,000 years (kyr) B.P. and largely completed by 7 kyr B.P., it was Earth's latest great extinction event. The QME was the only major extinction that took place when humans were on the planet, and it also occurred at a time when human populations were rapidly expanding during a global warming episode. Thus, the QME takes on special significance in understanding the potential outcomes of a similar but scaled-up natural experiment that is underway today: the exponential growth of human populations at exactly the same time the Earth is warming at unprecedented rates.

Causes of the QME have been explored primarily through analyzing the chronology of extinction, geographic differences in extinction intensity, timing of human arrival vs. timing of climate change, and simulations that explore effects of humans hunting megafauna (Martin, 1967; Martin and Wright, 1967; Martin and Klein, 1984; MacPhee, 1999; Alroy, 2001; Roberts et al., 2001; Grayson and Meltzer, 2003; Barnosky et al., 2004; Trueman et al., 2005; Koch and Barnosky, 2006; Wroe and Field, 2006). Results of past studies indicate that human impacts such as hunting and habitat alteration contributed to the QME in many places, and that climate change exacerbated it. Potentially added to those megafauna stressors was the explosion of a comet over central North America, which may have helped to initiate the Younger Dryas (YD) climatic event, and which may have caused widespread wildfires, although those ideas are still being tested (Firestone et al., 2007).

Whatever the cause of the QME, one thing is clear: there was a dramatic change in the way energy flowed through the global ecosystem. The energy that powers ecosystems is derived from solar radiation, which is converted to biomass. Before the extinction, the energy needed to build megafauna biomass was divided among many species. After the extinction, increasing amounts and proportions of energy began to flow toward a single megafauna species, humans.

Humans are, by definition, a megafauna species, with an average body weight of «67 kg for modern Homo sapiens and 50 kg for Stone-Age people, placing us at the lower end of the body-size distribution for megafauna

FIGURE 12.1 Body-size distribution of mammals in North America. The black bars illustrate the distribution of species that went extinct in the QME. Note that humans are at the lower end of the distribution for species that went extinct. Illustration modified from Lyons et al. (2004); see that source for similar distributions of fauna from other continents.

FIGURE 12.1 Body-size distribution of mammals in North America. The black bars illustrate the distribution of species that went extinct in the QME. Note that humans are at the lower end of the distribution for species that went extinct. Illustration modified from Lyons et al. (2004); see that source for similar distributions of fauna from other continents.

as a whole (Fig. 12.1). Previous work has demonstrated that, as human biomass grows, the amount of solar energy and net primary productivity (NPP) available for use by other species shrinks, ultimately shrinking the amount of the world's biomass accounted for by those non-human species (Vitousek et al., 1986; Maurer, 1996; Vitousek et al., 1997; McDaniel and Borton, 2002). Therefore, growth of human biomass should be inversely related to biomass of other species in general and to other megafauna species in particular, given that large body size itself to a large extent depends on available NPP. Such energetically driven biomass tradeoffs provide a new way to explore the QME and have the potential of extracting general principles relevant to understanding the future. That is the approach I take here, one that necessarily has many caveats (see Methods), but that nevertheless leads to some interesting observations.

Details of the QME and debates about its causes are summarized in recent reviews (Barnosky et al., 2004; Lyons et al., 2004; Koch and Barnosky, 2006; Wroe and Field, 2006). Salient points include the following. It was a time-transgressive extinction, beginning by 50 kyr B.P. in Australia and largely ending there by 32 kyr B.P., possibly concentrated in an interval between 50 and 40 kyr B.P. (Roberts et al., 2001; Trueman et al., 2005; Wroe and Field, 2006). In northern Eurasia and Beringia, extinctions were later and occurred in two pulses, the first between 48 and 23 kyr B.P. and the second mainly between 14 and 10 kyr B.P. (Koch and Barnosky, 2006), although some species lingered later in isolated regions (Irish elk until 7 kyr B.P. in central Siberia and mammoths until 3 kyr B.P. on Wrangel

230 / Anthony D. Barnosky and St. Paul Island) (Guthrie, 2004; AJ Stuart et al, 2004). In central North America, extinctions corresponded with the second Eurasia-Beringia pulse, starting at 15.6 kyr B.P. and concentrating between 13.5 and 11.5 kyr B.P. (Koch and Barnosky, 2006; Waters and Stafford, 2007). In South America, the extinction chronology is not well worked out, but growing evidence points to a slightly younger extinction episode, between 12 and 8 kyr B.P. (Hubbe et al, 2007).

Extinction intensity varied by continent, with Australia, South America, and North America hard-hit, losing 88% (14 extinct, 2 surviving), 83% (48 globally extinct, 2 extinct on the continent, 10 surviving), and 72% (28 globally extinct, 6 extinct on the continent, 13 surviving), respectively, of their megafauna mammal genera. Eurasia lost only 35% of its genera (4 globally extinct, 5 extinct on the continent, 17 surviving). Africa was little affected, with only 21% loss (7 globally extinct, 3 extinct on the continent, 38 surviving), including at least three Holocene extinctions.

Humans evolved in Africa, and hominins have been interacting there with megafauna longer than anywhere else. Insofar as they are dated, there is no correlation between human arrival or climate change for the few African extinctions. In general, extinctions in Australia intensified within a few thousand years of human arrival «50 kyr B.P. but did not correspond with unusual climate change. Extinctions in northern Eurasia corresponded in time with the first arrival and population expansions of H. sapiens, but both pulses also were concentrated in a time of dramatic climate change, the first pulse at the cooling into the Late Glacial Maximum (LGM) and the second pulse at the rapid fluctuation of YD cooling followed by Holocene warming (Barnosky et al., 2004; Koch and Barnosky, 2006). Other species of Homo had been interacting with the megafauna for at least 400,000 years without significant extinctions before H. sapiens arrived. In Alaska and the Yukon, the first pulse of extinctions corresponded with LGM cooling but in the absence of significant human presence; the second pulse coincided with humans crossing the Bering Land Bridge and with the YD and Holocene climatic events. In central North America, extinction was sudden and fast, coinciding with the first entry of Clovis hunters, the YD-Holocene climatic transition, and the purported comet explosion. In South America, humans were already present by 14.6 kyr B.P., megafauna did not start going extinct until Holocene warming commenced some 11 kyr B.P., and species of ground sloths, saber cats, glyptodonts, and horses have seemingly reliable radiocarbon dates as young as 8 kyr B.P. (Hubbe et al., 2007).

Few islands ever had non-human megafauna sensu stricto. That, and the fact that even human biomass of islands is very small compared with the continents, caused me not to consider them in this analysis. However, it is important to note that, on nearly every island where humans have landed, extinctions (especially of birds) and wholesale habitat destruction have shortly followed.

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