In the Light of Evolution Volume II: Biodiversity and Extinction (2008) / Chapter Skim
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12 Megafauna Biomass Tradeoff as a Driver of Quaternary and Future Extinctions--ANTHONY D. BARNOSKY
12 Megafauna Biomass Tradeoff as a Driver of Quaternary and Future Extinctions--ANTHONY D. BARNOSKY
Pages 227-242
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From page 227... ...
Then, total megafauna biomass crashed, because many non-human megafauna species sud denly 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.
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From page 228... ...
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.
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From page 229... ...
. 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.
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From page 230... ...
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.
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From page 231... ...
RESULTS AND DISCUSSION Species Loss vs. Human Population Growth The numbers of megafauna species lost were modest until the human growth curve began its rapid exponential rise between 15.5 and 11.5 kyr B.P.
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From page 232... ...
However, global megafauna biomass crashed dramatically between 15 and 11.5 kyr B.P. The crash reflects the second pulse of extinction in Eurasia–Beringia and the major extinction pulse in North and South Amer ica.
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From page 233... ...
The unusual perturbations included increasingly sophisticated hunting of megafauna by people, habitat alteration by growing human populations, climate changes that would have decreased total global ecological energy at least temporarily, and possibly a comet impact. The potentially dramatic effects of hunting (so-called overkill)
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From page 234... ...
In the precrash state, megafauna biomass was distributed among many megafauna species, each with a relatively narrow ecological niche. In the postcrash alternative state, megafauna biomass concentrated in one species, humans, which has a very broad ecological niche.
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From page 235... ...
First, the buildup of human-associated megafauna biomass, even in the absence of the extinct megafauna, took ≈9,700 years to reach precrash levels. That indicates that recovering from global ecosystem shifts takes much longer than the shift itself.
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From page 236... ...
In essence, the QME begins to stand out not just as a major extinction event but also as an example of how threshold effects change the global ecosystem, and what new threshold events may be in sight. In the specific case of the QME, a global crash in megafauna biomass resulted when the coincidence of at least two events constricted the share of ecological energy allotted to each non-human megafauna species.
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From page 237... ...
, then depletion of fossil fuels without replacement by alternative energy sources would mean that a biomass crash is imminent, this one depleting human biomass and causing extinction in a wide spectrum of other species. Reliable projections on the number of years into the future that fossil fuels can sustain the global ecosystem at current levels vary, but generally are in the area of 50 more years for oil, 200 more years for natural gas, and 2,000 more years for coal (Galoppini, 2006)
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From page 238... ...
The energetic constraints that underlie the biomass tradeoff mean that, as human biomass grows, the only way other species can persist is through conscious stepped-up efforts to save them, by such actions as setting aside reserves, enforced protection of existing reserves, and efficient and sustainable food-production practices. It is particularly urgent to act upon the knowledge that the high level of megafauna biomass today, which means humans, can be sustained only by developing alternative energy resources to replace the dwindling supply of fossil fuels.
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From page 239... ...
+ 1.22. Second, megafauna species typically have geographic range sizes that average between 7% and 9% of the area of the continent on which they live.
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From page 240... ...
However, because the purpose of this part of the analysis was to see whether domestic stock compensated for a reduction in wild megafauna, the overestimation actually makes the conclusions more robust. Sensitivity Tests Sensitivity tests were conducted to assess how robust the general trends were to varying assumptions about density and geographic range size, on which the calculated biomass value for each species depends.
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From page 241... ...
Hadly and Pablo Marquet for discussions about these ideas; Claudio Latorre for discussions about extinctions in South America; and Paul Koch, Robert Feranec, and Alan Shabel for help in assembling information. Useful comments on the manuscript were supplied by John Avise, Jenny McGuire, Nick Pyenson, and two anonymous reviewers.
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