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baseline until we began to add to the global energy budget through mining fossil energy out of coal, oil, natural gas, and related sources. As soon as we began to augment the global energy budget, megafauna biomass skyrocketed, such that we are orders of magnitude above the normal baseline today.

CONCLUSIONS

When examined in the light of megafauna biomass tradeoffs, the cause of the QME becomes clearer, and implications for the future emerge. 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. One event was a time of rapid growth in human biomass, which meant an inordinate supply of NPP began to be consumed by a single megafauna species. The other was a probable temporary reduction of NPP as the YD cooling hit both of the Americas and northern Eurasia (Hajdas et al., 2003). Exacerbating the global energetic constraints were the first entry of humans into the Americas, increasingly sophisticated hunting strategies and wider disruption of habitats, and possibly a comet explosion over North America.

In the general sense, the QME has four lessons. First, the global ecosystem is in a fundamentally different state than before the megafauna biomass crash. In contrast to the distribution of resources among the 350-plus megafauna species that were alive before the QME, most of the energy available to megafauna species in the post-QME world was coopted by humans. What is left after that is being subdivided among only 183 (plus or minus) other non-human megafauna species. It is perhaps comforting from a biodiversity standpoint that those other 183 species have remained on Earth since the crash. That may speak to a reasonable amount of stability in the alternative state the global ecosystem reached after the QME threshold event, at least in pre-Industrial times. It is also consistent with the expectations of ecological threshold theory.

Second, the Industrial Revolution elevated Earth’s carrying capacity for megafauna biomass. However, despite that increase in carrying capacity, ≈50% (>90 species) of those megafauna species that persisted so well for the previous 10,000 years have become extinct, critically endangered, endangered, or vulnerable to extinction in the past few decades, including >40% of the megafauna species of mammals in Africa, the only continent that made it through the QME largely unscathed. For mammals



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