Also coincident with the megafauna biomass crash was rapid climatic cooling, then warming as the YD gave way to the Holocene. In the Americas, where most of the extinction was concentrated, the tail end of the LGM, then the YD cooling, depressed NPP in at least the Northern Hemisphere. A slightly earlier YD-like cooling did the same in South America, just as humans began to interact with the non-human megafauna. Likewise, YD cooling was pronounced in northern Eurasia at the time of the world biomass crash.

If the evidence for a comet explosion over North America stands the test of time, NPP available to megafauna would have been further depressed at the time of the big extinction pulse. Large tracts of land are thought to have burned, and the explosion itself may have triggered the YD cooling in the Northern Hemisphere through opening the way for massive amounts of cold glacial meltwater to flood into the North Atlantic (Firestone et al., 2007).

At the crash, megafauna biomass fell below its previous baseline value (Fig. 12.4). Then, beginning ≈10 kyr B.P., it began to build back up. By that time, the energy bottleneck that accompanied the crash was over. Global NPP was increasing as Holocene temperatures warmed, more land area was being exposed as glaciers melted, and there were fewer megafauna species on Earth among which to split the energy allocation. Even so, it took thousands of years for megafauna biomass to build back to precrash levels. The way it built back up was fundamentally different from the way it had been before, because virtually the entire recovery was by adding human biomass; the biomass of non-human megafauna remained virtually unchanged.

In terms of ecosystem dynamics under threshold models (Scheffer et al., 2001; Scheffer and Carpenter, 2003), the biomass trajectory suggests that the global ecosystem crossed a threshold when the crash occurred. 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. That means that ultimately humans were successful in coopting energy previously shared among other species with big bodies. It also means that not only are those extinct megafauna gone forever, but also there is no potential for new megafauna species to evolve into the “megafauna space” as long as humans are so abundant. In that respect, we have decreased biodiversity for as long as we remain abundant on Earth.