as a whole, 25% of the 4,629 species known on Earth fall in the critically endangered through vulnerable categories. This suggests that not only has all of the “extra” carrying capacity been used by humans, but also we are beginning, as happened during the QME crash, to steal from the part of the global energy budget allotted to other megafauna species. We are also going farther and using energy previously allotted to species in even smaller body-size classes. Under business-as-usual scenarios, the inevitable result will be another biomass crash that moves down the body-size classes relative to the QME event.
Third, that the normal biomass baseline was exceeded only after the Industrial Revolution indicates the current abnormally high level of megafauna biomass is sustained solely by fossil fuels. If biodiversity is actually a tradeoff between human biomass and other species’ biomass, as both the QME and theoretical considerations indicate (Vitousek et al., 1986, 1997; Maurer, 1996; McDaniel and Borton, 2002), 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). Thus, without technological breakthroughs, the next biomass crash could be in as little as a few human generations.
Fourth, it may be no coincidence that the QME did not occur until the intersection of growing human biomass and climate change that ultimately manifested as global warming. Climate change, either cooling or warming, itself produces adjustments in geographic range distribution and population size that can lead to extinction (Barnosky, 1986; Parmesan and Yohe, 2003; Root et al., 2003; Thomas et al., 2004; Parmesan, 2006; Pounds et al., 2006). Add to that the overall reduction of NPP that must have occurred with YD cooling, the indirect co-opting of energy by rapidly growing human biomass, and direct human displacement of megafauna by killing and habitat alteration, and the combination becomes particularly lethal. Today, we stand at a similar crossroads, because growth of human biomass in the past few decades has moved us to the point where we are beginning to co-opt resources from, further displace, and cause extinctions of species with whom we have been coexisting for 10,000 years. At the same time, Earth’s climate is warming even faster than the rates of climate change that characterized the QME.
Recognizing the tradeoff between human biomass, non-human megafauna biomass, and non-human biomass in general highlights the need for extraordinary efforts to conserve the world’s remaining biodiversity (McDaniel and Borton, 2002). Business as usual will not stave off severe