Carbon dioxide starvation, the development of C4 ecosystems, and mammalian evolution

The decline of atmospheric CO2 over the last 65 million years (Ma) resulted in the 'CO2-starvation) of terrestrial ecosystems and led to the widespread distribution of C-4 plants, which are less sensitive to CO2 levels than are C-3 plants. Global expansion of C-4 biomass is recorded in the diets of mammals from Asia, Africa, North America, and South America during the interval from about 8 to 5 Ma. This was accompanied by the most significant Cenozoic faunal turnover on each of these continents, indicating that ecological changes at this time were an important factor in mammalian extinction. Further expansion of tropical C-4 biomass in Africa also occurred during the last glacial interval confirming the link between atmospheric CO2 levels and C-4 biomass response. Changes in fauna and flora at the end of the Miocene, and between the last glacial and interglacial, have previously been attributed to changes in aridity; however, an alternative explanation for a global expansion of C-4 biomass is CO2 starvation of C-3 plants when atmospheric CO2 levels dropped below a threshold significant to C-3 plants. Aridity may also have been a factor in the expansion of C-4 ecosystems but one that was secondary to, and perhaps because of, gradually decreasing CO2 concentrations in the atmosphere. Mammalian evolution in the late Neogene, then, may be related to the CO2 starvation of C-3 ecosystems.