HOW THE RED QUEEN DROVE TERRESTRIAL MAMMALS TO EXTINCTION: THE IMPORTANCE OF THE FAILURE TO ORIGINATE AND THE SECONDARY IMPORTANCE OF EQUILIBRIAL (DIVERSITY DEPENDENT) PROCESSES

Most species disappear by the processes of background extinction, yet those processes are poorly understood. To better understand the dynamics of background extinction we analyzed the evolutionary dynamics of 19 Cenozoic terrestrial mammalian clades with rich fossil records that are now fully extinct or in diversity decline. To qualify for analysis each family had to be monophyletic, have at least 100 genus occurrences (average = 419), a total diversity of at least five genera, a longevity of at least eight stratigraphic stages, and an average preservation potential of at least 0.6 per genus per stage (average = 0.89). We found that their diversity loss was not just a consequence of “gamblers ruin” but resulted from the evolutionary loss to the Red Queen, a failure to keep pace with a deteriorating environment. Interestingly, diversity loss was driven equally by both depressed origination rates and elevated extinction rates. This pattern has been seen before, but the causes of depressed origination (evolutionary sterility, or the Entwives effect) have largely escaped analysis. Although we find diversity-dependent origination and extinction rates, we find that the diversity of each clade only transiently equaled the implied equilibrium diversity. Thus, the processes that drove diversity loss in terrestrial mammal clades were fundamentally non-equilibrial and overwhelmed diversity-dependent processes. Our results suggest that diversity-dependence plays a role in diversity dynamics similar to the role that friction plays in the dynamics of motion – while it must be accounted for in the dynamics of diversity change, the dominant forces of diversity change lie beyond the existence of diversity dependence.