One equation fits overkill: why allometry underpins both prehistoric and modern body size-biased extinctions

The higher extinction proneness of large bodied vertebrates, both in the past and during the modern global biodiversity crisis, has a fundamental explanation in allometry: maximal population increase is scaled to body mass (W) by W-0.25, whilst generation length scales by W0-15. Populations of any sized vertebrate can persist if their populations experience the same proportional reduction each generation, but if this chronic mortality occurs at an annual rate, then smaller short-lived animals are able to survive whilst larger animals are driven inexorably to extinction. On this basis, our interpretation of the empirical body mass-extinction risk evidence for both the Late Pleistocene extinctions and the contemporary biodiversity crisis is that human impacts are sufficiently rapid and ubiquitous to outstrip the capacity of natural selection in most large taxa, upsetting the highly evolved life history trade-offs that permit the maintenance of a diverse assemblage of different sized animals.