Relative demographic susceptibility does not explain the extinction chronology of Sahul's megafauna

Abstract

The causes of Sahuls megafauna extinctions remain uncertain, although multiple, interacting factors were likely responsible. To test hypotheses regarding plausible ecological mechanisms underlying these extinctions, we constructed the first stochastic, age-structured models for 13 extinct megafauna species from five functional/taxonomic groups, as well as 8 extant species within these groups for comparison. Perturbing specific demographic rates individually, we tested which species were more demographically susceptible to extinction, and then compared these relative sensitivities to the fossil-derived extinction chronology. Here we show that the macropodiformes were the most resilient to extinction, followed by carnivores, monotremes, vombatiform herbivores, and large birds. Five of the eight extant species were as or more susceptible than were the extinct species. There was no clear relationship between extinction susceptibility and the extinction chronology for any perturbation scenario, but body mass and generation length explained much of the variation in relative risk. Our models reveal that the actual mechanisms leading to extinction were unlikely related to variation in demographic susceptibility per se, but were driven instead by finer-scale variation in climate change and/or human prey choice and relative hunting success.