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

Abstract

The causes of Sahul's megafauna extinctions remain uncertain, although several interacting factors were likely responsible. To examine the relative support for 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 eight 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. Our models show that the macropodiformes were the least demographically susceptible to extinction, followed by carnivores, monotremes, vombatiform herbivores, and large birds. Five of the eight extant species were as or more susceptible than the extinct species. There was no clear relationship between extinction susceptibility and the extinction chronology for any perturbation scenario, while body mass and generation length explained much of the variation in relative risk. Our results reveal that the actual mechanisms leading to the observed extinction chronology were unlikely related to variation in demographic susceptibility per se, but were possibly driven instead by finer-scale variation in climate change and/or human prey choice and relative hunting success.

Sahul’s megafauna were vulnerable to plant-community changes due to their position in the trophic network

“…By considering the network position of all vertebrate species in the assemblage, a clear difference between extinct and extant species emerged — extinct species had fewer predators than did species that survived (mean number of predators: 0.2 versus 3.3 for extinct versus extant species, respectively; Figure 4B). This predator naivety, coupled with the species’ slow life histories, likely made megafauna especially vulnerable to new predators [34,79–81] and suggests that hunting by humans could have adversely affected megafauna. Thus, a network modelling approach to assessing extinction vulnerability suggests that bottom-up and/or top-down processes could have selectively removed the now-extinct species from the Naracoorte community.…”

Logistic‐growth models measuring density feedback are sensitive to population declines, but not fluctuating carrying capacity

“…Because the variability in population growth rates is driven primarily by variation in survival rates for species with slower life histories such as mammals (Oli & Dobson, 2003 ) and birds (Sæther & Bakke, 2000 ), we parameterised the simulated population dynamics of 21 long‐lived species of extant ( n = 8) and extinct ( n = 13) Australian vertebrates from five taxonomic/functional groups (herbivore vombatiformes [wombat suborder] and macropodiformes [kangaroo suborder], large omnivore birds, carnivores, and invertivore monotremes [echidnas]), spanning mean adult body masses of 1.7–2786 kg and generation lengths of 2.3–21 years (Bradshaw et al, 2021 ; Table 2 ). These species differ in their resilience to environmental change, and represent the slow end of the slow‐fast continuum of life histories (Herrando‐Pérez et al, 2012c ).…”

“…We chose this suite of species to cover a range of demographic types—it is the relative structure of the population and the particulars of the life histories that matter here, not the specifics of species A or B, or whether they are extant or extinct or live(d) in Australia or elsewhere. A full justification of the selection of our test species can be found in Bradshaw et al ( 2021 ).…”

“…We parameterised the base model with n 0 = AD M w for a closed population (dispersal = 0), where w is the right eigenvector of M (stable stage distribution), and A is the surface area of the study zone ( A = 250,000 km 2 ), so that the species with the lowest n 0 would have an initial population of at least several thousand individuals at the start of the simulations. Based on theoretical equilibrium densities ( D , km −2 ) calculated for each taxon (Bradshaw et al, 2021 ), we set the species‐specific carrying capacity K = DA .…”

“…(v) In this example, the system is in a state of nonstationarity because the K is declining over time. (n = 8) and extinct (n = 13) Australian vertebrates from five taxonomic/functional groups (herbivore vombatiformes [wombat suborder] and macropodiformes [kangaroo suborder], large omnivore birds, carnivores, and invertivore monotremes [echidnas]), spanning mean adult body masses of 1.7-2786 kg and generation lengths of 2.3-21 years(Bradshaw et al, 2021; Table…”