The Aftermath of Megafaunal Extinction: Ecosystem Transformation in Pleistocene Australia

Abstract

Giant vertebrates dominated many Pleistocene ecosystems. Many were herbivores, and their sudden extinction in prehistory could have had large ecological impacts. We used a high-resolution 130,000-year environmental record to help resolve the cause and reconstruct the ecological consequences of extinction of Australia's megafauna. Our results suggest that human arrival rather than climate caused megafaunal extinction, which then triggered replacement of mixed rainforest by sclerophyll vegetation through a combination of direct effects on vegetation of relaxed herbivore pressure and increased fire in the landscape. This ecosystem shift was as large as any effect of climate change over the last glacial cycle, and indicates the magnitude of changes that may have followed megafaunal extinction elsewhere in the world.

What caused extinction of the Pleistocene megafauna of Sahul?

“…The spores are abundant and so provide a continuous measure of activity of large herbivores that can be quantified as spore-influx rates or indexed relative to pollen counts. A dung-fungus record from north-eastern Australia showed no trend from 130 ka until a steep decline at about 41 ka [69]. This decline cannot be explained by climate, which was evidently stable at the time [70].…”

“…At the Lynch's Crater site, Rule et al [69] used counts of spores of dung fungi, pollen grains and charcoal particles to reconstruct environmental changes associated with megafaunal extinction. Before the decline of dung fungi at 41 ka, the vegetation around the site was a mixture of angiosperm and gymnosperm rainforests and dry sclerophyll forest with little or no fire.…”

Climate, not Aboriginal landscape burning, controlled the historical demography and distribution of fire-sensitive conifer populations across Australia

“…Although fossil evidence that extinct mammals ate Callitris is inconclusive [79], some introduced mammalian herbivores (sheep, goats and rabbits) can cause recruitment bottlenecks [31]. Our study finds no evidence of adverse effects on temperate Callitris populations, for example from putative increased burning resulting from fuel build-up following megafaunal extinction [15]. If this had been the case, then the increased fire frequency should have genetically impacted Callitris Australia-wide, whereas we detected regionally differential demographic changes and these changes matched the modelling of species' distributions.…”

“…On the other hand, there is a lack of congruence between human activity and fire records during 20-40 kya, a period of consistently low Aboriginal populations [9] and large changes in fire regimes [10], and recent studies argue for significant roles of climate change on vegetation shift and megafauna extinction [11][12][13][14]. Furthermore, studies of sedimentary records from the humid tropics and southeastern Australia suggest an ecological feedback, where relaxed herbivore pressure following extinction of megaherbivores resulted in a switch to flammable sclerophyllous vegetation, which increased fire activity [15,16]. Thus, there is still a controversy concerning whether climate or fire impacted vegetation more strongly, and the role of fire regime changes owing to human activity and megafaunal extinction during the last glacial period.…”

Landscape of the lost giants

“…But, where these records can be tied together, it seems that the link between habitat change and megafaunal extinction might not be as straightforward as previously assumed 1,2,3 . Instead of a change in vegetation removing the herbivores' preferred food source, the loss of the extensively grazing animals may have allowed brush to build up.…”