Causes of Late Pleistocene Continental Extinctions

Island extinctions since the Late Pleistocene were almost undeniably caused by humans, but what happened on the continents is much less clear. A meta-study (giant compilation and review of other studies) by Barnosky et al. (2004), assessed the role of humans and climate on each continent. Perhaps predictably, the dichotomy between humans/ climate and megafauna extinctions has been challenged more recently, as can be seen in this paper as well as in Koch and Barnoksy (2006) The State of The Debate.

Summary of the numbers of megafaunal genera that went extinct on each continent, the strength of the extinction chronology and a comparison of the timing of extinction with the arrival of humans and late Pleistocene climatic change.
From Barnosky et al. (2004)

Eurasia falls in the middle of the extinction body count with 9 genera extinct. Humans are thought to have arrived here around >30ky ago (Raff & Bolnick, 2014). The extinctions were found to be mainly climate induced, with only partial blame falling on the humans. However, this continent stands out in having provisional chronological data points.

Evidence from palaeontology, climatology, archaeology and ecology now supports the idea that humans contributed to extinctions on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere (Barnosky et al., 2004). 

Barnosky et al.'s 2004 paper suggests that the intersect of human impacts and climatic change drove the precise timing and geography of extinction in the Northern Hemisphere. In North America, the 33 genera that went extinct are thought to be mostly down to humans. Humans arrived here around 11.5 ky (thousand years ago) (Barnosky et al., 2004), with climate change beginning prior to human arrival.

South America lost a staggering 50 genera of megafauna, but the mechanism behind this needs more work, according to the paper, to come to a conclusion. Humans arrived here from 12.5ky according to Barnosky, but if settlers took a different route down the Americas, as suggested by Misarti et al. (2012) then this could change the debate, i.e., if people took the Pacific route then they might have arrived in South America much earlier.

'A hypothetical scenario for the peopling of the Americas, showing possible migration events (a–e) coloured according to putative region of origin (Beringia, blue; Siberia, shades of purple). 

 Shading depicts the extent of Beringia during the Last Glacial Maximum. (kyr, thousand years ago.)'

From Raff & Bolnick (2014)

The Southern hemisphere was found to be generally lacking in data. While new evidence from Australia suggests humans helped cause extinctions there, the correlation with climate is weak or untested. Here, 21 genera went extinct. Africa was also found to be in need of more data, with 8 genera extinct and humans appearing around 160ky.

So what does this pattern say about human induced extinctions? It would seem to roughly follow the path of human expansion, with increasingly high death tolls further away from Africa. Previously studies have suggested that this is down to the animals in new places being unaware/ unprepared for humanity, and therefore unable to defend themselves (Martin and Klein, 1984).

However, in North America alone this can be shown to be a shaky assertion. Why did humans tip the balance, when previously arriving predators such as the lion (Pathera leo atrox) and grey wolf (Canis lupus) hadn’t? It's possible it was due to their omnivory and prey switching, combined with an ability to avoid predation from the resident carnivores by using weapons/ dogs/ language (Ripple and Valkenburgh, 2010). According to Ripple & Valkenburgh (2010), humans played a different role within the large-predator guild. Unlike other mammalian carnivore systems, in which interspecific competition is known to affect species densities, humans were omnivorous and probably less subject to intraguild predation, allowing their numbers to increase independently of large-carnivore densities

In order to better understand the details of the late Pleistocene extinctions globally and the population dynamics of the species involves, more realistic ecological models, established chronologies and insight into region-level paleoecology of the time are required. (Barnosky et al., 2004)