The structure and dynamics of South American megafaunal assemblages and the aftermath of megafaunal extinctions in Patagonia.

PIRES, MATHIAS M.; KOCH, PAUL L.; FARIÑA, RICHARD A.; RINDEL, DIEGO; DOS REIS, SERGIO F.; GUIMARAES JR., PAULO R.; PEREZ, S. IVAN

Lima

Congreso; IX Latin American Paleontology Conference.; 2016

Extinctions were particularly severe in Australia and the Americas where most of the large mammals died out. In South America more than 50 genera of large-bodied mammals were lost. Studying the Pleistocene megafaunal extinctions provides insights on how human activities and climate change reshape ecological communities and help us understand the ecological consequences of diversity loss. We combined paleontological and archeological data, and a suite of techniques including mathematical modeling, network analyses, and coalescence analyses using DNA sequence data, to understand the changes in mammalian assemblages from the Pleistocene to the Holocene. Using information from South American fossil assemblages we reconstructed potential interaction networks of the Pleistocene and studied their properties. Our findings suggest Pleistocene mammalian assemblages were not particularly vulnerable to perturbation when compared to modern African analogue assemblages. However, the arrival of humans behaving as generalist predators would have impacted the dynamical properties of these ecological networks, increasing their vulnerability perturbations. As assemblages reorganized after the extinctions they became less vulnerable to perturbations, but very sensitive to species loss, due to the lack of ecological redundancy. We then focused on the southernmost region of South America, Patagonia, and used DNA data and coalescence analyses to assess the ancestral demography of two of the iconic survivors in this region, humans and guanacos (Lama guanicoe). Ancestral demographic analysis showed a rise in human densities that coincides with an increase in the guanaco population, between 7.5 and 5 kyrs ago, after the demise of other large mammals. The observed dynamics may be the result of two phenomena: (i) the Holocene changes in climate and vegetation (ii) and/or competitive release of the guanaco, which would have favored the increase in populations of huntergatherers. The zooarchaeological record suggests that the use of guanacos by hunter-gatherer populations increased during this interval. Although an increase in the use of guanaco is not sufficient evidence, it is consistent with the hypothesis that the demographics of hunter-gatherers in Patagonia responded to changes in the demographics of guanacos. Our findings show the potential of direct and indirect effects of ecological interactions to shape extinction episodes and their aftermath.