Bigger kill than chill: The uneven roles of humans and climate on late Quaternary megafaunal extinctions
Introduction
Since the 19th century, when science became aware of the sudden and geologically recent disappearance of many large-bodied animals, the late Quaternary Extinctions (LQE) have remained a great and controversial matter (Grayson, 2008). Starting around 50,000 years ago, about two thirds of all large terrestrial animal genera went extinct in a sequence that affected most continents (Koch and Barnosky, 2006). For a long time, two main hypotheses – attributing these extinctions either to climatic changes during the last glacial event or to the impacts of modern man's dispersal across the world – have divided the academic community. Many researchers also came to defend a synergy between both factors as a more plausible scenario for the extinctions (Barnosky, 2004, Nogués-Bravo et al., 2008, Lorenzen et al., 2011, Prescott et al., 2012, Lima-Ribeiro and Diniz-Filho, 2013), although controversies about the balance of climate and humans as extinction drivers still remain (Lima-Ribeiro et al., 2012, Prescott et al., 2012).
The late Quaternary megafaunal extinctions were a global phenomenon and we believe that a global approach is the best way to understand causal mechanisms. This would bring the full range of temporal and geographical variation in extinction times to bear, allowing one to disentangle the overall signal from regional trends. Most studies, however, have focused on particular continents and taxa (e.g. Alroy, 2001, Diniz-Filho, 2004, Johnson, 2006, Nogués-Bravo et al., 2008). A few global analyses have been presented (Lyons et al., 2004, Gillespie, 2008, Prescott et al., 2012, Sandom et al., 2014); but innovative and insightful as these studies have been, they carry some problems. While some works lack quantitative analyses of the proposed extinction causes (e.g. Lyons et al., 2004, Gillespie, 2008), others are based on crude and often unrealistic scenarios of human arrival and megafaunal extinction (Prescott et al., 2012; see Lima-Ribeiro et al., 2012 for details). Both Prescott et al. (2012) and Sandom et al. (2014) include non-quantitative variables in their models, as their hominin paleogeography variable is based on discrete human arrival scenarios. The most recent global analysis (Sandom et al., 2014) is based on global databases on extinct (and extant) mammals' distributions that are bound to be incomplete and/or to contain a proportion of untrustworthy data (as shown by the inclusion on the analysis of Africa and Southern Asia, regions with poor paleontological records). Additionally, their approach lacks a comparison of extinction dates with human arrival and climatic change focused on chronology (rather than geography).
Fossil dating allows the establishment of synchrony between extinction events and their potential drivers. In the last years, a growing number of dates have been published and reviewed around the world (see Supplementary References). Improved climatic models have been developed for the last 122,500 years (Andersen et al., 2004). These advances made a once unfeasible chronological global analysis of climatic changes, human arrival to each region and extinction of megafaunal taxa a concrete possibility, opening a promising path for resolving the extinction debate.
In light of these new chronometric advances, we performed an exhaustive gathering of data for human first appearance dates (HFADs) and last appearance of megafaunal genera (MLADs) on nineteen regions across the globe, together with climatic variation through the late Quaternary, to provide the first high-resolution chronological analysis of the LQE extinctions. We tested the hypotheses that human arrival or climate variance would be responsible for the extinction of megafaunal genera. This more detailed approach should advance the extinction debate, providing the first quantitative chronological test of the roles of anthropogenic impacts and climatic variation on the demise of the world's megafauna.
Section snippets
Data
The predictions of both hypotheses were compared in order to evaluate them. The environmental hypothesis predicts that extinctions would have occurred during or following intense climatic changes through the late Quaternary. The human impact hypothesis, on the other hand, predicts that extinctions would have followed human colonization of each landmass across the planet.
First, last appearance dates of megafauna (MLADs) species and first appearance dates of anatomically modern humans (HFADs) on
Results
A total of 2088 dates for 67 genera of extinct megafauna (58 mammals, 8 birds and 1 reptile) were considered reliable, totalizing 126 independent sampling units across 19 regions (Supplementary Table 1). Similarly, 762 human dates fulfilled the reliability requirements, and were used in the analyses (Supplementary Table 2).
Null models showed that most extinctions (85/126, or 67.4%; Fig. 2 and Supplementary Table 4) took place around the time of human arrival in each region, as it can be seen by
Discussion
Overall, our results indicate that human arrival was a necessary factor for the extinctions, whereas climate variation was a contributory one, enhancing regionally the effects of anthropogenic impacts in additive rather than synergistic ways. This conclusion builds upon the previous findings of the previous global analyses by Prescott et al. (2012) and Sandom et al. (2014), but it rests on a finer data base and it clarifies the causal relation between the two factors. The fact that Africa
Conclusion
The present study allowed a widened quantitative perspective on the relative roles of climate variation and human impacts in the LQE, with human impacts as a much stronger determinant of the number of extinct genera than climate variance. The extinction of hundreds of species across the whole planet is such a complex process that one could hardly expect a single factor to completely explain every aspect of it. It is highly likely that causal factors like human impacts and climate changes both
Acknowledgements
We thank Joaquín Hortal, Leonardo Ávila, Leopoldo Solbeizon, the late Paul S. Martin, the members of the Laboratório de Ecologia e Conservação de Populações – UFRJ and the members of the First Peopling of the Americas UNESCO Symposium at Puebla for discussions. We also thank Caio Kenup for his help with R programming, and Chris Johnson for his constructive commentaries on the manuscript. All authors except M.S.L.-R. were supported by personal grants from CNPq (Conselho Nacional de
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Laboratório de Macroecologia, Universidade Federal de Goiás, Campus Jataí, 75804-020, Jataí, GO, Brazil.