Quantitative global analysis of the role of climate and people in explaining late Quaternary megafaunal extinctions

Abstract

The late Quaternary period saw the rapid extinction of the majority of the world's terrestrial megafauna. The cause of these dramatic losses, especially the relative importance of climatic change and the impacts of newly arrived people, remains highly controversial, with geographically restricted analyses generating conflicting conclusions. By analyzing the distribution and timing of all megafaunal extinctions in relation to climatic variables and human arrival on five landmasses, we demonstrate that the observed pattern of extinctions is best explained by models that combine both human arrival and climatic variables. Our conclusions are robust to uncertainties in climate data and in the dates of megafaunal extinctions and human arrival on different landmasses, and strongly suggest that these extinctions were driven by both anthropogenic and climatic factors.

Fig. 1.

Observed and predicted extinction rates (proportion of megafauna that become extinct) for each region and time interval in the 700-ky analysis. Observed extinctions (open circle) are the mean of the 10,000 extinction scenarios to take account of date uncertainties. Colored circles show the extinction rates predicted by models containing climate only (green), human arrival only (orange), or both human arrival and climate (blue). The time interval in which humans have an effect is shaded. Prob., probability.

Fig. 2.

Proportion (Prop.) of deviance explained by climate only (yellow), human arrival only (red), and shared deviance (orange) for the 700- and 100-ky analyses. For the 100-ky analysis, the results for each of the 32 human arrival scenarios are shown. The deviance explained by climate variables (yellow + orange combined) is independent of human arrival scenario.

Fig. 3.

Strength of the effect of four climatic variables and human arrival in predicting extinctions. The absolute magnitude of the median standardized coefficients for each scenario is given by the diameter of the circles, which are scaled such that the largest coefficient is represented by a circle filling a whole square in the grid. The sign and magnitude of the coefficients are given by the color of the circles, according to the scale at the bottom of the graph (positive coefficients represent increased extinctions, negative coefficients decreased extinctions). Note that the size and color scales differ between models covering the past 700 ky vs. the ones covering the last 100 ky. Max, maximum; SD, standard deviation.

Fig. 4.

Observed and predicted extinction rates (proportion of megafauna that become extinct) for each region and time interval in the 100-ky analysis. Observed extinctions (open circle) are the means of the 10,000 extinction scenarios. Colored circles show the extinction rates predicted by models containing climate only (green), human arrival only (orange), or both human arrival and climate (blue). For time intervals in which there is uncertainty over the timing of human arrival, the interquartile range of predicted values is shown, with the circle representing the median of these predictions. The ranges of time intervals in which humans may have had an effect are shaded. Prob, probability.