What Killed The Megafauna?

We've seen a mass extinction of megafauna worldwide since humans started to develop technologies like bows and arrows and make their first forays out of Africa. 

How much was climate responsible for this shift and how much was due to overhunting by modern humans?

A new paper makes the case for overhunting as the dominant cause of megafauna extinction worldwide. But I'm not convinced that there is a single answer. The paper itself observes that there are regional differences and then largely ignores this observation:

Population declines varied across ecological realms, with Australasia and the Neotropics experiencing the least severe declines over the Quaternary period (95% HPDI: [-0.244, 0.044] and [-0.228, -0.070], respectively), compared to Indomalaya and Nearctic (95% HPDI: [-0.458, -0.299] and [-0.410, -0.227], respectively). 

Separation of species according to the biome they occupy resulted in the largest discrepancy of population size decline between polar (95% HPDI: [-0.317, 0.037]) and temperate-adapted species (95% HPDI: [-0.460, -0.296]), while insectivores (95% HPDI: [-0.228, 0.201]) experienced a small and non-significant decrease compared to hypercarnivores (95% HPDI: [-0.394, -0.223]). 

Lastly, species with ranges overlapping regions where Homo sapiens was the first and only hominin present, tend to have the lowest decline (95% HPDI: [-0.269, -0.155]), compared to species in regions where archaic Homo species arrived early (95% HPDI: [-0.380, -0.290]). 

Generally, non-African temperate regions with a relatively long history of hominin activity experienced the largest decrease in megafauna population sizes. In contrast, and with the exception of polar species, warmer biomes with only H. sapiens activity seem to have declined the least. 

However, this observation is most likely driven by an increase of megafauna population sizes in Neotropics and Australasia between 1.25 million and 100,000 years ago, prior to human arrival. Notably, population decline starting at approximately 100,000 years ago, and continuing towards the present, is ubiquitous across realms.

In the Americas and especially in North America, the Younger Dryas climate event appears to have been a powerful driver of species extinction relative to the overhunting that had occurred by then. 

In Australia, we see one wave of mass megafauna extinction when modern humans arrive and a secondary wave when the dingo enters the Australian ecosystem.

In Africa, where modern humans originated and to a lesser extent in tropical Southeast Asia, megafauna extinction was more measured.

In Northern Eurasia, the Last Glacial Maximum ice age surely played some part in extinguishing megafauna and modern humans alike from the region and resulted in a global reduction of modern human effective population size almost everywhere in the world.

This paper's bottom line conclusion may be correct, but I'm not convinced that the story can be completely told at this level of generality.

The worldwide loss of large animal species over the past 100,000 years is evident from the fossil record, with climate and human impact as the most likely causes of megafauna extinctions. To help distinguish between these two scenarios, we analysed whole-genome sequence data of 142 species to infer their population size histories during the Quaternary. 

We modeled differences in population dynamics among species using ecological factors, paleoclimate and human presence as covariates. We report a significant population decline towards the present time in more than 90% of species, with larger megafauna experiencing the strongest decline. We find that population decline became ubiquitous approximately 100,000 years ago, with the majority of species experiencing their lowest population sizes during this period. 

We assessed the relative impact of climate fluctuations and human presence on megafauna dynamics and found that climate has limited explanatory power for late-Quaternary shifts in megafauna population sizes, which are largely explained by Homo sapiens arrival times. 

As a consequence of megafauna decline, total biomass and metabolic input provided by these species has drastically reduced to less than 25% compared to 100,000 years ago. These observations imply that the worldwide expansion of H. sapiens caused a major restructuring of ecosystems at global scale.

Juraj Bergman, et al., "Worldwide late-Quaternary population declines in extant megafauna are due to Homo sapiens rather than climate" bioRxiv (August 15, 2022). doi: https://doi.org/10.1101/2022.08.13.503826