Assessing the impact of late Pleistocene megafaunal extinctions on global vegetation and climate

Abstract

The end of the Pleistocene was a turning point for the Earth system as climate gradually emerged from millennia of severe glaciation in the Northern Hemisphere. The deglacial climate change coincided with an unprecedented decline in many species of Pleistocene megafauna, including the near-total eradication of the woolly mammoth. Due to an herbivorous diet that presumably involved large-scale tree grazing, the mammoth extinction has been associated with the rapid expansion of dwarf deciduous trees in Siberia and Beringia, thus potentially contributing to the changing climate of the period. In this study, we use the University of Victoria Earth System Climate Model (UVic ESCM) to simulate the possible effects of these extinctions on climate during the latest deglacial period. We have explored various hypothetical scenarios of forest expansion in the northern high latitudes, quantifying the biogeophysical effects in terms of changes in surface albedo and air temperature. These scenarios include a Maximum Impact Scenario (MIS) which simulates the greatest possible post-extinction reforestation in the model, and sensitivity tests which investigate the timing of extinction, the fraction of trees grazed by mammoths, and the southern extent of mammoth habitats. We also show the results of a simulation with free atmospheric CO₂-carbon cycle interactions. For the MIS, we obtained a surface albedo increase and global warming of 0.006 and 0.175 °C, respectively. Less extreme scenarios produced smaller global mean temperature changes, though local warming in some locations exceeded 0.3 °C even in the more realistic extinction scenarios. In the free CO₂ simulation, the biogeophysical-induced warming was amplified by a biogeochemical effect, whereby the replacement of high-latitude tundra with shrub forest led to a release of soil carbon to the atmosphere and a small atmospheric CO₂ increase. Overall, our results suggest the potential for a small, though non-trivial, effect of megafaunal extinctions on Pleistocene climate

Megafauna and ecosystem function from the Pleistocene to the Anthropocene

“…Although it is well-known that animals are influenced by climate, an interesting question is the extent to which animals can themselves influence climate (89). Through consumption and digestion, megafauna can have impacts on biogeochemical cycling, including the release of greenhouse gases.…”

“…In regions with abundant winter snow cover, trees tend to warm the surface (92) because they are dark features that peek above highly reflective snow. Two studies have tried to estimate whether the extinction of high latitude megafauna impacted albedo and therefore global temperatures (21,89) and estimate albedo-related global warming impacts of up to 0.2°C after the extinctions. Therefore, at a global scale, the greenhouse gas and albedo effects of megafauna work in opposite directions.…”

Soil biological responses to, and feedbacks on, trophic rewilding

“…Here we focus on highlatitude ecosystems, as they are disproportionally important to global carbon cycling and are a target for some ambitious rewilding projects [109]. Modelling suggests that the shift from subarctic tundra to shrubland linked to the Pleistocene megaherbivore extinctions resulted in net carbon losses [110], but it is hard to predict whether reversing such habitat changes through rewilding would also reverse the effects on carbon cycling. Restoration of cold steppe biomes through the introduction of large herbivores in subarctic tundra can enhance soil fertility [39], yet it has been suggested that it would also reduce carbon losses under climate change, by mechanisms such as carbon storage in root biomass and reduced decomposition due to lower soil temperature [109,111].…”

The Rise of the Anthroposphere since 50,000 Years: An Ecological Replacement of Megaherbivores by Humans in Terrestrial Ecosystems?

“…Indirectly, megaherbivores have an impact on climate through the changes they produce on the landscape and vegetation cover, modifying the albedo (e.g., Doughty et al, 2010) and the local humidity level (e.g., Brault et al, 2013), as well as through indirect changes in carbon storage by plants (Doughty et al, 2016c). Moreover, emissions of methane by megaherbivores may have had an effect on greenhouse gas concentration in the atmosphere and therefore on climate (Smith et al, 2010b(Smith et al, , 2016aBrook and Severinghaus, 2011).…”

“…The ecological consequences of these extinctions are increasingly investigated (e.g., Bunzel-Drüke et al, 2001;Catling, 2001;Putshkov, 2003;Brault et al, 2013;McClure, 2013;Gill, 2014;Bakker et al, 2016;Doughty et al, 2016a). After the extinction of most terrestrial megaherbivores around 12,000 years ago (Stuart, 2015), various human populations developed independently in different parts of the world a new way of subsistence by producing their food items instead of collecting them from the surrounding environment (Crawford, 2009).…”