Impacts of Pleistocene extinctions on local mammal assemblages from across the globe

<p dir="ltr">The modern extinction crisis will have severe consequences for natural systems and human civilisation. Extinctions of even a few ecologically-important species can have devastating effects on biological communities, in turn fundamentally altering entire ecosystems. Large mammals are a key example. As ecosystem engineers, they play a critical role in community functioning, and are more prone to extinction due to several biological traits that make them particularly vulnerable to environmental and anthropogenic threats. Thus, understanding the impacts of large mammal extinctions is essential. The Late Pleistocene megafaunal extinctions serve as a valuable case study. While previous research has focused mostly on quantifying their impacts at global and continental scales, far less work has assessed impacts at the local scale. For the first time, this thesis assesses changes in large mammal communities over the late Quaternary by analysing 652 fossil assemblages from across the globe. Using a new dataset of site-based specimen counts for 562 large mammal species obtained from published literature and museum collections, key metrics including biomass, energy use, and species richness are calculated for each assemblage, in addition to 241 modern-day camera trap inventories that are also analysed. Differences in biomass and energy use are compared between pre- and post- extinction assemblages, and temporal changes in both metrics are assessed alongside changes in species richness. The thesis shows that assemblage biomass and energy use were both greatly reduced following the extinctions. Post-extinction assemblages were also fundamentally altered by humans due to the subsequent agricultural revolution that quickly followed. Furthermore, the high abundances of domesticates in many Holocene assemblages geographically altered global community energetics; continental variation in the Pleistocene was due to natural ecological differences, whereas such variation in the Holocene is due to differences in anthropogenic domestication patterns. Strong geographical variation in energy use and biomass is still seen in modern mammalian communities, and this closely matches variation in Pleistocene megafaunal extinction intensity. Dramatic decreases in energy use and biomass occurred mainly at the Pleistocene-Holocene boundary and did not correspond with declines in species richness, at least in the Old World where extinctions were far more gradual and where agriculture developed rapidly. Lastly, the more steady declines in richness seen in Eurasian assemblages generates greater compositional differences in communities through time than through space. The patterns described throughout this thesis therefore show that the Pleistocene extinctions had highly transformative effects on local mammal communities all around the world. These impacts were just as severe as those seen on global and continental scales, and thus the results emphasise the critical importance of conserving large mammals in order to preserve community and ecosystem functioning.</p>