Post-Last Glacial Maximum vegetation and ecosystem responses to climate change and human expansion in the Neotropical realm: land cover changes and megafauna extinctions

Author: Antoine Champreux

Champreux, Antoine, 2024 Post-Last Glacial Maximum vegetation and ecosystem responses to climate change and human expansion in the Neotropical realm: land cover changes and megafauna extinctions, Flinders University, College of Science and Engineering

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The post-Last Glacial Maximum period (~ 19,000 years ago to the present) was characterised by major changes in the global climate and atmospheric CO2 concentration, such as the general increase in global temperatures and changes in precipitation regimes. In the Neotropical realm, a biogeographical region spanning the southern edge of the USA to Tierra del Fuego in Chile in southernmost South America, this period coincided with widespread ecosystem upheavals, including vegetation changes, the extinction of more than 80% of South American megafauna species (i.e., species weighing > 44 kg), and the increase in population densities of Homo sapiens. However, quantifying how the relationships between these events shaped South America's past landscape remains a challenge. The overarching aim of my thesis was to quantify how past environmental changes associated with an increase in human pressure shaped South America's ecosystems. More specifically, I aimed to 1) synthesise knowledge and quantify the effects of post-Last Glacial Maximum climate change and the intensification of human activities on Neotropical ecosystems at broad spatial scales focussing primarily on changes in vegetation, and 2) determine whether climate-induced vegetation changes after the Last Glacial Maximum could have caused megafauna extinctions in the Neotropical realm. To that end, I combined the information provided by palaeo-environment and megafauna fossil records with a spatial and dynamic vegetation modelling approach driven by palaeo-climate simulation experiments. I showed that variation in precipitation, temperature, solar radiation, and atmospheric CO2 have potentially led to substantial, widespread, and asynchronous increases in regional tree cover, strongly affecting a third of the Neotropical realm mainly prior to the beginning of the Holocene (~11,700 years before present). While the effect of climate appears to be predominant in large-scale ecosystem changes prior to the Holocene, human activities played an increasingly important role during the Holocene, resulting in a progressive decoupling between vegetation patterns and climate, consequently also blurring our current understanding of climate-vegetation relationships. My results also support the hypothesis that climate-induced vegetation changes were likely not the main driver of the demise of most megafauna species in the Southern Cone (latitude > 30 °S) at the very end of the Pleistocene. Overall, my thesis provides a better understanding of the broad-scale mechanisms that shaped Neotropical ecosystems as we know them today, and provides insights to predict the response of these ecosystems to future climate change and the ongoing intensification of human endeavour.

Keywords: Quaternary, Pleistocene, Holocene, LGM, Neotropics, South America, Patagonia, Southern Cone, large scale, biogeography, ecology, palaeo-ecology, biodiversity, biomes, forest, tree cover, leaf area index, palaeoenvironment, megafauna, grazers, vegetation changes, climate change, palaeoclimate, extinctions, human impact, dynamic global vegetation model, DGVM, LPJ-GUESS, pollen, palaeoproxies, computer modelling, ecological modelling, vegetation modelling, remote sensing, biomisation, map

Subject: Earth Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2024
School: College of Science and Engineering
Supervisor: Corey Bradshaw