PhD defense by Filippo Botta

Title: "The ecology of abrupt climate changes during the last glacial period"

Abstract During the Last Glacial Period, several climate changes characterized by rapidity and broad geographical impact have occurred. In most cases, climate proxy records reveal patterns of general reorganization for atmospheric and oceanic currents, with abrupt modal switches among two or more states of equilibrium. However, no extinction event has been recorded following such dramatic environmental changes, with the notable exception of some megafauna species. This suggests that inspecting this past events might provide us of a deeper insight of the dynamics and the strategies that species and populations adopt to survive abrupt environmental change, a topic which appears to be of particular interest in the perspective of the present need to predict how nature will respond to the current climate change. Nevertheless, the short spatial and temporal scales of these events makes the study of such processes challenging. This project aims to investigate the underlying biotic mechanisms of response through analysis of paleo-archives and simulations realized by species distribution models.

As such, it is divided in three parts. The first part aims to collect and organize the existing knowledge about biotic responses to past abrupt climate changes, to achieve a general overview of the responses, and highlight the current gaps of knowledge on the topic. This is accomplished by a compilation of the published literature on the subject, with global geographic coverage and spanning the period comprising the Last Glacial Period  (~120-12 ka BP) and early Holocene. The collected information is classified by habitat and organization level, in order to infer modal responses and main mechanisms of adaptation. This analysis, included in the manuscript presented in Chapter 2, exposes the importance of microrefugia and minority population presence for communities to maintain equilibrium with rapid environmental shifts. The second part introduces a novel method of investigation of population mechanisms of adaptation to abrupt climate change, in the form of a species distribution model aiming to reproduce explicitly some of the key response dynamics. It is fully introduced in the manuscript presented in Chapter 3. The model takes climate simulation maps and paleo-archives of the taxa of interest as input; it subsequently produces a climate-driven distribution range simulation and evaluates its likelihood by comparing it against the paleo-archives. By testing the likelihood of the distribution range simulations against paleorecords of the taxa under study, the model incorporates a correlative approach to a mechanistic model. This hybrid nature aims to make the model a tool for studying and testing hypotheses about vi populations response mechanism, since it reproduces explicitly some of the key response dynamics being process-based; and, at the same time, for statistically reconstructing species traits by their paleoarchives. The model is tested through a case study of Abies populations in North America during the transition to Holocene. In the third part, displayed in the manuscript comprising Chapter 4, the model is applied to a fossile pollen database containing information about seven common North American tree taxa with the aim to analyse their response mechanisms to the environmental changes of last deglaciation. Simulations with different dispersal models are realized and compared to infer the colonization strategies of the taxa under rapid climate changes. The study shows a prevalent resort to long-range dispersal, with the frequency of such events inversely correlated with the ability of the taxa to maintain equilibrium with the environment. These findings illustrate the validity to recurring to process-based models for reproducing and analysing dynamics with brief spatial and temporal resolution as rapid climate-driven distribution range responses. Explicitly spatial models allow to take into account the stochasticity in the simulated events, which are an important factor at such short time scales. The maintenance to meta-population structures appears as an important feature for successful species responses to abrupt climate changes; in particular, long-range dispersal has played a significant role in tree populations responses to Late Glacial abrupt climate changes.

Supervisors
Professor Dorthe Dahl-Jensen, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen
Associate Professor Anders Svensson, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen
Professor Carsten Rahbek, Biologisk Institut, Macroecology, Evolution and Climate, University of Copenhagen
Associate Professor David Nogués-Bravo, Natur Historisk Museum, University of Copenhagen  

Assessment Committee
Professor Jørgen Peder Steffensen, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen
Research Associate  Sandra Nogue, Palaeoenvironmental Science for Geography and the Environmental Science at the University of Southampton
Associate Professor Alistair Seddon,  Department of Biological Sciences, University of Bergen

Please contact Filippo Botta (botta@nbi.ku.dk) for a copy of the thesis.