Eco-evolutionary dynamics of density-dependence in pushed expansion fronts

President of the jury :                       

Rapporteurs :                     

• Benoit Facon, DR, Centre de Biologie pour la Gestion des Populations (CBGP), Université de Montpellier, Montferrier sur Lez, France
• François Massol, DR, Centre d'Infection et d'Immmunité de Lille (CIIL), Université de Lille, Lille, France

Examiners

• Ellyn Bitume, CR, USDA Forest Service, PSW, Institute of Pacific Islands Forestry, Hilo, Hawaii
• Benjamin L. Phillips, PR, Spatial Ecology and Evolution Lab (SpELL), Curtin University, Perth, Western Australia

Thesis Director :

• Dr. Elodie Vercken, Institut Sophia Agrobiotech, Sophia Antipolis CEDEX, France

Abstract :

Population density, i.e. the number of individuals present in a given space, has a major influence on individual performance and ultimately population biology. The nearly ubiquitous presence of density-dependence and density-dependent traits within the Tree of Life makes it important to know more about how density-dependence can evolve and influence population dynamics. When a population is expanding over space, density varies dramatically over a short spatial scale from the already occupied, sometimes densely populated, core area to the empty spaces beyond the expanding edge. In this context, understanding how dispersal traits respond to density is essential to know as it will potentially lead to or shape various ecological and evolutionary changes along the expansion. Notably, positive density-dependence in dispersal (but also in population growth rates) can generate so-called "pushed" expansions, where individuals in populations well behind the leading edge mostly drive the spread. Such dynamics are compared to more "pulled" expansions, in which the spread is driven by individuals at the leading edge. Many studies on this pushed/pulled continuum ignore the possibility of an evolving positive density dependence, and how traits driving that density dependence may be correlated with other traits or each other. During this thesis, I combined experimental and simulated expansions to explore how the evolution of correlated density-dependent life-history traits could influence eco-evolutionary dynamics under the lens of pushed/pulled dynamics. First, I demonstrated that among different species of Trichogramma microwasps, each species' position on a pace-of-life continuum was partially correlated with how pushed or pulled the expansion is. Slower species generating more pushed expansions. Then, using an Individual-Based Model, I found, conversely to my expectations, that the strength of life-history trait correlation does not significantly influence overall pushed/pulled dynamics. However, there is evidence that dispersal costs can markedly reshape the relationship between neutral genetic diversity and density-dependence that is key to pushed dynamics. Finally, using simulated expansions again, I attempted to build predictive models that can infer key pushed expansion parameters from a set of metrics based on population genetics or demography that could be easily obtainable from empirical datasets or in the field. Our first proof of concept presented encouraging results, with good model performances when predicting the presence of positive density-dependence in dispersal or the spatial neutral genetic diversity. Overall, this thesis presents the importance of including the evolution of density-dependent traits within studies on pulled versus pushed expansions, as it may result in shifts within this continuum. The (co)evolutionary history also seems to influence how much the expansion is pushed or pulled, but not the correlation structure itself. Indications of divergent evolutionary trajectories between pushed expansions generated by positive density-dependence in dispersal or population growth open the door for further studies on the evolution of biological expansions, and on how to include it to make better predictions in real-life scenarios.

Keywords :

Population Biology; Ecology; Evolution; Modelling

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