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Abstract

Reaction-diffusion equations are commonly used to describe population propagation dynamics. They can be used to define two types of dynamics: pulled and pushed. The pulled dynamic results from a colonization led by individuals in small numbers located at the edge of the population, the pushed dynamic results from a colonization led by larger numbers from the core of the population. Biological mechanisms underlying the pushed waves are still poorly described, however, the presence of an Allee effect (characterized by a positive correlation between growth rate and population density for small populations) is known to lead to a pushed dynamic. Other mechanisms generating a dependence between population size and the probability of colonization are also likely to generate pushed waves, but this generalization remains to be confirmed.
Few studies have addressed the effects of environmental factors such as carrying capacity on population expansion rates. However, while for a pulled dynamic, the colonization of the few individuals at the front of the wave should not be influenced by the value of the carrying capacity of the environment, in the pushed case, individuals evolving at larger numbers, the carrying capacity could be a limiting factor of the expansion.
We have therefore attempted to describe (i) the effects of carrying capacity on population expansion dynamics, particularly through the prism of pulled/pushed dynamics, and (ii) the diversity of demographic mechanisms that can generate high dynamics.
For this purpose, we used individual-based models and microcosm experimentation on parasitoid : Trichogrammas. We used a species with a positive density-dependent dispersion resulting in an increase in the individual probability of migrating in large populations, thus leading to potentially high colonization dynamics.
Using these two approaches, we first studied the impact of carrying capacity on expansion velocity, validating our hypotheses that the dependence of expansion velocity on carrying capacity could be an indicator of the pushed/pulled nature of expansions. In a second part we studied the phenomena of Range pinning described for populations subjected to Allee effects and we tried to generalize this phenomenon to pushed dynamics in the presence of positive dependent density dispersion. In a third part we studied how a heterogeneous distribution in space of different carrying capacities would influence the dynamics of particularly high expansions.

Keywords

Carrying capacity, Expansion, Individual based models, Microcosm, Population, Trichogramma chilonis, pushed/pulled dynamics, colonization, Allee effect, Density dependant dispersal, stochasticity

• Président/te du jury : 
  • Hendrik Davi, Directeur de recherche, INRA Avignon
• Rapporteurs/trices :  
  • Christelle Robinet, Directrice de Recherche, INRA Val de Loire
  • Emanuel Fronhofer, Chargé de recherche, ISEM, de Montpellier                       
• Examinateurs/trices :
  • Frédéric Hamelin, Maître de conférence, INRA Agro Campus Ouest
  • Virginie Raybaud, Maître de conférence, Université Côte d’Azur
• Directeur/trice de Thèse :
  • Élodie Vercken, Chargée de recherche, INRA Sophia Antipolis   
  • Ludovic Mailleret, Directeur de recherche, INRA Sophia Antipolis
  • Lionel Roques, Directeur de recherche, INRA Avignon