An important prediction of spatio-temporal population dynamics models is that habitat quality, as determined by the carrying capacity of the environment, should have no in uence on the velocity of population expansion. This is for instance the case with the Fisher-KPP model, where the asymptotic spreading speed of solutions is entirely determined by the intrinsic population growth rate and the di usion coe cient. We show that this prediction is far from being generic. Using di erent modelling frameworks, ranging from reaction-di usion equations to stochastic individual based models, we highlight that many mechanisms can actually lead to a positive relationship between habitat quality and the velocity of population expansion. We have tested one of these mechanisms with minute size parasitoid wasps in laboratory microcosms. The experimental results con rm the relationship between habitat quality and the speed at which populations spread. To conclude, we discuss how this property relates to the pulled or pushed nature of the propagation front.