Not all populations expand as Fisher waves: Transition from pulled to pushed invasions with increasing intraspecific facilitation in an experimental population
The prevalence of the Allee effect (positive density dependence of the growth rate) is being increasingly documented in natural populations. This, coupled with the increasing risk of invasions and range shifts due to climate change, provide a strong motivation to better understand the effects of intraspecific facilitation on range expansions. The Allee effect is expected to affect the rate of invasion and the shape population profile compared to a classical Fisher wave. One consequence of these differences between invasions with and without Allee effect is the different rates of diversity loss due to the founder effect. Similarly, a trade-off between growth rate and carrying capacity has different outcomes depending on the presence or absence of intraspecific facilitation. However, many of these theoretical predictions are difficult to test empirically in field studies due to large variability and numerous unrelated abiotic factors and ecological processes. Here we use laboratory microbial populations as a model system to test these hypotheses quantitatively in a controlled way over a wide range of environmental conditions.
For populations with logistic growth, we found that the velocity of emergent waves is indeed in agreement with Fisher's predictions in spite of small day-to-day environmental and demographic fluctuations, including higher order effects due to patchy landscapes and seasonality. We also measured the shape of the wave profile over multiple orders of magnitude of density, and found that demographic fluctuations have a much stronger effect on the profile shape than on velocity. Upon gradually increasing the strength of the Allee effect, we see a transition from the 'pulled' to the 'pushed' regime, where the emergent wave properties (velocity and front shape) diverge from Fisher's predictions (pushed waves have a sharper profile and higher velocity than predicted). We also see some evidence of an accelerating wave front once the population density crosses the Allee threshold. In terms of the evolutionary impact, we have preliminary evidence indicating that genetic drift is lower in pushed populations compared to pulled ones. Broadly, our results provide guidance regarding the phenomena that might be observed in natural populations expanding into new territory.