Individual variation in dispersal, and its sources, shape the fate of pushed vs. pulled range expansions

Abstract

Ecological and evolutionary dynamics of range expansions are shaped by both dispersal and population growth. Accordingly, density-dependence in either dispersal or growth can determine whether expansions are pulled or pushed, i.e. whether expansion velocities and genetic diversity are mainly driven by recent, low-density edge populations, or by older populations closer to the core. Despite this and despite abundant evidence of dispersal evolution during expansions, the impact of density-dependent dispersal and its evolution on expansion dynamics remains understudied. Here, we used simulation models to examine the influence of individual trait variation in both dispersal capacity and dispersal density-dependence on expansions, and how it impacts the position of expansions on the pulled-pushed continuum. First, we found that knowing about the evolution of density-dependent dispersal at the range edge can greatly improve our ability to predict whether an expansion is (more) pushed or (more) pulled. Second, we found that both dispersal costs and the sources of variation in dispersal (genetic or non-genetic, in dispersal capacity versus in density-dependence) greatly influence how expansion dynamics evolve. Among other scenarios, pushed expansions tended to become more pulled with time only when density-dependence was highly heritable, dispersal costs were low and dispersal capacity could not evolve. When, on the other hand, variation in density-dependence had no genetic basis, but dispersal capacity could evolve, then pushed expansions tended to become more pushed with time, and pulled expansions more pulled. More generally, our results show that trying to predict expansion velocities and dynamics using trait information from non-expanding regions only may be problematic, that both dispersal variation and its sources play a key role in determining whether an expansion is and stays pushed, and that environmental context (here dispersal costs) cannot be neglected. Those simulations suggest new avenues of research to explore, both in terms of theoretical studies and regarding ways to empirically study pushed vs. pulled range expansions.

Colonisation debt: when invasion history impacts current range expansion

“…The maintenance of genetic variance in pushed invasions could therefore also facilitate a weakening of the mechanism on the front. Similarly, studies indicate that positive density-dependent dispersal is expected to be reduced along invasions, by evolving towards density-independent dispersal (Erm and Phillips, 2020;Travis et al, 2009), although recent results suggest that this evolution might not be systematic (Dahirel et al, 2022). Dispersal from the populations behind the front underlying the colonisation debt might therefore also enable the invader to evolve out of the density-dependence mechanisms generating the colonisation debt.…”

A gene-regulatory network model for density-dependent and sex-biased dispersal evolution during range expansions

“…In theoretical work, density-dependent dispersal can lead to accelerating range expansions (Travis et al, 2009), due to the evolution of decreased positive density-dependence of dispersal at range fronts. Yet, experimental studies have shown both, reductions (Fronhofer et al, 2017;Dahirel et al, 2021Dahirel et al, , 2022 and increases (Mishra et al, 2020) in positive density-dependence in dispersal during range expansion.…”