Landscape connectivity alters the evolution of density-dependent dispersal during pushed range expansions

Abstract

As human influence reshapes communities worldwide, many species expand or shift their ranges as a result, with extensive consequences across levels of biological organization. Range expansions can be ranked on a continuum going from pulled dynamics, in which low-density edge populations provide the "fuel" for the advance, to pushed dynamics in which high-density rear populations "push" the expansion forward. While theory suggests that evolution by spatial sorting, a common feature of range expansions, could lead pushed expansions to become pulled with time, empirical comparisons of phenotypic divergence in pushed vs. pulled contexts are lacking. In a previous experiment using Trichogramma brassicae wasps as a model, we showed that expansions were more pushed when connectivity was lower. Here we used descendants from these experimental landscapes to look at how the range expansion process and connectivity interact to shape phenotypic evolution. Interestingly, we found no clear and consistent phenotypic shifts, whether along expansion gradients or between treatments, when we focused on low-density trait expression. However, we found evidence of changes in density-dependence, in particular regarding dispersal: populations went from positive to negative density-dependent dispersal at the expansion edge, but only when connectivity was high. As positive density-dependent dispersal leads to pushed expansions, our results confirm predictions that evolution during range expansions may lead pushed expansions to become pulled, but add nuance by showing environmental context may slow down or cancel this process. This shows we need to jointly consider evolution and ecological context to accurately predict range expansion dynamics and their consequences.

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

“…However, that theoretical study and others typically allow only positive density-dependent dispersal to evolve, when negative density-dependent dispersal is just as likely in nature (Harman et al, 2020). This may explain mismatches with some of the few existing experimental studies (Dahirel, Bertin, Calcagno, et al, 2021; Fronhofer et al, 2017; Mishra et al, 2020). More importantly, the fact that dispersal becomes on average more density-independent across the range of densities tells us nothing in itself about the difference between average/maximal dispersal and specifically low-density dispersal.…”

“…Besides the extreme h 2 =0 and h 2 = 1 cases, the “intermediate” level of initial heritability was set to 0.33 to roughly match “average” heritabilities seen in dispersal traits (Saastamoinen et al, 2018). The growth rate r 0 was constant and set to a value that is within the range of plausible values for insects (Hassell et al, 1976), and worked well in previous simulations (Dahirel, Bertin, Haond, et al, 2021). K was chosen to match previous simulations (Dahirel, Bertin, Haond, et al, 2021; Haond et al, 2018), and is within an order of magnitude of typical population sizes seen during experimental range expansions in plants or invertebrates (e.g.…”

“…The growth rate r 0 was constant and set to a value that is within the range of plausible values for insects (Hassell et al, 1976), and worked well in previous simulations (Dahirel, Bertin, Haond, et al, 2021). K was chosen to match previous simulations (Dahirel, Bertin, Haond, et al, 2021; Haond et al, 2018), and is within an order of magnitude of typical population sizes seen during experimental range expansions in plants or invertebrates (e.g. Dahirel, Bertin, Haond, et al, 2021; Ochocki & Miller, 2017; Van Petegem et al, 2018; Williams & Levine, 2018).…”

“…However, these studies have so far mostly focused on variation in dispersal capacity (or the underlying enabling or enhancing traits). As a result, and despite some indications that individual variation in density-dependence itself may be subject to selection during range expansions (Dahirel, Bertin, Calcagno, et al, 2021; Fronhofer et al, 2017; Mishra et al, 2020; Weiss-Lehman et al, 2017), we mostly don’t know how the structure of individual variation in dispersal influences whether an expansion is and stays pushed or pulled. Indeed, the one theoretical study on the subject, which showed that genetic variation leads pushed expansions to become pulled with time, was focused on Allee effects only (Erm & Phillips, 2020).…”

Phenotypic evolution during range expansions is contingent on connectivity and density dependence

“…In a previous study, Dahirel et al (2021a) combined simulations and experimental evolution of the small wasps Trichogramma brassicae to show that low connectivity led to more pushed expansions, and higher connectivity generated more pulled expansions. In accordance with theoretical predictions, this led to reduced genetic diversity in pulled expansions, and the reverse pattern in pushed expansions.…”