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

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.

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Optimal Thinning of a Year-Class with Density-Dependent Growth

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/f86-110 ◽

1986 ◽

Vol 43 (4) ◽

pp. 889-892 ◽

Cited By ~ 3

Author(s):

Rögnvaldur Hannesson

Keyword(s):

Density Dependence ◽

Optimal Harvesting ◽

Early Date ◽

Density Dependent ◽

Moderate Density ◽

Class A ◽

Dependent Growth ◽

The Impact ◽

Harvesting Costs

I consider the impact of density-dependent growth on the optimal harvesting of a year-class of fish. In general, density dependence makes "thinning" of the year-class a desirable strategy. Moderate density dependence implies that thinning should be gradual, even in the case of zero harvesting costs where the optimal harvesting strategy would otherwise be instantaneous harvesting. Strong density dependence calls for an immediate thinning at an early date, in the case of zero harvesting costs.

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Evolution of positive and negative density-dependent dispersal

Proceedings of The Royal Society B Biological Sciences ◽

10.1098/rspb.2014.1226 ◽

2014 ◽

Vol 281 (1791) ◽

pp. 20141226 ◽

Cited By ~ 29

Author(s):

António M. M. Rodrigues ◽

Rufus A. Johnstone

Keyword(s):

Density Dependence ◽

Habitat Heterogeneity ◽

Temporal Stability ◽

Ecological Factors ◽

Positive Density ◽

Density Dependent ◽

High Dispersal ◽

Variable Environments ◽

Negative Density Dependence ◽

Density Dependent Dispersal

Understanding the evolution of density-dependent dispersal strategies has been a major challenge for evolutionary ecologists. Some existing models suggest that selection should favour positive and others negative density-dependence in dispersal. Here, we develop a general model that shows how and why selection may shift from positive to negative density-dependence in response to key ecological factors, in particular the temporal stability of the environment. We find that in temporally stable environments, particularly with low dispersal costs and large group sizes, habitat heterogeneity selects for negative density-dependent dispersal, whereas in temporally variable environments, particularly with high dispersal costs and small group sizes, habitat heterogeneity selects for positive density-dependent dispersal. This shift reflects the changing balance between the greater competition for breeding opportunities in more productive patches, versus the greater long-term value of offspring that establish themselves there, the latter being very sensitive to the temporal stability of the environment. In general, dispersal of individuals out of low-density patches is much more sensitive to habitat heterogeneity than is dispersal out of high-density patches.

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

10.1101/2021.03.03.433752 ◽

2021 ◽

Author(s):

Maxime Dahirel ◽

Aline Bertin ◽

Vincent Calcagno ◽

Camille Duraj ◽

Simon Fellous ◽

...

Keyword(s):

Range Expansion ◽

Landscape Connectivity ◽

Positive Density ◽

Low Density ◽

Trichogramma Brassicae ◽

Biological Organization ◽

Density Dependent ◽

Spatial Sorting ◽

Edge Populations ◽

Density Dependent Dispersal

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.

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Eco-evolutionary dynamics, density-dependent dispersal and collective behaviour: implications for salmon metapopulation robustness

Philosophical Transactions of the Royal Society B Biological Sciences ◽

10.1098/rstb.2017.0018 ◽

2018 ◽

Vol 373 (1746) ◽

pp. 20170018 ◽

Cited By ~ 11

Author(s):

Justin D. Yeakel ◽

Jean P. Gibert ◽

Thilo Gross ◽

Peter A. H. Westley ◽

Jonathan W. Moore

Keyword(s):

Evolutionary Dynamics ◽

Movement Ecology ◽

Evolutionary Model ◽

Basins Of Attraction ◽

Collective Behaviour ◽

Migratory Behaviour ◽

Dual Nature ◽

Density Dependent ◽

Dynamic Landscapes ◽

Density Dependent Dispersal

The spatial dispersal of individuals plays an important role in the dynamics of populations, and is central to metapopulation theory. Dispersal provides connections within metapopulations, promoting demographic and evolutionary rescue, but may also introduce maladapted individuals, potentially lowering the fitness of recipient populations through introgression of heritable traits. To explore this dual nature of dispersal, we modify a well-established eco-evolutionary model of two locally adapted populations and their associated mean trait values, to examine recruiting salmon populations that are connected by density-dependent dispersal, consistent with collective migratory behaviour that promotes navigation. When the strength of collective behaviour is weak such that straying is effectively constant, we show that a low level of straying is associated with the highest gains in metapopulation robustness and that high straying serves to erode robustness. Moreover, we find that as the strength of collective behaviour increases, metapopulation robustness is enhanced, but this relationship depends on the rate at which individuals stray. Specifically, strong collective behaviour increases the presence of hidden low-density basins of attraction, which may serve to trap disturbed populations, and this is exacerbated by increased habitat heterogeneity. Taken as a whole, our findings suggest that density-dependent straying and collective migratory behaviour may help metapopulations, such as in salmon, thrive in dynamic landscapes. Given the pervasive eco-evolutionary impacts of dispersal on metapopulations, these findings have important ramifications for the conservation of salmon metapopulations facing both natural and anthropogenic contemporary disturbances. This article is part of the theme issue ‘Collective movement ecology’.

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Virus-host interactions shape viral dispersal giving rise to distinct classes of travelling waves in spatial expansions

10.1101/2020.09.23.310201 ◽

2020 ◽

Author(s):

Michael Hunter ◽

Tongfei Liu ◽

Wolfram Möbius ◽

Diana Fusco

Keyword(s):

Cooperative Behavior ◽

Test Organism ◽

Host Density ◽

Reaction Diffusion ◽

Reaction Diffusion Equations ◽

Effective Density ◽

Density Dependent ◽

Phage T7 ◽

The Impact ◽

Density Dependent Dispersal

Reaction-diffusion waves have long been used to describe the growth and spread of populations undergoing a spatial range expansion. Such waves are generally classed as either pulled, where the dynamics are driven by the very tip of the front and stochastic fluctuations are high, or pushed, where cooperation in growth or dispersal results in a bulk-driven wave in which fluctuations are suppressed. These concepts have been well studied experimentally in populations where the cooperation leads to a density-dependent growth rate. By contrast, relatively little is known about experimental populations that exhibit a density-dependent dispersal rate.Using bacteriophage T7 as a test organism, we present novel experimental measurements that demonstrate that the diffusion of phage T7, in a lawn of host E. coli, is hindered by steric interactions with host bacteria cells. The coupling between host density, phage dispersal and cell lysis caused by viral infection results in an effective density-dependent diffusion rate akin to cooperative behavior. Using a system of reaction-diffusion equations, we show that this effect can result in a transition from a pulled to pushed expansion. Moreover, we find that a second, independent density-dependent effect on phage dispersal spontaneously emerges as a result of the viral incubation period, during which phage is trapped inside the host unable to disperse. Our results indicate both that bacteriophage can be used as a controllable laboratory population to investigate the impact of density-dependent dispersal on evolution, and that the genetic diversity and adaptability of expanding viral populations could be much greater than is currently assumed.

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