When higher carrying capacities lead to faster propagation

AbstractThis preprint has been reviewed and recommended by Peer Community In Ecology (https://dx.doi.org/10.24072/pci.ecology.100004). Finding general patterns in the expansion of natural populations is a major challenge in ecology and invasion biology. Classical spatio-temporal models predict that the carrying capacity (K) of the environment should have no influence on the speed (v) of an expanding population. We tested the generality of this statement with reaction-diffusion equations, stochastic individual-based models, and microcosms experiments with Trichogramma chilonis wasps. We investigated the dependence between K and v under different assumptions: null model (Fisher-KPP-like assumptions), strong Allee effects, and positive density-dependent dispersal. These approaches led to similar and complementary results. Strong Allee effects, positive density-dependent dispersal and demographic stochasticity in small populations lead to a positive dependence between K and v. A positive correlation between carrying capacity and propagation speed might be more frequent than previously expected, and be the rule when individuals at the edge of a population range are not able to fully drive the expansion.

Download Full-text

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.

Download Full-text

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.

Download Full-text

Pushed beyond the brink: Allee effects, environmental stochasticity, and extinction

10.1101/004887 ◽

2014 ◽

Cited By ~ 1

Author(s):

Gregory Roth ◽

Sebastian Schreiber

Keyword(s):

High Probability ◽

Geometric Mean ◽

Environmental Stochasticity ◽

Initial Population ◽

Positive Density ◽

Allee Effects ◽

Population Densities ◽

Density Dependent ◽

Environmental Fluctuations ◽

Predation Rates

To understand the interplay between environmental stochasticity and Allee effects, we analyze persistence, asymptotic extinction, and conditional persistence for stochastic difference equations. Our analysis reveals that persistence requires that the geometric mean of fitness at low densities is greater than one. When this geometric mean is less than one, asymptotic extinction occurs with high probability for low initial population densities. Additionally, if the population only experiences positive density-dependent feedbacks, conditional persistence occurs provided the geometric mean of fitness at high population densities is greater than one. However, if the population experiences both positive and negative density-dependent feedbacks, conditional persistence only occurs if environmental fluctuations are sufficiently small. We illustrate counter-intuitively that environmental fluctuations can increase the probability of persistence when populations are initially at low densities, and can cause asymptotic extinction of populations experiencing intermediate predation rates despite conditional persistence occurring at higher predation rates.

Download Full-text

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.

Download Full-text