The role of dispersal in life history and population dynamics: an experimental and theoretical approach

Abstract

Understanding the evolution and maintenance of phenotypic and genetic variation within populations is a key challenge in population biology. Discrete phenotypic variation such as alternative reproductive strategies and dispersal strategies are extreme forms of this. To understand phenotypic variations, such as male dimorphisms and dispersal expression, requires investigating the costs and benefits of these different phenotypes. In this thesis I do so. First, at the individual level, I determine trade-offs between life-history traits and phenotypic expression. The influence of male morph expression is assessed by determining whether male morph survival is frequency dependent. Costs of dispersal expression were assessed by comparing individuals that expressed the dispersal phenotype during their ontogeny with individuals that did not. Second, at the population level, I specifically investigate costs of phenotype expression to the natal population when dispersers fail to disperse. To determine any demographic costs to natal populations, I used structured integral population models to calculate population biology quantities, which I compared between populations that produced dispersers that fail to disperse and populations that produced no dispersers. I show that expressing a dispersal morph is costly to life-history traits and skews the male morph ratio, indicating that these two conditional strategies interact during ontogeny. This questions whether current models explaining single conditional strategies, such as the environmental threshold model, should consider interactions between different conditional strategies. In natal populations where dispersal is expressed, but dispersers fail to disperse, populations suffer reduced fitness and this demographic cost is enhanced in stochastic environments. These results do not include benefits of successful dispersal or other costs such as inbreeding. However, they do provide a cost of dispersal expression which indicates what the benefits of dispersal would need to be for dispersal to evolve. One aspect that the results do not inform on is possible eco-evolutionary dynamics in populations. Future work should look to incorporate eco-evolutionary feedback, within a metapopulation structure, to identify the maintenance and evolution of male dimorphism and dispersal