Ecological and evolutionary implications of shapes during population expansion
Coles, Christopher Lee
Thesis or dissertation
University of Exeter
Reason for embargo
To enable publication of the thesis elsewhere
The spatial spread of populations is one of the most visible and fundamental processes in population and community ecology. Due to the potential negative impacts of spatial spread of invasive populations, there has been intensive research into understanding the drivers of ecological spread, predicting spatial dynamics, and finding management strategies that best constrain or control population expansion. However, understanding the spread of populations has proved to be a formidable task and our ability to accurately predict the spread of these populations has to date been limited. Microbial populations, during their spread across agar plate environments, can exhibit a wide array of spatial patterns, ranging from relatively circular patterns to highly irregular, fractal-like patterns. Work analysing these patterns of spread has mainly focused on the underlying mechanistic processes responsible for these patterns, with relatively little investigation into the ecological and evolutionary drivers of these patterns. With the increased recognition of the links between microbial and macrobial species, it is possible that many of the ecological/evolutionary mechanisms responsible for these patterns of spread at a microbial level extrapolate to the spatial spread of populations in general. Through an interdisciplinary approach, combining empirical, computational and analytical methods, the principal aim of this thesis was to investigate the ecological and evolutionary basis of microbial spatial dynamics. The first section of this thesis utilises the Pseudomonas microbial model system to show that the rate of microbial spatial spread across agar plate surfaces is affected by both intrinsic and extrinsic factors, thereby causing the exhibited rates of spread to deviate from the predictions made by the classical models in spatial ecology. We then show the spatial dynamics of microbial spread depends on important environmental factors, specifically environmental viscosity and food availability and that these spatial dynamics (particularly the shape of spread) has conflicting impacts on individual- and group-level fitness. From this, we used a geometric framework representing the frontier of a population, combined with an individual based model, to illustrate how individual-level competition along the leading edge of the population, driven by geometric factors and combined with simple life-history rules, can lead to patterns of population spread reminiscent of those produced by natural biofilms. The thesis finishes by establishing that the spatial pattern of spread is not seemingly amenable to artificial selection, although based on other results in this thesis, we believe it remains likely that the patterns of spatial spread and the strategies responsible for them have evolved over time and will continue to evolve. Combined, the results of this thesis show that the array of evolutionary factors not accounted for by the simple ecological models used to help manage invasive species will often cause these models to fail when attempting to accurately predict spatial spread.
PhD in Biological Sciences