The ecology and economics of alien invasive species with a case study from the South African Fynbos

Biological invasion by alien species is a global phenomenon that is associated with an array of ecological and economic impacts. This thesis consist of three parts, each of which aims to improve our understanding of these impacts. Part I reviews the literature on invasive species and ecosystem services and provides an overview of Parts II and III. Part II investigates the impacts of alien species on biodiversity using theoretical neutral models of community ecology. Part III investigates the impacts of alien species on ecosystem function and ecosystem services with a case study from South Africa.;The central question in Part II of the thesis is whether macroscopic biogeographical patterns of invasion can be explained by simple neutral ecological models that do not invoke species-specific fitness differences. Part II lays the groundwork for rigorous tests of the classic spatially implicit model of neutral theory against data from invaded ecosystems. Chapter II.1 presents analytical results that relate the immigration parameter of the neutral theory to the parameters of dispersal kernels. This result is important because it assigns biological meaning to the immigration parameter and allows the spatially implicit model to be compared to data without relying on curve-fitting procedures. For a tropical forest community in Panama, I find acceptable correspondence between the estimate of the immigration parameter derived from dispersal kernels and previous curve-fitted values. Chapter II.2 presents an analytical proof that neutral and niche models predict asymptotically equivalent species abundance distributions in high-diversity ecosystems. This result is important because it suggests that neutral drift can govern species abundances even when strong niche structure exists. Thus, neutral models are robust, parsimonious models of diversity, although they cannot be used to infer an absence of niche structure or to predict ecosystem function. Chapter III.3 investigates the time-dependent solutions of the neutral model, which are essential for applying the theory to invasions. I present analytical expressions for the time-dependent solutions, and illustrate the results with numerical simulations. I present species abundance and richness data from chronosequences of sites in invaded grassland and forest ecosystems, but comparisons with the theoretical results are difficult because of confounding factors that vary systematically across the chronosequences. To facilitate future progress in the application of neutral theory to invasions, I propose that (i) the analytical time-dependent solutions need to be simplified to allow quantitative confrontation with data; and (ii) permanent plots should be established in invaded ecosystems to test theories from invasion biology and community ecology more generally.;The central question of Part III of the thesis is whether afforestation of a South African Fynbos catchment with alien invasive Pinus radiata trees is economically viable when the potential benefits of carbon sequestration and timber production are balanced against the potential losses to water supply. In Chapter III.1, I present field data showing that changes in soil carbon under plantations, hitherto a crucial unknown, are negligible in the context any economic analysis. In Chapter III.2, I use a dynamic ecological-economic model to show that afforestation is currently economically unviable from the perspective of society, but that it appears viable to the forestry industry because of water subsidies and that the future viability of afforestation is highly uncertain because of uncertainty in economic parameters. Part III concludes with recommendations that water tariffs be raised to align the incentives of forestry companies with those of society, that future studies on this topic should utilise non-probabilistic decision theory because of the large uncertainties in future economic conditions, and that global climate legislation should be explicit as to whether afforestation of native vegetation with alien trees is a legitimate climate-change mitigation strategy.