| dc.description.abstracteng | European lumbricid earthworms were introduced into northern North America by European settlers about 400 years ago. They are invasive across the continent and cause notable changes in native forest ecosystems. Human-mediated introductions and dispersal significantly contributed to the spread of European species in North America, which commonly are used as fishing bait and are often disposed deliberately in the field. During their range expansion they encountered harsher climatic conditions as compared to their native range in Europe. Variance of abiotic factors and genetic identity or diversity can be of particular importance for successful invasions, because environmental filtering and ecological tolerance of genotypes determines performance, establishment and spread of invasive earthworm species. However, it is unclear if climate or geographic dispersal barriers shape genetic structure of earthworm populations, and whether the successful establishment of populations is based on adaptation or selection. Genetic diversity or identity of invasive earthworms in North America was never analysed in combination with climate conditions or the impact on soil properties and microbial functions.
In my PhD project I investigated the drivers for dispersal of invasive European earthworms in northern North America by using molecular markers and a climate chamber transplantation experiment. Earthworms were collected on continental, regional and local scale, i.e. > 500 km, about 100 km and within 25 km distance, respectively, and analysed using molecular markers for phylogeographic, population genetic and experimental analyses. Each chapter of this thesis investigated specific aspects of earthworm invasion in northern North America, and combined results provide new insights into drivers of earthworm invasion, i.e. (i) connectivity among populations at continental and regional scale, and the importance of dispersal barriers (Chapter 2), (ii) connectivity among populations at local scale and genetic structure of a recent invasion (Chapter 3), and (iii) role of climate conditions and origin on genetic identity/diversity, and on performance as well as soil and microbial functions (Chapter 4).
In the first part (Chapter 2) I investigated two invasive earthworm species (Lumbricus terrestris and L. rubellus) that co-occur in the same habitats but differ in ecology and use as fishing baits. For both species I tested if dispersal barriers, climatic selection, or anthropogenic activities, such as fishing bait disposal, shape the dispersal of free-living earthworm populations and if dispersal mechanisms are the same for both species or if they differ between species. Both species were sampled in five transects ranging from the east coast to the west coast of northern North America, and the sampling design including two major dispersal barriers, three different climate zones. Additionally, the same species were purchased from bait shops near sampling locations to account for local introductions by bait disposal. Genetic diversity and structure within and among populations and bait shop individuals was assessed using four markers (COI, 16S rDNA, 12S rDNA, H3). Populations of both species are genetically diverse with little geographic structure, which was more pronounced in L. terrestris than in L. rubellus. Common haplotypes were present in all regions, but locally restricted haplotypes also occurred. Further, two distinct genetic clades of L. terrestris co-occurred only in two transects (Alberta and Minnesota). Genotypes identical to bait individuals were omnipresent in field populations of L. terrestris. Genetic diversity was high in both species, and invasive populations represented a genetic subset of European earthworms. Geographic and climatic dispersal barriers affected the less mobile species, resulting in differences in genetic structure between the two species. Results indicate common long-distance dispersal vectors that are valid for both species and specific vectors affecting only L. terrestris.
The second study (Chapter 3) investigated the genetic structure of L. terrestris in a 100 km range south of Calgary, Canada, an area that likely was devoid of this species two decades ago. Genetic relationships among populations, gene flow, and migration events among populations were investigated using seven microsatellite markers and the mitochondrial 16S rDNA gene. Earthworms were collected at different distances from the city and included fishing baits from three different bait distributors. The results suggest that field populations in Alberta established rather recently and that bait and field individuals in the study area have a common origin. Genetic variance within populations decreased outside of the urban area, and the most distant populations likely originated from a single introduction event. The results emphasise the utility of molecular tools to understand the spatial extent and connectivity of populations of exotic species, in particular soil-dwelling species, that invade native ecosystems and to obtain information on the origin of populations.
Lumbricus terrestris had been introduced into North America from different source populations in Europe for several hundred years and initiated severe changes in the invaded ecosystems. As a first step to disentangle the relative importance of genetic and environmental factors on earthworm invasions I investigated (Chapter 4) the performance of earthworm populations from climatically dissimilar locations in different environmental contexts as well as their impact on soil properties and the microbiome. I conducted a yearlong full-factorial transplantation climate chamber experiment with 180 individuals of L. terrestris, which were collected from three North American sites with distinct climate conditions, altitude, and history of European settlement. Four combinations of warm and cold temperature conditions, and wet and dry moisture conditions were simulated in a climate chamber, and genetic diversity and identity was determined of surviving individuals and offspring. The results indicated that seasonality of temperature and precipitation was the main determinant for earthworm biomass gain, offspring number, and activity. Further, results showed significant effects of earthworms on soil moisture and microbial functions that were differently influenced by burrowing and litter burying/feeding activity, respectively. Genetic diversity and identity did not show a clear correlation with earthworm performance and ecosystem functions under the different climate conditions.
By combining genetic diversity of species and population genetic data at different geographic scales with climatic, geographic and historical factors, this thesis exemplifies the utility of molecular markers to address general questions in invasion biology, ecological adaptation and population structure. It further shows the potential but also limitations and caveats of transplantation climate chamber experiments that take genetic information into account. This study addressed several aspects of earthworm invasion in northern North America, i.e. intraspecific variance and climatic adaptation, and provides important insights into dispersal and genetic structure and diversity for future research. In order to understand drivers of earthworm invasion, on continental, regional and local scale, main results of this project were (1) genetic diversity of the two species is reduced, but similar to its native range, (2) passive transport of earthworms is important at all scales but differs among geographic scales, (3) climate and disturbance were identified as additional factors that affect the genetic structure of earthworm populations, and (4) genetic diversity increases in vicinity to human agglomerations. Overall, genetic structure and diversity as well as the importance of dispersal vectors vary among species, ecological groups and the geographic scale, and have to be considered in future research. | de |