| Summary: | Earthworms are among the most crucial species for global food production and soil fertility. However, in soils and ecosystems that have evolved without worms, their introduction can lead to significant ecological change. Due to the eradication of soil fauna during the last glacial cycle, and slow recolonization, high-latitude soils generally lack large earthworms. But this situation is about to change as several species of earthworms are spread to northern habitats through human-mediated dispersal. In this thesis, I investigate the impacts on plant communities and carbon cycling that results from the dispersal of earthworms—primarily Lumbricus and Aporrectodea spp.—in the Fennoscandian Arctic. To achieve this, I combined data from a four-year mesocosm study with observations from earthworm-invaded soils in the Fennoscandian mountain range. My findings indicate that earthworm presence can make tundra ecosystems more graminoid-rich, and cause preferential grazing by rodents, likely due to the higher nitrogen content in plants growing in more fertile soil. My research has revealed that earthworms play a significant role in stimulating tundra plant biomass growth, particularly belowground. I attribute this increase in plant biomass to the extended growing season facilitated by earthworm activity and more plant available nitrogen. This growth enhancement was consistent across different vegetation types but only led to an increase in net ecosystem carbon (C) uptake in dwarf shrub-dominated tundra. In contrast, in meadow tundra, earthworms had no net effect on the ecosystem C pool, due to an increased mineralization of soil organic carbon (SOC), which counterbalanced the enhanced plant carbon sequestration. Furthermore, using species distribution modelling, I confirmed that earthworm dispersal in the Fennoscandian Mountains is likely driven by human vectors. I estimate that approximately 7% of this region currently consists of habitats that are both climatically suitable and prone to human-mediated earthworm ...
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