Abstract
In northern Europe, the tick Ixodes ricinus is of particular concern, as it is the principal vector of the spirochete Borrelia burgdorferi sensu lato (s.l.) which is known to cause Lyme disease in humans, among other pathogens. Small mammals such as rodents and shrews are found to be the main host of I. ricinus larvae and competent reservoirs for tick-borne pathogens. As such, it is necessary to better understand the ecology of these small mammal tick hosts in order to increase the knowledge on tick-borne pathogens and how they spread. This thesis therefore focused on three of the most important small mammal hosts for I. ricinus larvae in Scandinavia, the bank vole (Myodes glareolus), the wood mouse (Apodemus sylvaticus), and the common shrew (Sorex araneus). The main aim of this thesis was to investigate to what extent habitat affect the occupancy and abundance of different small mammal species and their tick loads. There were two study areas used in this thesis; Førde, which is located in Sogn of Fjordane, and Son, which is located in Akershus, both in Norway. As expected, there were differences between the rodents and shrews in regards of their population structure, and the common shrew dominated in terms of abundance in Førde and constituted about 54% of the catch in Førde, while the rodents were the most abundant in Son with the bank vole constituting about 42% of the catch. The abundances were found to vary between the years and season, but overall, the abundance of the common shrew was found to be more stable than the rodents’. The spatial distributions of the small mammals increased in years with increasing abundances suggesting density-driven occupancy patterns. In order to gain insight into how the occupancy, abundance and tick load of these small mammals is affected by the habitat, several variables were measured at each trapping station. Vegetation cover by different groups and species of plants was represented by two principal component axes (PCA), in addition to structural variables such as hole in soil, hole under stone and canopy cover. There was in general too low spatial variation to explain their occupancy by the habitat variables, consistent with these small mammals being habitat generalists. While their abundance could be partially explained by these habitat variables, there was however little consistency in which specific variables affected the abundance between species and regions. The load of I. ricinus larvae on the small mammals was found to fluctuate both yearly and seasonally, and it was generally higher in shaded areas. However, the variables affecting the tick load differed from those affecting the small mammal abundance species-wise, suggesting that the tick load spatially is more dependent on the tick densities rather than the host densities. My findings show that one cannot necessarily extrapolate results from one area to another, even regarding the same host species. The generalist nature and high abundance of the small mammal species found in the both regions make them important as host for I. ricinus, and hence they likely play important roles in the transmission cycle of tick-borne diseases.