Metals, pathogens, and islands. How the environment shapes the gut microbiota of wild bank voles

Wild animals inhabit a complex world that affects every aspect of their biology, including their gut microbiota – the community of microorganisms that inhabits their lower gastrointestinal tract. As the gut microbiota can affect many physiological processes of the host, such as their metabolism and immune response, it is important to identify the drivers of gut microbiota variation. While many studies have examined the different factors affecting the gut microbiota of humans, or domestic or laboratory animals, the determinants of wild animal gut microbiota are less known.
The work presented in this thesis shows how the gut microbiota of wild bank voles (Myodes syn. Clethrionomys glareolus) is affected by the three features of the host and its environment. First, inhabiting areas with low-level metal pollution (such as in Kemi, Tornio and Harjavalta) is associated with changes in the gut microbiota diversity and composition of bank voles. Second, infection by systemic pathogens can affect the gut microbiota of wild bank voles, despite the lack of direct interaction between the pathogen and gut microbiota, with important effects of possible coinfections. Third, the large-scale biogeographical patterns described in the theory of island biogeography, previously observed in plants and animals, may also be observed in the microbial communities within hosts, as seen in the gut microbiota of bank voles inhabiting the Porvoo archipelago.
Together the results of this thesis highlight the complexity of interactions between wild animals, their gut microbiota and the environment the animal inhabits. Importantly, the work in this thesis has raised further questions on the importance of gut microbiota in wild animal health, and on the potential mechanisms behind host-microbiota-environment interactions. Thus, it is increasingly evident, that the understanding of the effects of the environment and environmental change on wild animals will be incomplete without considering the impacts on their microbial symbionts.