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There are some organisms on Earth that have an especially close relationship with humankind, and one of them is Saccharomyces cerevisiae. This species, which is also known as the baker´s or brewer´s yeast, has been used for thousands of years, and almost all around the world, for the production of alcoholic beverages and bread. Today, we owe even more to this organism than wine, as it became one of the best studied model organisms in biology and is widely used in different scientific disciplines like Cell Biology, Biochemistry, Genetics and Molecular Biology. But surprisingly little is known about its natural ecology. The habitat of wild, undomesticated Saccharomyces yeasts cannot be reliably defined and contrary hypotheses exist. There is some evidence that the yeast is adapted to live in sugar rich environments like fruits, but on the other hand there is also indication that wild Saccharomyces yeasts are associated with oak trees, especially their bark. In both environments, yeast is only found in a small proportion of samples and most studies additionally suffer from different biases making it impossible to conclude which, if either, environment is truly the natural habitat of the yeast. In this thesis the natural ecology of Saccharomyces spp. was studied, with an emphasis on the natural environment. The oak bark environment was analyzed by characterizing the associated microbial community using culturing and pyrosequencing methods. S. paradoxus, the wild and undomesticated relative of S. cerevisiae, could indeed grow on nutrients present in oak bark but is only a rare member of the microbial community. Studying the influence of co-isolated oak bark microorganisms on S. paradoxus’ growth and survival in natural oak bark medium revealed a wide range of effects strongly dependent on temperature. Further experimental analysis of the interactions between the yeast and two of the bacteria gave insights into the diversity and complexity of natural microbial interactions. While one Pseudomonas spp. killed the yeast, another bacterium; Mucilaginibacter spp. promoted its growth. Saccharomyces’ metabolic specialty of fermenting under aerobic conditions (Crabtree effect) is taken as evidence that the yeast is adapted to sugary fruit environments. To test this, a Crabtree positive (“fermenter”) and its isogenic Crabtree negative mutant strain (“respirer”) were competed in laboratory media and grape juice, confirming that the Crabtree effect provides a benefit under resource competition in lab medium. Unexpectedly, this benefit was absent in the more natural grape juice, although adding natural microbial competitors restored the benefit of fermentation over respiration, perhaps by interference competition. Finally, the yeast / oak association could be confirmed by an intense sampling study, but Saccharomyces was found to be much more abundant in oak leaf litter than on bark. Oak leaf litter provides a stable habitat over the year from which yeast can be easily isolated and studied. This is a useful discovery for the ecology and evolutionary history of Saccharomyces yeasts, with great promise for future studies.