Main objective of the study was the description of Weighted Average Analysis (WAA) as appropriate mathematical method for modelling the demands of freshwater molluscs on their habitats. With this approach site-specific environmental factors defining a certainbiotope are weighted by the related population density of a selected organism. This allows the determination of optimum ranges of physical and chemical factors, within which highest probabilities of occurrence of the investigated organism may be expected. As an exemplary case ecological demands of the Austrian spring snail, Bythinella austriaca, were modelled by using WAA. In addition, a global marginality coefficient, which reflects the ability of the gastropod to occupy ecological niches, and a global tolerance coefficient, describing the width of the niche colonized by the organism, were computed for 10 selected environmental factors. As suggested by the modelling results, B. austriaca exhibits specialist behaviour with regard to water temperature, electric conductivity, dissolved oxygen, total water hardness, geographic altitude, biological oxygen demand, current velocity, and dissolved nitrate. Furthermore, the gastropod only tolerates small to intermediate fluctuations of these environmental factors, which remarkably reduces its ability to act as a pioneer species.