Fossorial rodents are through their burrowing activities important disturbance agents in grassland ecosystems. Disturbances change resources availability, and thus influence the overall structure and biodiversity in a community. Fossorial rodents are frequently considered as keystone species and ecosystem engineers. The European ground squirrel is endemic fossorial rodent to central and south-eastern Europe with decreasing population trend. In spite of scientific and conservation interest on this species, there is no information about the effect of this small fossorial rodent on diversity of grassland ecosystems. The aim of the dissertation thesis was to analyse the impact of the European ground squirrel on biotic composition of grassland ecosystem. We found that disturbed patches by ground squirrels showed higher diversity of vegetation with different plant communities in compare to undisturbed grassland vegetation accompanied with reduction of graminoid competitors, and support of forbs with different plant strategies on the mounds. Our results showed that mounds created by ground squirrels exhibit different soil properties as surrounding undisturbed soils and some soil properties are changing during the mound development. Our results indicate that ground squirrels can have significant effect on epigeic organisms that are sensitive to changes in vegetation structure like spiders and harvestmen. Our findings demonstrate the importance of strong biotic interactions in the view of management and reintroductions of fossorial rodents as keystone species.