The Influence of Plant Species Composition on an Endangered Grassland Specialist Reptile, the Hungarian Meadow Viper

) is one of the most threatened vertebrates in Hungary, whose populations are not growing significantly despite enormous conservation efforts. Previous studies suggested an influence of vertical vegetation structure on habitat use, while the role of horizontal vegetation structure is still poorly understood. In the present study, we used vegetation survey data to investigate the effects of variables related to the horizontal structure and functional composition of vegetation on the occupancy and density of the Hungarian meadow viper. During a spring survey period, we collected viper occurrence data in 59 sampling quadrats alongside plant community samples, then used single-season occupancy models and N-mixture models for analysis. After model selection, the best models included the moisture-related vegetation gradient, species richness, graminoid-forb ratio, and height of plants as explanatory variables for both occupancy and density. Wetter meadows with fewer plant species, a higher graminoid/forb ratio, and habitats with characteristically lower-growing plant species were more probable to be used by the vipers. Our results suggest that the horizontal structure of the vegetation influences the habitat use of vipers and also draw attention to the threats posed by more frequent droughts and heatwaves.