Effects of settlement size, urban heat island and habitat type on urban plant biodiversity

We studied three habitats with different disturbance regime in 45 Central European settlements of three different sizes. We sampled 1-ha plots in each habitat by recording all spontaneously occurring vascular plant species. We divided recorded species into groups according to their origin and residence time and according to their temperature requirements based on Ellenberg indicator values. We used ordination methods and ANOVA to detect that species communities in urban areas are generally more species rich in larger settlements than in small ones. These differences are mostly pronounced in residential areas. Increasing settlement size is significantly reflected by neophytes that are dependent on constant input of propagules caused by human activities and by native species that survive in remnants of semi-natural vegetation in urban environment. In contrast archaeophytes as a homogeneous group of species with similar traits are widespread equally through settlements of all sizes. We did not confirm the effect of urban heat island on species composition, indicating that species composition is significantly more affected by local habitat conditions than by urban size. Our results highlight the importance of urban size as important factor shaping biodiversity of native and alien plant communities in individual urban habitats and the important role of habitat mosaic for maintaining high species richness in city floras.