Sample-size-based rarefaction (solid line segment) and extrapolation (dashed line segment) curves for species richness (Hill number q = 0) with 95% confidence intervals of native (a) and alien (b) vascular plant species of the four studied regions. PolaPlai – Polish Plain, HighWeCa – High Western Carpathians, MatrWeCa – Matricum of Western Carpathians, Pannonian – Pannonian lowland.