Frequency of plant species in remnants of calcareous grassland and their dispersal and persistence characteristics

Why should some species be so much more common than others? Here, we propose that traits affecting dispersal and persistence can be used to predict the frequency of occurrence of a species in grassland remnants in the Swiss Jura Mountains. We established the frequency of occurrence of 112 plant species in 96 remnants of calcareous grassland in NW Switzerland. We determined the following eight traits for each species: (1) dispersal category (unassisted, ant-, adhesion- or wind-dispersed), (2) terminal velocity of diaspores, (3) plant height, (4) mass per seed, (5) seed shape (variance of length, width and height), (6) onset of flowering, (7) duration of flowering, and (8) life form (clonal perennials, perennials without pronounced vegetative reproduction, short-lived species). Then we tested whether there is a correlation among these traits with the frequency of occurrence of a species in grassland remnants using stepwise multiple regression with the original data as well as with phylogenetically independent contrasts (PICs) calculated with a phylogeny based on recent molecular analysis. Species with an early onset of flowering (p < 0.01, R-2 = 0.09), a clonal life form (p < 0.01, R-2 = 0.08), a long duration of flowering (p < 0.02, R-2 = 0.04) and with heavy seeds (p < 0.05, R-2 = 0.03) occurred more frequently than others. In total the model explained 26% of the variance in species' frequency. Results were confirmed by the analysis with PICs except for the one concerning life form. Our results suggest that traits enhancing persistence are more important for the frequency of occurrence of a species in calcareous grassland than traits affecting dispersal. This might imply that in grassland remnants colonisations by far-distance dispersal are insignificant because of a lack of diaspore exchange among populations.