Vascular flora of 71 artificial islands of varying agewas analyzed in 22 fishponds, Třeboň Basin, the Czech Republic. Data on species richness were interpreted in terms of Wilson’s (1969) hypothesis on the development of biotic communities. An increase in species richness during the non-interactive stage (one to two years) and a decrease in the interactive stage (three to six years) were both statistically significant. As predicted, by Wilson’s hypothesis, there was also an increase in species richness in the assortative stage (seven to >50 years), however, this trend was not significant. This successional pattern was confirmed by the vegetation development recorded on 34 re-sampled islands. For these islands the positive change in species richness during the assortative stage was significant based on both paired t- and binomial tests. Contributions of island area, elevation, and isolation during individual successional stages were evaluated. As a whole, this is probably the first clear confirmation of Wilson’s hypothesis for vascular plant communities on islands.
The intermediate disturbance hypothesis predicts that species diversity in biotic communities should peak at intermediate levels of disturbance. The vegetation of eight calcareous screes in the Northern Calcareous Alps, Austria was studied to test this hypothesis. Data on total vegetation cover and on cover of individual species were obtained from 0.25 m2 , 0.5 m2 , and 1.0 m2 plots. An index [100 – vegetation cover (%)] was used as a surrogate for direct quantification of disturbance (movement of stones and destruction of biomass). Its suitability was substantiated by the observation of position changes of marked stones after two years. In agreement with the intermediate disturbance hypothesis, significant unimodal responses of both species richness and species diversity to disturbance were found using plots of all three sizes. However, the maxima of species richness- and species diversity-disturbance curves were shifted towards higher vegetation cover (= lower disturbance) with increasing mean diameters of the scree-building stones. This shift can be interpreted either as a synergistic effect of disturbance frequency combined with disturbance intensity/extent, or as a result of an increase in spatial heterogeneity promoting species coexistence in successionally more advanced patches of vegetation.