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.
1_Insects feeding on the foliage of oak were studied on a mountain where species of Mediterranean deciduous and evergreen oak coexist. There were 58 insect species (54 Lepidoptera, 1 Coleopteran and 3 Hymenoptera) belonging to twenty families in the assemblage feeding on eight species of Quercus, two of which are introduced from nearby regions. The overlap in occurrence in time and of feeding niches of the insects feeding on the foliage of the different species of oak was determined using the: (a) Poole-Rathcke method, which tests phenological overlap and (b) Petraitis method, which tests niche overlap. This indicated that insect families partition seasonal time in a random and the entire assemblage in a regular way. All groups of insects partitioned season randomly except for the pairs of monophagous-oligophagous and Palearctic-Eurosiberian species, which partition season regularly. Oak folivorous insects correctly perceive the three subgenera of oaks with the exception of the planted Q. robur pedunculiflora. The folivorous insects recorded on the Mediterranean evergreen oaks (subgenus Sclerophyllodrys) differ from those on the other two subgenera (Quercus and Cerris) and co-occurring deciduous trees. The hypothesis of complete general overlap is rejected for groups based on feeding specialization, zoogeographical categories and taxonomic families. The same was the case when the entire insect assemblage was considered. The percentage of specific niche overlap of the folivorous insects is low and greatest among the monophagous species (13.8%) and those with a Mediterranean distribution (15.4%). Voltinism is not very important for this assemblage and only seven species are bivoltine of which four fed on a different species of oak in the second generation., 2_The overall conclusion is that the co-occurrence in space of these species is possible because they occur regularly at different times during the season whereas that of insect groups based on zoogeographical, taxonomic or feeding specialization are randomly dispersed in time., Maria Kalapanida, Panos V. Petrakis., and Obsahuje seznam literatury