Using standardized sampling designs from population ecology to assess biodiversity patterns of therophyte vegetation across scales
The analysis of diversity across multiple scales is hampered by methodological difficulties resulting from the use of different sampling methods at different scales and by the application of different definitions of the communities to be sampled at different scales. It is our aim to analyse diversity in a nested hierarchy of scales by applying a formalized sampling concept used in population ecology when analysing population structure. This concept involved a precise definition of the sampled vegetation type by the presence of a target species, in our case Hornungia petraea. We compared separate indices of inventory diversity (i.e. number of species) and differentiation diversity (i.e. extent of change in species composition or dissimilarity) with indices derived from species accumulation curves and related diversity patterns to topographical plot characteristics such as area and distance. Location
Ten plots were established systematically over a distance of 100 km each in the distribution centre of H. petraea in Italy (i.e. Marche and Umbria) and in a peripheral exclave in Germany (i.e. Thuringia and Saxony-Anhalt). Methods
We used a nested sampling design of 10 random subplots within plots and 10 systematically placed plots within regions. Internal α-diversity (species richness) and internal β-diversity (dissimilarity) were calculated on the basis of subplots, α-, β- and γ-diversity on the basis of plots in Italy and Germany. In addition, indices of inventory diversity and differentiation diversity were derived by fitting species accumulation curves to the Michaelis–Menten equation. Results
There was no significant difference in the internal α-diversity between German and Italian plots but the α- and γ-diversity were higher in Italy than in Germany. In Germany, the internal β-diversity and β-diversity were lower than in Italy. The differentiation diversity increased with increasing scale from subplots over plots to regions. The same results were obtained by calculating species accumulation curves. Significant positive correlations were encountered between the internal α-diversity and α-diversity in both countries, while the internal β-diversity and internal α-diversity showed a correlation only for the Italian plots. Similarity decay was found for German plots with respect to inter-plot distance and for Italian plots with respect to altitudinal difference and to a smaller degree to distance between plots. Main conclusions
The design chosen and the consistent analysis of species accumulation curves by the Michaelis–Menten equation yielded consistent results over different scales. The specific therophyte vegetation type in this study reflected diversity patterns also observed in other studies, e.g. a greater differentiation diversity in central than in peripheral habitats and a trend of increasing species richness towards lower altitudes. No asymptotic saturation of species richness between different scales was observed. Indications were found that the absolute level of inventory diversity at a particular scale and the completeness of the sampling procedure are the main clues for explaining the relationship between inventory and differentiation diversity at this particular scale.