The humped-back relationship in species diversity during succession was tested using vegetation in a coastal sand dune system of the German Wadden Sea island of Spiekeroog. Permanent plots were studied over 15 years along a spatial chronosequence from young grey dunes to old brown dunes. Species diversity, succession rate, and environmental indicator value were used to evaluate multitemporal dynamics of the ecosystem. Long-term development of the dune vegetation was reflected along the chronosequence, whereas fluctuations of plant communities were analyzed by short-term changes of each permanent plot.

The study confirmed the intermediate stress theory, whereas highest species diversity was reached at the transition zone of the environmental gradients. Total species richness showed humped-back relations along the xerosere. Hotspots in species diversity varied with the life form group. Highest richness of herbaceous plants was reached in semidynamic young grey dunes, whereas highest richness of bryophytes and lichens shifted due to competition effects to the driest parts of the dune gradient in older successional stages.

Moreover, the study confirmed the biodiversity-stability theory, with highest ecosystem stability at highest diversity. More stable environments showed lower succession rates than dynamic, disturbed environments. Thus, during secondary succession with probably repeated disturbance, the succession rate was higher and no clear direction of vegetation development was found in comparison to primary succession. The succession rate in a primary series represented a unimodal relation with total species richness. Thus, species-poor vegetation often dominated by one species, such as heathlands, as well as species-rich vegetation, showed lowest succession rates. These communities probably are more stable due to a balanced species combination represented by higher evenness.