Quantitative data are presented of a study of a combined height-pH gradient in a primary dune slack at Schiermonnikoog. A transect of 10 x 4 m was floristically and ecologically analysed per sq m. Species number was taken as a measure of species diversity and was found to be related to range of pH and height: highest diversities occurred in the middle range of the gradient. This confirms a general rule described by Odum. Species-area relation in the transect fits into the Preston model, the slope index being slightly under the theoretical equilibrium value 0,27. Ordination and classification of quadrats on a basis of information index I showed continuous variation of vegetation, which was mainly governed by pH variation. The twofold differentiation of the interrelated height and pH is termed ecological amplification and discussed. The relation between fioristic fall and gradient range and between fioristic difference of vegetations and their distance along a gradient are discussed. The diversity pattern found is interpreted in terms of niche differentiation governed by spatial and temporal variation of the environment. The possible relation between niche differentiation and gradient structure of the environment is discussed.