VARIATION IN SPECIES RICHNESS ON SMALL GRASSLAND QUADRATS - NICHE STRUCTURE OR SMALL-SCALE PLANT MOBILITY

In this study we make clear that the significance of deviations from a n expected variance in species richness as demonstrated in alvar grass land, is a function of spatial pattern at the scale richness was measu red, i.e. 10 cm(2). If corrected for spatial dependence, more than hal f of the significant cases become nonsignificant. As regards cases of significant variance deficit, which has been interpreted as an indicat ion of niche limitation, we suggest that there may be a simple physica l limitation to the number of species on the scale of observation in t he form of a low number of plant units which can find a place because of the modal plant size of the species involved. In most cases of sign ificant variance deficit the modal size of the plant units involved wa s probably bigger than average. Insofar as a significant variance defi cit can be demonstrated, the resulting species richness distribution c urve should be analyzed and we propose a skewness test, enabling us to differentiate between significantly left-skewed curves ('niche limita tion'), significantly right-skewed curves ('niche facilitation') and s ymmetrical curves. We present results obtained with the G-test, a log- likelihood ratio goodness of fit test. Only few cases of significantly left-skewed curves and a majority of symmetrical curves were found. A ttempts to demonstrate guild proportionality in grasslands suffer from the heterogeneity of usuary distinguished guilds, such as annuals vs. perennials or graminoids vs. dicotyledons. We observe that niche limi tation may occur in alvar grassland, but indications for niche facilit ation are stronger. Finally, we conclude that deviations in species ri chness variance are interesting indications of community structure, bu t only of spatial structure. Niche structure resulting from assembly r ules should be investigated through experiments.