RATES OF SMALL-SCALE SPECIES MOBILITY IN ALVAR LIMESTONE GRASSLAND

Small-scale species frequency and cumulative species frequency were st udied in four plots in limestone grassland of the Veronica spicara-Ave nula pratensis association on Stora Alvaret on the Baltic island of Ol and, Sweden. Species mobility was expressed as increase in cumulative species frequency in 20 subplots of 100 cm(2). Observed cumulative fre quencies from 1985 - 1989 in all four plots, and from 1985 - 1995 in o ne plot were compared with values following from two null models, a 'm inimal mobility' model and a random mobility model. In ca. 50 % of the cases the observed cumulative frequency was not significantly differe nt from the random expectation. However, in many such cases the mean a nnual frequency was either very high or very low. Three ways of calcul ating the mobility rate are presented though only one is used: (observ ed cumulative frequency lowest annual frequency)/expected cumulative f requency. Values x 100 range from 0 to 100. There were slight differen ces between the four plots which were interpreted in terms of differen ces in grazing intensity and soil depth. It is stressed that the idea of the Carousel model has never been meant to suggest that all species would show random mobility, which we now quantify, but that species d iffer in their mobility rate and that the mean rate is much higher tha n generally realized.