Fluctuations of dominance and the coexistence of plant species in limestone grasslands

In limestone grasslands in northern Switzerland some factors which cause fluctuations of species' dominance (cover) were investigated. It was assessed whether these fluctuations contribute to the coexistence of up to 40 different plant species per square metre. Weather fluctuations, particularly drought years, and irregularities of the mowing date are causes of the so-called exogenic fluctuations of species abundance. Endogenic fluctuations are e.g. those caused by the turnover rate of the populations and by the changes of population density of the common vole (Microtus arvalis Pall.). Examples for plant species favoured or inhibited by these different fluctuations are given. The fluctuations and their interferences mean that no species encounters optimal growth conditions for a prolonged period of time. Therefore no species can outcompete the others. Fluctuations are thus decisive for the coexistence of species. Other ecosystem structures and functions contributing to coexistence are mentioned.