Species dynamics of fungi (filamentous fungi and yeasts) on apple leaves were studied within the framework of the theory of island biogeography by following "immigration" and "extinction" patterns on individual apple leaf "islands" over time. Total fungi were censused on unmanipulated leaves collected throughout two seasons; filamentous fungi only were monitored additionally for several weeks in one season on newly created, axenic, model (seedling) islands introduced to the orchard, and on surface-sterilized, preexisting leaves. Analyses based on both the natural and the surface-sterilized systems showed that an equilibrium in species number was reached and turnover in species composition occurred in both. Immigration and extinction events were strongly related to number of species present on each island. The balance between immigration and extinction implies that species number on leaves and "real" (oceanic) islands is determined by a common mechanism, and emphasizes the need to regard leaf microbial communities as dynamic.