Plant biodiversity is currently declining swiftly in many European regions and ecosystems, as documented by the declining number of recorded populations and increasing number of plant species on Red Data Lists. An estimated 20-30% of the native flora of The Netherlands is now considered seriously at risk or actually has become extinct in recent years. This follows from an analysis of distribution maps over the period 1960-80 (Weeda et al. 1990). This figure corresponds well with similar data for Germany (Korneck & Sukopp 1988), Switzerland (Landolt 1991), the USA (Falk 1992) and even for the whole of the vascular plant flora of the world (Raven 1987). There is strong concern about the loss of diversity because diversity is frequently regarded as a measure of ecosystem quality and there is growing evidence that biodiversity is important for maintaining ecosystem functions (Naeem et al. 1996; Tilman et al. 1996). Moreover, because there is a time-lag before the deterioration of conditions is expressed in the extinction of populations, the recorded decline in regional biodiversity may represent only the forerunner of future larger losses (Tilman et al. 1994). This time-lag may be especially large in long-lived plants.