ABSTRACT

The relative importance of demography, genetics and reproductive biology for the viability of small populations was assessed forthe rare perennial herb Gentiana pneumonanthe. Based on a previously published analysis, a stochastic matrix projection model was constructed to simulate population dynamics during heathland succession after management-induced disturbance (sod-cutting). Demographic data were used mainly from one population that has been censused from the invasive to the regressive stage of population succession. In earlier studies, I established the relationships between population size, selfing rate and inbreeding depression, as well as between population size and reproductive success. Three variants of the basic, demographic model were made, incorporating (1) the relationship between population size and reproductive success (RE variant), (2) the relationships between population size, selfing rate and inbreeding depression (ID variant) and (3) both of the above relationships (RE + ID variant). Reduced reproductive success did not have a significant effect on population performance. Inbreeding depression, however, resulted in significantly lower peak population sizes, a more rapid extinction and a lower growth rate. The latter effects were even stronger in the RE + ID model. In this variant, the time to extinction, TE, of small populations decreased by six years relative to the basic model. Significant differences in TE between the basic model and this variant persisted up to an initial population size of 250 plants. Further simulations with various time intervals between sod-cutting again showed that the RE variant differed very little from the basic model. The ID variant differed only for initial population sizes of <50 individuals.