Population size and the risk of local extinction:: empirical evidence from rare plants

Due to habitat fragmentation many plant species today occur mainly in small and isolated populations. Modeling studies predict that small populations will be threatened more strongly by stochastic processes than large populations, but there is little empirical evidence to support this prediction for plants. We studied the relationship between size of local populations (number of flowering plants) and survival over ten years for 359 populations of eight short-lived, threatened plants in northern Germany (Lepidiumcampestre, Thlaspiperfoliatum, Rhinanthusminor, R. serotinus, Melampyrumarvense, M. nemorosum, Gentianellaciliata and G. germanica). Overall, 27% of the populations became extinct during the study period. Probability of survival of a local population increased significantly with its size in all but one species (R. minor). However, estimated population sizes required for 90% probability of survival over 10 years varied widely among species. Survival probability increased with decreasing distance to the nearest conspecific population in R. serotinus, but not in the other species. The mean annual growth rate of surviving populations differed greatly between species, but was only for G. germanica significantly lower than 1, suggesting that there was no general deterministic decline in the number of plants due to deteriorating habitat conditions. We conclude that the extinction of populations was at least partly due to stochastic processes. This is supported by the fact that in all species a considerable proportion of small populations survived and developed into large populations.