Population persistence and offspring fitness in the rare bellflower Campanula cervicaria in relation to population size and habitat quality

Data from several animal species and a few plant species indicate that smal l populations face an elevated risk of extinction. Plants are still underre presented in these studies concerning the relation between population size and persistence. We studied the effect of population size on persistence am ong natural populations of the rare bellflower Campanula cervicaria in Finl and We monitored 52 bellflower populations for 8 years and found that the m ean population size decreased from 24 to 14 during this period. Small popul ations with less than or equal to 5 individuals were more prone to losing a ll fertile plants than were larger ones. Reduction in population size was n evertheless unrelated to the degree of population isolation measured as the distance to the nearest known population To test the hypothesis that offsp ring fitness is lower in small populations, we germinated bellflower seeds from different-sized populations in a laboratory and found that seed germin ation ability was independent of population size. The seedlings raised from seeds of small populations grew faster than those taken from larger popula tions. Population size was negatively related to the amount of shade in the habitats. In conclusion decreasing population sizes of C. cervicaria seeme d not to be caused by lowered germination ability or growth rate in small p opulations; rather, population size reductions appeared to be due to closin g of vegetation in the habitats.