Abstract
Plant population size and density can influence the interactions between plants and pollinators, and affect plant reproductive success. We investigated the effects of local conspecific density on insect visitation and fertilization success in the rare, cliff-dwelling, self-incompatible Centaurea corymbosa, in which fecundity plays a key role in variation in population growth rate among years and among the six extant populations. Plant size, capitulum size, the abundance of co-flowering species, and the weather conditions were added as covariates in the analyses. Over three populations and two years (1995 and 2002), fertilization rate was positively related to the number of flowering conspecifics within 10 m. Fertilization rate varied among populations, but this variation over all six populations in 2002 could not be attributed to differences in population size. Observations in one population in 2003 showed that there was no difference in insect visitation per capitulum between plants in sparse vs dense patches whilst plants from sparse patches suffered reduced fertilization rate. Visitation and fertilization rates were not affected by plant size and the abundance of co-flowering species, but weather conditions at the time of flowering had a strong effect on both variables. Capitulum size had a positive effect on visitation rate, but an effect on fertilization rate only in 1995 and 2002 and not in 2003. Our results suggest that pollen limitation affects fertilization rate in C. corymbosa due to limited compatible mate availability rather than pollinator limitation. They agree with previous genetic results derived from paternity analysis. Whether or not the benefits of local spatial agregation to reproductive success result in increased individual fitness will depend on the relative reduction in survival of vegetative stages due to intra-specific competition.