Pollination success and reproductive output in experimental populations of the self-incompatible Primula vulgaris

We studied the effects of population size, plant density, and interpopulation distance on the process of sexual reproduction in experimental populations of the perennial self-incompatible Primula vulgaris ( Primulaceae). Decreasing population size significantly reduced the proportion of flowers setting fruit, the seed : ovule ratio, and the overall seed production per plant, whereas no density effect was observed for any of the measured reproductive components. The seed : ovule ratio decreased more strongly and significantly with decreasing population size when populations were more spatially isolated. Increasing interpopulation distance also significantly reduced seed production per fruit and per plant. We found indications that skewed morph frequencies, associated with population size, may further reduce reproductive success as supplemental hand pollinations revealed that a lack of compatible pollen significantly reduced fruit and seed production, especially in individuals belonging to the morph in majority. Although the effects of morph bias and population size could not be investigated separately, we conclude that reproductive success in small populations of P. vulgaris is significantly affected by combined effects of reduced pollinator services and limited availability of compatible morphs ( deficit in compatible pollen). As such, this study exemplifies the importance of a plant breeding system in determining reproductive success in fragmented populations of self-incompatible species.