Reduced reproductive success in small populations of the self-incompatible Primula vulgaris

1 Habitat fragmentation and the resulting decline in population size can affect biotic interactions and reproductive success of plant species. We investigated the impact of habitat type, population size, morph type and frequency, plant density, floral display and predation on different reproductive components in 16 populations of the distylous self-incompatible perennial herb, Primula vulgaris, a rare, declining species in Belgium. 2 Although habitat type accounted for significant variation in population size, we did not find any relation between habitat type and either reproductive and vegetative characteristics. Population size, however, strongly affected reproductive success, such that plants in small populations produced significantly fewer fruits per plant and seeds per fruit, and therefore fewer seeds per plant. 3 No significant difference was found between morph types for any reproductive characteristic, nor an interaction with population size. However, when morph frequency was strongly biased (greater than or equal to1:3), the proportion of flowers setting fruit and the number of seeds per fruit were significantly lower in individuals of the common morph type. 4 Within populations, individual plants varied tremendously in size and floral display. Total number of fruits per plant significantly increased with floral display, but the highest fruit set per flower was found at intermediate flower number. 5 The proportion of fruit suffering pre-dispersal predation per plant significantly increased with floral display, but this did not offset the potential fitness gains of producing a large display. Furthermore, the absolute number of predated fruits per plant was significantly and positively affected by the interaction of the total number of fruits per plant and the density of the population.