Female reproductive success, fruit and seed‐set and factors affecting them were measured for 3 years (1995–1997) for 24 populations of Primula sieboldii E. Morren, a threatened Japanese plant species in a highly fragmented landscape in southern Hokkaido. The smaller populations (genets <=; 3) set almost no seeds consistently during the study years. In larger populations (genets >= 7), proportions of failed flowers without damage from fungi or herbivores were relatively high, but fruit set per flower varied among the populations in 1995. Correlations of fruit (r = 0.589, P = 0.011) and seed‐ set (r = 0.688, P < 0.01) with population pollinator availability were highly significant. In 1995, pollination failure would be the most plausible reason for the variation in reproductive success among the populations. However, pollinator availability was relatively high in all populations in 1996 and 1997. In these years, higher proportions of fruit failed because of antagonistic biological interactions. In the populations with poor pollinator availability in 1995, mean fruit and seed‐sets were greater in the long‐styled morph than in the short‐styled morph. Such between‐morph differences are thought to be caused by partial self‐compatibility of the long‐styled morph and the results of hand pollination experiments support this hypothesis.

Under a semi‐natural setting the between‐morph pollen exchange patterns were studied in distylous Primula sieboldii flowers by measuring pollen removal from the anthers on a single visit by a Bombus diversus tersatus queen, and stigmatic pollen deposition along the sequence of the visitation of the opposite‐morph flowers by the bee. Despite the twofold larger number of pollen grains produced in a single flower of the long‐styled morph compared to that of the short‐styled morph, no significant difference in pollen removal from a flower was found between the morphs. The stigmas of the long‐styled morph received significantly more opposite‐morph pollen grains than those of the short‐styled morph on a single visit by the bee. Sufficient legitimate pollen grains, surpassing the ovule number, were loaded on the stigmas of 27% and 17% of visited flowers of the long‐ and short‐styled morphs, respectively. The short‐styled morph could more efficiently donate pollen to the opposite morph stigmas than the long‐styled morph.

In an age of deepening biodiversity crisis, plant species biological studies integrating ecological and genetic approaches, especially exhaustive studies with a model plant species, are urgently needed for both assessing the present status and implementing effective conservation measures, as a comprehensive understanding of demographic/ genetic interactions involved in the vicious cycle of plant population extinction is a prerequisite for any precise prediction regarding plant conservation. In this article, we summarize the major contributions to conservation ecological studies on a heterostylous clonal herb Primula sieboldii, focusing on gene flow and reproductive success, which are dependent on the life-history traits of the species and biological interactions with its effective pollinators, long-tongued bumblebees.