Pollinator limitation causes sexual reproductive failure in ex situ populations of self-compatible Iris ensata

Background: The absence of pollinators may prevent sexual reproduction and affect the distribution and persistence of individual plant populations, but the role of pollinators in shaping the patterns of distributions of plant species or populations has not been well studied. Aims: Here, we tested the hypothesis that failure of sexual reproduction due to the absence of pollinators may contribute to the disjunct distribution of Iris ensata. Methods: We assessed floral traits and pollinator visitation frequencies in three native and two ex situ populations. We also conducted controlled pollination experiments and examined progeny performance to estimate pollen limitation (PL) and inbreeding depression. Results: Hand-pollinations indicated that I. ensata was a partially self-compatible outcrosser. Sexual reproduction in our study region depended on Bombus trifasciatus. We observed inbreeding depression for seed set and germination, but not for seedling survival. Native populations had higher frequencies of pollinator visitation and lower pollen-limitation of seed set than ex situ populations. Among populations, the magnitude of PL was negatively related to pollinator visitation rate (P = 0.04). Conclusions: Pollinator limitation and inbreeding depression may both restrict the southern distribution of I. ensata, preventing its northward expansion, thus explaining the disjunct distribution of I. ensata in East Asia.