Arctomecon humilis (dwarf bear-poppy) is an endangered plant species restricted to gypsum-rich soils of the Moenkopi Formation and occur within 16 km2 of the St. George area in southern Utah. Extirpation of the rare oligolectic bee Perdita meconis and decline of other native pollinators within the last 20 years has been largely attributed to the advent of invasive Africanized honey bees (AHB), which have become the predominant pollinator for A. humilis. Subsequently, previous reproductive output studies have reported reduced fruit set for A. humilis, particularly for the small, isolated population at the Shinob Kibe Preserve. In 2018, hand pollination experiments were carried out at Shinob Kibe to determine efficacy of hand pollination to augment potentially limited pollinator activity, as well as determine if interpopulation crosses would affect fruit set. Drone support was utilized to analyze interplant distance, which has historically been shown to have a significant effect on reproductive output prior to AHB arrival. In 2019, reproductive output studies were conducted across 4 A. humilis populations including Shinob Kibe. In both years, field data collected included number of flowers and fruit set per plant. Fruits were collected and analyzed to determine seed set and average seed weight per fruit.
Results/Conclusions
At Shinob Kibe, fruit set from open-pollinated flowers was exceptionally high (85%) compared to a previous year (12%), quelling concerns of pollinator limitation. Hand pollination significantly increased fruit set only with interpopulation crosses (96%). There was no treatment effect on seed set, although seed production was uncharacteristically low (~12 seeds/fruit). Both interpopulation and intrapopulation crosses resulted in significantly smaller seeds than the control group. There was no effect from interplant distance or flower number on reproductive output, suggesting that sparse A. humilis populations may benefit from pollinators with larger foraging ranges such as the AHB. Reproductive output studies in 2019 showed nearly 100% fruit set across all study populations, further supporting evidence of adequate pollinator visitation. Seed production ranged from 12-25 viable seeds per fruit, with most populations on the lower end of the reported range for A. humilis. Seed size did not significantly vary across populations. The consequences and sustainability of this relatively young relationship between A. humilis and the AHB remains unclear. However, results show that the AHB is currently capable of adequately fulfilling pollinator services for A. humilis in lieu of native bees. Ensuring healthy populations of pollinators is critical for the persistence of this rare plant.