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14 December 2018 Failure of Pollen Transport Despite High Bee Visitation in an Endangered Dioecious Shrub
Elsa Youngsteadt, Clyde E. Sorenson
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Dioecy is rare among flowering plants, and is associated with a high frequency of threatened species. Dioecious plants are often pollinated by wind or insects, but are susceptible to pollination failure should male and female plants become spatially separated, or should pollinator abundance decline. Here we characterize the plant–pollinator interactions of Rhus michauxii Sarg (Sapindales: Anacardiaceae), an endangered dioecious shrub endemic to the southeastern United States. Working in the sandhills region of North Carolina, we detected a diverse community of arthropods visiting R. michauxii flowers, including 55 species or morphospecies, with moderate niche overlap between male and female flowers. Although most visitors acquired pollen from male flowers, pollen loads were greatly reduced or diluted on visitors to female flowers; conspecific pollen was completely absent at all-female sites. Bees in the genus Megachile appear to be the most important pollen vectors in this system because of their abundance and pollen load composition. We constructed a regional pollen transport network involving 73 arthropod species and 46 pollen species/morphotypes, in which R. michauxii participated in 10% of links and attracted 38% of individual visitors, suggesting that it competes successfully with other plants for visitation. Finally, time-lapse videography revealed that female inflorescences were visited about six times less often than male inflorescences, but at similar times of day. Despite overall high rates of bee visitation, pollen movement from male to female plants was uncommon, and restoration of sexual reproduction in this species may require hand pollination or translocation of suitable mates to single-sex sites.

© The Author(s) 2018. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail:
Elsa Youngsteadt and Clyde E. Sorenson "Failure of Pollen Transport Despite High Bee Visitation in an Endangered Dioecious Shrub," Annals of the Entomological Society of America 112(3), 169-179, (14 December 2018).
Received: 13 June 2018; Accepted: 8 November 2018; Published: 14 December 2018

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endangered species
plant–pollinator interaction
Rhus michauxii
time-lapse video
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