Reproductive biology, hybridization, and flower visitors of rare Sclerocactus taxa in Utah's Uintah Basin

Authors: TEPEDINO, VINCENT - ARS COLLABORATOR; Griswold, Terry; BOWLIN, WILLIAM - RETIRED

Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2009
Publication Date: 10/4/2010
Citation: Tepedino, V.J., Griswold, T.L., Bowlin, W. 2010. Reproductive biology, hybridization, and flower visitors of rare Sclerocactus taxa in Utah's Uintah Basin. Western North American Naturalist. 70(3):377-386.

Interpretive Summary: The Uintah Basin of eastern Utah is currently undergoing rapid change due to energy development. The Basin is home to many plants that occur nowhere else in the world, some of which are listed under the Endangered Species Act. This study reports on the mode of reproduction and pollinators of two threatened species of barrel cactus in the genus Sclerocactus. We found that to produce seeds, the flowers of both cactus species must be visited by bees. The bees that frequent the flowers are several species of sweat bees that nest in the ground and that also visit the flowers of numerous other plant species to collect pollen and nectar. These bees are also the agents responsible for hybridization between plants in populations of the two cactus species that are near to one another. Land managers responsible for the wellbeing of these cactus species must plan for the health of bee populations as well as other flowering plant species which serve as alternate resources for bees. While permitting energy development, they must also be careful stewards of the ecosystem.

Technical Abstract: The mating system and flower visitors of two threatened species of Sclerocactus (Cactaceae) were studied in the Uintah Basin of eastern Utah, an area undergoing rapid energy development. We found that both S. wetlandicus and S. brevispinus, as well as a third presumptive taxon (undescribed) which we dub S. wetlandicus-var1, cannot produce seeds unless their flowers are visited by pollinators. The two named taxa are predominantly outcrossed and are effectively self-incompatible. The undescribed taxa is fully self-compatible. We found tentative evidence for pollinator limitation of fruit set in S. brevispinus but not in the other taxa. Pollinators are largely species of native ground-nesting bees in the subfamily Halictinae. These bees are indiscriminate collectors of pollen and nectar from Sclerocactus flowers as well as other plant species. The wellbeing of Sclerocactus populations requires that land managers ensure the preservation of halictid bee habitat, which includes both nesting sites and a diversity of floral resources. These bees are also undoubtedly the pollen vectors responsible for the production of hybrids in the zones where S. wetlandicus and S. brevispinus meet. We experimentally confirmed that reciprocal crosses between S. wetlandicus and S. brevispinus are indistinguishable from each other or from pure crosses in fruits/flower, seeds/fruit and seed weights. We suggest that hybridization preceded energy development, that it attests to the ongoing process of evolution in