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Comparisons of the pollination biology of members of a number of genera (Prosopis, Helianthus, Opuntia, and Krameria) widespread in the arid American Southwest are made between sites in the Sonoran Desert of southern Arizona and the dry oak-juniper grasslands of central Texas. As in the majority of cases studied to date in the dry regions of the Southwest, solitary bees are the dominant pollinators in all of the systems examined. Rich arrays of oligolectic bees are associated with Prosopis, Helianthus, and Opuntia, but none with Krameria which offers oils rather than pollen and nectar as the primary floral reward. Nevertheless, Krameria appears to have the most restricted pollination system as none of the other taxa are obligately dependent on their specialist bees. Reward production and bee foraging activity were examined in Opuntia and Helianthus. In Helianthus, bimodal pollen presentation, but near constant nectar production, results in different activity patterns of the specialist and generalist bees visiting the flowers. Reward production is unimodal in the Opuntia species studied, but diurnal phenological differences can result in apparent partitioning of floral resources by foraging bees.

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© 1987 Beryl B. Simpson, John L. Neff

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