Stratum, Tree, and Flower Selection by Tropical Bees: Implications for the Reproductive Biology of Outcrossing Cochlospermum Vitifolium in Panama

Abstract

Forager behavior in three patches of flowering Cochlospermum vitifolium (Malvales, Cochlospermaceae) and fruit, flower, seed, and ovule number on trees in six sites were used to test two incipient paradigms and study the role of bee pollinators in tree reproductive success. Antho- phorid bees of the genus Centris were the most important pollinators. Behavior of both small and large Centris species indicated no consistent preference for flowers either in the densest floral arrays or within the stratum containing most flowers in the local flowering population. Flowers 7-12 m in height were never visited more often than flowers '_4 m from ground level. However, within groups of 5-33 flowers, Centris visited on average at least 50% of the flowers during a foraging bout. Polli- nating bees were indiscriminate in selecting inflorescences within floral arrays, but they appeared to maximize the rate of resource harvest once a small group of flowers was entered. The fruit of Cochlospermum contained an average seed number equal to 13% of the average ovule number per flower. Seed-set per fruit was no higher among flowers visited more often by pollinators than among flowers visited less frequently. Female competition for pollinators was not detected, but male com- petition may have occurred at one site where bees preferred flowers that were most abundant in the patch as a whole. Efficient outcrossing of a tree is in direct conflict with the optimal foraging of its pollinators. Cochlospermum may manipulate pollinators by (1) presenting small numbers of open flowers, (2) presenting large, conspicuous flowers and no leaves, and (3) providing only pollen con- cealed in poricidal anthers as a reward. If pollinators rather than the physiological capacity of a tree limited reproductive success, this probably arose from deposition of incompatible pollen on stigmas, rather than intertree competition or low pollinator abundance.