VARIATION IN POLLINATOR ABUNDANCE AND SELECTION ON FRAGRANCE PHENOTYPES IN AN EPIPHYTIC ORCHID

Extraordinary floral variation is common among some orchids that emplo y deception to attract pollinators. This variation may be maintained b y frequency-dependent selection where rare phenotypes are preferred. O ver a 2-yr period, 1993-1994, we monitored the reproductive success of Tolumnia variegata, an obligately outcrossing epiphytic orchid, at th ree localities in Puerto Rico that differed in pollinator service. Pla nts varied in floral morphology and fragrance characteristics. Artific ial arrays of varying frequencies of scentless and fragrant phenotypes were established to test for frequency-dependent selection. Where pol linators were rare (Cambalache, range of census average = 0-0.2 bees/h ), 0.9-1.2% of the flowers were effectively visited (pollinarium remov als and pollinations). At Tortuguero where 0.4-1.1 bees/h were observe d, 4-9.2% of the flowers were visited. At Pi (n) over tilde ones where bees were the most abundant (1.4-5.2 bees/h), 20.9-25.0% of the flowe rs were visited. A significant portion of the variance in all measures of reproductive success (male, female, and combined) was explained by differences among populations, which we attribute mostly to variation in pollinator abundance. Neither the fragrance phenotype nor its freq uency had a significant effect on success as revealed by a split-plot ANOVA. There was a significant interaction between population and phen otypic frequencies in all our measures of reproductive success, but on ly for the 1994 flowering season. Thus, variation in floral fragrance phenotypes is not likely maintained by frequency-dependent selection. High levels of variation remain unexplained.