Does flower color variation matter in deception pollinated Psychilis monensis (Orchidaceae)?

Abstract

Negative frequency dependent selection (FDS) had been proposed as a mechanism for the maintenance of the high levels of variability in floral traits of rewardless plants. Thus far the evidence has been equivocal for discontinuous traits. We experimentally tested the FDS hypothesis for continuous variation in flower color of Psychilis monensis, a rewardless, epiphytic orchid of Mona Island, Puerto Rico. P. monensis flowers all year long with a peak in June and July. Plants are self-incompatible. Individual flowers last 9-15 days if unpollinated. Over a 9-month observation of an unmanipulated population, a high percentage of plants showed pollinarium removals (79%) and fruit initiation (63%), but the actual percentage of flowers visited was very low (12%) and final fruit set was as low as the lowest recorded for orchids (2.4%). In a FDS experiment using a Latin Square design, we manipulated flower color in three populations. Over 50% of the variation in either male or female reproductive success was explained by time and site with no significant effect of treatment except as part of a three-way interaction of time x site x treatment. Paired comparisons with the controls gave equivocal results for both male and female measures of reproductive success. Major community changes had occurred during the FDS experiment with flower activity falling dramatically and by the third run of the experiment, only P. monensis was in flower. Coincidentally, the numbers of effective visits increased with time, presumably as pollinators became less discriminating in search of new food resources. Thus, negative frequency dependent selection is either sporadic or non-existent. Reproductive success was statistically related to locality and date, which was reflected in the flowering phenology of the local communities. High natural levels of color variation may be more influenced by drift than selection.