A Test of the Pollinator Limitation Hypothesis for a Neotropical Herb

Abstract

We hand—pollinated all flowers in inflorescences of the neotropical herb Calathea ovandensis (Marantaceae) and compared fruit and seed set of treatment plants to that of open—pollinated controls. Hand—pollination increased the number of initiated fruits by 24%, the number of mature fruits by 6.8%, and the number of seeds by 12.6%, but only the first of these differences was statistically significant. The proportion of flowers that matured fruits was very low compared with data reported for other species: 8.1% for the hand—pollinated treatment and 7.4% for open—pollinated controls. The proportion of flowers that initiated fruits was also quite low: 24.5% for hand—pollinated plants and 20.2% for open—pollinated plants. Mature fruit and seed production were significantly affected by nonpollinating animals that act at different developmental stages. Antguards attracted to extrafloral nectaries on the inflorescence appeared to stimulate flower production, while an ant—tended herbivore of reproductive tissues had a negative effect on fruit maturation. We searched for @`windows" of favorable biotic conditions during which full pollination could potentially improve fruit production, i.e., high antguard activity and low herbivore levels; while results were in the predicted direction, the effect was not significant. We conclude that while fruit initiation was pollen—limited, mature fruit and seed production were limited primarily by resources. The extremely low percentage of flowers maturing fruits may reflect selection for increasing fitness through male function. The remarkable pollination system of this species, with each flower having only a single opportunity to serve as a pollen donor, probably contributes to the unusually low fruit—to—flower ratio.