Pollination by Deceit in a Mass-Flowering Tropical Tree Plumeria rubra L. (Apocynaceae)

Abstract

Flowers of Plumeria rubra, a mass-flowering tropical tree, share many traits with hawkmoth-pollinated flowers, and Plumeria pollen was recovered from the tongues of 17 hawkmoth species in Costa Rica. Rates of visitation and fruit set were exceptionally low compared with other hawkmoth flowers sympatric with Plumeria, apparently because Plumeria flowers produce no nectar reward. Hawkmoths in a flight cage quickly learned to avoid nectarless Plumeria flowers, but readily fed from these flowers after artificial nectar was added. Although no specific model for Plumeria was found in Costa Rica, the presentation of generalized odor and visual cues that mimic typical hawkmoth flowers may facilitate deceitful pollination of Plumeria. FLORAL MIMICRY ENCOMPASSES a diversity of reproductive strategies used by plants to achieve pollination by animals (see reviews of Wiens 1978, Little 1983). Because of experience with a model, pollinators visit a floral mimic expecting a reward. However, the mimic provides no reward and pollination occurs through deceit. Despite the importance of floral mimicry as a reproductive strategy, little detailed information about such systems exists (Wiens 1978, Williamson 1982, Little 1983). Williamson (1982) stressed that floral mimicry should be defined in terms of reproductive fitness. However, showing that fitness of a floral mimic is increased by its interaction with a model has rarely been successful in floral mimicy systems (Boyden 1980, Bierzychudek 1981, Williamson, pers. comm.). The essential difference in selection pressure on floral and insect mimics suggests that floral mimics may be less dependent upon resemblance to a model than insect mimics. Failure of an insect mimic to deceive a predator usually leads to the mimic's death, but failure of a floral mimic to deceive a pollinatory only results in the loss of potential pollination event. Deceitful plants may also attract pollinators by presenting cues that the pollinators innately recognize, rather than relying on strong resemblance to a model. In some instances a specific model may be absent (Dafni and Ivri 1981a, b; Ackerman 1981, 1983). In addition, convergence within floral syndromes may further complicate analysis of floral mimicry systems (Brown and Kodrik-Brown 1979, Williamson and Black 1981, Little 1983). In this paper I describe the floral biology of Plumeria rubra L. (Apocynaceae), a tropical tree apparently adapted for hawkmoth pollination. Plumeria produces no floral nectar and may be a generalized mimic of other hawkmoth-pollinated flowers. The reproductive biology of this species differs from typical examples of floral mimicry in that Plumeria lacks a specific model, is a massively flowering tree with an extended blooming period, and may be the most abundant hawkmoth flower in tropical deciduous forest of Costa Rica during much of its blooming period. STUDY SITES AND METHODS From 1976-1982 I studied Plumeria in northwestern Costa Rica (Guanacaste Province) at Hacienda La Pacifica, 10 km north of Canias, and at the Comelco ranch, centered 10 km north of Bagaces. The lowland deciduous and evergreen riparian forests at these sites (50-100 m elevation) are described by Frankie et al. (1974, 1983), Opler et al. (1980), and Opler (1983). I also made observations near the community of Monteverde (northern Puntarenas Province) from 1979-1982 where Plumeria grows in semi-deciduous, moist forest on the Pacific slopes at 800-1200 m elevation. FLORAL BIOLOGY.-Flowering phenology is based on Frankie et al. (1974) and my observations from 19761982. I studied the floral behavior of Plumeria using the methods of Haber and Frankie (1982) and Frankie et al. (1983). I monitored nectar secretion intensively on bagged flowers of two trees over several days and checked for possible nectar production on about 20 additional trees by probing flowers with microcapillary tubes and inspecting dissected flowers. I counted ovules in dissected flowers and seeds in mature fruits from the lowland site. Fruit numbers were easily observed on leafless trees during the dry season at