THE IMPACT OF FLORAL PARASITISM IN TWO NEOTROPICAL HUMMINGBIRD‐POLLINATED PLANT SPECIES

Many angiosperm flowers produce substances (e.g., nectar, pollen, lipids) which serve as rewards for animals which are morphologically and behaviorally adapted to effect pollination by transferring pollen in sequential visits to flowers of the same species (Faegri and van der Pijl, 1966; Baker and Baker, 1975). It has long been realized that these floral rewards are frequently also attractive to other animal species which, due to morphology or behavior at particular flowers, do not effect pollination (e.g., Kerner, 1896; Free and Butler, 1959; Proctor and Yeo, 1973; Inouye, in press). The relationships among plant, pollinators, and floral parasites have been documented to some extent for several insect-pollinated plant species, usually involving pollen transfer by longtongued bees and nectar parasitism by shorter-tongued species (Hawkins, 1961; Macior, 1966; Koeman-Kwak, 1973; Rust, 1977). For the closed-flowered Calathea species, Kennedy (1978) has found that the identities are reversed, with pollination by short-tongued bees and nectar parasitism by longer-tongued species. Recognition of the remarkable suite of corresponding characteristics shared by hummingbirds and the flowers they pollinate (Faegri and van der Pijl, 1966; Grant and Grant, 1968), has fostered the assumption that once the plant-pollinator(s) "unit" has been identified, the floral biology of the plant is understood. Observation of many tropical hummingbirdpollinated plants suggests that a more complicated relationship frequently exists involving floral parasitism by a variety of animals (Skutch, 1954; Lyon and Chadek, 1971; Colwell et al., 1974; Gentry, 1974; Janzen, 1975). While earlier work (e.g., Skutch, 1954) generally reported parasitism as occuring only rarely, our observations indicate that visits by both pollen and nectar parasites to flowers of many long-tubed, nectar rich, hummingbirdpollinated species are relatively frequent and very effectively remove resources. The interactions between these parasites and pollinating visitors may be both direct and, through their impact on the availability of floral rewards, plant-mediated. The ultimate effects of these interactions can be viewed in terms of changes in the relationship between a plant species and its pollinators, and, concomitantly, in the reproductive success of the plants. In this paper we quantify visitation by both pollinating animals and non-pollinating animals to flowers of two neotropical hummingbird-pollinated plant species. Several aspects of the more complex problem of determining the effect of floral parasitism on the reproductive biology of the plants are also explored. The impact of the utilization of floral resources by nonpollinators is presented in terms of the proportion and diurnal pattern of flowers visited and damaged by parasites. Since the effect of this parasitism on visitation by pollinators will depend upon its impact on the availability of floral rewards, we compare patterns of nectar availability in flowers which have been parasitized (pierced) and in those which have not been visited or have been visited only by pollinators. A very important aspect of the plant-mediated interactions among flower visitors is the relative ability of different species using different visitation modes to extract the nectar available. We investigated this problem indirectly, by determining the amount of nectar remaining in