Floral Ecology and Hummingbird Pollination of Combretum farinosum in Costa Rica

Abstract

The pollination biology of Combretum farinosum, a hummingbird-pollinated, outcrossing liana of the dry forest in Costa Rica, was investigated in order to determine the evolutionary interactions between nectar rewards and pollinator foraging behavior. Plants generally produced many flowers, arranged in dense inflorescences which were highly synchronized in development, and with large nectar output per flower. Inflorescences change color from green on the first day, to greenorange (day 2), orange (day 3), and red (days 4 and 5). Only green, first-day inflorescences secrete nectar, and these were visited preferentially by all hummingbirds. Red inflorescences contained little nectar, were rarely visited (2% of all visits), and probably serve as flags to attract pollinators. The hummingbirds Chlorostilbon canivetii and Archilochus colubris foraged as transients, moving primarily between plants, while Amazilia saucerottei and A. rutila foraged as tenants at single plants and periodically defended small sections of large plants. The visitation rate per inflorescence increased with effective plant size (= number of nectar-producing inflorescences) for tenants, but not transients. Mean seed set per inflorescence decreased with effective plant size, but seed production per plant increased. Seed set was not limited by hummingbird visitation rate per se, as expected seed set per inflorescence (calculated from visitation rate and the number of flowers probed per visit) increased with plant size. Despite the higher visitation rate of tenants to large plants, seed set per infloresconce did not increase due to the low outcrossing potential of tenants as compared to transients, and the occasional removal of transients by territorial tenants. I suggest that the large nectar production of Combretum has evolved to satiate potentially territorial species and reduce the advantages of patch defense-increasing the number of outcrossing visits per plant. This pollinator-satiation strategy is an evolutionarv parallel of that found in seed-dispersal systems; in both cases foraging behavior is altered through resource abundance. PLANT REPRODUCTIVE CHARACTERISTICS result fronm selection operating on life history and breeding system in the context of maximizing reproductive success. The evolutionary advantages of sex (Maynard Smith 1971, 1978; Williams 1975) have effected an astonishing diversity of floral characters designed to manipulate the pattern and process of pollen movement. With the exception of autogamy, all forms of sexual reproduction in angiosperms require the intraspecific movement of pollen to a receptive stigma via some external agent. When animal vectors perform this transfer a reward is required, except in cases of dupery in which plants trap pollinators (Proctor and Yeo 1973) or induce pseudocopulation (Kullenberg 1961). The strategies and tactics utilized by plants in the evolutionary development of their floral attractiveness to pollinators are largely functions of the pollinators' nutritional and energetic demands (Heinrich and Raven 1972, Baker and Baker 1975). The level of reward offered can markedly affect pollinator movement patterns and, as a result, plant reproductive success (Willson and Rathcke 1974, Frankie et al. 1976, Carpenter 1976). Thus, plants will evolutionarily manipulate the spatial distribution, quantity, and quality of reward to optimize pollinator movement (Janzen 1977). This can be achieved by varying floral distribution on an individual and by regulating reward level through variations in the 1) numbers of flowers/plant, 2 ) nectar volume/flower, 3) nectar composition, and 4) the rate and duration of nectar flow. The extent to which resource abundance and distribution affect reproductive success is highly dependent on breeding system, with self-incompatible species most affected by sedentary pollin-