Nectar Availability and Bee-Foraging on Ipomoea (Convolvulaceae)

The relationship between nectar scheduling and availability and the patterns of a pollinating guild's foraging on two species of co-occurring morning glory was investigated in disturbed habitats of the lower montane rain forests at Monteverde, Costa Rica. Ipomoea indica and Ipomoea batatas partition pollinating resources, presumably according to tongue length, with long-tongued bees visiting I. indica. Bees visiting 1. batatas show staggered visitation times with larger bees visiting in the early morning when nectar is most available and smaller bees visiting in the late morning and afternoon when nectar availability is at its minimum. The larger bee species of the early morning return in the late afternoon after nectar availability has in-creased from its early afternoon low. Since no aggression was observed between bee species, small bees may be prevented from foraging in the early morning by cold temperatures. The larger bees probably do not forage when there is very little nectar available. I. indica shows no such pattern. Due to the deeper effective corolla of I. indica, nectar may be inaccessible to most bees in the habitat, consequently, no pattern should be expected. However, lack of a pattern may also be the result of this plant's recent introduction to the area. ANY UNDERSTANDING OF COMMUNITY STRUCTURE ultimately depends upon an understanding of the dynamics of the resources upon which the organisms in that community rely. In what manner do resources vary in abundance and how do the shifting patterns of their availability mold the behavior and structure of the supported organisms? Pollination systems are ideal for addressing such problems since the requisite resources are known and easily monitored along temporal and spatial gradients (Heinrich 1975, Heinrich and Raven 1972, Hocking 1968, Feinsinger 1978). Changes in the behavior of individual pollinating species with changes in nectar availability have been documented for bees (e.g., Waddington and Holden 1979, Pyke 1978, Heinrich 1976), butterflies (e.g., Gilbert 1975), and nectarivorous birds (e.g., Waser 1978, Gill and Wolf 1977), but the influence of shifting patterns of energy availability on the structure of pollinating guilds is still poorly understood. Feinsinger (1976, 1978) observed substantial daily and seasonal changes in the guild structure of hummingbirds and could correlate many of these changes with changing patterns of nectar availability and production. Gilbert (1975) similarly observed changes in the foraging patterns of butterflies associated with diurnal variation in nectar production. Such variation in nectar availability may be commonplace along a variety of time scales (Stiles 1977, Waser 1978, Heithaus 1974, Parrish and Bazzaz 1976). Sequential diurnal changes in the species of bees visiting flowers have been observed often (C. D. Michener, pers. comm.; Schlissing 1970; K. D. WadlPresent address: Department of Zoology, North Carolina State University, Raleigh, North Carolina 27650, U.S.A. 64 REPRODUCTIVE BOTANY 64-69 1981 dington, pers. comm.). However, the reasons for such changes are not well understood. Schlissing (1970) observed a regular replacement of bees visiting molrning glories in Costa Rica. Similar sequential changes can be observed in bees visiting temperate morning glories (K. D. Waddington, pers. comm.). METHODS The study was conducted in lower montane rain forest at Monteverde, Costa Rica, between early January and late February 1978. Monteverde is a small dairy community located at about 1400 m elevation on a Pacific-facing slope of the Sierra de Tilaran in northwest Costa Rica. The climate and topography of Monteverde are described in Feinsinger ( 1976). Two species of morning glory, Ipomoea batatas (L.) Lam, the "camote" or sweet potatoe, and Ipomnoea indica (Burm.f.) Merrill, co-occur and were abundant along road cuts, embankments, and disturbed secondary vegetation. Both species are trailing vines whose flowers last a single day. Ipomoea batatas flowers from October to July and is pantropical in distribution. The corolla is light lavender with a dark-purple throat about 4-7 cm long. The sweet potatoe is probably a native lowland species and was introduced to high elevations many centuries ago (D. Austin, pers. comm.). I. indica, on the other hand, is most likely a lowland species which has only recently escaped from cultivation at the study site (D. Austin, pers. comm.). The corolla is blue-purple about 5-7 cm long with lavender nectar guides. Both species attract a large variety of bees which gather nectar and/or pollen. This content downloaded from 157.55. 39.136 on Thu, 19 May 2016 05:42:52 UTC All use subject to http://about.jstor. org/terms Bees were collected from the two species over an eight-day period at hourly intervals from 0600 to 1630. Samples between days were pooled, but hour of collection was kept distinct. Bets were collected only if they actually entered the corolla and thus potentially affected pollination. All bees were covered with yellow pollen which was shed profusely throughout the day. The pollination effectiveness of the different bee species cannot be determined from present data. Presumably, large bees are most effective since they invariably come in contact with the stigma while removing nectar. Small bees, on the other hand, can remove nectar without contacting the female reproductive parts. Many of the small bees, however, crawl around the anthers while collecting pollen and consequently contact the stigma. Nectar quantity and quality available to a visiting pollinator were determined by measuring sugar concentrations and nectar volumes from flowers selected randomly from patches of the two species. Nectar samples were collected from open flowers at hourly intervals over several days. A total of 20 samples for each hour interval was collected for each species. Nectar was drawn up and measured using Drummond micro-capillaries. The nectaries of both species were inaccessible from the front, and sampling necessitated removing the flower from the vine. Sugar concentration in percent sucrose equivalents by weight was measured using a National hand refractometer. Buds of flowers expected to open the following day were bagged at night to exclude insect visitors. Flowers were bagged with light muslin to allow air flow and temperature equilibration. The following day subsamples of the bagged flowers were measured for nectar quantity and concentration. Subsamples were taken at two-hour intervals and were used to determine nectar accumulation over the day. RESULTS INSECT VISITORS. -The pattern of bee visitation according to time of day and species of bee is presented in figure 1 for I. indica and figure 2 for I. batatas. The principal visitors to the two species are not shared; I. indica appears to be commonly visited by a species of Thygator, probably analis, and an Euglossa bee of the cordata group. I. batatus, on the other hand, is visited by the larger bees Bombus volucelloides and Xylocopa tabaniformis in the early morning, and by large numbers of the smaller stingless bees Melipona fasciata and Trigona cupira in late morning and afternoon. Not only are the insects that visit the two Ipomoea species different, but the overall pattern of activity is different as well. 1. indica has a single peak in visitors at around 0930 hrs. This peak also corresponds to the time when the diversity of visitors is greatest. Very little pollination activity occurs at noon, and none was observed past 1400 hrs. I. batatas, however, shows a bimodal distribution of pollinator activity with a distinct afternoon peak in visitation frequency. For this species the greatest diversity of bee visitors occurs during the afternoon peak. That the two flower species do not share prinBee-Foraging on Ipomoea 65 Coenougo chlor o sp