Pollinator Reward Research Articles

Pollination in a cryptically dioecious plant Decaspermum parviflorum (Lam.) A. J. Scott (Myrtaceae) by pollen-collecting bees in Sulawesi, Indonesia

Decaspermum parviflorum is a common shrub or treelet in clearings in eastern Sulawesi, Indonesia. It is widely distributed in SE Asia. It produces numerous white flowers and small blue-black fruits. Individual shrubs were of two types in 1:1 ratio, one bearing staminate flowers, the other bearing flowers with both stamens and carpels. The ‘hermaphrodite’ flowers were shown to have sterile pollen so the species is functionally dioecious. The flowers and floral sprays of the two sexes differed in several measurements: the male flowers had more anthers and were larger and more numerous, making the corporate visual image of male sprays more than twice as large as that for sprays of female flowers. Flowers, individuals of which last only one day, opened only every second day, when all bushes flower synchronously. When the anthers burst in the morning, pollen and sterile pollen was collected vigorously by a variety of bees, mostly Apis dorsata and Nomia spp. Most of the pollen was collected within 1 h of anthesis. Anthesis in males takes place about 20 min earlier than in females; insect visitation follows this pattern. The minute quantity of nectar was collected by only few insects and mostly after the pollen had gone, if at all. Fruits were eaten by birds. The species displays many features typical of dioecious tropical plants with the unusual feature of pollen being the main food reward for pollinators. It was pollinated effectively in Sulawesi.

Mimicry and Deception in Pollination

Many flowers produce rewards, usually nectar or pollen or both. C. K. Sprengel (165), the founder of modern floral biology, was the first to report that flowers do not always reward pollinators; his observations were on the genus Orchis. Subsequent authors (7; 51; 66; 97; 99; 105; 186, p. 21; 195) have discussed various aspects of deceitful flowers that attract pollinators by resembling rewarding flowers or other objects.

DISTRIBUTION OF DEFENSE NECTARIES IN IPOMOEA (CONVOLVULACEAE)

The structure and distribution of defense nectaries in the genus Ipomoea (Convolvulaceae) were investigated. These nectaries do not reward pollinators and probably contribute to antiherbivore defense. Of 22 species sectioned, 15 had defense nectaries on the sepals. Of 12 other species observed, ten had sepal nectaries and two did not. Structurally, 14 of the species sectioned had crypt sepal nectaries and one had a basin nectary. Of the 14 species with crypt nectaries, two had invaginated spaces adding greatly to the internal area of these nectaries, and forming the most complex nectaries that have been reported. We term these labyrinthine crypt nectaries. All three types of nectaries are lined with secretory trichomes along the proximal surfaces of the crypts. Species with defense nectaries on the sepals tend to have petiolar defense nectaries as well, but the two locations may have different nectary types; e.g., basins on the petiole and crypts on the sepals. Since most reports of the function of these nectaries have shown antiherbivore defense by nectar feeders, the distributions of defense nectaries with respect to region and life history of the species were sought. Plants without sepal nectaries were found to have significantly smaller seeds than plants with sepal nectaries; they were also more frequently annuals. No significant relationship was found between region or breeding system and defense nectaries.

THE ROLE OF RESIN IN ANGIOSPERM POLLINATION: ECOLOGICAL AND CHEMICAL CONSIDERATIONS

Members of at least two unrelated genera of plants, Dalechampia (Euphorbiaceae) and Clusia (Guttiferae), attract pollinators by secreting resins from floral structures. Bees that pollinate these flowers collect and use the resin in nest construction; these include Euglossa, Eulaema, Eufriesea, Trígona and, Hypanthidium in the neotropics, and Heriades in Africa. Floral resins are slow in hardening as compared with many other plant resins; this facilitates collection and storage by bees, hence probably enhancing the attractiveness of the flowers. Floral resins are probably of great utility and dependability and may be an especially important resource to certain bees, which in turn are major pollinators of many species of tropical plants. Bees appear to forage floral resin using a “strategy” of energy efficiency; large bees collect resin only from copious sources, smaller bees collect resin from sources with small amounts as well as from sources with large amounts of resin. Resin secretion in flowers may have originated as defense against herbivores and secondarily assumed the role of pollinator reward.

Chiropterophily and ornithophily inFreycinetia (Pandanaceae) in Samoa

In Samoa, field observations, experiments, and SEM micrographs of hair and feather samples indicate the dioecious lianaFreycinetia reineckei to be both chiropterophilous and ornithophilous with the principal pollinators being an endemic flying fox,Pteropus samoensis, and an endemic starling,Aplonis atrifuscus. Nutritional analysis of staminate and pistillate inflorescenses indicate qualitative and quantitative differences between the rewards offered by the two sexes. No nectar is presented, the principal pollinator rewards being fleshy bracts and pollen. As a result, staminate and bisexual inflorescenses sustain higher levels of pollinator damage, a factor which may tend to maintain dioecism within the population.

EUGLOSSINE BEE POLLINATION OF THE ORCHID, COCHLEANTHES LIPSCOMBIAE: A FOOD SOURCE MIMIC

Cochleanthes lipscombiae is pollinated by large male and female euglossine bees. The flowers lack pollinator rewards, but attract bees searching for nectar. The euglossines extend their long tongues and crawl into the gullet‐flower. The bees probe the back‐swept lateral sepals for nectar. Pollination occurs as a pollinarium laden bee backs out, deposits pollinia on the stigma, and obtains a new pollinarium load by dislodging the anther. Some related orchid species have similar morphological characteristics as those essential to the pollination mechanism of C. lipscombiae. These features may have taxonomic significance at the generic level. Cochleanthes lipscombiae may be a floral mimic of a sympatric legume, but may also receive exploratory visits by bees searching for food resources. The latter may be young, recently emerged naive bees, or individuals seeking new nectar hosts during a period of rapid host species turnover.

Euglossine Bee Pollination of the Orchid, Cochleanthes lipscombiae: A Food Source Mimic

Cochleanthes lipscombiae is pollinated by large male and female euglossine bees. The flowers lack pollinator rewards, but attract bees searching for nectar. The euglossines extend their long tongues and crawl into the gullet-flower. The bees probe the back-swept lateral sepals for nectar. Pollination occurs as a pollinarium laden bee backs out, deposits pollinia on the stigma, and obtains a new pollinarium load by dislodging the anther. Some related orchid species have similar morphological characteristics as those essential to the pollination mechanism of C. lipscombiae. These features may have taxonomic significance at the generic level. Cochleanthes lipscombiae may be a floral mimic of a sympatric legume, but may also receive exploratory visits by bees searching for food resources. The latter may be young, recently emerged naive bees, or individuals seeking new nectar hosts during a period of rapid host species turnover.

Inter- and intra-floral heterogeneity of nectar production in Helleborus foetidus L. (Ranunculaceae)

Three southern Spanish populations of Helleborusfoetidus L. (Ranunculaceae) were sampled for nectar content in the absence of nectarivorous flower visitors. Nectar volume was measured in individual nectaries of flowers at the same stage in the anthesis cycle. Total nectar content per flower was extremely variable between plants within populations and between flowers within plants, but much less so between populations. Average sugar content per flower was roughly similar in the three populations sampled. High variances in nectar abundance occurred also among nectaries within the same flower. Heterogeneity in pollinator rewards thus occurs simultaneously at several spatial scales, namely between nectaries, between flowers and between plants. This implies a strong component of uncertainty to foraging pollinators, which may therefore tend to avoid Helleborus flowers.

A bonanza-blank pollinator reward schedule in Delphinium nelsonii (Ranunculaceae).

Data indicate a non-random distribution of nectar among plants in a population of Delphinium nelsonii Greene. More plants than expected had no nectar (blanks) or large amounts of nectar (bonanzas), while fewer plants than expected had intermediate amounts of nectar. This bonanza-blank pollinator reward schedule suggests that a high proportion of the nectar obtained by pollinators comes from occasional bonanza plants encountered in the population.

THE POLLINATION ECOLOGY OF ZENOBIA (ERICACEAE)

Zenobia is a shrub endemic to the coastal plain of the Carolinas and southeastern Virginia. The sole species, Zenobia pulverulenta, appears to be weakly self‐compatible and partially self‐pollinating. Fruit set was reduced, but not eliminated by excluding insect floral visitors. Insect visitors were collected and their pollen loads examined for Zenobia and foreign pollen. Analysis of these pollen loads and observations of insect‐foraging behavior indicate that workers of three species of bumblebee (Bombus bimaculatus, B. griseocollis and B. impatiens) are. the principal pollinators of Zenobia. However, pollinator density and composition appear to be strongly influenced by the habitats in which Zenobia grows. Pollen and nectar serve as the primary rewards for pollinators.

Wind-Pollination in the Understorey of a Rain Forest in Costa Rica

SUMMARY (1) In Trophis involucrata, a dioecious tree in the understorey of a Lowland Tropical Rain forest in Costa Rica, the anther filaments are held under tension, and as the flower opens the filaments spring out violently and the anthers concurrently release pollen in a small cloud. The pistillate flowers are green and inconspicuous, and lack not only perianth but also nectar, and are thus devoid of attractants and rewards for pollinators. (2) In addition to these morphological features, a number of other observations and experiments suggest that Trophis involucrata is wind-pollinated. (3) Analysis of the dispersion pattern of plants indicates that the average distance between staminate and pistillate plants is small (6-6 ? 3.3 m), and that the two sexes are randomly distributed with respect to each other. It is suggested that both factors should facilitate the capture of pollen by pistillate plants. (4) Other examples of wind-pollination in lowland tropical humid forests are described, and the factors responsible for the evolution of anemophily in such forests are discussed.

FLORAL MIMICRY BY EPIDENDRUM IBAGUENSE (ORCHIDACEAE) IN PANAMA.

Mimicry in animals is a well-known field of research with an extensive literature. In contrast, the subject of mimicry in plants is an area which has received little attention by investigators. Wickler (1968) and Barlow and Wiens (1977) give examples of plant mimicry in their recent reviews, but experimental data demonstrating deception and its advantage to the plant are, in most instances, scant or lacking. An interesting case of floral similarity occurs in Central America which involves three plant species: Lantana camara L. (Verbenaceae), Asciepias curassavica L. (Asclepiadaceae) and Epidendrum ibaguense HBK (Orchidaceae). These species are frequently sympatric, have flowers that are similar in color (orange and yellow) and are visited primarily by butterflies and hummingbirds and to a lesser extent, bees (Dodson, 1962; van der Pijl and Dodson, 1966). The flowers of L. camara and A. curassavica have nectar while those of E. ibaguense do not. These facts have led researchers (C. H. Dodson, H. G. Baker and others, pers. comm.) to hypothesize that E. ibaguense may be a mimic of the other two species, although no published accounts are available. By deceiving flower visitors with mimicry, plants of E. ibaguense are, in theory, spared the cost of producing nectar as a reward for pollinators. While examining populations of these species in Panama in 1976, I had an opportunity to test this hypothesis. Specifically, I wanted to discover what rewards were offered by these plants, whether flower visitors were deceived by the mimicry, if such it was, and if there was a means by which pollen could be transferred to the correct species. I sampled flowers of these three species for nectar near Boquete, Panama. Sugar content of the nectar was measured in the field with a Bausch and Lomb hand refractometer. Only new flowers were sampled. The nectar of L. camara averaged 19% sugar by weight (N = 25), while that of A. curassavica averaged 35% by weight (N = 32). I obtained no nectar from the flowers of E. ibaguense (N 30). I witnessed only two visits to E. ibaguense flowers by insects in the Boquete area. Both of these visitors were monarch butterflies (Danaus plexippus L.). To determine whether or not individuals were visiting flowers of all three species, I captured ten monarchs and examined them with a dissecting scope. Each of these butterflies had pollen of L. camara on its head and thorax; one or more pollinia of A. curassavica attached to its legs; and one or more pollinia of E. ibaguense attached to the upper one-third of its proboscis. In all individuals the pollinia of A. curassavica and E. ibaguense were confined to these specific areas. Pollen of L. camara was generally on the head and ventral part of the thorax of D. plexippus but could sometimes be found on other parts of the body. These observations, although limited, suggest that E. ibaguense may be a bona fide mimic of L. camara and A. curassavica and that monarch butterflies are deceived by the mimicry. The species-specific placement of the pollinia (and pollen) of these three species on different areas of the body of D. plexippus imply proper intraspecific pollen transfer, data which

AN EVOLUTIONARY EXAMINATION OF THE FLORAL DISPLAY of CATALPA SPECIOSA (BIGNONIACEAE).

Those plant species that rely on animals to mediate the transfer of pollen usually attract pollinators with visual and/or aromatic stimuli and then reward them with pollen, nectar, or some other food (Faegri and van der Pijl, 1971; Proctor and Yeo, 1973). Under some circumstances (i.e., competition for pollinators) there may be selective pressures to increase the size of the attractants and rewards in order to secure more visits by the pollinators. One possible response of a plant to these pressures is to cluster flowers in time and space. This would make the plant more conspicuous to pollinators and it would also increase the pollinator's reward by decreasing the search time and distance between food sources (see Burtt, 1961; Janzen, 197 la; Heinrich and Raven, 1972; Gentry, 1974; Willson and Rathcke, 1974; Heinrich, 1975). There are many plant species that regularly produce inflorescences consisting of many flowers but initiate only a few fruits. Species with this flowering and fruiting pattern include Albizia lebbeck (Maheshwari, 1931), Aesculus spp. (Percival, 1965; Benseler, 1975), Cassia grandis (Janzen, 197 la), Eucalyptus spp. (Carr et al., 1971), various orchard trees (see Kozlowski, 1973), Asclepias spp. (Willson and Rathcke, 1974; Wilbur, 1976; Willson and Price, 1977) and Catalpa spp. (Stephenson, 1978). Because each flower on these species has a low probability of setting fruit, there is the possibility that inflorescence size has been selected for factors other than a flower's ability to set fruit (see Willson and Price, 1977). In this study, I use Catalpa speciosa (Warder ex Barney) Engelm. (Bignoniaceae) to test which of three alternative hypotheses explains the proximate factors involved in a high ratio of flowers to fruits and then to test several ultimate factors that infer the adaptive significance of this pattern.

Floral Conservatism in Neotropical Malpighiaceae

Floral Conservatism in Neotropical Malpighiaceae

Additional Notes on the Floral Biology of Neotropical Lecythidaceae

Observations on the floral biology of seven species of New World Lecythidaceae, including the first report of bat pollination for neotropical members of the family, are presented. The shift from actinomorphic flowers with many stamens to zygomorphic flowers with fewer stamens and the concomitant change from pollen to nectar as the pollinator reward are discussed and related to the respective pollinators. Several mechanisms which operate to reduce competition for pollinators between sympatric species of the family are suggested.

Breeding systems of dominant perennial plants of two disjunct warm desert ecosystems.

A comparison of the floral syndromes, flower biomasses, pollen and total sugar production of the dominant perennial species of two climatically similar but disjunct desert scrub ecosystems was made to assess the degree of convergence in breeding systems. Results indicate that the dominants at the northern site in the Sonoran Desert near Tucson Arizona, USA, possess more diverse floral types, utilize more reproductive methods, produce a greater annual biomass of flowers, provide more rewards for potential pollinators and employ a wider array of pollen vectors than those at the southern site near Andalgalá, Catamarca, Argentina. The discrepancies in these features can be best explained in terms of the differences in the annual dispersion of rainfall at the two sites. However, when compared to the dominant species of two Mediterranean scrub ecosystems, the breeding systems of the dominants of the desert scrub sites proved to be more similar to one another than to those of a neighboring but different ecosystem type.