Passerine Pollination of Erythrina Megistophylla Diels (Fabaceae)

Floral morphology and behavior, nectar composition and observational data suggest that Erythrina megistophylla is adapted to passerine pollination. It has short, open-mouthed, backwardly directed flowers on horizontal racemes. The primary visitors are members of the Coerebidae (honeycreepers), passerine birds which perch on the inflorescence rachis while probing the flowers. The nectar has a low sugar concentration (6-9% W/V), is hexose dominant, and contains a high concentration of amino acids. Pollination of Old World Erythrina species by birds (Passeriformes) has been known for some time (Ali, 1932; Docters van Leeuwen, 1932; Porsch, 1924; Singh, 1929), although similar pollination of species of this genus in the New World has gone largely unnoticed, probably due to the vast majority of hummingbird-pollinated species. Fifty-two of the 64 New World species of Erythrina are adapted to hummingbird pollination (as presented by Toledo, 1974). This estimate is based on descriptions and illustrations provided in Krukoff & Barneby's (1974) treatment of the genus. This leaves 12 species which show adaptations to or passerine birds. Observations have been made on visitations to some of these species by several authors (Leck, 1974; Raven, 1974; Skutch, 1954; Snow & Snow, 1971; Steiner, unpubl.; Timken, 1970). Cruden & Toledo (1977) and in this symposium Feinsinger et al., Morton, and Toledo & Hernaindez have considered the syndrome for New World species of Erythrina in detail. Their studies have helped demonstrate that in the New World a perching bird syndrome based on floral morphology and nectar composition definitely exists. The nectar of Erythrina breviflora A. DC. studied by Cruden & Toledo (1977) differed from that of a typical hummingbird Erythrina (E. coralloides A. DC.) on the basis of sugar concentration, ratio of the sugars (glucose and fructose/sucrose), and on the concentration of amino acids. The analysis done by Irene Baker showed that E. breviflora had hexosedominated nectar that was less concentrated and contained a greater proportion of amino acids than the sucrose rich nectar of the hummingbird adapted E. coralloides. Since Erythrina breviflora is considered by Krukoff & Barneby (1974) to be phylogenetically primitive, the syndrome is most probably a primitive condition that has been retained by some New World species. Perhaps the most important adaptive change that has occurred facilitating hummingbird pollination among Erythrina species has been the morphological switch from 1 This study was supported by University Research Expeditions Program, Berkeley. I am grateful to Adrianne Borgia and Andrea Sessions for their excellent field assistance and Wendy O'Neil for support and assistance throughout the preparation of this manuscript. I would especially like to thank Irene Baker for analyzing the nectar. Dr. Herbert G. Baker and Dr. Grady L. Webster provided valuable comments on the manuscript. ' Botany Department, University of California, Davis, California 95616. ANN. MIssouRI BOT. GARD. 66: 490-502. 1979. 0026-6493/79/0490-0502/$1.45/0 This content downloaded from 157.55.39.111 on Sat, 17 Sep 2016 06:04:00 UTC All use subject to http://about.jstor.org/terms 1979] STEINER-POLLINATION OF ERYTHRINA MEGISTOPHYLLA 491 short, open-mouthed corollas directed inwardly to long closed corollas directed outwardly. The more primitive short corollas are borne in horizontal racemes whereas the more specialized long corollas are borne in vertical racemes. Clearly, one permits perched probing for nectar while the latter makes hovering in mid-air necessary. These morphological changes reduced the number of birds that could successfully obtain nectar and therefore cut down on unspecialized opportunistic feeders that may have been less efficient pollinators than hummingbirds. In addition to the obvious morphological changes, there has been a change in nectar composition associated with the transition from perching-birdto hummingbird-pollinated Erythrina species. The nectar of perching-bird-pollinated Erythrina species may have less concentrated, hexose-dominated nectar with significantly more amino acids (Cruden & Toledo, 1977; Baker & Baker, 1980; Baker & Baker, this symposium). Thus, visitation and nectar data were collected for Erythrina megistophylla, a species morphologically similar to Erythrina breviflora, in order to provide further evidence for the existence of a passerine pollination syndrome in the New World. STUDY SITE AND METHODS This study was carried out at the Rio Palenque Science Center located 65 km south of the equator, between Santo Domingo and Quevedo in the Pacific lowlands of central Ecuador (see Dodson & Gentry, 1978, for further details). Field observations and measurements were made on Erythrina megistophylla during two periods between 20 July 1978 and 2 Aug. 1978. Erythrina megistophylla is an understory tree of up to 4 m and is a fairly common component of disturbed and mature portions of the wet forest (sensu Holdridge) at the Science Center (Dodson & Gentry, 1978). In addition to its characteristic floral morphology, it is distinguished by the presence of a single-seeded, baseball-sized fruit and very large (ca. 30 cm) leaflets. During each period of observation, floral visitors and nectar secretion patterns were recorded for Erythrina individuals. Observations totaling 60 hours from 0600 to 1700 EST were made on 4 trees at the Science Center. Three of these were located in forest edge habitats while the fourth was in the forest. Two different trees on two different days (one week apart) were utilized for continuous nectar sampling (see Figs. 1-3). The same inflorescences on each tree were sampled both times. Data from tree 3 are an average of 10 flowers from two -inflorescences, while only 5 flowers from a single inflorescence were measured for tree 2. Continuous nectar sampling consisted of hourly samples beginning at 0630 EST and ending when nectar secretion ceased (which was usually between 1350 and 1550) (Fig. 1). Five individual flowers were numbered on each inflorescence enabling the pattern of secretion for each flower to be followed. Nectar was easily removed repeatedly without injury to the flowers. Except when taking measurements, inflorescences were covered with nylon mesh bags, beginning prior to anthesis, to eliminate nectar removal by visitors during the sampling period. Volume measurements were obtained using calibrated micropipets. Nectar This content downloaded from 157.55.39.111 on Sat, 17 Sep 2016 06:04:00 UTC All use subject to http://about.jstor.org/terms 492 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 66 Tree 3 Tree 3 Tree 2 4.0 7-26-78 8-2-78 8-2-78