- Pollination Biology of Gentianales in a Southern Ecuadorian Montane Forest (2005)
- The pollination and reproductive biology of mostly small flowered, taxonomically related species of the monophyletic order Gentianales (Apocynaceae including Asclepiadoideae, Gentianaceae, and Rubiaceae) was investigated in a tropical montane forest in southern Ecuador. This study is part of an interdisciplinary project on the diversity and functioning of a montane ecosystem, and provides important results with respect to pollination biology. Such data are rarely collected for such a large number of taxa in a way that permits asking questions concerning both community ecology and evolution. This thesis investigates the contribution of diurnal floral visitors to the reproductive success of the night flowering "sphingophilous" Isertia laevis (Rubiaceae), and discusses the value of pollination syndromes in characterizing the pollinator spectrum of a given plant. Isertia laevis possesses flowers that are morphologically adapted to pollination by sphingids, but diurnal flower observation showed that nine different hummingbirds (Trochilidae) were frequent visitors. At night, only a few sphingid individuals were observed. Pollination experiments showed that flowers exclusively presented to nocturnal pollinators had a low fruit set (14%) but a high seed set (59%). Flowers accessible only to diurnal floral visitors showed a high fruit set of 63%, but a low seed set of 14%. Efficiency of pollination (fruit set X seed set) was equal for both diurnal and nocturnal pollinators. This shows that frequently occurring, but not very effective pollinators contribute substantially to seed production when the expected pollinators are scarce. This study is one of the first examining the pollination biology of Apocynaceae-Asclepiadoideae, tribe Asclepiadeae (other than the temperate genus Asclepias), providing basic information on a plant's reproductive biology, focusing on animal-plant interactions of these mostly twining, highly scattered inhabitants of forests and forest margins, possessing very small, inconspicuous flowers. A large variety of floral visitors was observed on the flowers, however, pollinaria were carried by only four insect species. The flowers showed a comparatively low pollinaria removal rate with an average of 0.32 ± 0.13, and an even lower average was recorded for the pollinia insertion rate at 0.13 ± 0.07. The percentage of inserted pollinia to removed pollinaria was comparatively high with an average of 42.7% ± 22.3%. This shows that if an insect achieves pollinia transfer, it does so very effectively. The derived floral structure and pollinating system in the Apocynaceae-Asclepiadoideae has often been characterized as a very close flower-pollinator interaction. The present study, however, suggests limited specialization, with some degree of generalization. Sucrose is the predominant floral nectar sugar in the order Gentianales (although the order possesses flowers morphologically adapted to ornithophily, sphingophily, chiropterophily, melittophily, and myiophily), suggesting that nectar sugar composition is a conservative characteristic. The only significant differences in sugar composition were found in chiropterophilous and myiophilous flowers, which had a significantly lower sugar ratio than sphingophilous flowers. A separation of chiropterophilous and myiophilous flowers from the other pollination syndromes was further substantiated by non-metric, multidimensional scaling using the CNESS index of dissimilarity, based on nectar sugar compositions. Further, matrix tests revealed no correlation of observed floral visitors to nectar concentrations, whereas a weak significant correlation was found between floral visitors and nectar sugar compositions. Therefore, some degree of adaptive convergence of floral nectar compositions to principal pollinator type within the constraints set by phylogenetic history is likely. Matrix tests revealed a correlation between floral visitors and nectar volume of covered flowers, and to a lesser extent, of uncovered flowers. The nectar volumes of covered and uncovered flowers were related to, and differ significantly among, pollination syndromes. Therefore, the driving force to visitation appears to be the volume of nectar the visitor can expect to consume. A phylogenetic study of Arcytophyllum (Rubiaceae) based on Internal Transcribed Spacer (ITS) was conducted and compared with an earlier study based on cpDNA. To understand the expression of heterostyly in the genus, we analyzed the inter- and intraspecific variation in floral morphology, nectar, pollen-ovule ratio, and seed set of ten species in eleven populations. Stigma and anther levels differed significantly between the morphs in the species/populations investigated except in A. filiforme. Different expressions of heterostyly in Arcytophyllum seemed independent of phylogenetic relationships, however, there was a tendency towards a higher percentage of sucrose in the more derived species of the genus. Nectar sugar composition was similar between the morphs. Pollen dimorphism, both with regard to number (long-styled > short-styled) and to size (short-styled > long-styled) was observed in all taxa investigated except for A. filiforme. This shows that both features in the expression of heterostyly and the reproductive system may vary among closely related species.