Effective pollination of Aeschynanthus acuminatus (Gesneriaceae) by generalist passerines, in sunbird-absent East Asia

Kai-Hsiu Chen,Jing-Yi Lu,Chun-Neng Wang

doi:10.1038/s41598-019-53035-2

Abstract

Aeschynanthus (Gesneriaceae), a genus comprising approximately 160 species in subtropical Southeast Asia, has red, tubular flowers, typical of a sunbird pollination syndrome. A. acuminatus, the species that is distributed extending to the northern edge of the genus, where the specialized nectarivorous sunbirds are absent, possesses reddish-green flowers and a wide-open corolla tube, flowering time shifts from summer to winter and the species achieves high fruiting success. This atypical flower led us to investigate the pollination biology of this species. Three species of generalist passerines, Grey-cheeked Fulvetta (Alcippe morrisonia, Sylviidae), White-eared Sibia (Heterophasia auricularis, Leiothrichidae) and Taiwan Yuhina (Yuhina brunneiceps, Zosteropidae), were recorded visiting A. acuminatus flowers. Pollination effectiveness was quantified via conspecific pollen presence on stigmas and natural fruit set. The significantly high natural fruit set (60%) and conspecific pollen transfer rate (94%) indicate high reproductive success facilitated by the accurate pollen placement on the birds. The existence of copious (61 µL) and highly diluted (7%) hexose-dominant nectar, together with a major reflectance peak of corolla lobe in the long-wavelength red color spectrum, is consistent with the pollination syndrome of generalist passerines. The high pollination effectiveness of A. acuminatus due to the recruitment of generalist passerines as pollinators, and the specializations of floral traits to match generalist bird pollination, appear crucial in the successful colonization on islands such as Taiwan that lack specialized bird pollinators.

Highlights

The degree of evolutionary specialization between nectar-feeding birds and their food plants varies greatly among regions
Based on specimen records and our field observation, the fruit set of A. acuminatus is abundant, and individuals are spreading throughout the broadleaf forest, indicating that the species can adapt to various habitats and achieve high reproductive success
The fulvetta was observed at QSB and MYY and the Yuhina was recorded at all three study sites, usually in large flocks; while the Sibia was commonly seen on A. acuminatus in only one to two individuals at XT

Summary

Introduction

The degree of evolutionary specialization between nectar-feeding birds and their food plants varies greatly among regions. In temperate Asia, the nectarivorous bird specialist (sunbird) absence region, there were growing numbers of reports that opportunistic nectar-feeding birds (generalist passerines) act as alternative pollinators especially for winter flowering plants. In temperate Asia, generalist passerines have been demonstrated the most effective pollinators for winter flowering plants such as Brandisia hancei, Camellia spp., Eriobotrya japonica and Rhododendron spp[14,17,21,22,23] These opportunistic nectar-feeding birds had been acted as seasonally specialist birds for these winter flowering plants to achieve reproductive success, because in winter these flowers were visited almost exclusively by these generalist passerines despite their omnivorous diet over the remainder of the year[14,24]. We tested the following hypotheses: (1) In the absence of sunbirds, A. acuminatus exhibits a pollinator shift, adopting different pollinators different from other bird-pollinating Aeschynanthus species. (2) If opportunistic nectar-feeding birds such as generalist passerines act as alternative pollinators for winter-flowering A. acuminatus, this pollination may not be effective. (3) If the pollination effectiveness is high, the observed changes of pollination syndromes, the floral color, floral shapes and nectar properties, may match the alternative pollinators to facilitate reproductive success

Methods

Results

Conclusion