Abstract
When insect activity is limited at low temperature, birds may be comparatively more important pollinators than insects for flowering plants. It has been thought that many large-flowered Rhododendron species are pollinated by local birds in the Himalayan regions because most of these species flower in spring at high elevation with cool atmospheric temperature. However, experimental evidence for the role of bird pollination in this hyperdiverse genus remains scarce. To examine the role of birds and insects in pollination, we observed floral visitors to 15 Rhododendron species with different floral sizes and abundant flowering individuals in the eastern Himalayas, Southwest China. To examine the role of birds and insects in female reproductive success in each species, cages were used to exclude birds but not insects from visiting flowers and net bags were used to exclude all floral visitors. Inflorescences where visitation was excluded did not produce fruits in any of the Rhododendron species, indicating that sexual reproduction in these species depended on pollinator visitation. Bird visits were generally less frequent than bee visits in the studied species. However, in the nine species on which bird visitors were observed, fruit and/or seed set were greatly reduced in inflorescences caged to exclude birds but not bees, compared to open-pollinated inflorescences. In the other six species on which bird visitation was not observed, fruit and seed set did not differ significantly between caged and open inflorescences except in one species (R. wardii). Manipulations to achieve selective exclusion of visitors demonstrated that birds could be effective pollinators for 10 out of 15 studied Rhododendron species in the eastern Himalayas. Floral characteristics of these Rhododendron species and weather conditions might favour the evolution of bird pollination systems in the East Himalayas.
Highlights
It has been estimated that 87.5 % of flowering plant species rely on animals for pollination (Ollerton et al 2011)
To test the effectiveness of birds and insects as pollinators of 15 Rhododendron species, we addressed the following specific questions: (i) Given that low temperature limits insect activity, do warm-blooded birds act as floral visitors in some Rhododendron species? (ii) How do birds contribute to pollination in these species? (iii) Given that a morphological fit between floral morphology and pollinator body could facilitate pollen transfer, do birds tend to visit relatively large Rhododendron flowers? We investigated pollinator species and their visitation frequency in 15 Rhododendron species that had abundant flowering individuals with a wide variation in flower size
Bird visitation was observed in nine Rhododendron species which generally had large flowers (Tables 1 and 2)
Summary
It has been estimated that 87.5 % of flowering plant species rely on animals for pollination (Ollerton et al 2011). Huang et al – Bird pollination in Rhododendron et al 2006; Wilson et al 2007) Of these two groups of pollinators, birds tend to be larger and can be more active in cool temperature than insects. It has been proposed that bird pollination enhances plant reproductive success at low atmospheric temperature when weather conditions are unfavourable for bees (Cruden 1972). One hypothesis for the shift from bee to bird pollination is the high ratio of pollen receipt to pollen removal (Wilson et al 2007). Insects are generally short-lived and overwinter as immature stages or in hibernation (i.e. many bees, see Goulson 2003), and are rarely available for plants that flower in winter or early spring when the temperature is low
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