Seasonal dynamics of floral composition and flower visitors in a subtropical alpine ecosystem in Taiwan

Abstract

Flowering phenology of alpine plant communities and seasonal dynamics of flower visitors have been scarcely studied in the tropical/subtropical alpine regions. We report flowering phenology, flower production, and flower-visiting insects in the alpine site of central Taiwan. Throughout the research period (2017–2018), we recorded flowering phenology of 130 plant species, flower production of 81 species, and 15,127 insects visiting alpine flowers. Most of the alpine plants were visited by dipteran insects and/or hymenopteran insects. The seasonal patterns of flowering were more apparent in bee-visited plants compared to fly-visited plants in which the flowering of bee-visited plants clearly increased as the season progressed. About 63% of flower visitors were dipteran insects (syrphid and non-syrphid flies), and 30% were hymenopteran insects (mostly bumble-bee workers). Although the seasonal trend in fly abundance was less clear between years, bumble-bee abundance consistently increased in the middle to late seasons, reflecting colony development. There was a positive correlation between bee abundance and the number of flowering species of bee-visited plants, but there was no correlation between fly abundance and the number of flowering species of fly-visited plants throughout the season. These results suggest that the flowering phenology of subtropical alpine communities is influenced by the seasonal availability of pollinators. Bumble bees, syrphid flies, and non-syrphid flies had wide ranges of foraging flowers, but their niche overlap was relatively small. Because cold-adapted bumble bees are threatened by climate change in Taiwan, plant–pollinator interactions may be disturbed by global warming.

Flower colour and size-signals vary with altitude and resulting climate on the tropical-subtropical islands of Taiwan

“…A limitation of the current study is that whilst measuring flowering plant colouration across Taiwan, it was not possible to simultaneously capture a detailed survey of the insect species at the respective altitudes as this type of data in itself requires different methodologies (Shrestha et al, 2019b and reference within). We did however observe that honeybees, bumblebees, and several other bee species were frequently present across Taiwan (Starr, 1992;Sung et al, 2006;Lu and Huang, 2023;and Kudo et al, 2024); thus, the colorimetric analyses implemented was relevant at all field sites. Whilst for fly pollinators, new analyses tools like for colour similarity judgements have emerged (Garcia et al, 2022), it currently remains an open question how fly pollinators may use chromatic and/or achromatic contrasts for flower detection and subsequent recognition.…”

“…For example, whilst bee pollinators promote a variety of salient flower colours across the colour space of bees ( Figure 7 ), plants pollinated by flies are most frequently a dull yellowish green colour and have loci in a different region of colour space compared to bee pollinated flowers ( Garcia et al., 2022 ; Dyer et al., 2023 ). Interestingly, recent evidence suggests that in some mountain environments, fly visitors may become relatively more frequent than bee pollinators due to climatic conditions ( McCabe and Cobb, 2021 ; Kudo et al., 2024 ). Even between bee species colour preference experiments reveal some evidence of consistent preferences, such as for short wavelength rich “blue” colours, but also some differences between species that might influence the frequency with which particular plants are pollinated ( Giurfa et al., 1995 ; Koethe et al., 2016 ).…”

“…Moreover, similar to the patterns observed in Taiwan, flower size decreasing along elevation was reported at both species and community levels in China, Nepal, and Norway (Zhao and Wang, 2015;Garcia et al, 2021), which may be because of the resource-cost compromise in harsh alpine environments. Kudo et al (2024) suggested that the higher frequency of flies compared to bees that visited flowering plants in the Mt. Hehuanshan area (Nantou County), 3000m a.s.l., in central Taiwan may be due to the harsh environmental conditions.…”

“…relatively more frequent than bee pollinators due to climatic conditions (McCabe and Cobb, 2021;Kudo et al, 2024). Even between bee species colour preference experiments reveal some evidence of consistent preferences, such as for short wavelength rich "blue" colours, but also some differences between species that might influence the frequency with which particular plants are pollinated (Giurfa et al, 1995;Koethe et al, 2016).…”

“…One recent study in Taiwan ( Tai et al., 2020a ) showed that flower signals do show the same general patterns of conforming to pollination by bees as has been established in other countries ( Chittka and Menzel, 1992 ; Dyer et al., 2012 ; Bischoff et al., 2013 ; Shrestha et al., 2014 ). In addition, honeybees and bumblebees are frequently observed across Taiwan ( Sung et al., 2006 ; Kudo et al., 2024 preprint). In the current study, we thus employed the analysis techniques of understanding how bee colour vision optimally finds flowers depending upon chromatic and/or achromatic signals, and the respective size of flowers.…”