Plant-pollinator network in the subtropical alpine ecosystem
The network analysis between alpine plants and flower-visiting insects revealed that most alpine plant species in Taiwan depended on bumble bees, syrphid flies, and/or non-syrphid flies. Furthermore, niche overlap between these insect groups was relatively small (0.30), indicating that each insect group has its own linkage to the specific plant species. Interestingly, the similarity of foraging flowers between syrphid and non-syrphid flies was relatively low (0.57). This means that floral preference varies even within dipteran insects. As syrphid flies have relatively high floral constancy and high pollination efficiency (McGuire & Armbruster, 1991; Kearns, 1992; Fontaine et al., 2005), they will be important pollinators in the subtropical alpine ecosystems.
Bumble bees are the most important pollinators in alpine ecosystems due to their high pollination efficiency, floral constancy, and wide floral-use capacity (Bingham & Orthner, 1998; Fang & Huang, 2012). In the subtropical climate zone of Asia, species diversity and abundance of bumble bees increased with higher elevation, e.g., 3000–4000 m in the Himalayas (Saini et al., 2012). There are nine bumble bee species in Taiwan and most of them are seen in high mountain regions (Starr, 1992). Although we did not discriminate bumble bee species in the present study, the study of plant-bumble bee network conducted in the Himalaya-Hengduan Mountains in southern China reported that wide linkages between bumble bees and alpine plant species were formed by the intraspecific variation in floral choice (Liang et al., 2021). There are many studies demonstrating that the floral choice of bumble bees strongly depends on the body size and glossa length (e.g., Inouye, 1980; Harder, 1985, Pyke et al., 2011). To clarify how bee-visited plants share pollinators during the active period of worker bees, further studies are necessary.
Plant-pollinator networks may differ among subtropical/tropical alpine ecosystems located in different geographic regions. In the tropical alpine communities in the Venezuelan Andean paramo (3000–4200 m a.s.l.), for instance, flowering of most species occurred during the rainy season (May–November) although some species bloomed throughout the year (Pelayo et al., 2019). Major flower visitors were bumble bees (36.5%) and hummingbirds (43.5%), while dipteran insects were less common (4.1%). Bumble bees and hummingbirds were specialized to specific plants for foraging (low niche overlap), and flowering progressed continuously among plant species during the rainy season. Thus, the taxonomic composition of flower visitors and the flowering pattern of alpine plant communities in the Andean paramo were very different from the subtropical ecosystem in Taiwan. Dominance of dipteran insects (generalist pollinators), high humidity, and the existence of winter (December to April during which minimum temperature often decreases below zero) may characterize the plant-pollinator networks in the subtropical alpine ecosystem of Taiwan.