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.