Introduction
Alpine ecosystems in higher latitudes are characterized by cool and
short growing season, during which flowering of alpine plants progress
rapidly. Many alpine plants depend on insects for pollination, and
pollen limitation is a key factor affecting seed production of alpine
plants (Kudo, 2022). Because temporal matching of flowering time and
pollinator activity, and insect abundance are influenced by climate
change, plant–pollinator interactions in alpine ecosystems are
predicted to be disturbed by climate change (Inouye, 2015).
Bumble bees, syrphid flies, and non-syrphid flies are the most common
and dominant pollinators in alpine ecosystems (Kearns, 1992; Pyke et
al., 2011; Inouye, 2020). However, their frequencies and foraging
activities vary among geographic regions and along elevational or
latitudinal gradients. For instance, bumble bees are absent in New
Zealand, where solitary bees and syrphid flies are important pollinators
of alpine plants (Bischoff et al., 2013). The foraging range of floral
resources of bumble bees becomes wider at higher elevations in the
Colorado Rocky Mountains because of higher flowering overlaps among
alpine plant species during a short summer (Miller-Struttmann & Galen,
2014). Furthermore, the importance of fly pollinators commonly increases
with elevation and latitude due to higher tolerance to cool conditions
in dipteran insects compared to hymenopteran insects (Strathdee & Bale,
1998; McCabe & Cobb, 2021). Thus, relative importance of the two major
flower-visiting insect groups, i.e., flies and bees, as pollinators of
alpine plants may vary among mountain regions at a geographic scale.
Humid tropical/subtropical alpine regions are the most sensitive
ecosystems to climate change, where plant communities are composed of
many endemic species within isolated mountain areas (Buytaert et al.,
2011). Alpine ecosystems in lower latitudes are characterized by longer
growing periods and moderate seasonal fluctuations in temperature
compared to the alpine ecosystems in higher latitudes. Thus, flowering
phenology and seasonal dynamics of flower-visiting insects may be
different from the alpine ecosystems in higher latitudes. However,
phenological studies on the tropical/subtropical alpine ecosystems are
limited (e.g., Kudo & Suzuki, 2004; Pelayo et al., 2019, 2021). To
predict the climate change impacts on the tropical/subtropical alpine
ecosystems, therefore, clarification of the basic features of flowering
phenology at community scale and flower-visiting insects is crucial.
The previous studies conducted in northern Japan (Kudo, 2016; Mizunaga
& Kudo, 2017) reported that the abundance of bumble bees showed clear
seasonality reflecting the colony development process in which only
overwintered queens emerged in the early season and worker abundance
increased abruptly in the middle of the season. In contrast, the
frequency of flower visitation by dipteran insects was positively
related to ambient temperature, but often has no clear seasonality, and
they showed outbreaks at unpredictable times. Responding to the seasonal
pattern of bumble bees, the group of bee-visited plants tended to have a
bimodal flowering pattern, that is composed of early-flowering species
and late-flowering species, although actual flowering periods strongly
depended on snowmelt time. On the other hand, the group of fly-visited
plants showed a unimodal flowering pattern in which many species
flowered in the middle of season when the ambient temperature was high.
These results suggest that different selective forces may act on
flowering behavior between bee-visited and fly-visited species even
within the same alpine plant communities. To test this possibility,
comparisons of flowering phenology and flower visitors across multiple
alpine ecosystems are necessary.
In the present study, we recorded flowering phenologies of alpine plant
communities and seasonal dynamics of flower-visiting insects in the
Hehuanshan area of central Taiwan. Taiwan is a mountainous island
located in the tropical/subtropical climate zone, where more than 200
mountains exist above 3000 m elevations. The mountain flora of Taiwan is
characterized by a high proportion of endemic species (60%: Hsieh,
2002). A recent study reported that alpine vegetation in Taiwan has been
altered by climate change (Chou et al., 2011), and it is expected that
plant–pollinator interactions in the alpine ecosystems may also be
influenced by climate change. Moreover, one recent study showing
overdispersion in the flower color among closely related species in the
alpine flora of Taiwan stresses the importance of plant-pollinator
interaction at the community level (Tai et al., 2020). However, there is
no information about the flowering patterns of alpine plant communities
and the seasonal dynamics of flower-visiting insect communities. In
order to clarify the interactions between flowering phenology and
seasonal dynamics of flower visitors, we observed seasonal changes in
the flower production of plants and the frequency of flower-visiting
insects over two years. We expect that hymenopteran insects (mainly
bumble bees) and dipteran insects are major flower visitors in the
alpine ecosystem of Taiwan as well as mid-latitudinal alpine regions in
the northern hemisphere. Our hypotheses are as follows:
1) Flowering of bee-visited plants may be concentrated during the active
season of worker bees if bumble bees are major flower visitors in the
alpine environment of Taiwan.
2) Flowering of fly-visited plants may vary among species if the
seasonality of fly activity is less clear and unpredictable due to the
stable temperature conditions during the flowering season.