Community structure of pollinating insects and its driving factors in different habitats of Shivapuri‐Nagarjun National Park, Nepal

Abstract Insect pollinators are important means for a stable ecosystem. The habitat types play a crucial role in the community composition, abundance, diversity, and species richness of the pollinators. The present study in Shivapuri‐Nagarjun National Park explored the species richness and abundances of insect pollinators in four different habitats and different environmental variables in determining the community composition of the pollinators. Data were collected from 1,500 m to 2,700 m using color pan traps and hand sweeping methods. Non‐Metric Multidimensional Scaling (NMDS) and Redundancy Analysis (RDA) were conducted to show the association between insect pollinators and environmental variables. The results firmly demonstrated that species richness and abundances were higher (158) in Open trail compared to other habitats. The distribution of the pollinator species was more uniform in the Open trail followed by the Grassland. Similarly, a strong positive correlation between flower resources and pollinators' abundance (R2 = .63, P < .001) was found. In conclusion, the Open trail harbors rich insect pollinators in lower elevation. The community structure of the pollinators was strongly influenced by the presence of flowers in the trails.

Honeybees are better-known bees for pollination in comparison with wild pollen bees (Losey & Vaughan, 2006;Potts et al., 2010) while flies and butterflies are the least known as pollinating insects (Jennersten, 1984;Larson et al., 2001). Several studies show that there is decline in pollinators globally (Carvalheiro et al., 2013;Dirzo et al., 2014). Especially, the population of bumblebees (Cameron et al., 2011;Fitzpatrick et al., 2007) and butterflies (Van Swaay et al., 2010;Warren et al., 2001) is shrinking in the world due to natural as well as anthropogenic threats. The major drivers of pollinator loss are recognized as habitat loss, landscape modification, intensification in agriculture, and even climate change (Kearns et al., 1998;Kovács-Hostyánszki et al., 2017;Potts et al., 2010). Hence, maintaining pollinator diversity in the given landscape requires an understanding of a clear pattern of pollinator diversity along with the habitat types.
The species richness, diversity, distribution, and community structure of pollinators depend upon the local environment (Neumüller et al., 2020;Williams et al., 2010). Availability of flower resources, humidity, and temperature need to be taken into account especially while comparing the pollinators among habitats (Neumüller et al., 2020). The activity of pollinators is strongly correlated with air temperature, plant species richness (Hudewenz et al., 2012), and humidity (Pellissier et al., 2010). Bees and butterflies prefer warmer temperatures than flies (Kühsel & Blüthgen, 2015).
Furthermore, elevation also determines the abundance and community structure of insect pollinators (Adedoja et al., 2020). There is an interesting distribution pattern among different groups of insect pollinators. Hymenoptera is the dominant pollinator in the lowland while Lepidoptera and Diptera dominate the high land (Warren et al., 1988). This kind of distribution in a range of habitats is probably for fulfilling their ecological requirements and these ecological necessities are mostly species or guild specific (Proesmans et al., 2019). For instance, bees prefer to forage the flowering plants close to their nesting area (Gathmann & Tscharntke, 2002;Greenleaf et al., 2007).
They also construct the nest in deadwood (Sydenham et al., 2016) and the sun-exposed soil ground (Everaars et al., 2011). However, hoverflies and butterflies fly away from the egg-laying areas for foraging and do not construct the nest. Aphidophagous hoverflies such as Episyrphus, Sphaerophoria depend on agricultural habitat Pinheiro et al., 2015), while saproxylic hoverflies (Xylota) are benefitted from the forest (Reemer, 2005). Similarly, butterflies can make a flight over greater distances (Herrera, 1987).
They make such range of flight in search of different kinds of flowers for pollen (Gilbert, 1972) and nectar (Tiple et al., 2005). Additionally, oviposition-plant's location signifies habitat selection for smaller and less mobile butterflies, such as the 'blues' while the larger butterflies like Erebia epipsodea and Colias probably have large ranges of the search for their widespread ovipositing plants (Sharp et al., 1974).
The above examples show that pollinators share different habitats and hence the habitat types potentially impact the community composition, abundance, diversity, and species richness of the pollinators.
Many studies have evaluated how environmental factors influence pollinator composition. However, the effects of many environmental factors on pollinator composition can be very different among climate/vegetation zones (Neumüller et al., 2020;Senapathi et al., 2017). Shivapuri-Nagarjun National Park (SNNP) lies in the southern foot of the Himalayas, a mountainous area with a complex landscape. The unique landscape of the park could harbor interesting pollinator fauna and number of environmental factors may differently influence their composition. So, it is necessary to test the roles of these factors on pollinators in SNNP. The general understanding about natural habitat is, if freed from pesticides, it probably possesses more diversity than the managed habitat. However, the habitat heterogeneity would also affect the pollinator community as the interaction between plant-pollinator is specific (Oliver et al., 2010;Rundlöf et al., 2008;Weibull et al., 2000).
We examined species richness, abundance, and the community composition of insect pollinators in different habitats along the elevation gradient. We hypothesized that there is an effect of habitat types in the diversity, distribution, and composition of pollinating insects. Our research questions were as follows: 'What is the distribution pattern of pollinating insects? Which environmental factors significantly influence this distribution pattern, diversity, and community composition of pollinators?'.

| Study area
Shivapuri-Nagarjun National Park (Figure 1) is the only protected area in the mid-hill region of Nepal. The Park covers an area of 159 square kilometers with a buffer area of around 118.61 square km. It lies within 27°45′ to 27°52′N Latitude and 85°16′ to 85°45′E Longitude. It has an altitude ranging from 1,360 m to 2,732 m above sea level. The Park is rich in freshwater resources with abundant biodiversity and cultural heritage (SNNP, 2017

Number of transects Description
Forest trail 1,500-1,700 11 Forest trail is denoted as the forest with canopy coverage of more than 70% with a walking trail of 15 m width 1,800-1,900 12 2,000-2,700 10 Grassland 1,500-1,700 3 Grassland habitat is herb-dominated in the transect with sparsely located trees 1,800-1,900 6 2,000-2,700 5 Open trail 1,500-1,700 4 Open trail habitat of the forest means an area with no tree coverage within 15 m width on either side of the survey transect 1,800-1,900 5 2,000-2,700 6 Managed habitat 1,500-1,700 7 The Open trail of managed habitat is the transect along the walking trail of the managed habitat with no canopy tree within 15 m width of the trail that incorporates the buffer zone area of the National Park. The area is a human settlement with cultivated lands. The major crops cultivated during spring are mustard and coriander while mustard, buckwheat, balsam apple, and squash are in autumn 1,800-1,900 2 2,000-2,700 4 Note: The fourth column of the

| Pollinators' sampling
Pollinators here mean flower-visiting and nectar-feeding insects.
The transect line was fixed in walking trail of Forest, Open forest, and Managed habitat, while in the Grassland transects were drawn at the edge and the middle of the Grassland with 20-m inter-transect distance (Stanley, 2013). The survey was performed in the spring and autumn seasons for the consecutive years 2018 and 2019. In each season, the pollinators were sampled for 3 consecutive days in sunny weather between 9 am and 4 pm (Pollard & Yates, 1994). The pollinators were collected using hand sweeping and color pan traps. White, yellow, and blue color pans were used for insect sampling. During each sampling day, a transect walk of 30 min was made along the 100 m of the trail in the Open trail of the forest, Forest trail, and trail of the Managed habitat, while in Grassland the transect walk was made at the edge and the middle . The transect walk method was used to sample butterflies (Pollard & Yates, 1994), bumblebees (Fussell & Corbet, 1992), hoverflies, and other bees (Proesmans et al., 2019). During the walk, all insect pollinators, which fed on flowers within 2 m of the observer, were captured, except for those that could be identified to a species level in the field (Neumüller et al., 2020). Unidentified insect pollinators were collected in separate vials, transferred in the icebox, and brought to the laboratory for identification.
Similarly, colored pan traps were deployed in each transect. This method aids in the simultaneous sampling of multiple locations, coverage of a large number of sites, and is the efficient method of bee sampling (Nielsen et al., 2011;Westphal et al., 2008). The pans were plastic bowls of about 15 cm in diameter and painted with non-toxic three different colors; white, yellow, and blue (LeBuhn et al., 2003).
Each pan was attached to a post using a metal clamp adjusting the bowl in the rim. The pan was filled with 400 ml of detergent water.
Three posts were deployed at a 100-m distance, 20 m apart from each other. The traps were visited for collecting the fallen insects after 24 h and were transferred in labelled vials with 70% ethanol.

| Survey of flower resources
The survey of flower resources was carried out in the spring and autumn season during the pollinator survey in the same transect.
We made five quadrats of 10 m × 5 m in each sampling transect.
For an estimation of abundance flower resources, we scanned insect pollinating herbs and shrubs in each quadrat and identified the genus and species. The cover of flower resources in each quadrat was ranked between 1 and 6 (Szigeti et al., 2016). Rank: very scarcely = 1, scarce = 2, more or less scarce = 3, more or less abundance = 4, abundance = 5, and extremely abundance = 6. The abundance of flower resources for each sampling transect was calculated

| Humidity and temperature
The humidity and temperature during each sampling time were measured with a digital Thermo-hygrometer (HTC-2).

| Identification of insect pollinators
Unidentified insect pollinators in the field were identified to species level in the laboratory using relevant keys. Bingham (1897)  Pollinator's community compositions of different habitats (FT,

GL, OT, and MH) were analyzed by Non-metric Multidimensional
Scaling (NMDS) of the abundance data employing the function 'metaMDS' which is incorporated in the statistical package 'vegan' (Oksanen et al., 2013) and NMDS result with sample plots of different abundance scores were fitted with different habitats using the package 'ggplot2' (Wickham et al., 2016).

| Insect pollinators in Shivapuri-Nagarjun National Park
During the total sampling period, 8720 insect pollinators were Species accumulation curves for pollinators showed saturation in all habitat types indicating adequate sampling effort (Figure 3).

| Diversity and Distribution pattern of insect pollinators
Diversity indices in alpha level (Species richness, Shannon index) show that the Open trail was comparatively more diverse, followed Whittaker Beta diversity showed the high species turnover between Forest trail and Managed habitat, whereas accounted for low species turnover between Managed habitat and Open trail (Table 3).
Overall, Shannon Diversity of pollinators in SNNP (Gamma diversity) was 4.683 with 167 species (Supplementary 1).  there were no such differences (

| Abundance and species richness of pollinators in different habitats
Our present findings show that there are differences in the abundance and species richness of pollinators in the habitat types of SNNP.
Such influence of habitat types in pollinators has also been reported in a tropical megacity, Bangkok, Thailand (Stewart et al., 2018). Our F I G U R E 2 A range of insect pollinators collected from four different habitats of Shivapuri-Nagarjun National Park (Bees: a-d, Butterflies: e-h, and Hoverflies: i-l). (a) Andrena gorkhana Tadauchi and Matsumura, 2007; Brunetti, 1908 (j)  Leins, 2013; Masters & Emery, 2015). Adult hoverflies require high energy for hovering flight that could be obtained from the local landscape with abundant flowers (Haslett, 1989;Meyer et al., 2009;Proesmans et al., 2019). In contrast, the presence of low flower resources also accounts for the decline in pollinator abundance and species richness in Forest trail with canopy. Canopy cover increases shade in the understory herb and shrub of the forest lowering flower blooms and limiting pollinator's movement (Proesmans et al., 2019).
Sampled areas of the Managed habitat in SNNP were open, inhabited, and disturbed by some human activities. Some previous study also shows that the diversity of butterfly is negatively influenced by this factor of human disturbances (Kambach et al., 2013).

| Diversity and distribution pattern of insect pollinators
Furthermore, our result depicts that Open trail is more diverse followed by Managed habitat. There is low species turnover between these two habitats. This could again be explained by the influence Most of the pollinators show a strong preference for the structurally different land-use type that add variety in resources they required (Bates et al., 2011;Matteson & Langellotto, 2010). However, flower resources are comparatively less in the Forest trail and Grassland, which could be the probable reason for less diverse pollinators and similar evenness of pollinators between these two habitats.
The distribution pattern of pollinators is varied among different species in the present study. Among all pollinators, 61 species were common in all the studied habitats. This result reflects the different needs of pollinators for different activities. There is a heterogeneous habitat choice of wild bees and a wider temporal range of activities of flies (Willcox et al., 2019). Many bee groups require different habitats for nesting and foraging (Franzén et al., 2009) and also different spatial foraging distances made by bees like Bombus spp.
and Xylocopa spp. (Greenleaf et al., 2007). This could be a probable biotopes (Gittings et al., 2006). Furthermore, the abundance of hoverflies increases with proximity to the forest (Moquet et al., 2018) and some flies are confined to only one kind of habitat like Volucella trifasciata and Baccha maculata which were recorded from forest habitats only. The latter species usually prefer the shady area of forest (Coe, 1964;Hassan et al., 2019). Butterflies too, show a different kind of habitats preference in their life cycles (Janz, 2005). Females dwell in grassland for oviposition while forage in flower-rich habitats (Evans et al., 2020).

| NMDS
There is an influence of environmental variables in the community composition of pollinators. In our findings, the measured environ-  (Carreck & Williams, 2002).
The complex environmental gradient of high altitude affects the abundance of different taxa of pollinators (Warren et al., 1988). As expected, with the elevation, the abundance of pollinators declined.
The reason could be either decreasing of bee and fly attracted flowers or their limited number in higher altitude (Arnold et al., 2009).
The plant communities at the high altitude limit the vascular plant and thus, availability of insect pollinators (Mani, 1962 (Gathmann & Tscharntke, 2002;Vulliamy et al., 2006) so that they could gather maximum nectar to support the large bee colony (Potts et al., 2003). 2007) and humid areas to avoid hotter day temperature (Willmer, 1983). Aglais caschmirensis is the most abundant and frequently seen butterfly in all kinds of habitats (Irungbam et al., 2017). This could be the reason for its presence in the forest as well as Managed habitat in our study area. Similarly, the association of Episyrphus balteatus in the Managed habitat indicates the preference for flowers of vegetables such as coriander, buckwheat (Pinheiro et al., 2015) as their host plant.

| CON CLUS IONS
We studied the diversity, distribution, and community structure of insect pollinators in different habitats of SNNP, Nepal. Overall, habitats, humidity, atmospheric temperature, abundance of flower resources, and elevation played a significant role in the diversity, distribution, and community structure of pollinating insects. Insect pollinators were strongly associated with flower resources which were highly recorded in an Open trail. Shannon Index and evenness were high in an Open trail and similar evenness was found in the Forest trail and Grassland. Conservation of plant diversity in the walking trails of SNNP is important to conserve the community structure of insect pollinators. guiding to prepare a map in Arc GIS. The first author also thanks all the students who supported in the fieldwork. The first author is also grateful to the Department of National Parks and Wildlife Conservation, Nepal for providing permission for the collection of insects.

CO N FLI C T O F I NTE R E S T
The authors declare that they have no conflict of interest.