Butterfly diversity and community dynamics in the central Himalayas: Species composition, richness, abundance, and seasonal variation of butterflies (Lepidoptera: Papilionoidea) in Bhorletar, Nepal

Butterflies are among the most effective bioindicators of climate change; however, their diversity in many rural areas of the Central Himalayas remains understudied. This study provides an assessment of butterfly diversity in the foothills of Bhorletar, Madhya Nepal Municipality, Lamjung District, Nepal, within an elevation range of 420–600 m. Conducted between July 2019 and January 2021, the survey involved opportunistic observations and photography of adult butterflies in their natural habitats, with sampling occurring six times each month. The study aimed to investigate the species composition, richness, and abundance of butterflies across the survey period and identify seasonal changes in species composition and richness. A total of 94,009 individuals across 226 species, 129 genera, and six families were documented. During this study, Halpe arcuata Evans, 1937 and Hasora taminatus bhavara Fruhstorfer, 1911 were recorded for the first time in Nepal. Additionally, Halpe filda Evans, 1949 and Ctenoptilum vasava vasava (Moore, [1866]) were recorded for only the second and third times, respectively, in Nepal, following a gap of approximately three decades. The most abundant species was Pieris canidia indica Evans, 1926 (Relative Abundance [RA] 2.55%), followed by Pseudozizeeria maha maha (Kollar, [1844]) (RA 2.13%). Species richness showed an annual bimodal distribution, peaking in April (180 species) and August (161 species), while the lowest richness was observed in January and February, with 68 and 75 species, respectively. Diversity indices included a Shannon–Wiener index of 4.71, Pielou's J index of 0.87, an effective number of species of 111.24, and Margalef's richness index of 19.65, indicating high species diversity with a well-balanced mix of species evenness and richness. This study offers the first peer-reviewed checklist of butterflies from Bhorletar, providing crucial baseline data for future research and conservation efforts, and highlights the remarkable seasonal and species diversity within the region.

Methods
A survey of adult butterfly fauna was conducted in the foothills of Bhorletar (28°09′20.1″ N, 84°14′15.1″ E ) (Fig. 1) from July 2019 to January 2021 within the elevational range of 420 to 600 meters. The survey was conducted within an approximate 3-square-kilometer area. The survey followed an opportunistic design mostly owing to the variable landscape of the study area often altered by seasonal changes. While this method may lack consistency (Van Strien et al. 2013), such as in following a fixed transect throughout the survey (Pollard 1977, 1979), we maintained uniformity by ensuring an equal number of observation hours each survey session. The survey was conducted six times each month by the first author covering various habitats of the study area, including riverbanks, forest trails, streams, clearings, hilltops, residential areas, and open agricultural fields. Observations were conducted between 10 am and 5 pm Nepal Standard Time (NPT), except when inclement weather or unforeseen circumstances necessitated a pause. Although butterflies could be spotted as soon as temperatures rose, 10 am was deemed the optimal start time for the survey owing to the surge in their activity around this hour (Pollard 1977, Gupta et al. 2019); evening hours were chosen to align with the flight period of crepuscular skipper species (Chiba 2005, 2009). Any lost hours were made up on subsequent days to ensure comprehensive data collection. Butterflies were photographed in their natural habitats using a Sony Cyber-Shot DSC-HX90V camera from 2019 to 2020, and a Canon 7D Mark II camera coupled with a Canon EF 100 mm f/2.8L Macro IS USM lens from 2020 to 2021. All butterfly and habitat images (Figs 2–31) presented in this paper were photographed by the first author. The images were automatically geotagged by the cameras. Three species, viz., Acraea terpsicore (Linnaeus, 1758), Cethosia biblis tisamena Fruhstorfer, 1912, and Delias hyparete indica (Wallace, 1867) could not be photographed well during the study and are therefore represented by photographs taken from alternative locations. Specimens that were challenging to identify were collected using a hand net. Following each survey, species counts and abundances were recorded on a survey sheet and subsequently transferred to MS Excel 2019 for data consolidation and monthly analysis. It is important to note that estimates for species with high daily counts (typically exceeding 10 individuals) may not be as accurate as the exact counts possible for species with lower counts (fewer than 10 individuals). To facilitate data presentation, abundance figures have been rounded as follows: tens to the nearest ten, hundreds to the nearest fifty, and counts exceeding 1,000 to the nearest hundred. Given the high butterfly abundance in our study area, there is a possibility that some individuals may have been counted multiple times within a day or month. However, to minimize this risk, efforts were made to avoid recounting individuals that had already been observed. Most butterflies were observed engaging in relatively stationary activities such as feeding on flowers, perching on leaves, or mud-puddling, which helped reduce the likelihood of multiple counts. We also made efforts to avoid recounting individuals that had already been observed, by noting distinctive markings or behaviors. For accurate identification of cryptic species, genitalia analysis was performed using an Olympus Stereo-microscope Model SZ2-ILST in the science lab of Ishaneshwor Secondary School, Bhorletar. Genitalia were pretreated by soaking in 10% potassium hydroxide (KOH) solution in Petri dishes for a minimum of 12 hours to facilitate dissection and examination. Identifications were made with the help of reference materials such as Evans (1927, 1932, 1949, 1957), Talbot (1947), Kehimkar (2008, 2016), Smetacek (2011, 2015, 2017), Varshney & Smetacek (2015), and online resources (www.flutters.org, www.ifoundbutterflies.org, http://yutaka.it-n.jp). Previous distribution records in Nepal were compiled from Gough (1935), Bailey (1951), Smith (1994, 2010, 2011a, b), and Van der Poel & Smetacek (2022 first draft). Data analyses were performed using MS Excel 365; the study area map was created using QGIS software (version 3.32.3 ‘Lima’ 2024) and Google Earth (https://earth.google.com). Climatic data, comprising monthly average temperature and precipitation, were obtained from the POWER project (version 2.3.6) (NASA Power 2024) for the study period (Fig. 35). A correlation analysis was performed to investigate the relationship between monthly species richness and two key climatic variables: average monthly temperature and precipitation (Fig. 36). The Shannon-Wiener index (H') (Shannon & Weaver 1949) (H' = -Σ (pi * ln(pi)), where Σ represents the summation over all species, pi is the proportion of individuals belonging to species i, and ln is the natural logarithm) was employed to quantify species diversity, encompassing both species richness and abundance, for each family individually and for the overall species assemblage. Additionally, Pielou’s J index (J') (Pielou 1969) (J' = H' / ln(S), where H' is the Shannon-Wiener index, and S is the total number of species) was utilized to calculate species evenness, which measures the distributional uniformity of species on a scale of 0 to 1, where 1 denotes maximum evenness. Furthermore, Margalef’s Richness Index (D) (Margalef 1958) (D = (S - 1) / ln(N), where S is the total number of species, and N is the total number of individuals) was calculated to estimate species richness, providing a simple yet effective measure of the number of species present in each family while accounting for sample size. The effective number of species (E) was determined using the formula E = e^(H'), which employs the Shannon-Wiener index to provide a more nuanced measurement of diversity (Jost 2006). Relative abundance was quantified in percentage for each species recorded, rounded to three decimal places. A local abundance index was developed by the first author to objectively assess the status and rarity of observed species, based on observational data; however, it is important to note that some species, although common in nature, may be elusive and therefore less frequently recorded during surveys. The index categorizes a species as follows:

vF (Very Frequent): Observed in every survey during its flight season.

F (Frequent): Observed more than 10 times but not in every survey during its flight season.

fR (Fairly Rare): Observed 3–10 times throughout the study.

R (Rare): Observed only twice throughout the study.

vR (Very Rare): Observed only once throughout the study.