Seasonal diversity of butterflies and their larval food plants in the surroundings of upper Neora Valley National Park, a sub-tropical broad leaved hill forest in the eastern Himalayan landscape, West Bengal, India

Authors Contribution: PS was involved in the sampling, identification of studied species; statistical analysis of recorded data, manuscript preparation and site management. KKB was involved in data interpretation, literature survey and framing up of research questions. NG was engaged in the sampling, identification of studied species; statistical analysis of recorded data, data interpretation, framing up of questions and hypothesis along with manuscript preparation.


INTRODUCTION
Studies in India (Kunte 1997;Padhye et al. 2006;Bhusal & Khanal 2008) have established a relationship between butterfly species richness, density and diversity with respect to seasonality.For instance, tropical butterflies have been shown to be sensitive to seasonal changes in rainfall (Barby 1995;Hill et al. 2003).Wynter-Blyth (1957) documented 835 species from the eastern Himalaya in sharp contrast to only 415 species from the western Himalaya.The lowland forests of Bhutan harbour a rich and unique diversity of butterflies with maximum number of species recorded during spring and minimum number during the monsoons (Singh 2012).Saikia et al. (2010), during their study on 109 species from Rani-Garbhanga Reserve Forests recorded seasonality of butterflies with differences in the butterfly abundances as well defined dry and wet season forms due to distinct plant phenological state in different seasons of the year.Although a list of butterflies from the Darjeeling District of West Bengal (Maude 1949) is available, studies on the butterflies inhabiting the rich and diverse Himalayan landscape of Neora Valley National Park (NVNP) are lacking.NVNP is located at the trijunction of West Bengal, Sikkim (India) and Bhutan on the north and northeast.Rechila danda, the highest point of this National Park is situated at 3,170m (Mallick 2010).Therefore, work was carried out to document diversity of butterflies in different seasons from the fringe regions of the upper range of NVNP.The diversity and seasonality of butterflies probably reflect the phenophases of their host plants (Kunte 1997).Therefore an attempt was also made to record the larval food plants of butterfly species.

MATERIALS AND METHODS
The present study was conducted in the adjacent areas of the upper range of the NVNP (26 0 52'-27 0 7'N & 88 0 45'-88 0 55'E) located in the Kalimpong sub-division of the Darjeeling District, West Bengal, India (Fig. 1).It was notified as a protected area in April 1986 and was gazetted in December 1992.The park authorities divided Neora Valley into two ranges, namely the upper range with its headquarters at Lava, serving as its western entry point A C B and the lower range with its headquarters situated at Samsing, the park's eastern entry point (Mallick 2010).
The phytogeography of NVNP includes subtropical broad leaved hill forest, montane wet temperate forest along with subtropical pine forest (Champion & Seth 1968).Rodgers et al. (2002) placed NVNP in the biogeographic zone 2. The park has a wide altitudinal range varying from183m in the plains to 3,200m in the hills (Mallick 2012).The climatic condition varies between tropical/subtropical in its lower range to temperate in its upper range (Mallick 2010).The forest structure at the study site was mostly undisturbed.The surrounding terraces had cultivated fields of forest adjoining dwellers.
Four trail-cum-trekking routes (total length: 16km) (Table 1) were selected as study sites (i.e., NVNP-1, NVNP-2, NVNP-3 and NVNP-4) (Table 1).The survey was conducted between June 2011 and May 2012, following the Pollard-Walk Method (Pollard 1977) at eight randomly selected line transects (approximately 500m length and 8m breadth) located in each of the study sites.Butterflies were observed twice a day, (06:00-13:00 hr in the morning and 14:00-17:00 hr in the afternoon) by walking at a constant pace at each transect.Less time was devoted for sampling in the afternoon due to reduced butterfly activity at that time of the day.Separate days were devoted to sample each transect in each study site weekly for a month with the help of two trained field assistants.The sampling procedure was repeated at an interval of 30 days.As far as possible, surveys were conducted on sunny days with less than 30% cloud cover, as butterfly activity is suppressed on windy or cloudy days (Weiss et al. 1988).The sampling days missed due to inclement weather conditions were recorded.
The butterflies were observed (using Bushnell binoculars) and photographed occasionally (using Nikon COOLPIX-P90) for subsequent identification from literature (Evans 1932;Wynter-Blyth 1957;Haribal 1992;Kunte 2000;Kehimkar 2008) and reference collection at Zoological Survey of India.For better interpretation of collected data the year was divided into five seasons (viz., Spring: March; Summer: April-May; Monsoon: June-September; Autumn: October-November; Winter: December-February).The division of seasons was based on the variation of rate of precipitation and temperature.Larval host plants were recorded in each transect and also identified from the adjoining areas of transect.These plants were identified from published literature (Cowan & Cowan 1979;Polunin & Stainton 2005;Maity & Maiti 2007;Das et al. 2008) along with assistance from plant taxonomists.Meteorological data (i.e., temperature, precipitation) were collected during the study period.
The diversity of butterfly species across seasons was calculated using Shannon index of diversity given by the equation, H´=Σpi (ln pi), where, pi=ni/N; ni is the number of individuals of i th species and N=Σni.The Shannon index, which combines the number of species within a site with the relative abundance of each species (Shannon 1948;Magurran 1988) was determined using vegan package of "R".Margalef's species richness was used to compare the species richness across seasons.This index was calculated using equation R=(S-1)/ln N, where S is the number of species and N is the number of individuals (Magurran 1988).Evenness of species reveals how their relative abundance is distributed in a particular site or sample (Pielou 1969;Magurran 1988).This index is given by the equation, E=H´/ln S, where H´ is the Shannon index of diversity and S is the number of species.Rank abundance diagram was plotted to represent the distribution pattern of species abundances across each season during the study period (Whittaker 1965).Month-wise variation in the number of species sampled during the study period was represented graphically.
As November to February was marked by a number of foggy days (Table 4), sampling was carried out mostly on sunny days.July had the maximum number of rainy days (Table 4).Thus, a total of 192 days of sampling was carried out during the entire study period, each day devoted to two transects studied in the study site (Table 4).
The number of butterfly species and the total number of individuals recorded is shown in Table 5.The maximum number of butterfly species (158) and the maximum number of individuals (2480) was recorded during the monsoons.Shannon index of diversity (H´=4.968)along with the evenness index of species distribution (E=0.981) also exhibited highest values during this season (Table 5) as compared to summer (H´=4.819;E=0.974), autumn (H´=4.714;E=0.961), spring (H´=4.282;E=0.914) and winter (H´=3.872;E=0.811).The season wise species richness values are recorded in Table 5. Species richness showed maximum values during autumn (21.78), summer (21.58) followed by monsoon (20.09) (Table 5).Additionally, the rank abundance curve plotted to represent the distribution pattern of butterfly species, was least skewed during the monsoons, as supported by highest values of Shannon diversity and Evenness index during this season (Fig. 2).In contrast, winter was associated with a most highly skewed species abundance relationship as evident by lowest values of Shannon diversity and Evenness index (Fig. 2).However, rank abundance curve showed intermediate skewness in case of summer, autumn and spring.The curve representing the month-wise change in the number of species showed an increasing trend from March, through April, and reached its peak in June due to increased number of species with the approaching monsoon.This curve was almost steady throughout this season, and formed a second shorter peak during September-October followed by a decrease in the number during late autumn and winter gradually (Fig. 3).
A total of 143 species of plants belonging to 44 families were recorded as the larval host plants of the butterflies (Table 6).An overwhelming number of butterfly larvae fed on dicotyledons rather than on monocots.The only two groups associated with the monocotyledons were Satyrinae subfamily of Nymphalidae and Hesperiinae subfamily of Hesperiidae butterflies.Nymphalidae utilized 25 plant families and thereby exhibited highest host plant diversity (number of plant families used per butterfly family) in this study site (   Table 3. Meteorological data of the study site during study period *Source: Indian Meteorological Department, Govt. of India as their larval resources.Lauraceae and Rutaceae were their predominant larval food plants (Table 6).

DISCUSSION
Among the butterflies of the Himalayan region, 80% are recorded as forest species of which 60% occur below 3000m elevation (Uniyal & Mathur 1998).The     4,7,9,10,17,13,12,15,148 Arundinaria maling Gamble Shrub 8,5,6,4,3,11,19,13,14 ,16,112,113,147,148 Cymbopogon Wynter-Blyth (1957) identified two periods (March-April and October) as peak season of butterfly abundance in India.Kunte (1997) threw light on the abundance and species diversity of butterflies based on seasonality in four tropical habitats in Northern Western Ghats.Butterfly diversity at local or regional scales is closely related to their host plant density (Gutierrez & Mendez 1995;Cowley et al. 2001).A Rank-Abundance curve with steep gradient indicated low evenness (Magurran 2004) and low species diversity (Kunte 2008), in contrast to a curve with shallow gradient which represented high evenness (Magurran 2004) along with high species diversity (Kunte 2008).A similar trend is evident in the present study (Fig. 2).Maximum species diversity along with highest species evenness as observed during the monsoons could be correlated with the abundant distribution of luxurious vegetation which was said to be in suitable phenophase to support the growth of the larval stages of these butterflies.The monsoons were also associated with a greater abundance of species that had occurred in low frequency during summer (Atluri et al. 2011).Pöyry et al. (2009), stressed the importance of local habitat quality to explain species richness.Higher values of species richness as observed during autumn, summer and monsoon could be indicative of the presence of specific butterfly larval host plants during this season.This pattern is consistent with that of Wynter-Blyth (1957), Kunte (1997) and Padhye et al. (2006).Month wise fluctuation in the sampling size of butterflies could be attributed to the distinct changes from the wet season (May-October) to the dry season (November-April) forms (Emmel & Leck 1970;Saikia et al. 2010) in butterflies.Along with a distinct surge in butterfly distribution as observed during the monsoons (Atluri et al. 2011), butterflies are said to form peaks at transition periods between the wet season and the dry season (Emmel & Leck 1970).
The higher host plant diversity seen amongst the Nymphalid and Lycaenid butterflies in this zone of the National Park are probably due to the greater host plant diversity as previously reported from amongst the South East Asian Nymphalidae and Lycaenidae (Fiedler 1998).The preference for Poaceae hosts observed among Satyrinae larvae in this and other studies (Wynter-Blyth 1957;Haribal 1992;Munguira et al. 1997;Peñal & Wahlberg 2008) are significant.Himalayan distribution of the Heliconiinae subfamily of butterflies (Uniyal 2007;Borang et al. 2008;Singh 2009) also supported their presence in this study site.Occurrence of Glochidion sp., a common tree of the middle to upper Himalayan region along with Lonicera sp.(Cowan & Cowan 1979), a shrubby climber, probably sustained the larval population of Athyma sp. and Parasarpa sp respectively.The relationship between Euthalia lubentina-Moraceae, E.aconthea-Moraceae and Anarcardiaceae and E. sahadeva with Fagaceae threw light on the difference in the food plant preference by Euthalia butterflies (Wynter-Blyth 1957;Kehimkar 2008).Ulmus sp. and Celtis sp.(Ulmaceae) which constitute the essential part of the broad leaved forest of higher elevations (Maity & Maiti 2007) supported the larval population of Apatura sp and Hestina sp of butterflies.Wide scale distribution of Urtica sp., Debreagesia sp., Girardiana sp., Boehmeria sp. and Elatostemma sp.(Urticaceae) (Cowan & Cowan 1979;Maity & Maiti 2007) sustained the larval population of Symbrenthia hypselis, S. hippoclus, S. niphanda, Vanessa indica, V. cardui, Araschnia prorsoides and Aglais cashmiriensis (Nymphalinae subfamily) (Wynter-Blyth 1957;Haribal 1992;Kehimkar 2008).The association between Grewia sp.Libythea lepita, Celtis tetrandra, L. narina and C. cinnamomea with both L. myrrha and L. narina also stressed the importance of specific food plants for the butterflies (Haribal 1992;Kehimkar 2008).Daninae butterflies fed on Apocyanaceae, Asclepiadaceae and Moraceae, all plants possessing a milky fluid (Erhlick & Raven 1964).
According to the Singh & Pandey (2004) model, Lycaenidae, should represent 29.5% of the total number of species sampled in northeastern India.Although being the second most (27.95%)abundant family in this study site, Lycaenidae still appears to be slightly under represented in this study, which points towards the need for further investigation.The larvae of Heliophorus sp. and Lycaena phlaeas fed largely on the Polygonum sp. and Rumex nepalensis, respectively, throughout its range in the Himalayan region (Uniyal 2007;Borang et al. 2008;Singh 2009).Myrisinae (Ardisinia solanacea) served as larval resource of Nacaduba kurava (Polyommatinae subfamily).Besides this, legume feeding was prevalent amongst other Polyommatinae larvae (Wynter-Bylth 1957;Haribal 1992;Kehimkar 2008).Among Riodininae, Dodona adonira, D. eugenes and Zemeros flegyas were the butterfly species of northeastern India (Borang et al. 2008).Other Himalayan species, Abisara fylla D.egeon and D.ouida (Uniyal & Mathur 1998) were also associated with Maesa chisia plants.D. dipoea was reported due to the distribution of its host plant, Arundinaria maling which formed an important part of this forest habitat.Overhanging parasitic flora along with Rhododendron sp served as the food plants of a majority of Theclinae subfamily of lycaenid larvae (Wynter-Blyth 1957;Kehimkar 2008).
Besides association of Gonepteryx rhamni with Vaccinium sp., Fabales were decidedly the most important food plant of other Coliadinae butterflies (Ehrlich & Raven 1964).The extensive cultivation of Brassica juncea in the adjoining areas of the National Park may be the supportive larval host plant of Pieris butterflies.Pieris larvae are known to detoxify and eliminate, rather than sequester, the degradation products of glucosinolates (present in Brassicales) (Müller et al. 2003).
The marked reduction in the abundance of Hesperiidae in this study in accordance to that previously stated by the Singh & Pandey (2004) model for northeastern Indian hesperids, probably generates an urgent need for their further study in similar areas.
The association between black-bodied papilionids with Rutaceae and red-bodied papilionids with Aristolochiaceae were similar to observations made on Assam papilionidae (Barua et al. 2004).While Lauraceae -Magnoliaceae served as the food resource for Graphium eurous and Chilasa slateri larvae, Meandrusa payeni, Chilasa agestor and Graphium sarpedon depended solely on Lauraceae to sustain their larval population (Wynter-Blyth 1957;Haribal 1992).A report on the occurrence of Kaiser-I-Hind Teinopalpus imperialis, from Darjeeling District (Kehimkar 2008) also confirms their record in this study.Species such as Bhutanitis lidderdalei and Teinopalpus imperialis were strictly seasonal and found on wing between April-November.Such a seasonal trend could be attributed to synchrony with phenology of their food plants (Spitzer 1983).

Figure 1 .
Figure 1.A -Map of Neora Valley National Park; B -Map of India showing the location of West Bengal; C -Map showing Kalimpong subdivision of Darjeeling District of West Bengal.

Figure 3 .
Figure 3. Month-wise seasonal changes in the number of species

Table 2 . Seasonal sightings of butterfly species recorded in the surroundings of the upper range of Neora Valley National Park Journal of Threatened Taxa
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Seasonal diversity of butterflies of Neora Valley National Park Sengupta et al.
Araschnia sp, Symbrenthia sp., Vanessa sp. and Aglais sp.(Nymphalinae subfamily), while Euploea larvae predominantly depended on Moraceae plants.Lycaenidae showed the second highest host plant diversity and utilized 20 plant families as their larval resource (Table6).Fabaceae, Ericaceae, Myricaceae and Loranthaceae were the major food plants of larval lycaenids.A total of six families encompassing 20 species were recorded as the host plants of Pieridae butterflies.While Coliadinae fed predominantly on plants of Fabales, Pierinae butterflies chose Brassicales and mistletoes as their larval resource (Table6).28species of plants belonging to 13 families served as food plants for larvae of Hesperiinae, Pyrignae and Coeliadinae.Although Hesperiinae larvae fed on Poaceae and Pyrignae utilized Acanthaceae, Coeliadinae butterflies used plants of families Combretaceae, Moraceae, Euphorbiaceae, Sabiaceae (Table6).Four plant families were used by the Papilionidae butterflies