The seasonality of butterflies in a semi-evergreen forest: Gibbon Wildlife Sanctuary, Assam, northeastern India

Author Details: Arun P. Singh is working on the ecology and conservation of biodiversity of the Himalaya and northeastern India with special reference to butterflies and birds. Presently, he heads the Ecology and Biodiversity Conservation Division, Rain Forest Research Institute (ICFRE), Jorhat. Lina Gogoi is an environmental science post graduate from Tezpur University, Assam had worked on weathering geochemistry of Lohit River, Dibang River and Dibru Saikhowa National Park. Also worked on ecological studies of butterflies in Arunachal Pradesh at the Rain Forest Research Institute for a short period. Currently working in Tezpur University as a project fellow in biochar related project. Jis Sebastian did her MSc in forestry from FRI University in Dehradun, Uttarakhand. She has working experience in Wildlife Trust of India and as JRF in the Rain Forest Research Institute on ecological studies of butterflies in Arunachal Pradesh. Currently perusing PhD research in botany at Sacred Heart College, Cochin, Kerala.


INTRODUCTION
The northeastern region of India, that lies south of the Brahmaputra River, is part of the Indo-Burma biodiversity hotspot on the globe. It is located at the trijunction of Indo-Chinese, Indo-Malayan and Palaearctic biogeographic realms exhibiting a profusion of habitats characterized by diverse biota with a high level of endemism (http://www.biodiversityhotspots.org/xp/ hotspots/indo_burma/Pages/default.aspx).
More than 50% of the butterfly species found in India occur in the northeast, also called the "Papilionidae-rich zone" in the 'Indo-Burma hotspot' as per IUCN (New & Collins 1991). The high species richness and endemism make this an important region for conservation of biodiversity in India.

Study Area
The Gibbon Wildlife Sanctuary (GWS) 26 0 40'-26 0 45'N & 94 0 20'-94 0 25'E, lies in Jorhat District in upper Assam in northeastern India. It is today an isolated forest patch covering approximately 21km 2 of mainly lush green 'tropical semi-evergreen forest' sparsely interspersed with 'wet evergreen forest' patches, classified as 'Assam plains alluvial semi-evergreen forests (2B/ C1a)' (Champion & Seth 1968). Dipterocarpus retusus (Hollong) is the predominant element in the forest. The associated species are Ailanthus integrifolia, Altingia excelsa, Artocarpus chama, Castanopsis purpurella, Cinnamomum bejolgheta, Dysoxylum gobara, Mesua ferrea, Michelia champaca and Vatica lanceafolia (Baruah & Khatri 2010) with most of the tree species being utilized by the Western Hoolock Gibbon Hoolock hoolock here (Barua & Gogoi 2012). The altitudinal range of GWS varies between 100-120 m above sea level, while the average temperature ranges from 18.95 -27.9 0 C, the average humidity varies between 64.5% and 94.5% and the annual rainfall of the study area being ~250cm. The sanctuary was carved out of Hollongapar Reserve Forest set aside in 1881 named after the dominant tree species -Hollong (Dipterocarpus retusus). Subsequently, more forest areas were added to this RF and by 1997 the total area of the Hollongapar RF increased to 2098.62ha. The Government of Assam declared this entire RF area as the Gibbon Wildlife Sanctuary in 1997. GWS is surrounded by mostly tea gardens and small villages. The Bhogdoi River flows from Nagaland (south) to Assam (north-west) and distinctly demarcates the eastern boundary of this sanctuary as a permanent physical barrier (Image 1). GWS was once contiguous with a large forest tract that extended to Dissoi Valley Reserve Forests of Nagaland in the south and are now separated by a vast stretch of tea gardens presenting a barrier in the effective migration of wildlife such as elephants (Bhattacharjee 2012). GWS today is still a home to many species of animals of global concern namely, Hoolock Gibbon Hoolock hoolock (Endangered; Brockelman et al. 2008); Capped Langur Trachypithecus Image 1. Gibbon Wildlife Sanctuary and its surrounding areas.  (Bordoloi 2010). The published literature on the butterflies of the GWS is scanty. Senthilkumar et al. (2006) recorded 37 species from GWS. A blog by Abhijit Narvekar (http:// butterflyinggibbonwls.blogspot.in/) lists 31 species from GWS, recorded in May 2013. Besides these, there are no other published records of butterflies from GWS. The authors hereby report the results of a three and a half year study carried out by them in the GWS.

Sampling
Twenty-eight sampling surveys covering all the months were carried out in Gibbon WS from 4 August 2010 to 26 April 2014. Sampling was carried out along forest trails up to 5m on both sides along a stretch of 3.5km from the village Melang Grant to the Gibbon Forest Rest House (FRH) and along the two parallel trails that goes from the FRH towards river Bhogdoi in the east (Fig. 1). The 'Pollard walk' (Pollard & Yates 1993) method was used for sampling butterflies. Sampling was carried out between 08.00hr to 15.00hr mostly on sunny days, but the sampling hours varied in different samplings from 1.5-3 hours. The taxa encountered were recorded in each sampling. The data on abundance, however, could not be recorded for each survey, but species occurring in exceptionally high numbers (peak abundance) were noted. A total of ~65 hours of sampling was carried out. Butterflies were identified from photographs and using field guides (Evans 1932;Wynter-Blyth 1957;Haribal 1992;Smith 1989Smith & 2006Kehimkar 2008;Sondhi et al. 2013 and websites: www.flutters.org/ and www. ifoundbutterflies.org/).

Butterflies in Gibbon Wildlife Sanctuary
Singh et al.

Data Analysis
Data for the number of species recorded in each survey was pooled. Species accumulation curve was then plotted from the first to the last sampling to see the rate of species accumulation during the study period. The Sorensen's similarity index or β was calculated to see the species similarity in butterflies between four different seasons meeting different seasons [premonsoon (March-May), monsoon (June-Sept), postmonsoon (October-November) and winter (December-February)]in this semi-evergreen forest.
β= 2c/ (S1+S2) here, S1 = the total number of species recorded in one season/site S2 = the total number of species recorded in different season/site C = number of species common to both seasons/ sites The Sorensen's similarity index (Sorensen 1948) is a very simple measure of beta diversity, ranging from a value of zero, where there is no species overlap between the communities to a value of one, when exactly the same species are found in both communities.
The seasonality of butterflies in GWS was then compared with trends available in other studies in other forest habitats in the Himalaya and the northeast to see the variation in this forest type.

Species richness
Amongst the 211 species belonging to 115 genera recorded during 28 sampling surveys (Appendix 1), 19 species were of the family Papilionidae. This suggests that species richness of the area could be as high as 257 species based on the family proportion model (Singh & Pandey 2004), by taking Papilionidae's proportion as 7.4% of the total for northeastern India (Wynter-Blyth 1957). The present sampling thus represents about 82% of the species found in the study area. Families Lyceanidae and Hesperiidae proportions are less than those of the northeastern region, these two families are thus under-represented (Table 1) in the present surveys and there is a need to look for more species among these two families in GWS.

Species accumulation
An increasing trend in the species accumulation curve shows that new species were added during every sampling up to the last sampling at a prominently higher rate just after the monsoon rains (Aug-Sep) until pre-monsoon (March), every year (Fig. 2). The trend obtained during the last six samplings suggests that new species were still being discovered until the end (mainly Lycaenidae and Hesperiidae).

Seasonality
Maximum number of species were recorded during the 'post monsoon' season in the region (Fig. 3). The first peak in species richness (102 species) during March and April was smaller than the second peak in September to October (118) when most of the species are in flight in GWS. The two peak seasonal trends in butterflies is very typical of the Himalaya and northeastern India. In GWS, which is a semi-evergreen forest, the second peak is higher than the first peak, however. This pattern differs considerably from the sub-tropical lowland forests in Bhutan ( Fig. 4; Singh 2012) lying between 100-220 m,  where both the peaks are high but the first peak in April is slightly greater than the second peak in December (Fig. 4). The reason for the first peak being smaller than the second peak in GWS may be related to the pattern of rainfall here. The reason for the first peak being smaller than second peak higher in GWS may be related to the pattern of rainfall here. In GWS the onset of early rains is early in spring (from April), monsoons are less severe, there is short dry (moderate) winter in comparison to rains arriving relatively late in May-June, severe monsoon and a longer winter season in Bhutan.

Species similarity among seasons
Sorensen's similarity index between seasons varied from 0.25-0.55. This suggests that, the species composition varied in GWS all over the seasons of the year. However, the highest similarity was noticed between post-monsoon and autumn, post-monsoon and spring, winter and spring, spring and autumn, respectively. In other words from post-monsoon to spring the species composition in GWS showed much similarity. The similarity index was least between spring and pre-monsoon followed by monsoon and winter, respectively (Fig. 5). This suggests that major changes in species composition in the semi-evergreen forests occurs between these seasons, which may be related to the life history patterns of these butterflies. The number of species in flight during rainy season were few in comparison to the dry season. This could be due to proximity and continuity of Jeypore RF with Himalayan foothills of Arunachal Pradesh from where these species come down and non-connectivity of GWS forest with the nearest hills in Nagaland and no freshwater mountain streams inside the GWS. Besides, 30 papilionids have been recorded in Garo Hills (Sondhi et al. 2013) of which 10 have not been recorded at GWS, but Garo Hills have diverse habitats under at least three forest types and a large altitudinal gradient when compared to GWS. The Papiliondae species similarity between these three forests (Table 2) all having semi-evergreen forest component in common also have at least 53 percent papilionid species common among them.

Significant records
A dead female of the Great Blue Mime Papilio paradoxa telearchus, a rare species (Evan 1932), crushed by a vehicle on the forest road was recorded on 25

Between Sites
Jaccard's coefficient of similarity    March 2011 was also a species with a distribution in the Naga Hills, Siam and Borneo (Evans 1932). Norman (1956), however, had recorded S.w. woolletti Riley from Sibsagar District of Assam that was previously also known from Manipur. The record of Snowy Angle, Darpa pteria dealbata on 4 August 2012 (Image 6), is the second photographic record of this species from India. Earlier, it had been recorded from the forests of Jeypore-Dehing in Assam between April 24 and 29, 2011, the distribution of the species being further south through Burma, Thailand, Laos, Malay Peninsula, Tioman, Borneo, Sumatra, Java, and Palawan, Phillipines in South-east Asia (Karthikeyan & Venkatesh 2011).The Constable, Dichorrhagia nesimachus (Image 7) a very rare species (Evans 1932)  the sunshine and on wet mud inside the forest. Sylhet Oakblue Arhopala silhetensis (Images 9,10) is another rare species (Evans 1932)

CONCLUSION
Being a remnant forest of 21km 2 , GWS supports a rich diversity of butterflies found in northeastern India. The seasonality and diversity of butterflies of a 'semi evergreen forest' is unique from that of lowland subtropical forests of the lower Himalaya.  have also found that rainfall has a strong correlation with the abundance of some papilionids in northeastern India besides a strong seasonality in continental South-east Asian butterfly assemblages. GWS, besides supporting butterfly diversity, also needs to be preserved as a gene bank biodiversity of flora and fauna (birds, mammals, herpetofauna, orchids, canes, bamboos, etc.) unique to northeastern India and functions as an island habitat for movement of large mammals and birds between larger protected areas in the landscape. Also, better accessibility and location of GWS with the national highway in the region, proximity to Jorhat town, lying in the plains and having a rest house, increases its potential for attracting tourists for -butterflyinclusive eco-tourism in a natural semievergreen forest habitat. Using local villagers as guides to generate livelihood for communities involved thereby reducing biotic pressure on one hand and conserving this magnificent forest on the other along, with the researchers and students, GWS can easily be taken up as a role model in conservation biology.

Butterflies in Gibbon Wildlife Sanctuary
Singh et al.