Butterflies of Sundarban Biosphere Reserve,
West Bengal, eastern India: a preliminary survey of
their taxonomic diversity, ecology and their conservation
Soumyajit Chowdhury 1,2
1 Centre for Biodiversity and Ecological
Studies, P1/1B, Garia Park, Kolkata, West Bengal
700084, India;
2 Department of Zoology, V.J.R. College
(affiliated to the University of Calcutta), 8/2, Bejoygarh,Jadavpur, Kolkata, West Bengal 700032, India
wildlifesc@gmail.com
Abstract: The Indian Sundarbans,
part of the globally famous deltaic eco-region, is little-studiedfor butterfly diversity and ecology. The present study reports 76 butterfly species belonging to five
families, which is a culmination of 73 species obtained from surveys conducted
over a period of three years (2009–2011) in reclaimed and mangrove
forested areas and three species obtained from an earlier report. Six of these species are legally
protected under the Indian Wildlife (Protection) Act, 1972. Random surveys were employed for both
the study areas, supplemented by systematic sampling in reclaimed areas. The reclaimed and forested areas
differed largely in butterfly richness (Whittaker’s measure of β diversity
= 0.55). For sample-based
rarefaction curves, butterfly genera showed a tendency to reach an asymptote
sooner than the species. Numerous monospecific genera (77.19% of the taxa) resulted in a very
gentle but non-linear positive slope for the species-genus ratio curve. A species-genus ratio of 1.33 indicated
strong intra-generic competition for the butterflies of the Indian Sundarbans. Mangrove areas were species poor, with rare species like Euploea crameri, Colotis amata and Ideaagamarshchana being recorded in the mangrove
area; while Danaus genutiawas found to be the most frequent butterfly. Butterfly abundance was very poor,
with no endemic species and the majority (53.9% of the taxa; n=41) were found
locally rare. The changing
composition of butterflies in the once species-poor mangrove zone of the
fragile Sundarbans may interfere with their normal
ecosystem functioning.
Keywords: Asclepiadaceae,
butterfly, Danaus genutia,
India, Mangrove, reclaimed area, Sundarbans.
doi: http://dx.doi.org/10.11609/JoTT.o3787.6082-92 | ZooBank: urn:lsid:zoobank.org:pub:4F1E7FBB-0BEA-4A43-AED1-2372A5AF57D8
Editor: B.A.
Daniel, Zoo Outreach Organisation, Coimbatore,
India. Date
of publication: 26 July 2014 (online & print)
Manuscript details: Ms # o3787
| Received 28 September 2013 | Final received 05 July 2014 | Finally accepted
08 July 2014
Citation: Chowdhury, S. (2014). Butterflies of SundarbanBiosphere Reserve, West Bengal, eastern India: a
preliminary survey of their taxonomic diversity, ecology and their conservation. Journal
of Threatened Taxa 6(8): 6082–6092; http://dx.doi.org/10.11609/JoTT.o3787.6082-92
Copyright: © Chowdhury 2014. Creative Commons
Attribution 4.0 International License. JoTTallows unrestricted use of this article in any medium, reproduction and
distribution by providing adequate credit to the authors and the source of
publication.
Funding: Centre for Biodiversity and Ecological Studies [Ref
No: CBES/2010/S-PI)]
Competing Interest: The author declares to have financial support from the
Centre for Biodiversity and Ecological Studies (CBES) for the submitted work,
and no other relationships or activities that have inspired the same.
Author Details: Dr. Soumyajit Chowdhury is a faculty member of the Department of Zoology,
V.J.R. College, Kolkata and founder member of Centre for Biodiversity and
Ecological Studies (CBES). He has been working on the biodiversity and ecology
of Indian insects and authored several research articles. He also initiated ‘BIOME india’, an online resource on the ecosystem-based
conservation programmes of India.
Acknowledgement: The author is indebted to the Centre for Biodiversity and Ecological Studies for supporting, in part, the survey work on lepidopteranfauna and different mangrove and mangrove-associated floral components in Indian Sundarbans. Nihar Mondal, Tanmoy Mondal and Amit Das among several others deserve special thanks for logistic support during field trips in different parts of central, western and eastern Sundarbans.
For figures, images, tables -- click here
Introduction
Sundarbans forms a unique eco-region in the vast deltaic region on the Bay of
Bengal, hosting one of the most extensive mangrove forests of the world,
spreading over India and Bangladesh. This region is also the only mangrove tiger land in the world, securing
the endangered Royal Bengal Tiger Panthera tigris tigrispopulations with inimitable cases of tiger-human conflict. The human populations in this fragile
eco-region have sustained themselves by harvesting forest products and by
reclaiming the forest land for agricultural purposes,
over the last two centuries. The
usual flushing of tidal water and natural inundation has been greatly affected
by embankment constructions in the Sundarbans,
resulting in quick succession or shifting in plant communities (Sanyal et al. 1984).
Butterflies are recognized as focal
species of conservation in several areas of the world (New 2011). These widely studied insects show
significant ecological contributions in different ecosystems through herbivory and pollination services. The Indian Sundarbans,
being a unique deltaic region with a rich biorepository,
suffered from inadvertent reports on butterflies, accordingly lacking a
detailed survey for the same. Literature reviews indicated a meagre eight species (Mandal& Nandi 1989; Mandal & Maulik1997; Kehimkar 2008) to be precisely reported from
this region. Hence, an inventory on
the butterfly fauna in Indian Sundarbans and their
mode of habitat association was required for this globally important
eco-region.
Materials and Methods
Study Site
The Indian Sundarbans,
comprising an area of 9,630km2 is part of the delta of the
Ganga-Brahmaputra-Meghna basin in Asia; and along with the Bangladesh Sundarbanshosts one of the largest continuous mangrove forests in the world. The landscape is
characterized by a web of tidal water systems. Only the Hugli and Ichamati-Raimangalrivers carry freshwater flow of some significance (Dandaet al. 2011). The Indian Sundarbans are located between 21031’–22030’N
and 88010’–89051’E, spreading over the southern parts
in the districts of North and South 24-Parganas of the state of West Bengal (Naskar & Mandal 1999) and
demarcated by the ‘Dampier and Hodges Line’ in the north. This area has received the status of Sundarbans Biosphere Reserve (SBR) in 1989, by being the
world’s first mangrove forest to be brought under scientific management. Mangrove forests cover 4,264km2and include a tiger reserve covering 2,585km2, a national park
covering 1,330km2 and three wildlife sanctuaries, namely Sajnekhali WS (362km2), Lothian Island WS (39km2)
and Halliday Island WLS (5.8km2). The remaining area, covering 5,366km2,
comprises reclaimed land (Chaudhuri & Choudhury 1994; Zockler et al.
2005).
The region experiences a humid, tropical,
maritime climate, with an average annual rainfall of 142.5mm. Mean annual maximum and minimum
temperatures are about 300C and 230C respectively. Humidity is high, 80% on average due to
proximity to the Bay of Bengal (Chowdhury 2011). Recent reports suggest that air
temperature over the Sundarbans and adjacent parts of
Bay of Bengal are gradually increasing (Huq et al.
1999; Agrawala et al. 2003).
Based on the vegetation pattern and extent
of exposure to saline water regimes, two broad categories of eco-zones were
recognized among the 18 sites that were sampled in the eastern, central and
western Sundarbans (Fig. 1): 1 - Mangrove area and 2
- Reclaimed area. The former is
exposed to saline tidal submergence with luxuriant mangrove vegetation (Image
1), while the latter is retained for human settlements and agricultural
practices with low saline influence and non-mangrove vegetation (Image 2).
Butterfly sampling and data analyses
Butterflies were sampled by visual
estimation surveys for three years: 2009–2011; with samplings repeated
for pre-monsoon, monsoon and post-monsoon months of the study period. Sampling of butterflies varied according
to habitat patterns over the deltaic region. In the mangrove
forested regions, butterflies were broadly sampled in a random manner
along the forest edges and trails available. Whereas in the reclaimed areas, surveys
were carried out especially in the greener areas (viz., gardens, agricultural
fields, fragmented wooded areas) so as to encounter butterflies more frequently
as compared to the highly degraded and modified locales. Both random searches and systematic
sampling (along definite trails) were carried out in the latter. Butterflies were recorded from
0800–1700 hr each day. Species were observed while perched on
surrounding vegetation, in flight and puddling as
well as nectar feeding were recorded and photographed in situ for
reference. Species identifications
were done following available literature (Evans 1932; Wynter-Blyth
1957; Kehimkar 2008). Taxonomic classification of butterfly
families was followed after van Nieukerken et al.
(2011).
The presence-absence data for butterflies
in the sampling sites in the western, central, and eastern Sundarbanswere pooled to form a species X sample incidence matrix. Taxon sampling curves (sample-based rarefaction
curves) for butterfly species and for the genera to which they belong were
generated using Estimate S version 9.1.0 (Colwell 2013); their species-to-genus
ratios were plotted along with so as to deduce levels of competitive
interactions among species within genera (Simberloff1978; Järvinen 1982).
Estimation of β-diversity (diversity
between habitats) between the mangrove forests and reclaimed areas of the Sundarban deltaic region were done using Whittaker’s
measure βW (Whittaker 1960) for presence/absence data (as in
the present case):
βW = S / α ….……………….. (1)
where S = the total number of species recorded in both sites, and α = the average sample richness. The obtained result was calculated on
the 0 (minimum β diversity) to 1 (maximum β diversity) scale by
subtracting 1 from the obtained answer (Magurran2004).
Frequency of occurrence (FO) of the
butterfly species was calculated by considering the number of sampled sites in
which the species occurred in relation to the total number of samples (or
sites) surveyed (Severiano et al. 2012). Such an analysis was viewed into the
following categories when FO > 70% [very frequent (VF)]; ≤70% – >45%
[frequent (F); ≤45% – >15% [Infrequent (IF)]; ≤ 15% [Rare].
Results
A total of 76 species of butterflies have
been reported hitherto from Indian Sundarbans (Table
1). Of these, 73 species were
recorded following the three years (2009–2011) field survey. The remaining three species, viz., Graphium nomius, Eurema brigitta andIdea agamarschana, were recorded by Mandal & Nandi (1989) in Sundarbansbut not recorded in the present survey. All the following analyses have
been done with respect to the 76 species.
Among the five recorded butterfly families
in Sundarbans, Nymphalidaeappears to be most species rich (n=24, 31.6%), followed by Lycaenidae(n=19, 25%). The family with lowest
species richness is Papilionidae (n=9, 11.8%). Pieridae and Hesperiidae were intermediate with 17.1% (n=13) and 14.5%
(n=11) of the total species. Regarding genus richness, the butterfly families followed nearly the
same trend as their species; nymphalids (n=16, 28%)
outnumber the others while papilionids ranked last
(n=3, 5.2%) in the list (Table 2).
The distribution of butterfly species
across genera was found to be highly skewed. A large proportion of genera (44 out of
57) recorded in the present study were represented by single species, whereas
the remaining eight genera by two species, four genera by three species and one
genus by four species (Fig. 2). Figure 3 illustrates the most prevalent genera, that is, those with
three and four species.
Figure 4 depicts sample-based species and
genus rarefaction curves for the butterfly dataset. Both the curves do not reach a plateau
with increasing sample numbers. However,
the number of genera shows a tendency to reach an asymptote sooner than the
number of species. Moreover, a very
low species-genus ratio (S/G=1.33) exists for the butterflies of Indian Sundarbans delta.
A fairly high beta diversity for the butterfly fauna was indicated by the
obtained value (βW=0.55) of the Whittaker’s measure between the
mangrove forests and reclaimed areas of Indian Sundarbans. All the butterfly families show a higher
species richness in the reclaimed areas than in the mangrove forest zones (Fig.
5). Whereas Nymphalidae,Lycaenidae and Hesperiidaefall away by about 50% in mangroves as compared to reclaimed areas, a marked
drop can be seen in Lycaenidae and Hesperiidae (by < 20%) in mangrove forests as against
reclaimed areas.
Of the 76 species of butterflies, most are
‘common’ and ‘generalist’ as none of the species are threatened globally as per
the IUCN Red List (2011). However,
regarding their frequency of occurrence in the Sundarbans,
the majority of butterflies were classified as ‘rare’ (n=41; 53.9% of the
taxa), as they were recorded only in one or two sites. Most of the rare species belonged to Lycaenidae and Hesperiidae (Fig.
6).
No butterfly species in the Sundarbans were recorded as ‘very frequent’ during the
study period. A meagre 10.5% of the
species (n=8) were found to be ‘frequent’ in both the reclaimed areas and the
mangrove forests (Fig. 6). Hesperiidae and Lycaenidaefamilies remained devoid of any frequently observed species (Fig. 6). Three species of Tiger butterflies, viz.,Danaus genutiaCramer, D. chrysippus Linnaeus and Tirumala limniace Cramer were
recorded with FO>60%. Striped
Tiger, Danaus genutiawas the only species to occur in most of the sampled sites (12 out of 18
sites). Besides this, the deltaic
region does not hold any endemic species of butterflies.
Of the 76 butterfly species in the Indian Sundarbans, six are legally protected under the Indian
Wildlife (Protection) Act, 1972 (Table 3).
Discussion
As compared to an inventory of 73 butterfly
species from the Indian Sundarbans during the present
study, earlier workers like Mandal & Nandi (1989)
recorded eight species only. Species sampled during the field surveys when compared to that obtained
from literature surveys revealed that three species, viz., Graphium nomius, Eurema brigitta and
Idea agamarschana were not recorded by the author
during his present study. These
three butterflies were reported to occur in both, the dense and the scanty
mangrove forests of the Indian Sundarbans (Mandal & Nandi 1989). The present article thus includes these
three species (marked with * in Table 1) along with the species inventoried to
obtain a total of 76 butterfly species in the Indian Sundarbans(Table 1).
Butterfly species recorded in the Indian Sundarbans were found to be distributed among five
families: Papilionidae, Hesperiidae,Pieridae, Lycaenidae and Nymphalidae, under a single superfamily Papilionoidea(van Nieukerken et al. 2011). Large variations have been found to
exist among the butterflies at both their generic and specific categories as
reflected in Table 2. Such
variations can be attributed to the habitat heterogeneity and diversity of the
range of larval host plants that have colonized the delta from the mainland
over the years, primarily due to reclaiming of the previously occurring dense
mangrove forest lands for colonization, agriculture and aquaculture practices.
The tendency of the sample-based species
and genus rarefaction curves, not to reach a plateau with increasing sample
numbers, indicates an addition of some newer species with further
sampling. However, the tendency of
the number of genera to reach an asymptote sooner than the number of species
sampled reflects a pattern inevitable for any case of category-subcategory
sampling curves (Gotelli & Colwell 2001). Although the genus has fewer members
than the species in the present case, the respective rarefaction curves are
closely placed because of the occurrence of numerous monospecificgenera (n=44; 77.19% of the taxa) of butterflies in the Indian Sundarbans. This unique relationship is reflected in the species-genus ratio curve
(Fig. 4) revealing a very gentle but non-linear positive slope.
The species-genus ratio has long been in
practice to describe community patterns as well as to surmise levels of
competitive interactions among species within genera (Gotelli& Colwell 2001). A very low
species-genus ratio (S/G=1.33) for the butterflies of the Sundarbans,
following Elton’s (1946) proposition, indicates strong intra-generic
competition, limiting in turn congeneric coexistence as hypothesized by Darwin
(1859). Widespread observations
revealed that species-genus ratios were usually smaller for island than
mainland communities (Elton 1946; Simberloff1978). As most of the Indian Sundarbans delta is comprised of island complexes, the
present findings remain consistent with the aforementioned propositions.
Interference in intrusion of saline water
(resulting from tidal influx) and reclamation of forest lands for the ever-growing
demand for human settlement and agricultural practices in the upper and middle
estuarine regions of the Sundarbans has resulted in
modifying the floristic spectrum, indulging the intensification of a diverse
array of larval food plants as well as nectar plants for adult
butterflies. Such a phenomenon
facilitated an immigration of diverse butterflies from inland
areas—thereby changing their community composition in the butterfly-poor
mangrove ecosystems and the non-forest areas of the Indian Sundarbansdelta. Deficient food sources,
particularly in terms of non-nutritious mangrove leaves and stressed
environmental conditions, like strong sunlight, high temperature and
desiccation (Kathiresan & Bingham 2001) may have
made their species richness in such ecosystems correspondingly low. This is reflected by
their fairly high beta diversity existing between reclaimed and mangrove
forested areas of the Indian Sundarbans. Earlier studies also indicate fewer
species to be restricted to mangroves in other parts of the world (Corbet & Pendelbury1992). Thus, many of the species
recorded in the Sundarban mangrove forests in the
present study can be suggested to be visitors that frequently visit the
mangrove flowers for the rich nectar sources.
Although all butterfly families show a
higher species richness in reclaimed areas, prominent contribution towards
their difference among the reclaimed and mangrove areas were largely attributed
to the lycaenids and hesperiids(Fig. 5). The underlying reasons
for such an event seem to be twofold: (i) the extreme
deficiency in larval food plants for these two families in the mangrove forests
and (ii) the increased abundance of various cultivated and decorative plants (Arecaceae, Combretaceae, Fabaceae, Mimosaceae, Poaceae, Rhamnaceae, Rubiaceae, Rutaceae, Zinziberacaeae and others) in reclaimed areas of the IndianSundarbans that serve as preferred food plants for
their larvae.
Forty one species of butterflies in Sundarbans have
been considered rare in terms of their frequency of occurrence in the deltaic
region. Rare species like the
Spotted Black Crow Euploea crameri Lucas is a resident of the mangrove belt (Mondal & Maulik 1997),
although it was mentioned to be accidentally introducedby shipping on a small, flat alluvial island of Sundarbans(Fruhstorfer 1910). Mangrove Nymph I. agamarshchanaC. & R. Felder is rare in the Indian Sundarbans,
from where it was first reported by Mondal & Maulik (1997); although Talbot (1949) mentioned it hitherto
as very common in the mangrove forests of the Bangladesh Sundarbans.
Another rare butterfly–the Small Salmon Arab Colotis amata Fabricius, being
a ‘specialist’ and monophagous on Azima tetracantha (a semi-saline, mangrove-associated
species belonging to family Salvadoraceae) was found
to be locally threatened because of habitat modifications and rapid depletion
of the host plant populations (Chowdhury & Soren 2009) in some reclaimed areas of the Sundarbans (Image 3).
The deltaic environment of the Sundarbans proved hostile for butterfly abundance as no
butterfly species were found to be ‘very frequent’ (Fig. 6). Only 8 species were found to be
‘frequent’ in both the reclaimed areas and mangrove forests ,viz., the papilionid Papilio polytes Linnaeus, the pierids Catopsilia pyrantheLinnaeus and Eurema hecabeLinnaeus, and the nymphalids Euploeacore Cramer, Elymnias hypermenstra Linnaeus, Tirumala limiace Linnaeus, Danaus chryssipus Linnaeus and D. genutiaCramer. Hesperiidaeand Lycaenidae families remained devoid of any
frequently observed species. Tiger
butterflies (Danaus genutiaCramer, D. chrysippus Linnaeus, D. melanippus Cramer and Tirumala limniace Cramer) (Image 4) were found to be
unique to the mangroves. Except for
the White Tiger, D. melanippus, the other
three species were found to be more widely distributedin the Sundarbans. Moreover, by virtue of the presence of
mangrove-associated plant species belonging to the Asclepiadaceaefamily: viz., Hoya R. Br. and TylophoraR. Br. (Banerjee et al. 2002) that are preferred by the Striped Tiger, D. genutia as their larval food plants, the latter has
been found to be widely frequent in the mangrove forests of the Sundarbans as well.
In the butterfly-poor Sundarbansdeltaic region of eastern India, six of the recorded 76 species, approximately
8% of the butterflies are legally protected under the Indian Wildlife
(Protection) Act, 1972 (Anonymous 1997). Of these, two are Schedule I, three are Schedule II and one is a
Schedule IV species; the former two gain extreme importance because of the
highest level of legal protection they receive in the country. Table 3 shows the taxonomic breakdown of
these scheduled butterflies. Among
the scheduled species, the Spotted Black Crow E. cramerihas been recorded only from the deltaic regions of the Sundarbansand the mangrove and secondary forested regions of Orissa (Kehimkar2008). Moreover, legally protected
species like Gram Blue Euchrysops cnejus and Pea Blue Lampides boeticus are considered ‘pests’ for a variety of
crops (NBAII 2013) and many-a-times eradicated in large numbers (mainly at
their larval stages) along with other insect pests due to the indiscriminate
use of pesticides in different agricultural tracts of the reclaimed areas of
delta.
Conclusion
Habitat modifications and change in local
climatic conditions (Danda et al. 2011), primarily
due to human interventions, are the potent factors for organizing the butterfly
community in the fragile ecosystems of Sundarbans. Such a change, as initiated in the
otherwise lepidoptera-poor mangrove ecosystems of Sundarbans may interfere with their normal ecosystem
functioning. Under the present
scenario of increased stress towards these deltaic mangrove ecosystems, further
studies on the exploration of larval food plants and niche specificities of the
unique species (like the Tiger butterflies, Danaus genutia) in this region may help, in the long
run, to consider them as focal species (viz., indicator species) for
conservation programmes and ecosystem management of this highly threatened and
bio diverse deltaic region.
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