Distribution
and abundance of pit vipers (Reptilia: Viperidae) along the Western Ghats of Goa, India
Nitin S. Sawant 1,
Trupti D. Jadhav 2 &
S.K. Shyama 3
1 Research Scholar, 3 Associate
Professor, Department of Zoology, Goa University, Goa 403206, India
2 H.No. 359-A, St.Inez, Altinho,
Panaji, Goa 403001, India
Email: 1 nitinnature@yahoo.co.in, 2 truptijadhav2@gmail.com, 3 shyamask2001@yahoo.co.in.
Date of publication (online): 26 September
2010
Date of publication (print): 26 September 2010
ISSN 0974-7907 (online) | 0974-7893 (print)
Editor: S. Bhupathy
Manuscript details:
Ms
# o2489
Received
22 June 2010
Final
received 30 August 2010
Finally
accepted 13 September 2010
Citation: Sawant, N.S., T.D. Jadhav & S.K. Shyama (2010). Distribution and abundance of pit vipers (Reptilia: Viperidae)
along the Western Ghats of Goa, India. Journal of Threatened Taxa 2(10): 1199-1204.
Copyright: © Nitin S. Sawant, Trupti D. Jadhav & S.K. Shyama 2010. Creative Commons Attribution 3.0 Unported License. JoTT
allows unrestricted use of this article in any medium for non-profit purposes,
reproduction and distribution by providing adequate credit to the authors and
the source of publication.
Author Details: Nitin S. Sawant employed with WWF-India, as State
Director of the Goa State office, with responsibilities of designing, planning
and executing environmental awareness and conservation programme in the state
and to address issues pertaining to environment and wildlife. Actively involved
in rescue operations of snakes and other wild animals in association with the
forest department of Goa.
Trupti D. Jadhav founder member of Nirmiti, a local
NGO working towards documentation of biodiversity and its conservation. Pursued MSc. in zoology through Goa
University. Presently involved in wildlife conservation activities.
Dr. S.K. Shyama, Associate Professor, Department of Zoology, Goa University.
Author Contribution: NSS and TDJ have contributed in the field workand writing of the manuscript. SKS
contributed in the final editing of the manuscript.
Abstract: The distribution
and abundance of pit vipers in the Western Ghats namely Trimeresurus
gramineus (Bamboo Pit Viper), T. malabaricus(Malabar
Pit Viper) and Hypnale hypnale (Hump-nosed Pit Viper) was
investigated in five wildlife sanctuaries of Goa from 2005 to 2009. Seasonal day-night data was collected
based on band transect methods. All the pit viper species showed
specific habitat preferences and their abundance changed with season. They were most abundant during monsoon.H. hypnale extended its
range to the adjoining cashew plantations during the post monsoon and winter.
Keywords: Habitat preference, Hypnale, seasonal variation, Trimeresurus
For figures, images & tables -- click here
Introduction
Goa
(14051’-15048’N & 73041’-74020’E)
is a maritime state along the central west coast of Indian peninsula. The Western Ghats of India is one of
the 34 biodiversity hotspots in the world (Myer et al. 2000). Goa (3702km2) occupies about
2% area of Western Ghats (Joshi & Janarthanam 2004) and its biodiversity is under
threat due to deforestation (Myer 1990; Menon & Bawa 1997; Jha et al.
2000). Reptilian fauna is largely
dominated by the Indo-Chinese element, relicts India’s geological history. Approximately, out of 530 species of
reptiles presently reported from India 197 comprises endemics, of these 98
endemics out of 260 species are reported from the Western Ghats (Daniel 2002). In spite of this high endemism, herpetofauna
in India has received poor attention and has not been studied in detail
(Vasudevan et al. 2001) and it is possible that a few of them have already been
lost even before being reported (Dar et al. 2008). Pit vipers belong to the family Viperidae and subfamily Crotalinae,
which is represented by 21 genera. Nineteen species of pit vipers have been
reported from India (Bhide 2001) including seven from the Western Ghats (Kumar et al. 1998). All these species barring Trimerusurus gramineus are endemic to Western Ghats (Whitaker
1969; Whitaker & Captain 2004; Khaire 2006). Information on the distribution, abundance and present
conservation status of pit vipers in Western Ghats is scanty.
Materials
and Methods
The
present study was conducted in Mhadei Wildlife Sanctuary (MWS: 208.48km2), Bhagwan Mahaveer
Wildlife Sanctuary and National Park (BMWS & NP: 241km2), Bondla
Wildlife Sanctuary (BWS: 8km2), Netravali Wildlife Sanctuary (NWS:
211.05km2), Cotigao Wildlife Sanctuary (CWS: 86km2) and
in cashew Anacardium occidentale plantations within and adjoining areas of these protected areas
(PA) (Fig. 1). Altitude of the
study areas ranged from 20 to 800 m and consists of west coast tropical
evergreen, cane brakes, wet bamboo brakes, west coast semi-evergreen, moist
bamboo brakes, lateritic semi-evergreen forest, slightly moist teak forest,
southern moist deciduous forest, southern secondary moist mixed deciduous
forest, south Indian subtropical hill savannah woodland and southern
subtropical hill forest (Champion & Seth 1968; Table 1). The ambient temperature usually fluctuates between 15 and30 0C.
The
distribution and abundance of pit vipers was studied in all the PAs mentioned
above using band transect following Dahanukar & Padhey (2005). Surveys were carried out on foot in
different seasons (summer - March to May, monsoon - June to October and winter
- November to February) during June 2005 to January 2009. Surveys were conducted during both day
and night in predetermined paths or roads (2500x20 m). Geographical position of each study
area and location of each observation of the snake was recorded using a
hand-held geographical positioning system (GPS). Relative
humidity and ambient temperatures of the observation sites were recorded using
a hygrometer and mercury thermometer respectively.
Transects
were traversed everyday for a week at each site by the researcher with the help
of local volunteers. The transects were repeated five times per season. Visual inspection of shrubs, trees,
ground and leaf litter was carried out for locating the snakes. All pit vipers encountered during the
surveys were identified up to species level following Smith (1943), Murthy
(1990) and Daniel (2002). The
individuals were not marked, the snakes sighted on any single transect on
successive days were identified based on the scale count and was not
included to calculate the abundance. Abundance (number of individuals sighted in each study area) is
represented as mean ± standard error. The mean abundance of the three species in each of the study sites and
across seasons (summer, monsoon and winter) was tested to know whether the
seasonal variation in each study site has any influence in abundance of the
species, using two-way ANOVA.
Results
In
all, 45 transects (CWS - 7, MWS - 12, NWS - 7, BWS - 4 and BMWS & NP - 15)
were sampled to assess the species distribution and abundance of pit
vipers. Three species of pit
vipers, T. gramineus, T. malabaricus and H. hypnale were recorded during this study
(Images 1, 2, 3). A total of 356
pit vipers were observed during this study. H. hypnale was the most abundant species
contributing (46.63%, n = 166) followed by T. gramineus (28.09 %, n = 100) and T. malabaricus (25.28%, n = 90). The
abundance of pit vipers varied in different study locations (Table 3).
All
three species of pit vipers were observed in all the study locations, except
the BWS, where only T. gramineus was found. However,
the locals report the presence of T. malabaricus in BWS. The forests type preferred by each
species of pit vipers is listed in (Table 2). The temperature and humidity of
the area during the present study ranged from 20.88 ± 5.25 0C to
32.44 ± 0.88 0C and 53 ± 4% to 93 ± 2% respectively.
T. malabaricus was observed at an altitude above
200m and exhibited occurrence at higher elevation in all the sites except in
CWS; where it was recorded below 200m thus occupying the range between 123 and
765 m. Trimeresurus gramineus and H. hypnale were found from 37 to 672 m and 35 to 672 m respectively (Fig.
2). T. malabaricus and T. gramineus were observed at (mean & SD)
1.53 ± 1.6 m and 1.45 ± 0.68 m above ground respectively, whereas, H. hypnale was mostly found on the ground
(beneath leaf litter), but, occasionally found on shrubs and herbs (up to
0.39m) as well.
Distribution
of pit vipers varied among seasons. During monsoon they were found in all 45
transects, whereas, in summer and winter they were observed in 35 transects
which include transects having water bodies in the vicinity and transects in
the cashew plantations, and were restricted to patches near moist areas. H. hypnale moved to adjoining cashew
plantations during post monsoons (September and October) and winter, their
abundance was high during monsoon in all transects (Table 4). ANOVA showed that the abundance of pit
vipers varied significantly with seasons [T. malabaricus (F = 3.20, p = 0.00058), T. gramineus (F = 3.20, p = 0.00028) and H. hypnale (F = 3.20, p = 0.015)]. However, the abundance did not change
significantly during each season when compared between sites.
Discussion
Species
in a reptile assemblage are not randomly distributed in space either
horizontally or vertically, but occupy discrete microhabitat (Heatwole
1982). The distribution and
abundance of all three species of pit vipers varied as their habitat-use was different. The T. malabaricus and T. gramineus are arboreal, whereas, H. hypnale is terrestrial in nature, however, a semi- arboreal behavior was
also observed in H. hypnale. The present results
augment the report of Murthy (1990). It was observed that most of the pit vipers were encountered
in the regions having cool climate (segments of the transects with lower
temperature and higher humidity) suggesting that the species prefer cool and
moist places. Daltry et al. (1998)
reported that the pit vipers typically remain motionless in areas with dense
cover of undergrowth, suggesting this species is hygrophilic in nature. The three species remain highly
camouflaged in their habitat and move very rarely exhibiting ambush behaviour.
Trimeresurus malabaricus predominantly occupied tropical semievergreen forests and rarely
used moist deciduous forests. Trimeresurus gramineus and H. hypnale were observed largely in cane brakes, wet bamboo brakes, tropical semievergreen and
moist deciduous type of forest (Table 2). It was seen that all the three species were distributed in all study
locations, barring BWS, where T. malabaricus and H. hypnale were not recorded during the present study. This could be due to the anthropogenic
activities in and around the sanctuary as human disturbances can affect the
snakes in terms of their distribution (Greene 1988; Peterson 1990; Brown 1993;
Parent & Weatherhead 2000). Terrestrial and arboreal habitats differ profoundly in many ways
including the types and amount of food availability, vulnerability to
predators, and physical factors such as temperature and humidity. Those differences have resulted in many
taxa becoming highly specialized for either terrestrial or arboreal life and rarely
venturing to alternative habitat (Plummer 1981; Luiselli et al. 2000; Vilt et
al. 2000). Trimeresurus malabaricus and T. gramineus remain camouflaged in the thick
canopy of the trees at an average height of 3-5 m from the ground, which could
help avoiding avian predators. Sometimes they were also encountered while crossing the roads at night
hours during the monsoon. This
could be due to the thigmothermic response to warm surfaces, such as asphalt
roads at night in nocturnal crotalines (Klauber 1972). This excursion to the ground could be
driven by the availability of prey, especially frogs which are important food
of pit vipers and mate searching, pheromonal trail following may result in
males traveling further and longer to locate females (Shine et al. 2004). Further, H. hypnale was occasionally sighted among
small herbs exhibiting semi-arboreal habit. The occasional use of arboreal habitat offers a good
opportunity for the snake as it might reduce the efforts or energy needed to
thermoregulate or search prey. Similar observations are reported by Oliveira & Martins (2001)
and Shine et al. (2005). H. hypnale and T. gramineus preferred a broader range of
altitude as compared to T. malabaricus, however T. malabaricus was also found at lower altitude in CWS suggesting that it is the
ideal habitat with suitable hygrothermal profile and prey base which is the
basic factor influencing distribution of these species. Some researchers (Campbell &
Solorzano 1992; Huang et al. 2007) consider ambient temperatures to be the most
important environmental factor limiting the altitudinal distribution of
reptiles.
The
remarkable change in the shift in habitat by H. hypnale was seen during the present study, wherein it was observed that
during post monsoon and winter they occupy the cashew plantations (n = 45) adjoining
the PAs. It is mainly due to the
leaf litter, thick bushes which provides ideal
microhabitat, suitable hygrothermal conditions, prey availability, and predator
avoidance. Cool and humid
environment below leaf litter provides good microclimatic condition for the
forest floor reptiles (Kumar 2001), which is the major prey base for pit vipers. According to Block & Morrison (1998)
leaf litter depth is an important factor in habitat selection in amphibians and
reptiles. Since snakes are
predatory in nature, therefore their local distribution might be influenced by
distribution of prey abundance (Dar et al. 2008). It was seen that all the three species exhibited patchy
distribution during summer and winter, confined to areas in close proximity of
water bodies, whereas, in monsoons they were distributed in all transects.
It
was also seen that the abundance of all the three species varied in different
seasons. The abundance was highest
in monsoon compared to summer and winter. The highest abundance during monsoon was due to the suitable climatic
conditions such as low temperature (22-24 0C), high humidity and
rich prey base. In summer and
winter the prey base (such as frogs) and suitable climatic conditions are
restricted to patches in the vicinity of water bodies in the PAs. According to Sun et al. (2001) prey
availability and abiotic factors, especially
temperatures are the cues that drive seasonality in snakes.
It
seems evident from the present study that pit vipers species are habitat
specific and abiotic factors within the habitat such as seasonal changes in
temperature and humidity, have influence on the distribution of these
snakes. Hence, the protection of
habitat is an important aspect in conservation of these species.
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