Population
and prey of the Bengal Tiger Panthera tigris tigris (Linnaeus,
1758) (Carnivora: Felidae) in the Sundarbans, Bangladesh
M. Monirul H. Khan
Department of Zoology, Jahangirnagar University, Savar, Dhaka
1342, Bangladesh
Email: mmhkhan@hotmail.com
Date
of publication (online): 26 Febryary 2012
Date
of publication (print): 26 February 2012
ISSN
0974–7907 (online) | 0974–7893 (print)
Editor: L.A.K.
Singh
Manuscript details:
Ms
# o2666
Received
02 January 2011
Final
received 14 November 2011
Finally
accepted 29 December 2011
Citation: Khan,
M.M.H. (2012). Population and prey of the Bengal Tiger Panthera tigris tigris (Linnaeus, 1758) (Carnivora: Felidae) and their prey in
the Sundarbans, Bangladesh. Journal
of Threatened Taxa 4(2): 2370–2380.
Copyright: © M.
Monirul H. Khan 2012. Creative
Commons Attribution 3.0Unported 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: The author is a wildlife biologist
specializing in research and conservation of tigers. Currently, he serves as
Associate Professor of Zoology in Jahangirnagar University, Bangladesh, and his
activities include teaching and research on various aspects of wildlife and
wildlife habitats.
Acknowledgement:I sincerely acknowledge the financial
support from the Save the Tiger Fund, National Fish and Wildlife Foundation,
USA. Thanks to the Forest
Department of Bangladesh for giving the official permission, and providing the
local support, that made the project successful. The Zoological Society of London (ZSL) has provided
administrative support, and Sarah Christie, Chris Carbone and Marcus Rowcliffe
of ZSL have provided technical support to this project. My sincere thanks to Zahangir Alom and
all other field assistants who were an integral part of the fieldwork of this
project.
Justification for delayed
publication: Submission of this article to the journal after
completion of the fieldwork was delayed because there was an attempt to further
enrich the content by inputs from two other carnivore experts, but that
ultimately did not work out -- M. Monirul H. Khan.
Abstract: The results from intensive small scale surveys are
often difficult to extrapolate to wider spatial scales, yet an understanding at
such scales is critical for assessing the minimum densities and populations of
rare and wide ranging species. In
this paper, the minimum size of population and minimum density estimates of
Bengal Tigers Panthera tigris tigris and its prey were conducted from 2005 to 2007 using
camera traps for 90 days and using distance sampling surveys for over 200 days,
respectively. The results were
extrapolated from the core study area in Katka-Kochikhali, southeastern
Sundarbans, to five additional sites using indices of abundance. With the use of 10 camera-traps at 15
trap-points, field data provided a total of 829 photos, including seven photos
of five individual tigers. A total
of 5.0 (SE = 0.98) tigers (adults and sub-adults) are thus estimated in the
core area with an estimated density of 4.8 tigers/100km2. Distance sampling surveys conducted on
large mammalian prey species obtained an overall density estimate of 27.9
individuals/km2 and a biomass density of 1,037kg/km2. Indices of abundance were obtained by
using tiger track sighting rates (number of tracks/km of riverbank) and the
sighting rates of the prey species (number of prey/km of riverbank) in the core
area and in five additional sites across the region. The densities of tiger tracks and sighting rates of prey
were strongly correlated suggesting a wide scale relationship between predator
and prey in the region. By combining
the estimates of absolute density with indices of abundance, an average of 3.7
tigers/100km2 across the region is estimated, which given an area of
5,770km2, predicts a minimum of approximately 200 tigers in the
Bangladesh Sundarbans.
Keywords:Camera-trapping, distance sampling, Panthera tigris,
prey density, Sundarbans, tiger density track survey.
For
figures, images, tables -- click here
INTRODUTION
The Sundarbans of
Bangladesh and India is the world’s largest tidal mangrove forest (Chaudhuri
& Choudhury 1994; Khan 2002) and represents a region of international
importance (Seidensticker 2004). It has been identified as a Level I Tiger Conservation Unit (TCU),
because the habitat offers the highest probability of persistence of tiger
population in the long term (Wikramanayake et al. 1999) and holds one of the
two largest tiger populations globally (Seidensticker et al. 1999; WWF 1999; Khan
2002, 2004a). Because unfragmented
mangrove habitat is naturally inaccessible, this region offers a protected
environment with the potential for the long-term conservation of tigers.
The Bengal Tiger is
catagorized as Endangered globally (Chundawat et al. 2011) and Critically
Endangered nationally (in Bangladesh) (IUCN-Bangladesh 2000). It is listed in the third schedule of
the Bangladesh Wildlife Act of 1974, implying its full protection by
interdicting killing and capturing (MoEF-Bangladesh 2004).
Despite its importance for
tiger conservation, there have been a few studies which have used robust and
repeatable methods to estimate the abundance of tigers in the region. MoEF-Bangladesh (2004) has reviewed the
previous attempts to measure tiger population in Bangladesh and used the
pugmark tracking method extensively during 2004. The method used is an extension of the ethnic methods used
by tribal and shikaris in India. Mr. Saroj Rai Choudhury (Choudhury 1970, 1972), a forester from Orissa
is responsible for scientifically establishing this postulate (MoEF-Bangladesh
2004). A number of practicing
wildlife biologists have further intensified its use (Panwar 1979) or have
refined the technique (Singh 2000).
Previous attempts to
measure tiger population in this area from pugmark censuses or interviews
(Hendrichs 1975) have been shown to be unreliable (Karanth et al. 2003; Khan
2004b). Other studies in the
region have been based on indirect evidences (Seidensticker & Hai 1978;
Seidensticker 1986, 1987; Tamang 1993; Reza 2000; Khan 2004b) or extrapolations
from telemetry studies (Barlow et al. 2009). This paper presents the first estimate of tiger density
based on camera-trap surveys in the Bangladesh Sundarbans.
Since tigers depend on
large mammalian prey, the population density of large mammals should be
assessed in order to understand the carrying capacity and long-term
conservation status of tigers (Sunquist 1981; Karanth & Sunquist 1995;
Sunquist et al. 1999). Large
mammals including Spotted Deer Axis axis, Wild Boar Sus scrofa and Rhesus Macaque Macaca mulatta together comprise 95% of
the biomass consumed by tigers in the Sundarbans (Khan 2008). This study uses estimates of abundance
of these species to make inferences about tiger abundance in the wider region.
Camera-traps are becoming
established as one of the major tools in wildlife monitoring (Rowcliffe et al.
2008) and have been extremely effective at monitoring individually marked
species like tigers (Karanth & Nichols 1998; Karanth et al. 2006). However, most camera-trap studies focus
on relatively small areas (e.g., typically under 300km2) (Carbone et
al. 2001; Karanth et al. 2004). Ideally, we need information on wider spatial scales for wide ranging
and rare species. Under such
circumstances, it is useful to develop methods to extend camera-trapping
results to wider spatial scales through the use of calibrated indices such as
track counts (Stander et al. 1997; Stander 1998; Karanth et al. 2003; Stephens
et al. 2006).
In this paper I present the
results of intensive monitoring in the core study site, using mark-recapture
analysis of data collected from camera-traps (Otis et al. 1978; White et al.
1982; Rexstad & Burnham 1991) and estimates of the main prey species based
on distance sampling (Eberhardt 1978; Burnham et al. 1980; Buckland et al.
1993). Then using an index-based
survey of tiger tracks and sightings of their main prey species, I have
extended these results to the wider region.
MATERIAL AND METHODS
Study Area
The Sundarbans is a
mangrove swamp comprising mainly holophytic trees with the average canopy
height of less than 10m (Hussain & Acharya 1994). The forest floor is approximately 0.9–2.1 m above the
mean sea level (Tamang 1993). The
Bangladesh Sundarbans covers an area of 5,770km2, of which 1,750km2is covered by rivers and creeks
(Hussain & Acharya 1994). The
banks along the shores are cleared by tidal cycles twice per day providing
ideal conditions for tiger track counts. All tracks sighted are guaranteed to be relatively fresh (maximum five
days) because old tracks are washed away by the tides in about five days.
The study was undertaken
across six sites in the Bangladesh Sundarbans, of which three are in wildlife
sanctuaries (Sundarbans East, Sundarbans South and Sundarbans West) that form a
UNESCO World Heritage Site with a total area of 1,397km2. The camera-trap survey was conducted in
the Sundarbans East Wildlife Sanctuary (total area of 312km2,
between 21049”–21056”N & 89044”–89052”E),
covering only the southern part of the sanctuary. In five additional sites, and in the core study area, lower
intensity monitoring methods based on relative abundance of tiger tracks and
prey sightings along the riverbanks were used to assess relative
abundance. All of the additional
sites were of roughly equal size, approximately 170km2 (Table 1 and
Image 1).
Field study
The field study was
conducted for more than 200 days from October 2005 to January 2007 (camera-trap
survey was conducted for 90 days from 06 September to 04 December 2006), but
some of the data on prey were collected from September 2001 to February 2003. Tigers were identified using their
stripe patterns (Schaller 1967; McDougal 1977; Karanth & Nichols 1998)
(Image 2), but Goyal & Johnsingh (1996) experienced problems in identifying
camera-trapped tigers. An analysis
of the capture history was used to estimate capture-recapture analysis (Otis et
al. 1978; White et al. 1982; Rexstad & Burnham 1991). This technique as well as others based
on the use of camera-trap data has been shown to be effective at extremely low
population (Simcharoen et al. 2007; Lynam et al. 2008).
The location of camera-trap
points were selected to maximize the chances of obtaining tiger photos, based
on the presence of earlier tiger signs (tracks, scats, kills, scrapes, scent
deposits, etc.) and the intersections of trails (Karanth & Nichols
1998). The trap-points were set
approximately 2km apart, typical of other tiger surveys (Karanth et al. 2004)
so that it was unlikely that any area in the camera-trapping plot had a zero
probability of capturing a tiger (Karanth & Nichols 1998). All trap-points were marked on a map
using a GPS unit (eTrex Vista C; accuracy: ±15m). The survey area was surrounded on three sides by large
rivers. On the northern side,
however, I assumed the survey area included a boundary strip of 2km, based on
the movements of two recaptured tigers which had crossed traps of about 4km
distance (see Karanth & Nichols 1998). The total survey area surrounded by the rivers and
accounting for the boundary strip in the north was approximately 105km2(Image 3).
A total of 10 commercially
made Wildlife Pro (made by Forestry Suppliers, Inc.;
www.forestry-suppliers.com) camera-trap units were used in the survey
area. The camera-traps have
protective water-proof housing (with camouflaging colouration). Inside the
housing there is a Canon Super Shot fully-automatic 35mm autofocus camera and a
motion sensor for triggering the camera. The camera-traps were mounted on wooden posts or on tree trunks where
available, about 350cm away from the trail at a height of 45cm (Karanth & Nichols
1998).
During the sampling period
(06 September to 04 December 2006) the camera-traps were systematically shifted
in three camera-trapping sub-plots (Kochikhali, Katka and Chita Katka) in order
to cover all the potential trap-points by limited number of camera-trap units
(Image 3). The 90-day (24-hour)
survey period, was subdivided into two 45-day phases, occasion 1 (when the
photographed individual tigers were identified or ‘marked’) and occasion 2
(when both ‘marked’ and ‘unmarked’ individual tigers were photographed). For each occasion the camera-traps were
deployed in three consecutive sub-plots, for 15 days each (Image 3). Cameras were placed in pairs at each
trap site in order to get photos of both sides of a tiger. Therefore, each sub-plot contained five
trap-points with a total of 15. See Table 2 for details of photo captures.
Typically for tiger
surveys, a maximum of two months is recommended (Karanth et al. 2002), but more
time was required in this study because of the limited number of camera-traps
and the difficulty of obtaining photographs of tigers. Trapping rates may have been reduced by
the absence of obvious trails in the Sundarbans which lowers the chances of
predicting their routes of travel. Since the tiger is a relatively long-living and slow-breeding animal
(Nowell & Jackson 1996), I assumed that there was no significant change in
the dynamics of tiger population during the 90-day sampling period. The camera-traps were checked once
every day in order to record the date and location of each photographic ‘capture’.
The capture history data
were analysed by using CAPTURE2 software programme (www.mbr-pwrc.usgs.gov)
following M0 model since the capture probability for all adult tigers were the
same. This software was developed to implement closed-population
capture-recapture models. Since it was only possible to cover a relatively
small part of the Sundarbans with the camera-trap survey, tiger track surveys
were used to approximate tiger density over a wider area.
The Sundarbans provides
ideal conditions for track surveys because the tidal cycles make it easier to
assess new and older tracks. Tigers in this region frequently cross the rivers, especially those that
are not very wide. Thus, track
counts represent an estimate of recent tiger activity in the area. Counts along riverbanks were used to
estimate the relative density of tigers in the core study area and in each of
the additional study plots. Since
the tiger tracks are visually identifiable (Van Sickle & Linzey 1991; Palomares
et al. 1996), especially in the muddy riverbanks, all the fresh tracks (maximum
five days old; age assessed on the basis of reference observations of the
change of conditions of pugmarks and human footprints with time) were counted
from the riverbanks. Wide rivers
and narrow creeks present a problem with observation and navigation and thus
were not surveyed. The survey took
place from a dinghy driven slowly at a relatively constant speed. My field assistants and I searched for
fresh tracks on both banks of the river. However, the same track, i.e. the same crossing, on two sides of the
river was treated as one observation. Binoculars were used whenever necessary for searching tracks and for
general observations. Since the
rivers were not straight, the speed of the boat (by using a GPS unit) and the
total time of observation were recorded in order to convert the travelling
distance into equivalent straight distance.
Sighting rates of track
were compared against the density estimate of tigers obtained from camera-traps
in Sundarbans East Wildlife Sanctuary to provide a rough calibration between
track sighting rates and tiger density. This was then used to extend my estimate of tiger density in the wider region.
The population density of
large mammalian prey in the Sundarbans East Wildlife Sanctuary was estimated
using distance sampling (Eberhardt 1978; Burnham et al. 1980; Buckland et al.
1993). The transect line length
was measured by using a GPS unit. Since the Sundarbans is generally flat, the aerial distance was a close
representation of the actual distance covered in line transects. A total of 352 transects of variable
lengths was placed that covered a total of 466.8km length. The sampling effort was uniform for
different seasons of the year. My
field assistants and I walked along transects at a roughly uniform speed of
1.3km/h and concentrated on detecting the large mammalian prey at their initial
locations. For each observation
the sighting distance of the animal (when solitary), or of the centre of the
group (when in group), was recorded by using a rangefinder (Bushnell Yardage
Pro 800; accuracy: ±1.8m). The sighting angles were recorded by using a
compass. The work was mainly
conducted in the mornings (0600–1000 h) and afternoons (1500–1900
h) when the prey animals were most active and visible. Animal groups were used as the
analytical unit since individual data tend to underestimate the true variance
(Southwell & Weaver 1993). DISTANCE 4.0 software (www.ruwpa.st-and.ac.uk/distance) was used to
analyse the data derived from line transects to determine the individual
density.
The relative densities of
large mammalian prey in six sites were estimated by counting them along the two
banks of rivers in combination with the counts of tiger tracks. Since the vegetation conditions along
the riverbanks were similar, it was assumed that the visibility of prey was
uniform. As with the tiger
estimates, relative sighting rates at Katka-Kochikhali were used to calibrate a
density estimate for the wider area across the remaining five study sites. Sighting rates of large mammalian prey
from the river surveys were also made across all six sites and these indices of
prey abundance were compared against the tiger track sighting rates.
RESULTS
A total of 829 photographs
of different species was obtained from Katka-Kochikhali site during the survey
period, of which there were seven photographs (three in occasion 1 and four in
occasion 2, with two ‘recaptures’ in occasion 2) of five individual tigers
(Table 2). Using the ‘capture’
history data in CAPTURE2 software programme it was estimated that the absolute
number of tigers (adult and subadult) in the 105km2 area in the
southeastern end of the Bangladesh Sundarbans is 5 (SE=0.96, capture
probability or p-hat=0.70). This
means that the tiger density in the area covered by camera-trap survey is 4.8
tigers/100km2. Due to
the complexity and lack of correctness of estimating the variance of estimated
area sampled by camera-trapping, the standard error for this density estimate
was not calculated. However, due
to the fact that the sampled area (105km2) was very close to 100 km2,
it is assumed that the standard error for the density estimate is very close to
0.96. This is the first estimate
of the tiger population density in the Bangladesh Sundarbans that is based on
camera-trap survey (Table 3).
Based on tiger track counts
the relative density of tigers in six different sites was estimated (Table
4). The average of these six sites
represents the average for the entire Bangladesh Sundarbans, which is 0.44
tracks/km of riverbank surveyed. The three sites in three sanctuaries clearly had higher densities of
tiger tracks than the three sites outside the sanctuaries. The track densities, i.e. relative
densities of tigers, were then converted to an estimate of absolute density
through extrapolation (Table 4). The average of six sites provides an estimate of 3.7 tigers/100km2as an average for the entire area. Since the Bangladesh Sundarbans is an area of 5,770km2 it is
inferred that, to a rounded figure, the total tiger population size would be
approximately 200. Assuming that
the tiger density in the Indian Sundarbans (4,263km2) is similar to
that in the Bangladesh Sundarbans, we might expect around 150 tigers in the
Indian part, forming a single population of around 350 tigers in the entire
region.
In Katka-Kochikhali the
overall density of large mammalian prey (Spotted Deer, Wild Boar and Rhesus
Macaque) was estimated at 27.9 large prey/km2. The average number of large mammalian
prey along riverbanks in six sites, i.e., the relative density of prey in the
Bangladesh Sundarbans is 4.2 large prey/km of riverbank. The relative density was converted to a
rough estimate of absolute density, which is 17.3 large prey/km2 or
1,730 large prey/100 km2. Based on this estimate the total population of three species of large
mammalian prey (Spotted Deer, Wild Boar and Rhesus Macaque) in the Bangladesh
Sundarbans is inferred at, to a rounded figure of, 99,800.
The absolute densities of
tigers and three large mammalian prey in the Bangladesh Sundarbans were
converted to biomass densities and were found that it is 542kg/100km2for tigers and 102,430kg/100km2 for three large mammalian prey
combined (Table 3). Therefore, the
biomass ratio between tigers and prey is 1:189. The biomass densities of tigers
and prey show strong relationship (R2 = 0.896) across the six sites
(Fig. 1).
DISCUSSION
It is always difficult to
estimate the population density of a shy and secretive animal like the tiger,
which is thinly distributed throughout a large tract. It is even more difficult in the impenetrable swamp of the
Sundarbans where tigers are rarely seen by people. Therefore, most of the previous estimates used pugmark
census (Choudhury 1970, 1972; Panwar 1979; Singh 2000) and the figures of tiger
population in the Bangladesh Sundarbans (official estimates range from 350 to
450 tigers; MoEF-Bangladesh 2004) are much higher than what is estimated in
this study. The scenario is the
same in the Indian Sundarbans where, according to the official estimate
conducted in 2004, there are 274 tigers (Chowdhury & Vyas 2005), which is,
in the view of present findings, too optimistic. The wide availability of the pugmarks in the Sundarbans
(since the ground is soft) gives some the idea that the tiger density is very
high, which is not the case (Khan 2004a).
Based on the prey density,
and following Karanth & Stith (1999), and Karanth et al. (2004), there is a
previous estimate of tiger density in the Sundarbans East Wildlife Sanctuary
(Katka-Kochikhali area is the major part of this Sanctuary) (Khan 2004b) and
the estimated figure (4.3 tigers/100km2) is similar to that
estimated in the same area during this study (4.8 tigers/100km2). Notably, it is a well-established fact
that carnivores and their prey numbers show strong positive correlation in any
undisturbed area (Schaller 1967; Sunquist 1981; Seidensticker & McDougal
1993; Carbone & Gittleman 2002; Karanth et al. 2004).
Although there is no
previous estimate of tiger density in the Bangladesh Sundarbans based on
camera-trap survey, Karanth & Nichols (2000) reported the tiger density in
the Indian Sundarbans, which was based on camera-trap survey. The density (0.84 tigers/100km2),
however, was less than what was found in this study.
The results of
radio-collaring two tigresses for a few months in the southeastern Sundarbans
in Bangladesh estimated the home range sizes (14.6 and 12.8 km2) is
relatively very small, suggesting that the tiger density is very high (Barlow
et al. 2009). However, the
estimated prey density or other estimates of tiger density in the Sundarbans
(Karanth & Nichols 2000; Khan 2004b; Sharma 2009; Jhala et al. 2011; this
study) contradict this implication. In the Indian Sundarbans, one
radio-collared tigress was reported to roam in an area of approximately 50km2(Sharma 2009), which is very different from what was estimated for two
radio-collared tigresses in the Bangladesh Sundarbans (Barlow et al. 2009).
Despite some drawbacks, camera-trap
survey represents an effective method for surveying tigers. In this study, however, only seven
photos of the tiger were obtained, because there were very few or no trail in
the Sundarbans that are frequently used by tigers and other wild animals. The forest was very dense and there
were limited number of camera-traps.
Because of the tiger’s low
density and shy nature, other methods of animal population estimation like
distance or quadrat sampling (Buckland et al. 1993) are of limited value. Although radiotelemetry-derived data
can be used in estimating tiger density (Smith et al. 1987a,b; Quigley 1993),
the small number of tagged animals, the presence of untagged animals in the
population, and the excessive effort involved in capturing and radio-tracking
operations limit the usefulness of this method in tiger density estimation
(Karanth 1995).
The ratio of tiger and
large mammalian prey biomass densities (1:189) estimated for the Sundarbans is
different from those estimated (calculated from tiger and prey densities) for
tiger ranges in the neighbouring countries, e.g., 1:342 in Kanha, India (Schaller
1967; Newton 1987), and 1:391 in Chitwan, Nepal (Tamang 1982). This is an indication of insufficient
prey for tigers in the Sundarbans (Khan 2008).
The scientific estimate of
tiger and large mammalian prey population densities in the Sundarbans that was
done in this study will be the key factor in convincing different national and
international organisations and communities the potential of the tiger and prey
populations in the Sundarbans in the long term. The estimates, however, were largely extrapolations of the
absolute densities using indices of abundance. These are not robust estimates, but the indices are
correlated between predator and prey, suggesting that they represent a real
change in animal abundance across the region (Jhala et al. 2010). The estimates of absolute and relative
densities will be useful in temporal monitoring of population trends of tigers
and prey in Sundarbans, both inside and outside the sanctuary.
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