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Impacts of anthropogenic pressures on the
contemporary biogeography of threatened
crocodilians in Indonesia
KYLE J. SHANEY,AMIR HAMIDY,MATTHEW WALSH,EVY ARIDA
AISYAH ARIMBI and E RIC N. SMITH
Abstract The Greater Sunda region of South-east Asia sup-
ports a rich diversity of economically and ecologically im-
portant species. However, human pressures are reshaping
contemporary biogeography across the region. Megafaunal
distributional patterns have been particularly affected be-
cause of deforestation, poaching and human–wildlife con-
flict. Crocodilians are at the centre of these conflicts in
Indonesia and yet remain poorly studied across much of
the archipelago. We conducted population surveys of salt-
water crocodiles Crocodylus porosus and false gharials
Tomistoma schlegelii in Sumatra, and examined whether
crocodile abundance and distribution are correlated with
variations in human disturbance, fishing pressure, and habi-
tat type. We then used these data to model remaining suit-
able habitat for T. schlegelii across South-east Asia. We
found that abundance of T. schlegelii and C. porosus was cor-
related with distance from human settlements, and fish-
trapping pressure. We recorded the presence of T. schlegelii
in a river system in which it was previously unknown, thus
expanding the known range of the species. We also found
that the predicted remaining suitable habitat for T. schlegelii
in Indonesia is largely limited to areas of low human activ-
ity. From these empirical and modelling approaches we pro-
pose several key conservation priorities: () eliminate the use
of fish traps in remaining patches of T. schlegelii habitat,
() prioritize crocodile population surveys in remaining
suitable habitat, particularly in remote areas, () consider
T. schlegelii to be potentially Endangered locally in
Sumatra, and () expand existing reserves around the
Lower Kampar River and Berbak National Park/Sembilang
National Park areas of Sumatra.
Keywords Endangered, false gharial, fish trapping, refugial,
remote, reserve, Sumatra
Introduction
Agricultural practices across the Greater Sunda region
(i.e. Borneo, Java, Peninsular Malaysia and Sumatra)
are driving one of the world’s highest rates of deforestation
(Sodhi et al., ). Indonesia is at the forefront of contem-
porary global change in which habitat alteration and hunt-
ing pressure are reshaping species’distributions. In turn,
vertebrate populations are increasingly being forced into re-
mote, refugial habitat. Conversion of forest to oil palm, rub-
ber, tea and coffee plantations, in conjunction with a lack of
wildlife management resources (e.g. revenue and staff), has
led to unregulated overharvesting of natural resources
(Margono et al., ; Miettinen et al., ). Unregulated
hunting pressure for meat, skin and the pet trade in
Indonesia has also affected vertebrate populations across
the Greater Sunda region (Brooks et al., ; Brodie et al.,
). Although the impacts of human pressures on verte-
brates in the region have been addressed to some extent, im-
pacts on reptilian groups are largely unquantified.
Indonesia’s crocodilians are an excellent model system
for understanding the impacts of human pressures on con-
temporary biogeography. Furthermore, in light of human–
carnivore conflicts globally, crocodilians are a useful system
for developing long-term carnivore conservation schemes.
Salt-water crocodiles Crocodylus porosus are distributed
across South-east Asia and are considered to be a significant
threat to humans (CrocBITE, ). A large proportion of
attacks by C. porosus occur in Indonesia, and many of
these occur in the Greater Sunda region. Crocodylus porosus
is listed in CITES Appendix II (CITES, ), and in
Indonesia harvesting of wild adults is legal only from the
eastern province of Papua, although the collection of eggs
and juveniles from Kalimantan and Sumatra has been lega-
lized (Webb et al., ). Crocodylus porosus is categorized
as Lower Risk/Least Concern on the IUCN Red List
(Crocodile Specialist Group, ) but its population status
across most of Indonesia is unknown (Webb et al., ).
The false gharial Tomistoma schlegelii also inhabits the
Greater Sunda region and is considered to be one of the
least known crocodilians (Bezuijen et al., ,,;
Auliya et al., ; Stuebing et al., ; Simpson, ).
It primarily inhabits black-water, peat swamp forest,
although most lowland swamp forest in the region has
been lost as a result of deforestation (Sodhi et al., ;
KYLE J. SHANEY (Corresponding author), MATTHEW WALSH and ERIC N. SMITH The
Amphibian and Reptile Diversity Research Center and Department of Biology,
University of Texas at Arlington, 501 S. Nedderman Drive, Arlington, TX 76010,
USA. E-mail kjshaney@uta.edu
AMIR HAMIDY and EVY ARIDA Laboratory of Herpetology, Museum Zoologicum
Bogoriense, Research Center for Biology, Indonesian Institute of Sciences,
Bogor, West Java, Indonesia
AISYAH ARIMBI Wildlife Conservation Society, Indonesia
Received December . Revision requested March .
Accepted June . First published online November .
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Miettinen et al., ). Tomistoma schlegelii is listed in
CITES Appendix I (CITES, ) and is categorized as
Vulnerable on the IUCN Red List (Bezuijen et al., ).
Prior to an update in it was categorized as
Endangered but despite the status update, data on the spe-
cies’population status are sparse across much of its range,
particulalry in Sumatra. Although significant work has
been conducted on T. schlegelii (J. Cox et al., unpubl. data;
Bezuijen et al., ,,,), only c. confirmed
sightings of the species have been recorded in Sumatra
(Stuebing et al., ), and to the best of our knowledge
no crocodilian surveys have been conducted in Sumatra
since (Bezuijen et al., ).
We gathered population data on C. porosus and
T. schlegelii in areas of Sumatra that have and have not
been assessed previously for crocodilian abundance. We ex-
amined whether crocodilian abundance was negatively in-
fluenced by proximity to humans and whether crocodiles
were capable of persisting in areas of high human disturb-
ance. We also examined whether fish-trapping activity was
associated with declines in crocodilian abundance. We pre-
dicted that increased fish-trapping activity and proximity to
humans would be associated with declines in crocodile
abundance. We used species distribution modelling techni-
ques to identify potential remaining suitable habitat for
T. schlegelii across its range and suggest key areas for conser-
vation priority.
Study areas
Crocodylus porosus is distributed across northern Australia
and South-east Asia, whereas T. schlegelii is restricted to the
Greater Sunda region. Our work focused on these two spe-
cies in four study areas on the island of Sumatra (Fig. ): the
Air Hitam Laut River system in Berbak National Park (pre-
viously surveyed), the Lower Kampar River system (not pre-
viously surveyed), the Simpang Kanan River system (not
previously surveyed), and the Lalan River system (previous-
ly surveyed). We conducted surveys during June–August in
and . The Lalan River and Simpang Kanan River
systems were surveyed in , the Air Hitam Laut River sys-
tem was surveyed in , and the Lower Kampar River was
surveyed during both field seasons.
Berbak National Park, in the south-east of Jambi
Province (Fig. b), was surveyed previously by J. Cox
(, unpubl. data) and Bezuijen et al. (,). It in-
cludes one of the largest remaining tracts of peat swamp for-
est habitat in Sumatra. Two distinct seasons, wet and dry,
affect water levels in the peat swamp forests (lowland acidic
swamp) of the region. In Sumatra the wet season typically
lasts from October until late February, and large sections
of the forest become flooded during this time. In the dry sea-
son, during March–September, the forests progressively dry
up. By mid August only the main tributaries, billabongs and
lakes still hold water. The Park is intersected by the Air
Hitam Laut River system and encompasses branches of
the Batanghari River in the north (Air Hitam Dalam
Tributary, not surveyed here) and Benu River in the south
(not surveyed here). Logging and hunting activities are il-
legal within the Park boundaries; however, illegal activity
has penetrated the perimeter of the Park in multiple
locations.
The Lower Kampar River encompasses some of the last
remaining patches of primary peat swamp forest in Riau
Province (Fig. a) and has not been surveyed previously
for crocodiles. Multiple black-water tributaries originate in
the surrounding forests and enter the river at various
FIG. 1 (a) Lower Kampar River and Simpang Kanan study areas
(previously unsurveyed), and (b) Berbak National Park and
Merang River study areas (previously surveyed). Locations of
sightings of salt-water crocodiles Crocodylus porosus (black filled
circles) and false gharials Tomistoma schlegelii (white filled
circles) are marked, and all parks and reserves are delineated, as
are areas with potential for reserve expansion.
Human impacts on crocodilians 571
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locations immediately east and west of Teluk Meranti village
(Shaney et al., ). Major tributaries in the area include the
Kerumutan, Kutup, Serkap and Turip Rivers. Two small re-
serves are located along the Serkap River (Tasik Metas and
Tasik Serkap Reserves) and there is a larger reserve along the
Kerumutan River (Kerumutan Reserve). Reserves in the
area are rarely monitored by wildlife officials, and illegal log-
ging, fishing and hunting activity occur. We also surveyed
the Simpang Kanan River, which originates within the
peat swamp forest adjacent to the Kerumutan Reserve; how-
ever, the river drains directly into the ocean rather than into
the Lower Kampar River (Fig. a). The Simpang Kanan
River had not been surveyed prior to our study.
Our final survey area included the Lalan River system,
with a particular focus on the Merang River. The headwaters
of the Merang River originate near the Berbak National Park
study area; however, the river drains to the south, entering
the Lalan River system in South Sumatra Province (Fig. b).
Amongst Sumatran rivers the Merang River has received the
most survey attention for T. schlegelii activity in the past,
and the majority of historical T. schlegelii sightings in
Sumatra have been recorded in this river (Stuebing et al.,
). Bezuijen et al. (,,) and Shaney et al.
() provide detailed information about the study area.
Methods
Surveys
Surveys followed techniques from Bayliss () and
Bezuijen et al. (,). Night-time spotlight surveys
were conducted along sections of the main Kampar River
(.km surveyed) and on four tributaries of the Lower
Kampar River system: Kerumutan River ( km surveyed),
Kutup River (km surveyed), Serkap River (.km sur-
veyed) and Turip River (.km surveyed). We also surveyed
sections of the Air Hitam Laut River (.km surveyed) and
nearby Kumpe River (.km surveyed), as well as tributar-
ies of the Air Hitam Laut River system: Simpang Kubu
(.km surveyed), Simpang Melakka (.km surveyed)
and Simpang T (.km surveyed). We surveyed .km
of the Merang River (Lalan River System), km of the
main Lalan River and km of the Simpang Kanan River
(Fig. ). Start and end points of transects were recorded
using a global positioning system. Transect lengths were re-
corded and used to determine the number of crocodiles
sighted per km of river surveyed. Repeat surveys were con-
ducted only in three locations (Kerumutan, Serkap and
Turip Rivers) and we subtracted potential repeat sightings
from total counts. A total of .km of river was surveyed:
km by paddle boat and .km with the aid of small
motors. A total of survey nights and days were
spent in the study areas. Twelve survey nights were spent
on the Air Hitam Laut River system, on the Lower
Kampar River system, on the Simpang Kanan River and
on the Lower Merang River.
We used a wooden boat (m length) with a hp motor
on larger rivers, and canoes (m length) on small tributar-
ies. We traversed transects during the day to record eco-
logical and dependent variables for statistical analyses.
Surveys were typically initiated – minutes after night-
fall (.–.). In tidally influenced areas we altered sur-
vey start times accordingly. In upper tributaries unaffected
by tide, water levels were relatively low and did not extend
into fringing vegetation. We recorded pH levels at the begin-
ning and end of each transect, and when a crocodile was ob-
served we noted the pH measurement recorded closest to
the sighting. Crocodile eye-shine was detected using
, lumen headlamps. When we sighted a crocodile we
approached it and, when possible, identified the species and
recorded the age class. We defined crocodiles by the follow-
ing age classes: hatchling (young of that year, c. .–.m),
juvenile (not yet sexually mature, c. .–.m), and adult
(sexually mature, ..m). When crocodiles submerged be-
fore further identification could be made we recorded the
sighting as an eye-shine, or as probable species identification
if we were confident in the identification despite only catch-
ing a glimpse of the crocodile.
Statistical analysis
Survey results from the four major river systems and nine of
their tributaries were included in analyses. The dependent
variables associated with our crocodile data included con-
firmed crocodile species identification counts, probable spe-
cies identification counts, confirmed species identification
counts and daytime sign (description of daytime data
below), and probable species identification counts and day-
time sign. We also tested for varying effects on each age class
and combined species counts because of low sample size.
Poisson log-linear regressions were used to test the factors
associated with variation in crocodile abundance, using
SPSS v. (IBM, Armonk, USA). We tested whether croco-
diles were more likely to be found in remote areas, using dis-
tance from human inhabitance as a quantifiable measure.
We measured Euclidian, or straight-line, distance between
each sighting and the nearest village, and then we measured
the distance along the centre-line of rivers between each
sighting and the nearest village (i.e. river distance). We iden-
tified villages with . residents as a cut-off point for se-
lecting the nearest village to each sighting, following Stoner
et al. (). Next, we tested whether fish-trapping pressure
was negatively correlated with crocodile abundance. To ac-
complish this we counted fish traps along transects and used
the number of fish traps per km as a predictor variable. We
also identified habitat preferences, based on sightings of
572 K. J. Shaney et al.
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each species, and partitioned sightings into four habitat
categories: () primary forest (forest that appears to be un-
altered), () secondary forest (regrowth after logging),
() mangrove forest, and () no forest. We identified habitat
types along the edges of waterways where crocodiles were
sighted, and confirmed habitat types using land-cover data
in ArcMap .(ESRI, Redlands, USA). We used a two-
tailed t-test to test for interspecific habitat partitioning,
using pH levels associated with sightings (night-time sight-
ings only: n = for C. porosus and n = for T. schlegelii;
night and daytime sightings combined: n = for C. porosus
and n = for T. schlegelii). The daytime data included each
unique locality where crocodiles were seen during the day or
where we collected bones or eggshells, or saw slide marks
(see Plate for examples of both night-time and daytime
data). Datasets with and without daytime data were analysed
independently. We tested for possible multicollinearity be-
tween variables using collinearity statistics in SPSS.
Species distribution modelling
We used Maxent ..k(Phillips et al., ) to model suit-
able T. schlegelii habitat across the Greater Sunda region. We
focused only on T. schlegelii for this analysis because it is ca-
tegorized as Vulnerable across its range. We created four
species distribution models to compare and contrast vari-
able effects on potential distribution: () climate layers (
bioclim layers; WorldClim, ), () climate layers + land-
cover, () climate layers + human population density, () cli-
mate layers + land-cover + human population density.
Human population density and land-cover layers (all at
-second resolution) were acquired from DIVA-GIS
(). We used all confirmed records of T. schlegelii in
Auliya et al. (), Stuebing et al. () and Bonke
et al. (), data from East Kalimantan (A. Staniewicz, un-
publ. data), and our own survey data. Collinearity is not
considered to be problematic when using Maxent, and
therefore we did not exclude any variables after model test-
ing (Elith et al., ); however, we conducted jackknife tests
to determine which variables were most predictive of
T. schlegelii distribution. We used the Auto Features settings
provided, changed the number of iterations to ,, repli-
cated run type to subsample, and set random test percentage
to . We assessed the area under the curve (AUC) for each
model, to measure model performance using the presence
localities provided. AUC models are produced in Maxent
only when test values are provided. AUC values closer to
.indicate high performance of the predictive suitability
models. We then extracted areas from the species distribu-
tion models that intersected with rivers and inland water
bodies (DIVA-GIS, ), because T. schlegelii is aquatic.
We quantified the amount of suitable aquatic habitat across
the species’range with a probability of occurrence ..to
be conservative. We also distinguished other break points at
.,.and ., for context. Although T. schlegelii was re-
corded in disturbed habitat, the majority of sightings oc-
curred in unfragmented primary lowland forest.
Therefore, we also extracted suitable habitat from within re-
maining primary lowland forest areas and quantified those
areas separately. We restricted final quantifications with a
polygon mask, excluding biogeographical areas beyond the
species’range. We repeated these steps explicitly for
Sumatra.
Results
Survey data
A total of crocodiles (C. porosus and T. schlegelii com-
bined) were counted (eye-shine only) during night-time
surveys, and eight signs of crocodile presence were found
during the day (Table ). We recorded sightings on the
Air Hitam Laut River and on the Lower Kampar River.
No crocodilian sightings were recorded in the Lalan or
Simpang Kanan Rivers. In total, eight sightings were con-
firmed as T. schlegelii and were probably T. schlegelii,
and seven signs of T. schlegelii were recorded during the
daytime; an increase compared to previous records of
T. schlegelii in Sumatra. The Simpang T tributary of the
Air Hitam Laut River has not been mentioned in previously
published data, but this was where we recorded of the
T. schlegelii sightings in the Air Hitam Laut River system.
We recovered a single, dead juvenile T. schlegelii in
Kerumutan Village along the Kerumutan River, which is
the first confirmed record of T. schlegelii from the
Kerumutan River and from the Lower Kampar River system
in general. For details of all sighting localities see Shaney
et al. (). For a comparison of species densities from
this study with findings of previous studies conducted in
the Air Hitam Laut and Lalan River systems see Table .
Poisson regressions
Measures of collinearity indicated that fish-trapping activity
and remoteness (i.e. distance to human settlements) were
independent measures (collinearity statistics: tolerance
value = ., variance inflation factor = .). We ob-
served significant (P ,.) effects of remoteness and fish-
trapping activity on crocodile abundance (Table ). Fish trap
density was a significant predictor of crocodile counts for six
out of eight estimates of crocodile abundance, and remote-
ness was a significant predictor of crocodile counts in all
analyses (Table ). For all significant trends T. schlegelii
and C. porosus counts were positively correlated with in-
creasing distance from human inhabitance (high
Human impacts on crocodilians 573
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remoteness increases the likelihood of seeing crocodiles)
and negatively correlated with increasing fish trap density.
Age class
We evaluated separately the effects of human activities and
fishing pressure on age classes. Hatchlings and adults were
significantly more likely to be found in remote areas, and
hatchlings were more likely to be found in areas with low
fish-trapping activity (Table ). We could not test whether
fish-trapping activity had an effect on juveniles or adults be-
cause there was no variation in those data (e.g. zero fish
traps along transects where adults and juveniles were
found).
Habitat preferences and habitat partitioning
We identified habitat partitioning between crocodile species
(P ,.). Crocodylus porosus was more likely to be found
in secondary forest, whereas T. schlegelii was more likely to
be found in primary forest (Table ). Changes in pH in the
Air Hitam Laut River coincided with abrupt shifts in croco-
dile presence/absence (Fig. ); pH decreased from .to .
along the junctions of black water tributaries and saline en-
vironments. Tomistoma schlegelii was significantly more
likely to be found in water with low pH (.–.) and C. por-
osus was significantly more likely to be found in water with
high pH (.–.).
Habitat suitability
All four species distribution models for T. schlegelii identi-
fied approximately the same suitable areas and returned
similar AUC values (training data: .–.; test data:
.–.). Therefore, we assess the most conservative
model here (Model ), which modelled the largest amount
of suitable habitat and included all habitat variables
(climate + landcover + human population density). Model
identified ,. km
of suitable habitat across the
species’range (..probability of occurrence), of which
,.km
falls within remaining primary lowland forest
(Fig. ). Of this suitable habitat, ,. km
PLATE 1 Examples of crocodile
sightings or crocodile sign.
(a) Daytime slide mark in the mud,
(b) Tomistoma schlegelii eggshell
(measurements confirm this), (c) dead
T. schlegelii, (d) T. schlegelii skull,
(e) hatchling Crocodylus porosus,
(f) hatchling T. schlegelii.
574 K. J. Shaney et al.
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(,. km
primary lowland forest) falls within Indonesia,
,. km
(. km
primary lowland forest) within
Malaysia, and . km
(.km
primary lowland forest)
within Brunei. From these data we estimate that
c. , km
of suitable habitat remains in Sumatra, of
which only ,. km
falls within remaining primary low-
land forest. All values reflect only where suitable habitat and
waterways overlap. Our model returned four key areas for
T. schlegelii conservation in Sumatra and six in Borneo.
Discussion
Given the critical need for crocodilian conservation efforts
in Indonesia, we consider these data to be valuable to the
conservation of T. schlegelii and C. porosus, and more
broadly to the conservation and management of vertebrates,
considering the shifting biogeography of South-east Asia.
The Balai Konservasi Sumber Daya Alam (Natural
Resources Conservation Agency) currently regulates animal
harvest quotas in Indonesia and is in the process of consid-
ering new harvest regulations for C. porosus across the archi-
pelago. We suggest several key conservation priorities for
crocodilians in the Greater Sunda region.
Habitat partitioning
We identified clear habitat partitioning between crocodile
species, which confirms the findings of others (Bezuijen
et al., ; Auliya et al., ). Crocodylus porosus was
found along the coastline and in brackish environments,
whereas T. schlegelii was restricted to freshwater, in black
water tributaries. We found the species coincided along a
transitional pH zone (interspecific sightings only km
apart) on the Air Hitam Laut River. The location where spe-
cies composition changes along the river is where saltwater
Nypa palms transition into freshwater Pandanus palms, and
pH changes substantially (Fig. b). In many parts of its range
C. porosus is found far upriver in freshwater environments.
Conversely, T. schlegelii is not known to inhabit saline envir-
onments. Further investigation of the relationship between
C. porosus and T. schlegelii could yield information regard-
ing competition between these two large crocodilian species.
For example, how does the relationship between the two
species change seasonally (e.g. in the wet season, when C.
porosus often travels far upriver in other parts of its
range)? Also, do these species commonly engage in intra-
guild predation? Answering these questions could also in-
form future management approaches.
TABLE 1 Counts and densities of crocodiles (Tomistoma schlegelii and Crocodylus porosus; confirmed and probable combined) in the Lower
Kampar, Air Hitam Laut, Simpang Kanan and Lalan River systems in Sumatra, Indonesia (Fig. ), with additional records of eye-shines
(species unidentified).
Stretch of river surveyed (km)
No. of individuals Density (individuals km
−1
)
Eye-shinesT. schlegelii C. porosus T. schlegelii C. porosus
Lower Kampar River system
Serkap River 0–41.2 0 8 0 0.19 1
Main Kampar River 30–84.8 0 2 0 0.037 0
Kerumutan River 0–22 0 1 0 0.046 0
Turip River 0–11.2 0 3 0 0.27 0
Kutup River 0–200000
Total 131.2 0 14 1
Air Hitam Laut River system
Main Air Hitam Laut River 0–27.3 3 26 0.11 0.95 0
Simpang Melakka 0–11.4 0 1 0 0.09 0
Simpang Kubu 0–4.6 0 0 0 0 0
Simpang T0–8.7 12 0 1.38 0 0
Kumpe River* 0–16.1 0 0 0 0 0
Total 68.1 15 27 0
Simpang Kanan River system
Main Simpang Kanan River 30–75 0 0 0 0 0
Ocean mangroves Mouth of Simpang Kanan
River−16.1 south
000 0 0
Total 61.1 0 0 0 0 0
Lalan River system
Main Lalan River 65–90 0 0 0 0 0
Merang River 0–40.8 0 0 0 0 0
Total 65.8 0 0 0 0 0
Grand total 326.2 15 41 1
*River adjacent to main river system
Human impacts on crocodilians 575
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TABLE 2 Details of all crocodilian sightings during surveys of four river systems in Sumatra, Indonesia (Fig. ), (this study) and those re-
ported in previous studies, with length of river surveyed; numbers of hatchlings, juveniles, adults, and eye-shine; density (including and
excluding counts of eye-shine); and data source. Age classestaken from Bezuijen et al. (,,,) are adapted to age class from
foot class data. Bezuijen et al. (,) foot classes of .feet are included as adults in this table.
River
Stretch of
river
surveyed
(km)
No. of records Density (individuals km
−1
)
SourceHatchlings Juveniles Adults Eye-shine Total
Including
eye-shine
counts
Excluding
eye-shine
counts
Air Hitam Laut River
system
Air Hitam Laut River
1990 0–20.5 7 false gharials Tomistoma schlegelii seen, no
size reported
7 0.34 0.34 J. Cox (un-
publ. data)
1996 0–25 0 1 1 2 4 0.16 0.08 Bezuijen et al.
(1997)
2001 0–31 1 0 0 3 4 0.13 0.03 Bezuijen et al.
(2001)
2002 0–32 1 0 3 4 0.13 0.03 Bezuijen et al.
(2002)
2015 0–27.3 1 1 1 0 3 0.11 0.11 This study
Simpang Melaka Creek
1996 0–2 0 1 0 2 3 1.5 0.5 Bezuijen et al.
(1997)
2001 0–7.2 2 1 0 2 5 0.69 0.4 Bezuijen et al.
(2001)
2002 0–7.2 0 0 0 1 1 0.14 0 Bezuijen et al.
(2002)
2015 0–11.4 0 0 0 1 1 0.09 0 This study
Simpang Kubu
2015 0–4.6 0 0 0 0 0 This study
Simpang T
2015 0–8.7 3 5 4 0 12 1.379 1.379 This study
Lalan River system
Lalan River
1990 0–150 2 T. schlegelii seen, no size
reported
2 4 0.03 0.01 J. Cox (un-
publ. data)
1995 0–160 0 0 0 0 0 0 0 Bezuijen et al.
(1995)
2014 80–140 0 0 0 0 0 0 0 This study
Kepahyang 0–16.5 0 0 0 0 0 0 0 Bezuijen et al.
(1995)
Medak River
1990 0–8 0 0 0 0 0 0 0 J. Cox (un-
publ. data)
1995 0–53 0 0 2 0 2 0.03 0.03 Bezuijen et al.
(1995)
Medak River upper
tributaries
See
Bezuijen
et al.
(1995)
0 0 0 0 0 0 0 Bezuijen et al.
(1995)
Merang River
1990 0–45 1 T. schlegelii seen, no size reported 1 0.04 0.07 J. Cox (un-
publ. data)
1995 0–45 0 4 0 3 7 0.16 0.09 Bezuijen et al.
(1995)
1996 0–45 0 1 0 1 2 0.04 0.02 Bezuijen et al.
(1997)
576 K. J. Shaney et al.
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Human disturbance
Abundance of both C. porosus and T. schlegelii is correlated
negatively with proximity to humans (although C. porosus
was found in disturbed habitat). We also found that com-
mon fish-trapping methods throughout the region were
negatively associated with crocodile abundance. It is im-
portant to consider that the correlations we identified do
not necessarily indicate causation. There are other factors
that could potentially affect crocodilian abundance that
are cross-correlated with the independent variables we
tested. We interpret our results with this in mind. We sug-
gest future surveys target remote locations, and manage-
ment officials consider eliminating fish trapping from
areas of core crocodile habitat. Despite the difficulties in-
volved, trapping has already been successfully eliminated
from one section of Berbak National Park, along the
Simpang T tributary of the Air Hitam Laut River (Fig. b).
Because fish trapping yields important resources for many
local communities, regulations requiring woven box or fun-
nel traps (rather than netting) with access to air for crocodile
bycatch could be implemented, rather than complete elim-
ination of fish trapping in key areas.
Crocodilian population assessments and suitable habitat
Crocodylus porosus population densities were relatively low
across our study areas. Additionally, C. porosus densities in
the Air Hitam Laut River system mostly comprised hatchl-
ings, many of which were in close proximity to each other.
This suggests that many individuals were of the same
clutches, and given the low hatchling survival rate the actual
density of C. porosus is probably much lower, and more rep-
resentative of a few adults that had separate clutches along
the river. We recommend C. porosus population surveys be
conducted across other parts of Sumatra on an ongoing
basis. This is particularly important given recent changes
to C. porosus management in Indonesia, in which the taking
of eggs and juveniles from Sumatra will be permitted (Brien
et al., ). Sembilang National Park has never been sur-
veyed for crocodilian abundance but it could potentially
hold large populations of C. porosus, as it encompasses
large swaths of intact mangrove forest along the east coast
of Sumatra. Multiple C. porosus individuals were encoun-
tered in the Lower Kampar River, particularly at the mouths
of blackwater tributaries and in sections of mangrove near
the coast. Thus, we suggest surveys targeted on the far east-
ern portion of the Lower Kampar River could yield high
densities of C. porosus, and the area could be important
for the long-term viability of the species. Other regions,
such as the Bangka Islands and Riau province coastline,
seem to hold relatively large populations of C. porosus,
based on numbers of reported attacks on people, but require
scientific surveys for population density estimates. We also
suggest repeat surveys be conducted frequently on the Air
Hitam Laut and Lower Kampar Rivers.
Tomistoma schlegelii populations in Sumatra appear to
be fragmented, occurring in potentially fewer than five loca-
tions. Extant subpopulations are confirmed at only three lo-
cations in Sumatra (Air Hitam Laut, Lalan and Lower
Table 2 (Cont.)
River
Stretch of
river
surveyed
(km)
No. of records Density (individuals km
−1
)
SourceHatchlings Juveniles Adults Eye-shine Total
Including
eye-shine
counts
Excluding
eye-shine
counts
2001 0–45 0 4 0 0 4 0.09 0.09 Bezuijen et al.
(2001)
2002 0–45 0 1 0 0 1 0.04 0.02 Bezuijen et al.
(2002)
2014 0–45 0 0 0 0 0 0 0 This study
Merang River
1995 45–67 0 2 0 5 7 0.34 0.09 Bezuijen et al.
(1995)
1996 45–67 0 4 1 5 10 0.49 0.23 Bezuijen et al.
(1997)
2001 45–67 1 12 0 1 14 0.64 0.6 Bezuijen et al.
(2001)
2002 45–67 0 0 0 2 2 0.16 0 Bezuijen et al.
(2002)
Human impacts on crocodilians 577
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Kampar Rivers), which are all isolated by distance and ter-
rain, meaning it is unlikely individuals will travel across
large tracts of unfavourable habitat between populations
to increase gene flow. Although we recorded the first record
of T. schlegelii in the Kerumutan River, we confirmed the
presence of only a single individual. This suggests that
T. schlegelii densities may be relatively low in the Lower
Kampar River system. We suggest focusing future survey ef-
forts on the upper reaches of the Kerumutan River, includ-
ing a tributary suggested by local people, known as the Eka
River (Fig. ). The Eka River has never been surveyed but
may hold higher numbers of T. schlegelii than other sections
of the lower Kerumutan River. We also suggest the upper
reaches of the Serkap River, near Tasik Metas Reserve,
and the upper reaches of the Kutup River be surveyed
(Fig. ). Given time limitations and the logistical challenges
involved we could not survey the far upper reaches of those
tributaries, which could hold T. schlegelii populations (based
on remoteness and habitat characteristics). The Air Hitam
Laut River still has a relatively high density of T. schlegelii
and should be considered to be critical to the persistence
of the species in Sumatra. The upper Air Hitam Laut
River, particularly the Simpang T tributary, had the highest
densities of T. schlegelii recorded anywhere in Berbak
National Park, and these remote tributaries require contin-
ued protection. Overall, we recorded T. schlegelii densities
similar to those found in previous surveys conducted on
the main Air Hitam Laut River and lower Simpang
Melaka Creek (Bezuijen et al., ,,,).
Tomistoma schlegelii density in Simpang T tributary was
considerably higher than densities recorded in any other
tributaries of Air Hitam Laut River in the past.
As we spent only a single day and night on both the Lalan
and Simpang Kanan Rivers, we suggest those areas be tar-
geted for future surveys. Although we recorded no crocodil-
ian activity on these rivers, high population densities of
T. schlegelii have been reported for the Lalan and Merang
Rivers in the past (Bezuijen et al., ,,,).
Although this study was the first to survey the Simpang
Kanan River, its habitat characteristics and proximity to
the Lower Kampar River suggest it may be an important
river for both C. porosus and T. schlegelii populations, and
other neighbouring rivers and sections of mangrove forest
may also be important areas for future survey efforts
(Shaney et al., ).
Our models identified a maximum of ,. km
of
remaining suitable habitat and a minimum of , km
within remaining primary forest areas across the range of
T. schlegelii (, km
suitable habitat remain in
Sumatra, ,. km
within primary forest). Although
T. schlegelii has been found in disturbed habitat previously
(Bezuijen et al., ; Stuebing et al., ), we found the
species only in primary forest habitat (Table ). Therefore,
we believe our maximum model estimates could be an over-
estimation of remaining suitable habitat in the Greater
Sunda region (particularly in Sumatra). Furthermore, our
quantification included all modelled habitat with probabil-
ity of occurrence ..%, which is a conservative estimate.
Regardless, based on severely fragmented populations, low
local population density, when present, and severe habitat
fragmentation, we believe that T. schlegelii may be locally
TABLE 3 Statistical model outcomes after testing for effects of re-
moteness and fish-trap density on crocodilian counts, with sample
sizes (N) and P values. Data are separated according to whether
sightings were confirmed or probable, and by age class. Bold font
signifies statistically significant values.
Model data N P (Remoteness)
P (Fish-trap
density)
Confirmed T. schlegelii 8,0.01 ,0.01
Probable T. schlegelii 15 0.01 0.01
Confirmed T. schlegelii*200.01 0.01
Probable T. schlegelii*22,0.01 ,0.01
Confirmed C. porosus 32 0.01 0.1
Probable C. porosus 41 ,0.01 0.03
Confirmed C. porosus*330.01 0.112
Probable C. porosus*42,0.01 0.04
Hatchling crocodiles
(species combined)
38 0.02 0.05
Juvenile crocodiles
(species combined)
18 0.695
Adult crocodiles (species
combined)
10 0.02
*Partitioned data with daytime sign included in counts
TABLE 4 Species habitat preferences across all study areas, with
numbers in bold indicating a preference of C. porosus for second-
ary forest and of T. schlegelii for primary forest.
Species
Primary
forest (N)
Secondary
forest (N)
Mangrove
forest (N)
No forest
(N)
C. porosus
confirmed
523 10
C. porosus
probable
932 10
C. porosus
confirmed*
524 10
C. porosus
probable*
933 10
T. schlegelii
confirmed
10 00 0
T. schlegelii
probable
14 00 0
T. schlegelii
confirmed*
13 00 0
T. schlegelii
probable*
21 00 0
*Partitioned data with daytime sign included in counts
578 K. J. Shaney et al.
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Endangered in Sumatra. We cannot yet assess extent and
area of occurrence or make a range-wide assessment with-
out continued work on the island of Borneo and we acknow-
ledge that T. schlegelii may inhabit some areas that were not
modelled in our analyses. Future species distribution models
would benefit from additional data on the movement ecol-
ogy of T. schlegelii. Whether or not T. schlegelii commonly
inhabits other kinds of habitat or is capable of making ocean
crossings between freshwater systems requires further inves-
tigation. Such studies could also continue to fine tune habi-
tat suitability models. However, our species distribution
modelling approach indicated specific areas in Borneo and
Sumatra that should be considered for future population
surveys, and showed limited remaining suitable habitat
across the species’range (Fig. ).
Rödder et al. () used a similar modelling approach to
study T. schlegelii and identified similar key areas for popu-
lation viability. Our findings suggest that many of those
same locations remain important for T. schlegelii conserva-
tion. Based on our models, key areas that may be important
for T. schlegelii populations in Sumatra include the Air
Hitam Laut River system in Jambi; the Merang River system
in South Sumatra; the Lower Kampar River system and areas
surrounding the Simpang Kanan River system, Bukit Batu
and Giam Siak Kecil reserves and the Rokan River in
Riau. In Borneo, key areas include the lower and upper
Kapuas River in West Kalimantan, Tanjung Puting and
Sebangua National Parks in Central Kalimantan, Lake
Mesangat in East Kalimantan, and the Labi Forest Reserve
area along the Brunei–Sarawak border, including the
Belait River and other tributaries (Fig. ). Lake Mesangat
is one of the only known locations in Borneo to also hold
populations of the Critically Endangered Crocodylus sia-
mensis (Stuebing et al., ).
Reserve expansion
As there is still intact habitat around the Lower Kampar
River and Berbak National Park study areas, we suggest ex-
panding reserves to protect the remaining lowland habitat.
There are large areas of primary and secondary forest be-
tween Berbak and Sembilang National Parks, as well as be-
tween several reserves on the north bank of the Lower
Kampar River (Fig. ). We acknowledge that this may be dif-
ficult to accomplish. However, if attempts are not made to
initiate expansion of protected areas, remaining habitat will
be lost to continued habitat alteration across Sumatra’s low-
lands. Proposals to the Balai Konservasi Sumber Daya Alam
and the Indonesian Institute of Sciences are required first
steps, and involvement from stakeholders such as the vari-
ous IUCN specialist groups could aid significantly in this
process. Rödder et al. () also suggested other important
areas for reserve expansion, particularly in the lowlands of
West Kalimantan, which include most of the areas high-
lighted in our modelling results. Given the rapid and on-
going forest conversion and degradation across the
Greater Sunda region, reserve expansion in suitable habitat
for T. schlegelii would also benefit other threatened species,
including Sumatran rhinoceroses Dicerorhinus sumatrensis,
elephants Elephas maximus, tigers Panthera tigris sumatrae,
and clouded leopards Neofelis nebulosa.
Berbak National Park exemplifies the importance of pro-
tected areas in Sumatra’s lowlands, as evidenced by our sur-
vey data. The Park currently protects some of the last
lowland swamp forest in Sumatra, as well as some of the is-
land’s last remaining populations of Sumatran tigers, false
gharials and tapirs Tapirus indicus. The Park has been
well managed to date, and if a similar approach were used
in protected area expansion there could be a chance to
FIG. 2 Suitable habitat for Tomistoma
schlegelii in the Greater Sunda region
is modelled in white (Model ).
Numbers signify the distinct areas
that were found to have suitable
habitat for T. schlegelii in the model.
Human impacts on crocodilians 579
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protect more of Sumatra’s lowland biodiversity.
Nonetheless, limited management resources are still a chal-
lenge for the Park, and illegal logging activity occurs at its
periphery. It is a constant struggle to protect the remaining
biodiversity within the Park’s boundaries, and continued
support from the Indonesian government, NGOs, and the
public is critical to the future of the Park.
Acknowledgements
We are grateful to representatives of the Indonesian
Institute of Sciences at the Museum Zoologicum
Bogoriense for facilitating the study of crocodiles in
Indonesia, as well as field research permits, and to the
Ministry of Research and Technology of the Republic of
Indonesia for coordinating and granting research permis-
sions. The Forestry Department of Indonesia kindly pro-
vided research permits for areas under its jurisdiction. We
thank the local communities for their support, advice and
kindness during our travels; the members of the field expe-
ditions throughout Sumatra, especially Wayhu Trilaksono;
directors and officials at Berbak National Park for their pa-
tience and assistance; Agustinus Rante Lembang, Pak Dodi
Kurniawan, Pak Erwan, Riziko, Rini, Sismanto, Imron and
Rizal Andi Saputra for their help and support; and Pak
Kasno, without whom travel during the dry season would
not have been possible. We thank Alias, Bagas, Ferry and
Muslimin in Berbak National Park, and Khairil, Suandi
and Udi in the Lower Kampar River study area; and Bruce
Shwedick, Mark Bezuijen, Robert Stuebing and Agata
Staniewicz for their advice and assistance. This project
was funded by the IUCN Crocodile Specialist Group
(Tomistoma Task Force) and National Geographic. Many
people at the University of Texas at Arlington contributed
significant advice and feedback, including Elijah Wostl,
David Sanchez and Utpal Smart.
Author contributions
KJS obtained research permits, led the field work, conducted
analyses and contributed to the article. AH assisted with
permitting and field work, and contributed to the article.
MW assisted with statistical analyses and contributed to
the article. EA and AA assisted with field work and contrib-
uted to the article. ENS assisted with funding, permitting
and reviewing the article.
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Biographical sketches
KYLE SHANEY is interested in biogeography, conservation biology,
spatial ecology and human–wildlife conflict. His work focuses on
both game and non-game species, with an overall goal of improving
wildlife management and preservation of biodiversity. AMIR
HAMIDY and EVY ARIDA are interested in discovering and managing
herpetofaunal biodiversity. MATT WALSH has expertise in ecology and
statistics. AISYAH ARIMBI is focused on improving conservation strat-
egies in Indonesia. E RIC SMITH has a particular interest in discovering
tropical reptile and amphibian diversity and biogeographical patterns.
Human impacts on crocodilians 581
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