THE EFFECTS OF ILLEGAL LOGGING ON THE POPULATION. OF ORANGUTAN IN THE SEBANGAU TROPICAL PEAT SWAMP FOREST, CENTRAL
KALIMANTAN
Husson, S.J.1, Morrogh-Bernard, H.C.2, McLardy, C.S. 3, Driscoll, R.4, Fear, N.3, Page, S.E.5
1
2 Chartfield, Hove, East Sussex, United Kingdom (simon_husson@yahoo.com)
2
38 Ashton Road, Siddington, Cirencester, United Kingdom
3
72 Meadow Road, Wolston, Coventry, United Kingdom
4
The Bungalow, Shipley Country Park, Slack Lane, Heanor, Derbyshire, United Kingdom
5
Department of Geography, University of Leicester, Leicester, United Kingdom
Published in: Rieley J. O. & Page S. E. (eds.) Peatlands for People: Natural Resource
Functions and Sustainable Management. Proceedings of the International Symposium on
Tropical Peatland, 22-23rd August 2001, Jakarta, Indonesia
SUMMARY
Bornean orang-utan (Pongo pygmaeus) nest count surveys carried out in the Sungai Sebangau
catchment, Central Kalimantan, Indonesia have identified the presence of a large, globally
important sub-population of these great apes and thus demonstrate the importance of tropical
peat swamp forest for orang-utan conservation. The total population of the western Sebangau
catchment is estimated to exceed 6000 individuals, and is almost certainly the largest single
population of this species remaining in the world. This area has no protective status and is
subject to extreme levels of illegal logging. Illegal logging causes habitat destruction, provides
opportunities for poachers and extraction canals threaten the long-term stability of the peatland
ecosystem. This activity is shown to result in a large shift in orang-utan distribution, away
from logging operations and between areas of different habitat sub-type, often into areas of
sub-optimal habitat. Overcrowding occurs, which leads to increasing stress, juvenile mortality
and decreasing fecundity. Action is urgently needed to control illegal logging and habitat
conversion within the Sebangau catchment. Without firm commitment to address these threats,
the future of the Sebangau population remains uncertain.
Keywords: Orang utan, peat swamp forest, illegal logging,
INTRODUCTION
The population of the Bornean orang-utan (Pongo pygmaeus) is estimated to be between
10,200 and 15,500 individuals (Rijksen et al, 1995), although current knowledge about the
distribution, population size and conservation status is incomplete. The uncertainty
surrounding total population figures arises from an incomplete knowledge of species
distribution, combined with recent evidence suggesting that their population numbers are
undergoing continuous decline. Much of their habitat is disappearing owing to clearance for
agriculture and settlements. Fires in 1997/98 destroyed large areas of suitable orang-utan
habitat. In the remaining forested areas logging operations are reducing orang-utan densities
and illegal logging is rife throughout Kalimantan. This habitat-loss and degradation, combined
with hunting and capture for the pet trade, has increased the vulnerability of the orang-utan,
the wild population of which has declined by up to 50% in the last decade alone
(Environmental Investigation Agency (EIA), 1998). It is classified as ‘endangered’ by the
World Conservation Union (IUCN) and faces a high probability of extinction in the near
future.
Formerly distributed throughout the island of Borneo, the population is now heavily
fragmented and it has been estimated that only populations of 2000 individuals or more are
sustainable (Sugardjito & van Schaik, 1991), although even this figure may be an overestimate
(Yeager, 1999). The 1993 Orang-utan Population, Habitat and Viability Analysis (PHVA)
workshop identified only three protected areas in Kalimantan that met this requirement, but it
is suggested that significant populations may exist in the extensive unprotected peat swamp
forests of Central Kalimantan (Meijaard, 1997; Page et al, 1997). A 1996 study identified the
global importance of one such peat swamp forest, the Sebangau catchment, and suggested a
total population exceeding 6000 individuals (Morrogh-Bernard et al, unpubl.). Peat swamp
forests are a major global store of carbon, regulate hydrology over vast areas, provide valuable
sustainable forest products and contain a high biological diversity (Prentice & Parish, 1992;
Rieley et al, 1997). Illegal logging is widespread within this habitat in Central Kalimantan,
however, and the construction of timber extraction canals cause peat to dry out, thereby
promoting drainage and increasing the risk of fire. Illegal logging activity is also expected to
have an effect on orang-utan distribution and abundance. This study has examined the
movement of orang-utans following the commencement of illegal logging activities in the
northern Sebangau catchment.
METHODS
Survey Location
The study was undertaken in the Natural Laboratory of peat swamp forest, a 500km 2 semiprotected area located in the northern Sebangau catchment 15km south of the provincial
capital of Central Kalimantan, Palangkaraya. The Sebangau catchment covers an area of 6000
km2 and is predominately covered in tropical peat swamp forest. Four distinct habitat types
have been described here: sedge swamp (up to 1km from the Sebangau river); mixed swamp
forest (km1-km6); low pole forest (km6-km11); and tall interior forest (km12 onwards)
(Shepherd et al, 1997; Page et al, 1999).
The entire catchment is designated as production forest. Timber concessions within the region
ceased in 1997 to be allowed to regenerate for a period of 30 years. Since their departure the
area has been subject to extreme levels of illegal logging, which do not follow guidelines
regulating locations and sizes of extracted timber.
Field Survey
Orang-utan nest densities were estimated using counts along line transects (van Schaik et al,
1995). Permanent transects were cut in the Natural Laboratory study area and surveyed during
July-September 1999, then extended and resurveyed during July-September 2000. Densities
were estimated separately for each of the forest sub-types, with the exception of the mixed
swamp forest, which was divided into two regions: a) 1-3km and b) 4-6km from the Sebangau
river. This division was made as illegal logging was found to be most prevalent in the area
proximal to the river during 1999.
Observers walked slowly along the transects and, using standardised methods (van Schaik et
al, 1995), recorded all the nests encountered. The following information was recorded:






Length of transect
Forest sub-type
Perpendicular transect-to-nest distance in the horizontal plane for every nest observed
Decay stage of nest, in four classes:
A = fresh; leaves still green
B = older; nest still in original shape, firm and solid; leaves may be still attached but
brown
C = old; most leaves gone, holes appearing in nest
D = very old; twigs and branches still present, but no longer in original shape.
Disturbance level: no. of extraction skids and canals per kilometre.
Number of cut tree-stumps
Data Analysis
Orang-utan nest densities for each habitat type were calculated using the software program
DISTANCE 3.5 (Thomas et al., 1998). This automated technique uses distance sampling data
(in this case data on total transect length, number of nests and the perpendicular distance of
each nest from the transect) to estimate density and is reliable where transect lengths are
accurately estimated (Cassey & McArdle, 1999).
Nest densities are then converted to orang-utan densities using three known parameters:



p, proportion of nest makers in the population
r, rate at which nests are produced
t, time a nest remains visible
Orang-utan density d is calculated as :
N
/ (p x r x t) where N = nest density.
Estimating the parameters
Converting nest densities to orang-utan densities requires the knowledge of three parameters,
the nest building rate ‘r’, the proportion of nest-makers in the population ‘p’ and the nest
degradation time ‘t’. Other studies have attempted to estimate r and p. This was not possible in
our survey, so figures obtained elsewhere were used. Rijksen (1978) gave a value of 1.8 for ‘r’
estimated at Ketambe, Sumatra. Other estimated values of 1.7 and 1.6 were quoted by van
Schaik et al (1995), who decided to use the median value 1.7. These studies were carried out
in Sumatra and there may be differences in the Bornean species. No published data exists for
Kalimantan, although Galdikas (pers. comm.) reports an approximate value for ‘r’ in Tanjung
Puting National Park as 1.0 and Lacreman-Ancrenaz (Memo on the orang-utans in Sabah,
unpublished report) reports a figure of 0.9 in the Kinabatangan Wildlife Sanctuary, Sabah,
Malaysia based on extensive field surveys. Incidences of day-nest construction appear to be
considerably fewer within the Bornean species. In the light of this, 1.0 nests day-1 individual –1
has been used for ‘r’ in this study.
The above Sumatran studies estimate that approximately 10% of the population consists of
infants. This value is not expected to differ significantly between Sumatran and Bornean
populations, therefore the value for ‘p’ is 0.9 (van Schaik et al, 1995). Duration of nest
visibility ‘t’ is the most likely parameter to vary between habitat types and survey sites and
must therefore be estimated separately for each site. To estimate this, a technique was
employed that relies on re-recording the decay stage of nests of known initial state of decay
(see van Schaik et al, 1995; Morrogh-Bernard et al, unpubl. for methodology). In order to
calculate this variable, 53 nests in mixed swamp forest were marked on the initial survey and
revisited after a 90-day period. On each of these repeat visits the age of each nest was
reassessed with the additional nest age class, ‘gone’, added. It was not possible to repeat this
assessment in the low pole or tall interior forest sub-types owing to limitations on survey time.
This study was undertaken during August-November 1999.
Results from other surveys indicate a large variation in this figure. Rijksen (1978) estimated
this value to be 81 days and Djojosudharmo (cited in van Schaik et al, 1995) estimated the
value to be 90 days. Both studies were carried out at Ketambe, Sumatra. In contrast Singleton
(unpubl.) estimated the value to be 228 days in swamp habitats in Suaq Balimbing, Sumatra.
For Borneo, Lackreman-Ancrenaz (unpubl.) estimated this value to be approximately 250 days
in Sabah, whilst two studies using the matrix method reported values of 145 days in Danau
Sentarum (Russon, 2001) and 217 days in the Sebangau region (Morrogh-Bernard et al,
unpubl.). Van Schaik et al (1995) noted that their figure for t at Ketambe, Sumatra,
overestimated the true figure by approximately 30%, an inherent bias in the method. Thus we
have converted our figure by the same proportion, producing a ‘t’ value of 274 days.
RESULTS
Nest densities and associated orang-utan densities were calculated for four main areas within
the Natural Laboratory study area during 1999-2000. Parameters used in the estimation of
orang-utan densities are given in Table 1. These are directly comparable with densities
estimated in the same region during 1996 (Table 2.) The mixed swamp forest sub-type
supports the bulk of the orang-utan population. The 1996 observed density of 2.99 ind km-2 is
high and suggests a large population of orang-utans in this habitat-type (Morrogh-Bernard et
al, unpubl.). Illegal logging activities were concentrated here during 1998-1999, particularly in
the region 1-3km from the Sebangau river. Densities here decreased by over 300% between
1996 and 1999. Between 4 and 6km from the river the observed density is substantially higher.
This is consistent with orang-utans moving away from logging operations into the forest
interior.
Between 1999 and 2000 orang-utan density increased in the area nearest to the river. During
the same time-period illegal logging activities spread throughout the mixed swamp forest subtype and this change in observed orang-utan density indicates redistribution of orang-utans
throughout the mixed swamp forest. Densities in the tall interior forest are slightly lower in
1999 and substantially lower in 2000 than 1996, indicating displacement of orang-utans firstly
by fire in 1997/98 and subsequently by illegal logging activity which peaked in this area in
2000. Densities in the low pole forest are much higher than expected in 1999 and 2000,
indicating movement of orang-utans from the preferred mixed swamp and tall interior forest
sub-types into this sub-optimal habitat, which remains unlogged.
To fully demonstrate the shift in distribution caused by illegal logging, nest sighting frequency
was calculated for each transect and plotted against distance from the Sebangau river. Nest
sighting frequencies are approximately proportional to orang-utan densities, although nest
sighting ability does vary between habitat sub-types. Figure 1 shows the resulting graph for
1999, while Figure 2 shows 2000 results. Figure 3 shows the predicted nest sighting frequency
based on orang-utan densities obtained by Morrogh-Bernard et al, (unpubl.) during 1995/96.
Three major differences are apparent between nest sighting frequencies in 1999/2000 and
those predicted based on surveys in 1995/96: 1. Sighting frequency in mixed swamp forest during 1999 increases with increased distance
from Sg. Sebangau. Over the nine transects surveyed in mixed swamp forest there is a
highly significant correlation between the nest sighting frequency (no. of nests/km) and
distance from the river. (Spearmans Rank, rs = 0.95, n = 9, p<0.005).
2. Overall, sighting frequency in mixed swamp forest is lower than expected from previous
surveys.
3. Sighting frequency in low pole forest is higher than expected from previous surveys.
Figure 2 also demonstrates a marked peak in orang-utan distribution at 6km from the
Sebangau river. This is the boundary between the mixed swamp and low pole forest sub-types
and is the limit of logging in this habitat sub-type.
Illegal logging disturbance has a strongly negative effect on orang-utan density. There is a
significant correlation between the number of nests per kilometre and number of recently
constructed extraction skids and canals per kilometre in the mixed swamp forest (Spearman’s
Rank, rs = -0.83, df=8, p<0.05). A significant difference between orang-utan density in areas
with high and low numbers of felled tree stumps was also discovered (Chi-Squared, R2 = 0.83,
p<0.1).
DISCUSSION
Orang-utan densities in each of the three principal forest sub-types at the Natural Laboratory
have been estimated since 1996 and have altered substantially over this period. This change
has coincided with the commencement of illegal logging activity that has led to the shift in
distribution witnessed. Low densities near to the river may be attributed principally to a
decline in orang-utan numbers owing to intensive human activity in the region. Orang-utans
are known to move away from human activity (MacKinnon, 1990) and have moved
southwards from the Sebangau River into the forest at greater distances from the river. In
addition, the level of hunting of orang-utan is likely to have risen since official logging ended
in 1997. This may affect females more, as they have smaller, static home ranges and are thus
more vulnerable than males. Finally, some orang-utans will have died of natural causes
following forest degradation. The prolonged drought of 1997/98 may have hastened this
effect.
Sighting frequencies peak at km6, this is especially notable during the 2000 surveys. This is
the boundary between mixed swamp forest, the orang-utans principal habitat in peat swamp
forest; and low pole forest, a sub-optimal habitat naturally supporting a low density of orangutans. This is a probable result of females retreating to the limit of their home ranges. These
ranges are unlikely to extend far into the low pole forest.
A higher orang-utan density in the low pole forest sub-type indicates that many orang-utans,
probably males, have moved here to escape from logging. Males are more mobile and their
home ranges are larger than females, enabling them to retreat away from logging disturbance
into the low-pole forest. They can survive here, as they are not as reliant on high food
abundance as are females, which are usually accompanied by infants. These movements will
have caused stresses in the population, resulting in depressed fecundity and breeding rates.
Carrying capacities will have fallen following illegal logging, and overcrowding is likely to be
occurring.
If this distribution does not return to normal, it is clear that a substantial decline in numbers
will result. If illegal logging is stopped, the population will show a smaller decline consistent
with habitat degradation. However, illegal logging has become so rampant in this province
that regeneration of selectively logged forest may be unlikely (Rijksen & Meijaard, 1999).
Even in a selectively logged state it is clear that the Sebangau catchment supports a large and
globally important population of orang-utans, and it is crucial that the government does not
yield control of this area for uses other than forest management if this population is to be
maintained.
ACKNOWLEDGEMENT
This study was part funded through facilities provided as part of the EU INCO-DC Project
Contract Number ERB18CT980260.
REFERENCES
Cassey, P., MacArdle, B.H., (1999). An assessment of distance sampling techniques for
estimating animal abundance. Environmetrics, 10, 261-272.
Kleden, H.Y., Gaban, F. & Wicaksono (1999) Investigasi: Lahun Gambut Sejuta Nista Tempo,
Jakarta, 12 April
MacKinnon, J.R. (1990) Species Survival Plan for the Orang-utan in Borneo. In: G. Ismail,
M.Mohammed & S.Omar (eds.) Proceedings of the International Conference on
Forest Biology and Conservation in Borneo. Yayasan Sabah, Kota Kinabalu, Sabah pp
209-219
Meijaard, E. (1997) The importance of swamp forest for the conservation of the orang utan
(Pongo pygmaeus) in Kalimantan, Indonesia. In: J.O Rieley and S.E. Page (eds.)
Biodiversity and Sustainability of Tropical Peatlands. Samara Publishing, Cardigan,
UK. pp. 243-254.
Page, S.E., Rieley, J.O., Doody, K., Hodgson, S., Husson, S., Jenkins, P., Morrogh-Bernard,
H., Otway, S. & Wilshaw, S. (1997) Biodiversity of tropical peat swamp forest: A case
study of animal diversity in the Sungai Sebangau catchment of Central Kalimantan,
Indonesia In: J.O Rieley and S.E. Page (eds.) Biodiversity and Sustainability of
Tropical Peatlands. Samara Publishing, Cardigan, UK. pp. 231-242.
Page, S.E., Rieley, J.O., Shotyk, W. & Weiss, D. (1999) Interdependence of peat and
vegetation in a tropical peat swamp forest. Philosophical Transactions of the Royal
Society, Series B., 354, 1885-1897.
Rieley, J.O., Page, S.E., Suwido, H.L. & Winarti, S. (1997) The peatland resources of
Indonesia and the Kalimantan Peat Swamp Forest Research Project. In: J.O Rieley and
S.E. Page (eds.) Biodiversity and Sustainability of Tropical Peatlands. Samara
Publishing, Cardigan, UK. pp 37-44
Rijksen, H.D & Meijaard, E. (1999) Our Vanishing Relative: The status of wild orang-utans at
the close of the twentieth century. Kluwer Academic Publishers, Dordrecht.
Rijksen, H. D., Ramono, W., Sugardjito, J., Lelana, A., Leighton, M., Keresh, W., Shapiro, G.,
Seal. U. S., Traylor-Holzer, K. & Tilson, R. (1995) Estimates of orang-utan
distribution and status in Borneo. In: The Neglected Ape. Nadler, R. D. Galdikas, B. F.
M. Sheeran, L. K. & Rosen, N. (ed.). Plenum Press. New York & London, pp. 117122.
Russon, A.E., Erman, A. & Dennis, R. (2001) The population and distribution of orang-utans
(Pongo pygmaeus pygmaeus) in and around the Danau Sentarum Wildlife Reserve,
West Kalimantan, Indonesia. Biological Conservation 97, pp 21-28
van Schaik. C. P., Azwar. & Priatna, D. (1995). Population estimates and habitat preferences
of the orang-utan based on line transects of nests. In: The Neglected Ape. Nadler, R. D.
Galdikas, B. F. M. Sheeran, L. K. & Rosen, N. (ed.). Plenum Press. New York & London, pp.
129-147.
Shepherd, P.A., Rieley, J.O., Page, S.E., (1997). The relationship between forest structure and
peat characteristics in the upper catchment of the Sungei Sebangau, Central
Kalimantan. In: Rieley, J.O., Page, S.E. (Eds.), Biodiversity and Sustainability of
Tropical Peatlands. Samara Publishing, Cardigan, U.K., pp. 191-210.
Sugardjito, J. & van Schaik, C.P. (1991) Orang-utans: current population status, threats and
conservation measures. In: Proceedings of the Great Ape Conference, December 1522, 1991, Indonesia
Thomas, L., Laake, J.L., Derry, J.F., Buckland, S.T., Borchers, D.L., Anderson, D.R.,
Burnham, K.P., Strindberg, S., Hedley, S.L., Burt, M.L., Marques, F., Pollard, J.H. &
Fewster, R.M. (1998). Distance 3.5. Research Unit for Wildlife Population
Assessment, University of St. Andrews, UK.
Yeager, C. (Ed.), (1999). Orangutan Action Plan. WWF Indonesia.
Table 1 – Parameters used in orang-utan density estimates
(MSF = Mixed Swamp Forest)
Parameter
Region
Value
1999
Value
2000
Number of nests
MSF1-3
19
75
MSF 4-6
Low Pole
Tall Interior
53
41
122
73
85
61
Total nest count minus
outliers (>40m from transect)
“
“
“
Length of transects (km)
MSF1-3
MSF 4-6
Low Pole
Tall Interior
5.6
4.3
3.2
6.1
7.8
6.0
9.2
6.0
Measured distance
“
“
“
Effective strip width (m)
MSF1-3
28.90
27.94
MSF 4-6
Low Pole
Tall Interior
18.61
21.83
33.60
19.52
18.48
20.32
0.9
1.0
0.9
1.0
274
274
Proportion of nest builders ‘p’
Nest construction rate ‘ r’ (nests
day-1 ind-1)
Nest decay rate ‘t’ (days)
Comments
survey-based DISTANCE
estimate
“
“
“
van Schaik et al, 1995
unpublished estimates from
Borneo (see text)
Survey-based Markov model
estimate
Table 2. – Nest and orang-utan densities at the Natural Laboratory 1996-2000 ( S.E.)
Densities from 1996 taken from Morrogh-Bernard et al, unpubl.
(NB Morrogh-Bernard et al (unpubl.) estimated nest degradation time to be 217 days)
Region
& habitat subtype
Nest
Density
1996
(no/km2)
Mixed Swamp
(1-3km from
river)
Mixed Swamp
(4-6km from
river)
599 ( 78)
Low Pole
284 ( 59)
Tall Interior
636 ( 133)
OU
Density
1996
(ind/km2)
Nest
Density
1999
(no/km2)
OU
Density
1999
(ind/km2)
Nest
Density
2000
(no/km2)
OU
Density
2000
(ind/km2)
115 ( 46)
0.46 (
0.19)
344 ( 60)
1.39 (
0.24)
603 ( 157)
2.45 (
0.63)
623 ( 273)
2.53 (
1.11)
2.99 (
0.39)
1.41 (
0.29)
3.18 (
0.66)
501 ( 94)
618 ( 98)
2.04 (
0.37)
2.50 (
0.39)
558 ( 113)
339 ( 82)
2.26 (
0.46)
1.56 (
0.37)
Figure 1
Nests/km
Nest Sighting Frequency v. dist.
from river at Natural Laboratory
1999
30
25
20
15
10
5
0
0
2
4
6
8
10
12
14
Distance from Sungai Sebangau (km)
Figure 2
Nests/km
Nest Sighting Frequency v. dist.
from river at Natural Laboratory
2000
30
25
20
15
10
5
0
0
2
4
6
8
10
12
14
Distance from Sungai Sebangau (km)
Figure 3
Predicted Sighting Frequency v.
dist. from river
30
Nests/km
25
20
15
10
5
0
0
2
4
6
8
10
12
Distance from Sungai Sebangau (km)
14