evaluating the effectiveness of human–orangutan conflict mitigation strategies in sumatra

Evaluating the effectiveness of human–orangutan

conflict mitigation strategies in Sumatra

Gail Campbell-Smith1*, Rabin Sembiring2 and Matthew Linkie1,3

1Durrell Institute of Conservation and Ecology, University of Kent, Canterbury, Kent CT2 7NR, UK;2Human-Orangutan Conflict and Mitigation Programme, Orangutan Information Centre, Medan, North Sumatra,

Indonesia; and 3Fauna & Flora International, Jupiter House, Station Road, Cambridge CB1 2JD, UK

Summary

1. Crop raiding by great apes is an emerging conservation issue across their range. It is important

because it involves highly threatened species that can cause significant economic damage and be

killed in retribution. Yet, to date, no quantitative study has sought to test possible solutions for pre-

venting this form of human–wildlife conflict.

2. From February 2007 to August 2009, we monitored crop-raiding patterns across a Sumatran

agroforest landscape to determine background levels of human–orangutan conflict. We also inten-

sively monitored a subset of 50 farms to assess changes in farmer attitudes towards orangutan man-

agement; differences between farmer reported and independently enumerated monetary loss from

crop raiding; and the effectiveness of mitigation techniques in reducing orangutan crop raiding on

35 treatment farms (25 trialling noise deterrents and 10 trialling tree nets) in comparison with 15

control farms over a pre-trial (12 month) and a trial (18 month) phase. Five months after the trials

had ended, the ongoing use or uptake of the techniques were assessed.

3. Across the wider landscape, background levels of mean daily orangutan crop-raiding incidents

per month (±SE) farms did not significantly differ between the pre-trial (9Æ1 ± 3Æ7) and trial

(7Æ1 ± 4Æ3) phases, whilst on the 35 treatment farms it reduced significantly. Furthermore, crop

yield increased (+60Æ8%) on the netted trial trees, but reduced ()27Æ4%) on the control farm trees.

Despite this, there was no subsequent use of this technique, unlike those farmers (40%) who contin-

ued using the less-effective noise deterrents.

4. Farmer participation in the project yielded unexpected and positive attitude changes, from pre-

ferring orangutan removal (pre-trial) to in situ management with crop protection measures (post-

trial). However, project participation may have increased farmer expectations of receiving compen-

sation because the treatment farmers consistently overestimated their crop losses, unlike the control

farmers who did not.

5. Synthesis and applications. Whilst human–orangutan conflicts caused substantial losses to local

livelihoods, the identification of an effective mitigation method (nets) neither guaranteed its contin-

ued use nor uptake. Developing easy to install nets for valuable tree crops is therefore recom-

mended. Nevertheless, the project intervention efforts did create benign farmer attitudes towards

orangutan management, an essential prerequisite for managing large-bodied mammals in conflict

with people.

Key-words: crop protection, crop raiding, great ape, human habitat alteration, human–wild-

life conflict, large mammal conservation, Pongo abelii

Introduction

To avert the current biodiversity extinction crisis, conservation

practitioners require a clear understanding of the prevailing

threats and the strategies to mitigate them. Whilst the threats

are usually known and primarily relate to the competition

between humans and biodiversity for limited space and

resources (Sodhi et al. 2009), the recent calls for evidence-

based conservation indicates that the appropriate solutions

are not yet fully known (Sutherland et al. 2004; Ferraro &

Pattanayak 2006).*Correspondence author. E-mail: [email protected]

Journal of Applied Ecology 2012, 49, 367–375 doi: 10.1111/j.1365-2664.2012.02109.x

� 2012 The Authors. Journal of Applied Ecology � 2012 British Ecological Society

For large-bodied mammals living in the biodiversity-rich

tropics, mitigating human–wildlife conflicts is a conservation

priority. These conflicts have been widely documented from

each tropical continent and involve problem animals such as

elephants Loxodonta africana that crop raid in Kenya (Sitati

et al. 2003; Chiyo et al. 2011), leopards Panthera pardus that

kill livestock in Pakistan (Dar et al. 2009) and jaguarsPanther-

a onca that attack humans in Brazil (Zimmermann,Walpole &

Leader-Williams 2005). Given the magnitude of this human–

wildlife conflict, it is surprising that only a few studies have

sought to identify solutions (Sitati, Walpole & Leader-

Williams 2005). An important first step is to determine the

spatio-temporal conflict patterns and assess the effectiveness of

current mitigation strategies (Naughton-Treves et al. 1998;

Linkie et al. 2007). From this, appropriate and, where

necessary, alternative conservation intervention measures

should be developed, trialled and quantitatively assessed,

whilst controlling for the influence of confounding variables,

such as distance to human settlements (Sitati et al. 2003).

An emerging form of human–wildlife conflict in the tropics

that is beginning to receive greater attention is that involving

non-human great ape species. Reports of crop raiding exist for

chimpanzees Pan troglodytes in Uganda and Guinea Republic

(Reynolds 2005; Hockings, Anderson & Matsuzawa 2009),

gorillas Gorilla beringei in Rwanda (Hockings & Humle 2009)

and orangutans (Pongo abelii andPongo pygmaeus) in Indone-

sia and Malaysia (Meijaard et al. 2010; Campbell-Smith et al.

2010, 2011a,b).

Resolving human–great ape conflict is a conservation imper-

ative because these species are amongst themost threatened on

earth; their large body size means that they can cause substan-

tial economic loss to farmers through crop raiding; their high

visibility in farms may distort the perceived damage caused;

these attributes cause people to be fearful of them; they are

killed by farmers in retribution for crop raiding (Reynolds

2006); their varied diet makes it difficult for farmers to protect

crops with a single strategy and their advanced intelligence

means that they can quickly learn how to circumvent mitiga-

tion strategies. Comprehensive guidelines for mitigating

human–great ape conflict have been proposed but remain

untested in the field, making it difficult to apply the appropri-

ate conflict mitigation techniques (Yuwono et al. 2007;

Hockings &Humle 2009).

In this study, we perform the first quantitative assessment of

farm-based mitigation techniques that aimed to prevent great

ape crop raiding, in this case an isolated population of critically

endangered Sumatran orangutans living in a mixed natural

forest–agricultural landscape. First, we determine levels of

orangutan crop raiding across the entire landscape over

30 months. Secondly, we determine the various spatial

parameters that best explain crop-raiding patterns in farms.

Thirdly, we intensively monitor 50 farms during a pre-trial

phase and a trial phase, during which the effectiveness of local

and project-introduced mitigation strategies are compared

between 35 treatment farms and 15 control farms. Fourthly,

we test farmer reported monthly monetary loss caused by

orangutan crop raiding with that recorded by independent

enumerators and whether their participation in the project

elicited concurrent reporting with the enumerators. Finally, we

measure changes in farmer attitudes towards orangutan

management and the uptake of the mitigation trial techniques

by farmers, 5 months after the trial phase ended.

Materials and methods

STUDY AREA

Fieldwork was conducted from February 2007 to August 2009 in a

3234 ha closed agroforest system in Batang Serangan region

(3�43¢58Æ99¢¢N, 98�11¢41Æ99¢¢E), North Sumatra, Indonesia (Fig. 1).

The study area is enclosed by commercial oil palm Elaeis guineensis

Jacq. plantations, human settlements (>6000 people) and a large

river, all impassable to orangutans. Inside, there is a small-scale oil

palm plantation (450 ha) and pockets of degraded natural forest that

are intermixed with c. 1350 smallholder farms (2784 ha) containing

cash crop species of oil palm and rubberHevea brasiliensis, as well as

other cash and subsistence crop species (hereafter cultivated fruits)

such as jackfruit Artocarpus integer, durian Durio zibethinus, petai

Parkia speciosa and an Asian legume called jengkol Archidendron

pauciflorum.

Previous research from this study area identified an isolated orang-

utan population consisting of 16 individuals (eight adults, five adoles-

cents and three newborn infants; Campbell-Smith et al. 2011b).

Orangutans, which are primarily frugivorous in natural forest habi-

tats, were found to crop raid agricultural fruits and eat tree bark, as a

putative fallback food resource, in this area (Campbell-Smith et al.

2011a,b). Within this area, orangutans are primarily arboreal, as they

are in natural forests, but will descend to travel on the ground where

the canopy continuity of large-sized trees is disrupted.

The nearest neighbouring orangutan population is located in the

primary forests of the 2Æ75 million ha Leuser Ecosystem, c. 25 km

away (Wich et al. 2008). From Batang Serangan, the orangutan pop-

ulation comes into daily contact with the local farmers, who have

recently called upon the Department of Forestry to remove these

orangtuans. Based on this, a project (implemented by the University

of Kent, in partnership with the Indonesian Department of Forestry

and University of North Sumatra) decided to intervene by trying to

find alternative solutions for managing the orangutans in situ whilst

reducing farmer losses from their crop-raiding forays.

DATA COLLECTION

Data were collected over two phases: pre-trial (February 2007–Janu-

ary 2008) and trial (March 2008–August 2009). For this, nine enumer-

ators, from local communities, were trained over 4 weeks in the use of

orangutan crop damage and conflict mitigation datasheets, which

were modified from those produced by the IUCN ⁄ SSC African Ele-

phant Specialist Group (Hoare 1999). The enumerators also received

botanical training in wild and cultivated species identification. The

enumerators then visited the study area of 1350 farms on a daily basis

to measure crop raiding across the entire landscape, to determine

background levels for the 30-month project duration. To achieve this,

the landscape was systematically surveyed, whereby it was divided

into two sections and farmswithin each section were randomly visited

by one of two teams.

Data on the quantity and frequency of crop raiding were collected

through firstly speaking with the farmer to record perceived damage

and then independently assessing the farm for all records of damage.

368 G. Campbell-Smith, R. Sembiring & M. Linkie

� 2012 The Authors. Journal of Applied Ecology � 2012 British Ecological Society, Journal of Applied Ecology, 49, 367–375

When a crop-raiding incident was encountered, information was

recorded on its location (using a global positioning system unit), the

tree species raided, the tree part eaten (fruit or bark) and the amount

of damage (number of individual fruits or stripped bark). For the trial

phase, the study then focussed on the main crop species that had been

raided during the pre-trial phase.

MIT IGATION TRIALS

Based on the crop-raiding results from the 12-month pre-trial phase

and farmer willingness to participate, 50 focal farms were randomly

selected from the wider landscape (to cover the varying levels of crop

raiding recorded). These farms were then assigned as a ‘treatment’ or

a ‘control’ farm. The project offered the farmers no incentives to par-

ticipate because they had already willingly agreed to work with the

project. Next, for 35 treatment farms, one of two types of mitigation

techniques was introduced by the project, with no intervention being

made on the remaining 15 control farms (Fig. 1). These techniques

were based on those with proven success for large-bodied mammals

(Sitati &Walpole 2006) that seemed appropriate for great apes, based

on their behaviour, and that could be easily replicated within this

study and used by the farmers thereafter. The techniques were the fol-

lowing:

• Noise deterrents – Two types of non-projectile firing noise

makers were employed on 25 treatment farms. Hand-held

firecracker cannons that were locally manufactured from

bamboo and tin and used calcium carbide to produce a loud

noise. These cannons were only fired if orangutans were

found crop raiding within the treatment farms (excluding the

net trial farms). Locally manufactured hand-held bamboo

drums were used by the enumerators if the firecracker can-

nons proved unsuccessful, that is, did not cause the orangu-

tan tomove off the tree being raided.

• Tree nets – To prevent orangutans from raiding fruiting

trees, barrier nets of 5 · 5 cm square mesh stitching nylon

rope were placed in the canopy of 14 separate jengkol trees on

10 treatment farms. These tree barrier nets closed off the

arboreal travel pathways of the orangutans. Sometimes a net

would enclose the entire tree canopy and at other times just

part of the tree canopy depending on the position of the focal

tree with respect to other trees nearby. For example, if a focal

fruit tree had trees surrounding it thereby facilitating orangu-

tan access from all areas, the entire canopy was enclosed. If

the focal fruit tree only had a few trees to one side of it, then

only that side of the canopy was closed off. Jengkol fruit was

selected because it is the most important cash crop according

to the farmers, after jackfruit that was found to experience

low fruiting during the trial phase.

Over an 18-month trial phase, the 50 focal farms were monitored

intensively (visited twice a week). Each time a mitigation event

Fig. 1. Location of the orangutan crop-raidingmitigation in 35 treatment and 15 control farmlands in a 3234 ha closed lowland (28–165 m a.s.l.)

agroforest system in Batang SeranganDistrict, North Sumatra, Indonesia.

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Fig. 2. Comparison of control and treatment farmer reported mone-

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andmitigation trial phase.

Mitigating human–orangutan conflict 369


occurred (either firing a cannon or an orangutan attempting to forage

from a netted tree), the enumerators collected data on length of time

to carry out the mitigation (firing duration or foraging time), type of

technique used, distance travelled by an orangutan in response, and

whether successful or not.

Data on climate, food availability (cultivated and wild), farm pro-

file, crop protection strategies (local and project-introduced), mone-

tary losses from crop raiding and mitigation use after the trials were

collected from the 50 farms and their farmers.

CLIMATE AND FOOD AVAILABIL ITY

Rainfall data were collected twice a day (06Æ00 and 18Æ00 h) using a

standard precipitation tube rain gauge at a central location

(3�43¢577¢¢N, 98�11¢455¢¢E) in the study site. On a daily basis, the

smallholder farms were systematically monitored for fruit availabil-

ity on edible tree species. Unripe and ripe cultivated and wild fruits

were recorded as ‘present’ on a farm whether fruit availability was

above 50% of the canopy covered (Campbell-Smith et al. 2011b).

These data were compiled into fruit calendars that provided infor-

mation on daily and monthly availability of wild and cultivated

fruits on farms.

FARM PROFILE

For each focal farm, data were collected on its area (ha), wild and cul-

tivated fruit abundance, distance from the nearest village, and guard-

ing techniques in use. The traditional guarding techniques used on a

crop-raiding orangutan (throwing sticks or stones and using a bark-

ing dog or shouting) were present on all farms but over the duration

of the study were inconsistently and ineffectively applied. To deter-

mine absolute abundance and density of cultivated and wild tree spe-

cies, the total number of individual adult trees were counted for the

entire farm and divided by its size (ha). Tree size [diameter at breast

height (d.b.h.)], as a proxy for arboreal travel pathways, was mea-

sured along a 50 · 2 m transect, which was randomly placed within

the farm. We then focused only on jengkol as this was the main crop

being protected by the static mitigation trials. For the 10 treatment

farmers participating in the net trials, the farmers first estimated their

yearly yield (in kg). Similarly, the 15 control farmers were also asked

to estimate their yearly jengkol yields.

MONETARY LOSS

To compare perceived (farmer reported) against observed (enumera-

tor recorded)monetary loss fromorangutan crop raiding, a 24-month

study covering the pre-trial and trial phases (February 2007–January

2008 and September 2008–August 2009, respectively) was conducted

on the damage of the five main cultivated species (jackfruit, durian,

jengkol, petai and rubber) that the farmers identified as being most

important to them. To quantify this loss, the 50 focal farmers

reported their daily monetary loss from orangutans to the enumera-

tors. This was then related to the same damage that was indepen-

dently recorded by the enumerators and converted to a monetary (£)

value based on the then market prices and currency exchange rate.

We then focused only on jengkol fruit damage as this was the main

cultivated fruit being protected by the barriermitigation trials. The 10

participating farmers first estimated the volume of damage (in kg)

that was then converted to a monetary (£) value as above. The project

enumerators then estimated the volume of damage independently for

each crop-raiding incident, which was converted into a monetary

value using the same technique.

POST-PROJECT SURVEY

Changes in attitudes of the 50 focal farmers towards orangutan man-

agement were measured before and after the mitigation trials by ask-

ing (i) are orangutans dangerous and, if so, why? and (ii) what is your

preferred solution for reducing orangutan crop raiding on your farm?

To investigate opinions towards the mitigation techniques and to

assess whether farmers would continue to use them (including uptake

by control farmers) and why, the 50 focal farmers were interviewed

5 months after the formal trials had ended (Graham & Ochieng

2008).

DATA ANALYSIS

Spatial data were mapped using ArcGIS software v.9.2 (ESRI Inc.,

Redlands CA,USA), tabulated, where required, and then exported to

spss v.16 (SPSS, Chicago, IL, USA). The continuous variables were

normalized to reduce the disproportionate influence of outliers.

To examine orangutan crop-raiding patterns, an independent

crop-raiding incident was taken as the basic unit of measurement,

defined as a single crop-raiding event by an orangutan on the same

farm on the same day, irrespective of whether it raided more than

once (Naughton-Treves 1998). To compare crop raiding in the non-

focal farms over two project phases (12 months during pre-trial and

10 of the 18 trial months), which occurred over an unequal number of

months and also betweenmonths with a different number of days, the

mean number of daily crop-raiding incidents per month and per farm

was calculated (referred to hereafter as crop-raiding frequency).

Linear regression models were used to determine which combina-

tion of spatial factors, during the pre-trial and trial phases, best

explained overall crop-raiding frequency on each of the 50 focal

farms. Prior to this, a Pearson’s rank correlation coefficient (r) test

was performed to identify non-independence between factors. From

this, the independent factors included within the statistical models, in

various combinations, were farm size, mean distance to nearest

village, abundance of wild and cultivated tree species, wild and

cultivated tree size with d.b.h. ‡ 25 cm and use (presence ⁄ absence) oftraditional guarding. This method was repeated for the trial phase.

Linear regression models were used to test the additional relationship

between project-introduced mitigation methods (measured by their

presence or absence) and crop-raiding frequency on the 50 focal

farms.

The performance of the regression models was evaluated based on

their delta Akaike Information Criterion (DAIC) values, Akaike

weights (wi) and adjustedR2 values. In the spatial analysis, it was nec-

essary to test for non-independence caused by spatial autocorrelation

as farms close to each other may have shared similar characteristics.

The presence of spatial autocorrelation within the final models was

tested by calculating Moran’s I statistic adjusted for small distances

within Crime-Stat Software v.3.2 (N. Levine and Associates,

Annadale, VA,USA).

To compare control and treatment farm characteristics, manova

and t-tests were performed on the following variables: farm size, dis-

tance to the nearest village, use of traditional guarding methods,

abundance of large trees (d.b.h. ‡ 25-cm abundance) and key tree

species (jackfruit, jengkol, rubber, durian and petai), cultivated fruits

and crop-raiding levels during the pre-trial phase.

To test the effectiveness of the tree barrier nets, the crop yields (kg)

for treatment (n = 10 farms, 14 focal jengkol trees) and control

(n = 15 farms, 12 jengkol trees) during the pre-trial and trial phases

were measured and the effect of the nets measured (t-test). Next,

Wilcoxon signed-rank test was used to examine whether there was a



change in crop-raiding frequency over each of the two phases on both

treatment and control farms and the effectiveness of the mitigation

techniques and to examine perceived farmer crop loss with enumer-

ated crop loss for the 15 control farms and 35 treatment farms over

each of the two phases. To investigate how often each farm was re-

raided, crop-raiding intervals (number of days) per monthwere calcu-

lated for both the 15 control and the 35 treatment farms over each of

the two phases.

Next, to investigate the relationship between farmer perceptions

of orangutans and their conservation management, chi-squared

tests were performed both before and after mitigation trials.

Finally, the ongoing use and uptake of farm-based deterrents and

perceptions of effectiveness were analysed as the percentage of

treatment farmers that were still using project-introduced mitiga-

tion techniques 5 months after the study trial period had ended

and the percentage of control farmers that were now independently

using these techniques.

Results

BACKGROUND RATES OF CROP RAIDING

Across the entire study area, a total of 801 independent crop-

raiding incidents (frequency ± SE: 26Æ4 ± 5Æ2) were recordedby the enumerators on 179 farms during the pre-trial phase.

From the 12 cultivated fruit species recorded as raided

(Table 1), 4569 individual fruits and tree bark (mean daily fruit

volume per month ± SE: 124Æ2 ± 72Æ7 and bark:

26Æ5 ± 10Æ5) were damaged. Most (73%) incidents involved

consumption of five species that the farmers also reported as

their most important cultivated species: jackfruit (38%), jeng-

kol (15%), rubber (13%), durian (4%) and petai (3%). The

trial phase focused on monitoring these five crop species and

enumerators recorded 667 independent crop-raiding incidents

(frequency ± SE = 21Æ9 ± 4Æ1) that damaged 3604 individ-

ual crops and tree bark (mean daily volume per month ± SE;

fruits: 92Æ8 ± 32Æ3 and bark: 25Æ5 ± 6Æ8), on 143 farms (448

independent crop-raiding incidents in the 50 focal farms and

219 independent crop-raiding incidents in the 93 non-focal

farms). Excluding the 50 focal farms from both the 179 farms

in the pre-trial phase (n = 129 non-focal ⁄non-treatment

farms) and the 143 farms in the trial phase (n = 93 non-focal ⁄ -non-treatment farms), levels of mean daily orangutan crop-

raiding incidents per month (±SE) for the five main cultivated

fruits in 222 non-focal ⁄non-treatment farms did not signifi-

cantly differ between the pre-trial (9Æ1 ± 3Æ7) and trial

(7Æ1 ± 4Æ3) phases (n = 22 months, U = 44Æ0, Z = )1Æ056,P = 0Æ291).

COMPARISON OF CONTROL AND TREATMENT FARM

CHARACTERISTICS

Both the 15 control and 35 treatment farms shared similar

characteristics with no differences (manova) in traditional

guardingmethods (F1,49 = 0Æ092,P = 0Æ763), tree abundance(F1,49 = 0Æ435, P = 0Æ513), cultivated fruits, (F1,49 = 0Æ102,P = 0Æ751), distance to the nearest village (F1,49 = 0Æ209,P = 0Æ649) and tree d.b.h. ‡ 25 cm abundance

(F1,49 = 0Æ055, P = 0Æ815), as well as the abundances of jack-fruit (F1,49 = 3Æ335, P = 0Æ074), jengkol (F1,49 = 0Æ0240,P = 0Æ762) and rubber (F1,49 = 0Æ335, P = 0Æ565). Differ-

ences were found in the size of the farms (F1,49 = 6Æ013,P < 0Æ01), abundance of durian (F1,49 = 14Æ049, P < 0Æ001)and petai (F1,49 = 6Æ632, P < 0Æ01), and crop-raiding levels

during the pre-trial (F1,49 = 25Æ827, P < 0Æ001). Treatment

farms tended to be larger (t-test, d.f. = 48, t = 2Æ452,P < 0Æ01), had a higher abundance of durian (t-test,

d.f. = 48, t = 3Æ748, P < 0Æ001) and petai trees (t-test,

d.f. = 48, t = 2Æ575, P < 0Æ01), and during the pre-trial

phase received more mean crop raiding per month (t-test,

d.f. = 48, t = 5Æ082,P < 0Æ001) than the control farms.

SPATIAL PATTERNS OF CROP-RAID ING FREQUENCY

During the pre-trial phase, crop-raiding frequency in the 50

focal farms was primarily related to two key factors; farm

size and abundance of trees with d.b.h. ‡ 25 cm (Models 2Æ1and 2Æ2, Table 2) with no effect from spatial autocorrelation

(Moran’s I = )0Æ02, P > 0Æ1), distance of the farms to vil-

lages, abundance of wild and cultivated trees, abundance of

the five main raided crop species or traditional guarding

techniques employed. Orangutans tended to raid farms that

were small (<1–3 ha) and had a greater abundance of large

trees, whether cultivated or wild. In the trial phase, crop-

raiding frequency was primary related to project-introduced

mitigation techniques (Models 2Æ3, Table 2), with no effect

from spatial autocorrelation (Moran’s I = )0Æ02, P > 0Æ1),from the type of deterrent mitigation techniques employed

(i.e. noise deterrents or barriers), farm size, traditional guard-

ing techniques employed, distance of the farms to villages,

abundance of wild and cultivated trees or abundance of the

five main raided crop species. Orangutans tended to raid

farms where no mitigation techniques were employed even

though these farms were found to have higher levels of crop

raiding in the pre-trial phase.

Table 1. Cultivated species and parts eaten* by orangutans and the

plant parts that both field researchers and farmers stated as

constituting human–orangutan conflict

Crops grown Latin name Type

Areca nut Areca catechu BK*, BR*, YL

Asam Gelugur Garcinia atroviridis YL, FR

Bamboo Bambusa spp. BK, BR

Durian Durio zibethinus BK, FL*, FR*, YL

Jackfruit Arthocarpus integer BK, FR*

Jengkol Archidendron pauciflorum BK, FR*, YL

Jering Arichidendron sp. BK, FR

Mahogany Swietenia mahagoni BK, FR, BR

Mango Mangifera sp. FR*

Oil palm Elaeis guineensis FR*, YL*, BR*

Petai Parkia speciosa FL*, FR*

Rubber Hevea brasiliensis BK*, FL*, FR*,

LV, SD*, YL

BK, bark (inner cambium); BR, branch; FL, flower; FR, fruit;

LV, old leaf; SD, seed; YL, young leaf).



MIT IGATION TRIAL EVALUATION

Comparisons between the two project phases revealed a signifi-

cant decrease in mean crop-raiding incidents per month on the

35 treatment farms (Wilcoxon signed rank, Z = 2Æ007,P < 0Æ05) and a non-significant increase in the control farms

(Wilcoxon signed rank, Z = 0Æ188, P = 0Æ851, Table 3). Of

the 359 crop-raiding incidents on the 35 treatment farms, the

enumerators encountered orangutan crop raiding on 49 occa-

sions and used firecracker cannons on 32 of these occasions

(65%), bamboo drums on two occasions (4%) and both on 15

occasions (30%), which eventually drove orangutans away

most (84%) of the time. On average, it took 69Æ4 ± 36Æ5 min

(±SD) until the orangutan moved away a mean distance of

191Æ3 ± 117Æ7 m. In comparison, average orangutan feeding

time on cultivated trees if undisturbed was 81Æ4 ± 54Æ1 min,

suggesting that the firecracker cannons had limited effect.

During the pre-trial phase, there was no significant differ-

ence in jengkol yield between farms (n = 10) that later utilized

tree barrier nets (during the subsequent trial phase) and control

farms (n = 12) that did not (paired t-test, d.f. = 9, t = 0Æ602,P = 0Æ562). However, after the introduction of the nets (trial

phase), the treatment farms had an average jengkol yield

increase (60Æ8%) on the focal trees (mean yield ± SE,

pre-trial: 69Æ0 ± 14Æ8 kg; trial: 176Æ0 ± 39Æ0 kg), whereas the

control farms had an average decrease (27Æ4%; pre-trial:

64Æ3 ± 18Æ8 kg; trial: 46Æ7 ± 11Æ1 kg; paired t-test, d.f. = 9,

t = 2Æ502, P < 0Æ01). Similarly, across the 10 treatment

farms, crop raiding of jengkol in the pre-trial phase

(frequency ± SE = 3Æ1 ± 1Æ3) was significantly higher than

during the trial phase (1Æ2 ± 0Æ7; paired t-test, d.f. = 9,

t = 2Æ237, P < 0Æ05). The average time (days per

month ± SE) between crop-raiding events on the 35 treat-

ment farms increased by 37Æ3% from the pre-trial (5Æ9 ± 0Æ5)to trial (8Æ1 ± 0Æ7) phases, whereas on the control farms it

increased by 125Æ6% during these phases (from 3Æ9 ± 1Æ5 to

8Æ8 ± 1Æ0).

MONETARY LOSS

Comparing the effect of project intervention (Fig. 2), there

was no difference between control farmers estimated loss and

enumerated loss during the pre-trial phase (perceived,

£15 ± 5; enumerated £10 ± 3; Wilcoxon signed rank,

Z = 1Æ136,P = 0Æ256) or the trial phase (perceived, £17 ± 3;

enumerated £13 ± 3; Wilcoxon signed rank, Z = 1Æ307,P = 0Æ191), whereas the treatment farmers overreported dur-

ing both phases (pre-trial: perceived £48 ± 7, enumerated

£12 ± 2; Wilcoxon signed rank, Z = 4Æ996, P < 0Æ001 and

trial: perceived £19 ± 3, enumerated £12 ± 3; Wilcoxon

signed rank,Z = 1Æ940,P < 0Æ05).

FARMER PERCEPTIONS ON HUMAN–ORANGUTAN

CONFLICT MANAGEMENT

The trials did not change focal farmers’ fear of the orangutans,

as 40% of respondents claimed that they were dangerous

before the trials and 46% after the trials (v2 = 0Æ320,

Table 2. Linear regression models describing the spatial patterns of mean monthly crop-raiding incidents by orangutans in 50 focal farmlands

over two project phase, formodels within 10DAIC of the top-rankedmodel

Models K DAIC wi r2

Incidents (pre-trial phase)

2Æ1. T-tree abundance + T-farm size 3 0Æ00 0Æ621 0Æ3892Æ2. T-tree abundance 2 0Æ99 0Æ379 0Æ365

Incidents (trial phase)

2Æ3. Mitigation trials 2 0Æ00 0Æ230 0Æ5302Æ4. T-tree abundance 2 0Æ91 0Æ146 0Æ3502Æ5. T-tree abundance + T-farm size 3 2Æ81 0Æ056 0Æ170

T-TA is the transformed (normalized) tree abundance of wild and cultivated tress with a d.b.h. ‡ 25 cm data, T-farm size is the trans-

formed (normalized) farm size data, K is the number of parameters in the model, DAIC is the difference between each model and the

top ranked model and wi is the AIC model weight.

Table 3. Mitigation technique assessment and change in mean daily crop-raiding incidents per month (±SE) over a 12 month pre-trial phase

and an 18-month trial phase

n

Pre-trial

Mean incidents ⁄ farmTrial

Mean incidents ⁄ farm Mean change (%)

Farm type

Treatment 35 0Æ64 ± 0Æ06 0Æ57 ± 0Æ09 )10Æ9*Control 15 0Æ25 ± 0Æ02 0Æ33 ± 0Æ10 +31Æ7

Mitigation technique

Noise deterrent 25 0Æ60 ± 0Æ07 0Æ62 ± 0Æ12 3

Barrier 10 0Æ76 ± 0Æ13 0Æ45 ± 0Æ12 )41

*P < 0Æ05.



d.f. = 1, P = 0Æ572). However, the reasons related to fear did

change with some (40%) farmers citing large body size or

orangutan aggression (i.e. they would bite; pre-trial) and then

most (70%) citing crop damage after the trial (v2 = 10Æ571,d.f. = 4, P < 0Æ05). Pre-trial, most (58%) farmers thought

that crop raiding would be mitigated most effectively through

the removal of the orangutans, yet after the trials, most (58%)

thought that the mitigation techniques presented a better solu-

tion, with only aminority (28%) preferring orangutan removal

(v2 = 48Æ400, d.f. = 6,P < 0Æ01).After the trials, all mitigation strategies were considered to

have been effective by both treatment farmers (94% agreeing)

and control farmers (93%) with no difference in perceived

effectiveness of individual mitigation techniques (v2 = 7Æ049,d.f. = 4, P = 0Æ133). Whether a focal farmer experienced an

observed crop raiding decrease, or not, did not influence their

perception of technique effectiveness (v2 = 1Æ775, d.f. = 1,

P = 0Æ183). Most (90%) of the focal farmers asked to be con-

sidered for future mitigation trials, with a preferred combina-

tion of static and mobile methods (50%), tree nets only (35%),

or firecracker canons only (15%) and no preference for bam-

boo drums (0%). A minority (10%) of farmers did not wish to

be considered for future mitigation trials because they consid-

ered the techniques to be poorly suited for their planned plant-

ing of oil palm, which they stated as being unpalatable to

orangutans.

POST-PROJECT MIT IGATION TRIAL USE

From the 35 treatment farmers, some (40%) continued using

the mitigation techniques 5 months after the study, with all of

these choosing bamboo drums (even though previously cited

as being the least preferredmethod), because it was inexpensive

and the easiest to use. From those (60%) not continuing with

the mitigation techniques, most (72%) gave the reasons that

the techniques took too long to set up and were difficult to use,

or were too expensive to use (24%) or other (4%). From the 10

farmers trialling the tree nets, half preferred this method, but

only if the enumerators continued to manage the nets, because

they claimed not to understand the hangingmethod and, more

likely, that the process was too time-consuming.

Discussion

A common problem faced in most human–wildlife conflict sit-

uations, particularly those involving large-bodied mammals, is

that the successful identification of crop protection measures

enables further farmland expansion into the species’s range,

especially if it is not protected (Sitati & Walpole 2006). How-

ever, in the case of Batang Serangan, the problem ismore acute

because the isolated orangutan population co-inhabits a small

and finite space with the farmers. Managing these orangutans

therefore presents a dilemma.Whilst implementing an effective

crop protection strategy should, in turn, benefit local live-

lihoods, it is predicted to remove important food sources for

orangutans, with agricultural crops comprising 21% of

their diet (Campbell-Smith et al. 2011b). Prior to this study,

translocation of the orangutan population to a larger forest-

inhabiting populationwas being considered by theDepartment

of Forestry as it was the most realistic management option,

especially given the intensifying farmer pressure on the depart-

ment to do so.However, the positive change in recorded farmer

attitudes following project intervention presents a new oppor-

tunity to manage these orangutans in their current location.

The mitigation strategies that were trialled had mixed success;

hence, future work should focus on developing a simplified tree

net. It should be easy to install and locally accepted. It could be

used to protect the most commercially valuable trees, which

would exclude some, but not all of the cultivated fruits from the

orangutans’ diet. These trial results provide important insights

for conservationmanagers seeking to reduce conflicts with any

great ape species (i.e. tree nets) or other large-bodied mammal

species (i.e. firecracker cannons).

Numerous studies have found that crop raiding occurs more

frequently in farmlands located close to the forest edge

(Naughton-Treves 1997). In contrast, orangutans in Batang

Serangan lived within the agroforest system and spatial crop-

raiding patterns were, instead, explained by farmland size and

tree abundance. Small farms supporting a higher abundance

of large trees were more prone to crop raiding in the pre-trail

phase because they were more accessible to orangutans, con-

taining a denser network of arboreal travel routes (Thorpe,

Holder & Crompton 2007). The regression models also sug-

gested that additional explanatory factors might be missing.

For example, once orangutans were situated within the farm,

other factors such as human presence and calorific content of

the food that they were eating may have influenced how long

they stayed and howmuch they ate.

To successfully mitigate wildlife crop raiding, a combination

of preventative measures (i.e. early warning and direct crop

protection) and reactive measures (i.e. foraging disturbance)

based on the spatial patterns of crop raiding are recommended

(Sitati & Walpole 2006). No early warning systems, for exam-

ple, watch towers, were used or trialled in this study, because a

farmer, if on-site, would be able to detect an approaching

orangutan without difficulty. The tree nets proved effective in

preventing crop raiding on the focal trees, a view also shared

by most of the farmers. However, it is likely that these nets

caused orangutans to search for more profitable patches, that

is, the nearest fruiting trees without nets, thereby displacing the

conflict rather than reducing overall crop-raiding levels across

the landscape, as indicated by crop raiding decreasing on treat-

ment farms and increasing on control farms. Expanding the

net barrier technique to a greater number of trees would be pre-

dicted to result in a trade-off between the time an orangutan

spent searching for unprotected trees (that would require less

handling time) against trying to capture nutritious fruits from

the netted trees.

The socio-economic methodology applied in our study to

assess the long-term effectiveness of the different conflict miti-

gation techniques is transferable to other studies on large-bod-

ied mammal species. We found that most of the farmers

expressed their willingness to continue using the tree nets after

project intervention, but none actually did so. The initial cost



of purchasing a net (£58) may have seemed exorbitant to the

farmers, especially as this cost far exceeded the average annual

monetary loss incurred on an unprotected jengkol tree (£8).

Whilst this cost makes the nets an unlikely purchase for farm-

ers, it would certainly not be too expensive for a conservation

donor or government agency. Besides the financial investment,

the labour investment required may have deterred farmers.

Thus, to overcome the commonly cited problem that net instal-

lation was too time-consuming and difficult, training the farm-

ers in assembling the nets might help, but our field experience

found that it took 3–4 people c. 4 h to install a single net

around the entire tree canopy, and often at the cost of encoun-

tering aggressive wasps.

Motivating the farmers to use the tree nets was identified as

a main constraint to their use. This was certainly not the case

for the bamboo drums (40% used post-project by the treat-

ment farmers). These drums, although simple to use and inex-

pensive, provided negligible benefits to farmers, probably

because the orangutans were already used to varying levels of

human disturbance. Similarly, in Kenya, inexpensive chilli

smoke briquettes had the greatest uptake amongst farmers

(93%) trying to protect their crops from elephants, even

though the technique was not found to be effective (Graham&

Ochieng 2008). The firecracker canon deterrents also did not

guarantee a reduction in crop-raiding frequency. Only after

repeated firing (on average 60 min) by the enumerators, did a

crop-raiding orangutan leave the farm. Thus, an enhanced

scare tactic might test the effectiveness of iron canons, which

make a louder noise but are more expensive (£4) and slightly

heavier (1Æ5 kg) than the bamboo variety used (£0Æ65 and

1Æ0 kg). It is unknown whether this associated increase in dis-

turbance would precipitate an increase in orangutan stress, or

indeed, whether orangutans would quickly become habituated

to this false threat, as evidenced with crop-raiding elephants

and certain noise deterrents used in Namibia (O’Connell-

Rodwell et al. 2000).

Besides identifying mitigation technique effectiveness, or

otherwise, changes in farmer behaviour are recommended for

reducing crop loss. In Batang Serangan, farmers typically work

from 06Æ00 to 11Æ00 h and, most probably in response, orangu-

tans appear to wait until the afternoon to crop raid on the

unoccupied farms (Campbell-Smith et al. 2011b). This was

also found with crop-raiding yellow baboons (Papio cynoceph-

alus) in Kenya (Maples et al. 1976). For Batang Serangan, a

communal guarding system that works on a rotational basis

might be most effective, once a proven scare tactic technique

has been identified, but again this would greatly depend on

farmermotivation.

Several studies have used farmer estimates of crop losses, by

focusing on the volume of crops damaged (Hill 2000; Perez &

Pacheco 2006; Priston 2009) or the monetary value of the crop

loss (Wang, Curis & Lassoie 2005). Where these studies have

been accompanied by independent assessments of crop losses,

farmers have been found to overestimate their losses. In our

study, such overestimates were also recorded for the treatment

farmers. The enumerators and farmers used similar calculation

methods during the two project phases, so improved calcula-

tion accuracy is unlikely to explain why only treatment farmers

overestimated their crop loss during both phases. Whilst the

disparity between treatment farmers and enumerator reports

did lessen during the trial phase, it was still significant. A more

plausible explanation is that the farmers intentionally overesti-

mated in the hope that this would result in an equivalent form

of compensation, even though from the outset the enumerators

consistently explained that this would not happen. Surpris-

ingly, overreporting by control farmers was not found. These

farmers experienced lower levels of crop loss to orangutans

and this might have reduced their demands for compensation.

Conflicts between people and large-bodied mammal species,

for example, elephants (Sitati, Walpole & Leader-Williams

2005) and orangutans (this study), can pose a significant threat

to local livelihoods. Thus, the further identification of effective

crop protection techniques through treatment and control field

trials remains an important area for future research by practi-

tioners. Crop raiding can also reduce local tolerance towards

the conservation of the real or perceived crop pest species.

However, in our study, an initial (pre-trial) farmer view of the

orangutan as a dangerous species that should be removed to

mitigate conflict changed (post-trial) to it being considered as a

crop pest that should be managed through improved crop pro-

tection. This unexpected benefit resulting from farmer partici-

pation in the project suggests that future conservation efforts

should also explore ways to increase community participation

in a broader range of field activities and measure whether this

yields further benefits, or not.

Acknowledgements

We thank the Indonesian Institute of Sciences and the Director General of

Nature Conservation for the opportunity to conduct this research and the

Sumatran Orangutan Conservation Programme and the Orangutan

Information Centre for providing logistical support. We are grateful to

Professor Ir. Zulkifli Nasution and Hubert V.P. Simanjorang (University of

North Sumatra), our field assistants and the farmers in Batang Serangan for

invaluable assistance with our work. Funding was provided by the U.S. Fish &

Wildlife Service-Great Ape Conservation Fund (for the project ‘Bittersweet

knowledge: can people and orangutans live in harmony’, grant #31221208 and

#31221435 held by Nigel Leader-Williams), Born Free Foundation, Humane

Society International, SeaWorld and Bush Gardens Conservation Fund,

PanEco, People’s Trust for Endangered Species, Orangutan Republik

Education Initiative, Orangutan Foundation, Primate Society of Great Britain

and BorneoOrangutan Society-Canada.

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Received 1 August 2011; accepted 10 January 2012

Handling Editor: Julia Jones



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