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Innocent invaders? A preliminary assessment ofCecropia, an American tree, in JavaDouglas Sheil a b c & Michael Padmanaba ba Institute for Tropical Forest Conservation – Mbarara University of Science andTechnology, Kabale, Ugandab Center for International Forestry Research (CIFOR), Bogor, Indonesiac School of Environmental Science and Management, Southern Cross University, Lismore,AustraliaAccepted author version posted online: 08 Aug 2011.Published online: 24 Oct 2011.

To cite this article: Douglas Sheil & Michael Padmanaba (2011) Innocent invaders? A preliminary assessment of Cecropia,an American tree, in Java, Plant Ecology & Diversity, 4:2-3, 279-288, DOI: 10.1080/17550874.2011.610371

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Plant Ecology & DiversityVol. 4, Nos. 2–3, June–September 2011, 279–288

OPINION ARTICLE

Innocent invaders? A preliminary assessment of Cecropia, an American tree, in Java

Douglas Sheila,b,c∗ and Michael Padmanabab

aInstitute for Tropical Forest Conservation – Mbarara University of Science and Technology, Kabale, Uganda; bCenter for InternationalForestry Research (CIFOR), Bogor, Indonesia; cSchool of Environmental Science and Management, Southern Cross University, Lismore,Australia

(Received 25 June 2010; final version received 1 August 2011)

Background: Invasive alien species receive little attention in many tropical countries.Aims: We examine the Neotropical tree genus Cecropia in West Java and ask how eradication decisions should be judgedgiven limited resources.Methods: The distribution was determined based on field searches. The history, impacts and perceptions of Cecropia wereexamined through official records, observations, interviews and published literature.Results: At least four Cecropia species were planted in the Bogor Botanic Gardens, West Java Province, Indonesia, during thetwentieth century but only C. peltata appears to have naturalised extensively. Since 1943 Cecropia has spread over 1290 km2.Implied mean rates of spread range from 0.13 km year−1 to 0.68 km year−1, with the fastest rates occurring down-slope andalong river valleys. It has reached Jakarta to the north and the Mount Gede-Pangrango National Park to the south. It occursonly in open and disturbed locations. Local farmers consider Cecropia only a minor nuisance. We review 15 other reports ofCecropia naturalised outside the Neotropics.Conclusions: Without control, Cecropia may eventually become common throughout Java. There is no evidence that anyserious problem will, or will not result. We call for the development and implementation of low-cost and decisive assessmentprocedures for evaluating the control and management of naturalised organisms.

Keywords: alien species; botanic gardens; citizen science, invasive species; local views; low-cost survey; naturalised flora;precautionary principle; risk assessment

Introduction

Invasive alien species are highlighted amongst theharbingers of species extinction. They are considered amajor threat to native species and habitats, to ecosystemservices and to economic land-use (Ehrenfeld 2003; Hulme2003; Simberloff 2005). Efforts have been made to identifythe most dangerous of these organisms. For example, an ini-tiative by IUCN’s Invasive Species Specialist Group (ISSG)and others has listed ‘100 of the World’s Worst InvasiveAlien Species’ (Lowe et al. 2000, 2004). Cecropia peltataL. is one of the 32 terrestrial plants included on this list.

The Convention on Biological Diversity (article 8h)requires signatories to “eradicate those alien species thatthreaten ecosystems, habitats or species” (UNEP 1992).Such threats have received inadequate attention in vastareas of the tropics where conservation values are highand resources limited (Nunez and Pauchard 2010). In suchregions naturalised exotic species, even those highlightedas invasive elsewhere, only become recognised problemsafter they are widespread and the costs of control andelimination are prohibitive (Sheil 1994). To be useful inthese contexts assessments need to be simple, cheap anddecisive – and to lead to action when action is required.

Some argue that subjective views play too great a role inthe assessment of invasive exotics (Hettinger 2001; Colautti

*Corresponding author. Email: d.sheil@cgiar.org

and MacIsaac 2004) and that judging all aliens as problemsis misleading (Sax and Gaines 2008). Some note circum-stances where naturalised exotic species contribute to theecological recovery of degraded areas and the formation ofnovel ecosystems which provide valuable services (Lugoand Helmer 2004; Hobbs et al. 2006). Most accept thatsome naturalised alien species pose a greater degree ofthreat than others.

Cecropia peltata, a pioneer tree introduced from theAmerican tropics and subtropics (‘Neotropics’), is commonin parts of West Java, Indonesia (Whitten et al. 1996). TheISSG web site, http://www.issg.org (accessed 5 December2010), states that C. peltata “has been nominated as among100 of the ‘World’s Worst’ invaders”. This web site listsvarious other assessments and reports in which C. peltatahas been identified as a significant threat. Consultation withlocal agencies in Java (in 2006–2007) implied that there hasbeen no effort to either assess the threat posed by these alientrees or to control their spread. Western Indonesia is one theworld’s most species-rich regions (Mittermeier et al. 2004).Should we be concerned that there are no initiatives to pre-vent the ongoing spread of Cecropia around Bogor in WestJava? Here, we consider whether low-cost methods can helpformulate a satisfactory answer. Knowledge of distributionsis needed for planning control programmes (e.g. D’Evelyn

ISSN 1755-0874 print/ISSN 1755-1668 online© 2011 Botanical Society of Scotland and Taylor & Francishttp://dx.doi.org/10.1080/17550874.2011.610371http://www.tandfonline.com

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280 D. Sheil and M. Padmanaba

et al. 2008), but we also want to clarify if such controls arenecessary.

We made an inexpensive evaluation of Cecropia natu-ralised in Java. Where and how fast is it spreading? Howis it perceived (see for example Pfeiffer and Voeks 2008)?What problems does (or might) it cause? We were espe-cially concerned to note invasion into native vegetationand/or into protected areas. We also sought accounts ofCecropia as a naturalised alien from elsewhere on the basisthat these might offer information and insights regard-ing invasion success, impacts and management options.We also considered the wider implications of our low-cost approach for assessing alien invasive species whenresources are scarce.

Methods

The genus Cecropia

Cecropia is a taxonomically difficult genus of Neotropicaltrees, comprising mainly light-demanding pioneer species(Berg and Franco-Rosselli 2005). In the past the genus wasplaced in the Moraceae; more recently it was given its ownfamily Cecropiaceae, but phylogenetic analyses suggestthat it should be placed within the Urticaceae (AngiospermPhylogeny Group 2003).

In bright moist conditions most Cecropia speciesgrow quickly, are tolerant of damage and coppice read-ily (dos Santos et al. 2006). All Cecropia species aredioecious (each plant is male or female) and primarilywind-pollinated. Cecropia species typically produce largeamounts of seeds which can remain viable for years untilgermination is stimulated by forest disturbance (Brokaw1998; Dalling et al. 1998; Pearson et al. 2003). In theNeotropics, where Cecropia fruits are eaten by bats andbirds, seed dispersal is highly effective, with bats able tocarry seeds several kilometres (de Foresta et al. 1984; daSilva et al. 2008).

Cecropia species have various minor economic valueswithin their native range (Binggeli 1999; Csurhes 2008).They are also recognised as important in accelerating initialforest recovery in open areas (Slocum 2001; Lobova et al.2003). Many have become naturalised outside their nativerange, and the entire genus is regarded as ‘high risk’ aspotentially invasive aliens (Binggeli 1999; Csurhes 2008).

C. peltata is a typical, and variable, member of its genus(Fleming and Williams 1990; Brokaw 1998): it flowers at3–5 years; seldom exceeds 20 m in height or lives more than30 years. In the mainland Neotropics C. peltata is associ-ated with Azteca ants which feed on specialised Müllerian(glycogen rich) food bodies provided by the tree on specialtissue or ‘trichilia’ at the base of the leaf petiole in returnfor the ant’s protection (Sagers et al. 2000). These bodies,and the associated ants, are often absent on islands (see Putzand Holbrook 1988).

Site of introduction and wider context

In Indonesia, Cecropia trees appear to have been first intro-duced in the Kebun Raya Bogor (Bogor Botanic Gardens)

in the early 1900s. These gardens cover 87 ha in thecentre of Bogor City, West Java (UTM 9270387 69882548M) at 250 m altitude above sea level (Botanic GardensConservation International website, accessed in October2010) and it includes both formal and untended areas.Its 3449 registered species include many exotics/aliens(Registration Officer, pers. comm. 2008). With a mean rain-fall of approximately 4300 mm year−1 (Botanic GardensConservation International undated) the natural vegetationin Bogor would be evergreen rain forests (Whitten et al.1996). Soils are volcanic and fertile, but liable to erosionwhen exposed.

Oceanic islands, and other similarly restricted isolatedecosystems, are considered especially vulnerable to alieninvasive species (Sheil 1994). Despite its current island sta-tus, Java possesses a continental biota, having been part ofmainland Asia for most of the last 3 million years (Voris2000).

The Bogor Botanic Gardens lie within an urban envi-ronment. As stated in the provincial government web-site (www.jabarprov.go.id accessed in November 2008),the population in Bogor City, which covers an area of118.5 km2, is officially 844,778 residents or 7129 peo-ple km−2. The surrounding region supports peri-urban andindustrial land, but is dominated by intensive agricultureinvolving irrigated rice and diverse cash crops, fruits andvegetables. Most land is privately owned. Crops are oftenmixed, with tree cover, both planted and spontaneous, play-ing an important role in complex ‘home gardens’ (seeWhitten et al. 1996).

Records

We gathered available information about Cecropia fromregions outside the Neotropics, with specific interest inlocations where it had naturalised. We use the names givenin each source. We especially sought background informa-tion regarding the planting and status of Cecropia speciesin Java. We gained considerable help from the Sub-Divisionof Collection Registration at the Bogor Botanic Gardens,which is responsible for keeping a register of all treeplantings in the gardens.

Distribution in West Java

Initially, we relied on observations from roads, tracks andthe train. We used a Global Positioning System (GPS) torecord locations of any Cecropia seen either individuallyor in clumps. We excluded plants less than 1 m tall aswe required clear identifications which we were not certainwould be reliable for seedlings. We started the survey in2005. Each search generally involved one or two observerstaking records for 4–8 h. We combined opportunistic anddedicated searches. In total, we estimate that we managed28 separate searches, four of which involved searching onfoot around the Mount Gede-Pangrango National Park andMount Salak. The surveys were completed in the first halfof 2008 when most of roads in the city had been observedand we had sufficient samples of Cecropia distribution. We

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A preliminary assessment of Cecropia in Java 281

estimate, allowing for all sources of error and uncertainty,that for road- and rail-based records our GPS records wouldbe within 20 m, and when on foot within 6 m, of the truelocation in more than 95% of cases.

Based on our observations, we regularly updated ourdraft map of the presence and absence of Cecropia in WestJava. We focused new searches on areas not yet observedand double-checked all areas on our maps from where wehad no reports of Cecropia. We did not rely only on ourown observations but asked for and received accounts froma wider network of friends and colleagues as an exercise in‘citizen science’ – see Acknowledgements. In three caseswe could not verify a reported observation despite ded-icated searches (all these cases were outside the currentdistribution). These cases were suggestions that the treemight have been spotted by the road (1) near Sukabumi; (2)near Pelabuhan Ratu; and (3) at Cibodas Botanic Gardens,situated at approximately 38 km, 51 km, and 28 km,respectively, from Bogor City. Where roads were scarce wesearched paths and consulted local people. To determine theinvasion front we always continued our search for at least20 min beyond the last Cecropia recorded. We checked andupdated the observations which occur on the edge of themapped distribution in the first half of 2008.

All our GPS data were plotted onto a geo-referencedbase map using ArcGIS 9.2 (www.esri.com). This mapincludes rivers and altitude data (from Forestry PlanningAgency, Ministry of Forestry 2004 and the NASA ShuttleRadar Topographic Mission, 2000). For exploratory analy-sis we selected observations defining the outer edge of theCecropia population. Starting from a point population, anydeviation from a circular distribution implies heterogeneity(that is unevenness) in the invasion process (Lewis 1997).

Local views

We conducted short interviews to record local people’sperceptions regarding Cecropia. We asked if they werefamiliar with Cecropia, what they called it, how it wasused and valued, and whether there were any problems orconcerns associated with it. These interviews were rarelydone in strict isolation but often involved groups listeningin and agreeing with or making suggestions to the inter-view. Given the exploratory nature of this study we made noattempt to exclude such contributions, which often enrichedthe discussions.

Results

Record of introductions

Five separate introductions of Cecropia involving fourspecies have been documented in the Bogor BotanicGardens (Table 1). We found no records of any other intro-ductions, but references to the C. adenopus Mart. ex Miq.from ‘Lawang Gardens, East Java’ (Table 1) shows that atleast one other planting had occurred elsewhere in Java.

Table 1. Recorded Cecropia introductions in the Bogor BotanicGardens, West Java.

Recordedspecies Official records Persistence

C. peltata L. Origin ‘Brazil’. Twoindividuals ‘from ParisBotanical Gardens’ wereplanted at an unrecorded date.One was dead by 1935 whilethe other lasted until 1952.

Widelynaturalised

Two more individuals from the‘Theosophical Society’ inMadras, India were planted inMarch 1943. One died in 1949the other survived until 1990.

C. adenopusMart. ex Miq.

Two seedlings from the‘Lawang Gardens’, East Javaplanted in 1921. One died in1990 the other in 2005. In 1993two further individuals werepropagated from cuttings.

Two trees (oneclone) alive in2008

C. palmataWilld

Two individuals ‘from Africavia Kew England’ were plantedin 1904. One was dead in 1949and the other in 1971.

None

C. schiedeanaKlotzsch.

Two individuals planted ‘fromMexican seed’ – date notgiven. Both died in 1949.

None

Source: Sub-Division of Collection Registration at the Bogor BotanicGardens (pers. comm. 2008)

According to the records, C. peltata was introducedtwice to Bogor. Assuming the second planting would nothave been undertaken if the species was already regenerat-ing spontaneously, and that hybrid origins are unlikely, weconclude that naturalisation began after 1943.

Status by 2007/2008

Cecropia trees are common around Bogor (Figure 1). Theseplants are variable, but appear to conform to taxonomicdescriptions of C. peltata (Berg and Franco-Rosselli 2005).They lack Müllerian food bodies or symbiotic ants. Thesetrees generally seem healthy and seldom show signs of her-bivory or disease. Despite our searches, including insidethe Bogor Botanic Gardens, where C. peltata is abundant,we found no evidence that any other Cecropia species havenaturalised.

The spread of Cecropia from the Bogor BotanicGardens has not resulted in a circular distribution. Thegreatest spread has been downhill from Bogor towards thenorth, where it has reached 40 km to Jakarta. The spreadeast and west from Bogor has been more limited, 17 kmand 19 km, respectively, while towards the mountains tothe south individuals have reached as far as 20 km, but tothe south-west on the slopes of Mount Salak only 9 km.Accepting 1943–2008 as the period of expansion, thesedistances imply mean long-term rates of spread rangingfrom as low as 0.13 km year−1 to the south-west to 0.68km year−1 to the north. Spread has been most rapid at

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282 D. Sheil and M. Padmanaba

Figure 1. Verified records of Cecropia trees over 1 m tall in the vicinity of Bogor (2005–2008). Numbered locations are those selectedas representative ‘most distant occurrences’ based on searches beyond each.

lower altitudes: if for illustration we consider the 17 num-bered points marked on the map as ‘independent’, then byrank correlation we see that lower altitude observations arefurther from Bogor and higher altitude edge locations arenearer (Kendall’s tau-b = −0.81, P < 0.001).

The estimated area enclosed by the colonisation front in2008 was 1290 km2. If spread continued along each bearingat the same mean rate, just over 40 km2 would be addedeach year.

We observed that Cecropia was often very abundantin uncultivated land around the city such as river banks,drain channels, cemeteries and abandoned lots. In WestJava, when a house owner dies there is often a reluc-tance to use the location, which may subsequently becomeheavily overgrown; often now with abundant Cecropia. Inthe urban area around the Bogor Botanic Gardens manyCecropia were found on the banks of the Ciliwung andCisadane rivers. Cecropia is also common in and around the

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A preliminary assessment of Cecropia in Java 283

grounds of the Center for International Forestry Research(CIFOR) campus just outside Bogor City. There are anumber of well-established plantation forests around theCIFOR campus, but Cecropia, though common on the for-est edges, is absent in the shaded understorey. Epiphyticestablishment by Cecropia on trees, as well as on wallsand other built structures, is also common, especially inthe densely urbanised environments around Bogor City, butour observations suggest that these plants are small and sel-dom reach more than 2 m in height. In managed locationstoo, such as gardens and car park edges, Cecropia treeswere occasionally recorded. Based on our observations, thelocal distribution is clearly abundant on uncultivated land,relatively scarce elsewhere, and nearing the distributionboundary individuals become increasingly scattered.

We found only one Cecropia tree within a protectedarea. This was a 6-m high individual at 1000 m elevationin the Mount Gede-Pangrango National Park (see Figure 1point number 7). This tree, which we identified as C.peltata, was established in an area of re-growth amongst oldpine plantations, with a variety of other naturalised exoticplants of Neotropical origin including shrubby Brugmansia(Solanaceae), Calliandra (Fabaceae) and herbs includingClidemia hirta (Melastomataceae). Local people who col-lect firewood in the area were familiar with Cecropia, butsaid that they knew no other Cecropia in the National Park(but knew that there were many such trees in and aroundBogor City). Our searches (four visits to Mount Gede-Pangrango and Mount Salak and discussions with localpeople and park staff) recorded no additional observationsor reports that Cecropia is invading natural forest, whetherin gaps or otherwise.

Local attitudes

We interviewed 52 informants and gained additionalresponses from a similar number of bystanders who oftencontributed to the discussions. Twenty-five of the infor-mants considered their primary occupation to be farmer, sixwere security guards, four were helpers (doing odd jobs),four were gardeners, three were drivers and two were car-penters, while the rest had various occupations. In WestJava, Cecropia is known as ‘kibolong’, a reference to thehollow stems. ‘Ki’ from ‘kai’ means wood or branch, while‘bolong’ means hole or hollow. Two respondents used thename ‘kicopong’ which has the same meaning.

Farmers consider Cecropia a minor weed and clear itfrom their fields. No effort is made to eliminate it fromuncultivated ground. Seven informants noted that whenCecropia established on walls and buildings it could causedamage if not removed, but we heard no specific or seri-ous concerns regarding Cecropia’s spread. Local peoplereported that bats and birds eat the fruits and dispersethe seeds. Cecropia was recognised by the majority ofinformants as a low-quality firewood, with the foliage pro-viding fodder for goats. Two farmers noted the tree’s shaderole and its prevention of erosion on steep uncultivated

slopes. Eleven informants noted that the wood of largertrees could be used for light construction in a similar wayto bamboo, e.g. for roofing and even, in one case, as a‘tool box’. Children use the hollow stems as toy blowpipes.Cecropia are occasionally seen in well-tended gardenswhere they have a shade or decorative role. Outside of theBogor Botanic Gardens we found no reports or claims thatCecropia has been planted.

Cecropia outside its native range

We identified at least 18 documented occurrences, includ-ing Bogor in West Java, of Cecropia spp. having becomenaturalised outside the Neotropics (Table 2; n.b. accountsinclude archipelagos with Cecropia populations establishedon more than one island where the number of intro-duction and naturalisation events are unspecified). Wefound no reports of specific impacts aside from the localroadside displacement in Cameroon of Musanga cecropi-oides R. Br. ex Tedlie (McKey 1988). Musanga is closelyrelated to Cecropia and belongs to the same family, i.e.Cecropiaceae/Urticaceae.

We found no documented accounts of Cecropia hav-ing significant value for people outside of its native range.The reasons for the introductions are seldom noted; thesuggestion that Cecropia pachystachya in Singapore resultsfrom efforts to feed sloths in Singapore Zoo is an exception(Lok et al. 2010). Otherwise, most plantings appear to haveoccurred as part of larger botanical collections in publicor private gardens. We found one account of active con-trol (which an anonymous referee considered incorrect):a localised eradication in Queensland, Australia (Csurhes2008).

Discussion

Spread

By persisting, spreading, and being locally dominant, C.peltata is clearly invasive in West Java (criteria fromColautti and MacIsaac 2004). However, the nature anddegree of any threat posed remains unclear. Cecropia hasreached the Mount Gede Pangrango National Park but,based on our observations and consultations, it did notoccur in the natural forest there – not even in tree-fall gaps.

Local people noted that fruit-eating bats and birdsdisperse Cecropia seeds. This seems credible given theobservation of Lok et al. (2010) concerning Singapore’snaturalised C. pachystachya being “regularly visited bythe lesser dog-faced fruit bat (Cynopterus brachyotis) anda wide range of frugivorous and generalist bird species”,most of which are known as ‘good dispersers of seeds’.Most of these bird species also occur in Western Java.

From our own observations we know that within andaround Bogor Cecropia establishes primarily in the smallneglected sites within the complex land-use mosaics of thisdensely populated landscape. Based on these observations,we speculate that waterways provide the main avenue by

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284 D. Sheil and M. Padmanaba

Table 2. Records of Cecropia as a naturalised exotic outside of the Neotropical region (based on Binggeli online, Csurhes 2008, otheronline databases and specific sources as cited). All web sites accessed 5 July 2010. Note that some information is based on non-peerreviewed documents and databases.

SpeciesLocation (also date and origin if

known) Observations Source(s)

C. leucocoma Congo, Eala Botanical Garden “Naturalised in the vicinity” Bouharmont (1954)C. obtusifolia In several Hawaiian Islands Occurs in disturbed forests and pastures

up to 300 m elevation on the easternside of the Big Island.

Wetterer (1997) and www.hear.org

C. peltata Bogor Botanic Gardens, wherenaturalisation began after asecond planting in 1943

This study This study

C. peltata Limbe Botanical Gardens,Cameroon (between1900–1910)

By 1986 they had reached to Bombe andBonaberi, a distance of 54 km and 51km from Limbe, respectively. Treeshave been observed displacing thenative Musanga cecropioides alongroadsides.

McKey (1988) and distancesestimated by D.S. using GoogleEarth

C. peltata 1911 and 1912, Eala BotanicGardens, Congo, origin ofPuerto Bertani (Paraguay).

No data Leonard (1951)

C. peltata Peninsular Malaysia at ForestResearch Institute Malaysia andthe Sungei Buloh ForestReserve, 1953 (seeds fromBogor)

By 1982 plants were observed as far as0.6 km and 10.4 km from their originalsite of introduction, and are “amongthe most common early successionaltrees” in nearby plantations.

Putz and Holbrook (1988)

C. peltata Singapore Botanical Garden in1902

“at present only one small wildpopulation . . . has been found near theBotanic Gardens along the mid-pointof Tyersall Avenue . . . close to onelarge Cecropia peltata tree cultivatedinside the National Orchid Garden”.Now over most of the north and westof the Island (map in Lok et al. 2010).

See Lok et al. (2010) also Putz andHolbrook (1988)

C. peltata Congo No data Assi (1979)C. peltata Kenya No data Assi (1979)C. peltata Tahiti and Raiatea, French

Polynesia“Widely established in the forests to

several hundred metres elevation”.Global Invasive Species Database

www.issg.org and Pacific IslandEcosystems at Risk (PIER)www.hear.org/pier

Moorea Island, Raiatea (Havai)Island, Taha’a Island, Tahiti,Makatea (Ma’atea) Island, andRapa Island

Various records

C. peltata New Caledonia Various records www.issg.org, andwww.hear.org/pier

C. peltata Ivory Coast in 1910 at Aboisso Spread less than 20 km by 1958. Afterforest loss in 1960 plants spread by“more than 100 km”

Assi (1979)

C. palmata Congo No details Assi (1970, 1979)C. pachystachya Rarotonga, Cook Islands Various records cookislands.bishopmuseum.org

also www.hear.orgC. pachystachya Uncertain “wild populations of Cecropia

pachystachya in Singapore may haveoriginated from an introduction at theSingapore Zoo in 1992 as food forsloths”

Lok et al. (2010)

Cecropia sp. Queensland, Australia “To date, the only naturalised specimensof Cecropia in Queensland . . . havebeen destroyed . . . [but] gardenspecimens have been found in Mackay,Cairns and Brisbane” (accuracyquestioned by one anonymous referee)

Csurhes (2008), see also Low(2004), andwww.invasives.org.au

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A preliminary assessment of Cecropia in Java 285

which Cecropia has travelled north. This appears likelybecause river banks provide extensive untended areas, offera direct corridor for spread, and water may also carryfruit/seed (e.g. Csurhes 2008).

Our initial study plan had been to evaluate plant den-sities and associated size distributions more systemati-cally throughout the tree’s range, but this proved unfea-sible as plants were especially abundant in abandonedand often fenced private lots, where access was impeded.Nonetheless, we gathered many Cecropia observations andwere able to make informal judgements on local abun-dance. We observed that trees at the edge of the overall dis-tribution are usually solitary. We speculate that this reflectspollen limitation in this dioecious plant (see Kaufman et al.1998): the significance of occasional long-hop dispersalevents might thus be constrained by subsequent repro-ductive isolation. A recent global literature review of 133invasive plant species for which the invasion mechanismswere clearly established found that only one of these specieswas dioecious (Ren and Zhang 2009). Pollen flow amongisolated Cecropia trees, and the relationship of inter-treedistance with breeding success, would be a promising areafor future research.

Control and threat

Could Cecropia be eradicated readily if that option waschosen? Control is unlikely to raise major opposition andwould be simplified by the trees’ patchy and localiseddistribution. By focusing on dense populations seed setis likely to be reduced, even if some individuals persist.Challenges would include covering such a diverse areaof often private land, and ensuring adequate follow-upregarding re-growth and long-lived seeds. We have notattempted a full costing, but estimate that a team of onecoordinator along with 20 labourers working for 1–2 years,along with regular follow-up for a further 10 years couldachieve eradication. Cooperation among government agen-cies, along with public support would be important for anysuch efforts.

The nature of the threat posed by Cecropia in WestJava if it is left unmanaged is harder to clarify. Certainly,alien plants reduce the space available for native species(Denslow and Hughes 2004; Peh 2010). They may altercompetitive balances, facilitate invasion by additionalspecies, and modify key ecosystem properties such as nutri-ent dynamics (Ehrenfeld 2003). However, our preliminarystudy has provided no evidence that Cecropia poses a sig-nificant short-term conservation threat. As in Cecropia’snative range, the impact on soil and facilitation may evenbe viewed as positive (Slocum 2001; Lobova et al. 2003).If Cecropia were removed it would simply be replaced byother disturbance-dependent vegetation. Even in the MountGede Pangrango National Park it is arguably the distur-bance that is the real problem – if the shade-tolerant nativeforest were allowed to recover, all weedy species, Cecropiaincluded, would become minor components.

The long-term impacts are even more uncertain.Only a minority of naturalised species become harmful(Williamson 1996), but even those that do may first appearharmless for years or decades (MacDonald and Wilgen2002). As Cecropia spreads it will reach new locations,conditions, and communities – probably even other islandsin the region, as various other bat-dispersed Neotropicalplants such as the tree Piper aduncum L. (Piperaceae)have already done (Peh 2010). Populations will continueto evolve and to develop new ecological relationships, theimplications of which cannot be readily foreseen (for rel-evant accounts, see Prentis et al. 2008 and Buswell et al.2011).

If Indonesia pursues its commitments under theConvention on Biological Diversity (UNEP 1992) itrequires not only clearer procedures regarding alien speciesbut also an official body to take scientific leadership andresponsibility. The scientists working at the Bogor BotanicGardens (and associated herbarium) seem best placed toperform this role, given their expertise and experience inmanaging live collections, conducting taxonomic surveysand studying vulnerable ecosystems. Support from otheragencies, especially those with expertise and field pres-ence, would be valuable (such as the Forestry Researchand Development Agency of the Ministry of Forestry).Any such body, with suitable training and guidance, shouldbe able to conduct surveys and to formulate plans andimplement interventions as required. International agenciescould play a valuable support role in ensuring funds andtechnical assistance. Identifying and agreeing a need to actwould only be a first step. Engagement with and supportfrom local government, community leaders and the publicwould be essential for large-scale interventions.

Towards better decisions regarding exotic species

Our study was cheap and simple. This is appropriate tocontexts where resources are severely limited, but manyquestions remain unanswered. If resources were availableadditional information would be helpful. We would wantto clarify if any disturbed habitats vulnerable to Cecropiahave conservation importance in West Java. Genetic anal-yses could be used to clarify the origins of the trees. Wecould also perform carefully managed planting experimentsto clarify the conditions under which Cecropia may enterand disrupt forest communities. Trials would also help clar-ify the costs and challenges of a larger-scale control effort.There is no lack of potentially important and fascinat-ing research questions associated with invasive organisms.Unfortunately, resources and expertise to address suchquestions are limited.

For now the question remains: should we be con-cerned about the spread of Cecropia trees in Java, and ifso what are our priorities? Cecropia is certainly invasivearound Bogor, but our study is inconclusive concerningthe consequences. Is an organism best considered inno-cent until proved harmful, or is it the other way around?

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286 D. Sheil and M. Padmanaba

We have no compelling evidence that Cecropia is, or willbecome, a significant problem, but nor can we be sure thatit will not. Aside from displacing some shade-intolerantspecies there is no well-defined threat. Our literature searchtoo indicates nothing more. Absence of evidence is notevidence of absence, but neither can we prove a nega-tive. Doubters can insist, correctly, that something criticalmight have been overlooked. Indeed, even unlimited accu-rate data on the present cannot ensure perfect long-termpredictions.

What criteria can be used to assess harm? We chose: (1)invasion into native vegetation; (2) invasion into protectedareas; (3) the concerns of local people; and (4) availableevidence from elsewhere. We found that while Cecropia hasentered the Mount Gede-Pangrango National Park, the treehas not (yet) been found to invade native Javanese forests,local people did not have strong views about the tree, andwe failed to identify accounts of Cecropia causing signifi-cant harm elsewhere aside from displacing some abundantnative pioneers (McKey 1988).

So what next?

Some would argue for a precautionary approach withprompt eradication of all naturalised aliens: we can min-imise risks only by eliminating all naturalised species.Delays increase both risks and costs. Others will callfor more research: how can we judge what is best whenso much remains unknown? Various information-intensivestudies and schemes have been advanced to help guidethe management of alien invasives (Myers et al. 2000;de Wit et al. 2001; Bogich et al. 2008). Unless recog-nised priorities were to change radically, neither control norintensive study of all naturalised aliens currently appearsrealistic. Resources are insufficient. How then should weproceed?

Pragmatically, we might start by agreeing how harmshould be defined and judged in practice. Simberloff(2005), discussing this point for alien species more gener-ally, noted a lack of consensus even as to what we mean by‘harm’ and who should decide. Broad-based consultationand consensus would be useful.

As this paper was being finalised a new project fundedby GEF-UNEP, and executed by CABI, was launched.This project will focus on naturalised alien species(primarily suspected forest invasives) in Cambodia, thePhilippines, Vietnam, and Indonesia. The principal goalsare to strengthen policy, build capacity, create awareness,foster regional collaboration and develop ‘best’ manage-ment practices at selected pilot sites. Additional fundingwill be needed if the project is to achieve wide impact(A. Witt, pers. comm.). It is not yet clear if the project willaddress Cecropia in Western Java – but it offers a welcomeopportunity to confront such challenges more generallyand formulate and agree some practical guidelines forengagement.

Regarding information needs

It is striking that a recognised ‘high-risk’ genus likeCecropia has been so widely naturalised and so seldomsubjected to any reported management or control (Table 2).The practical significance of species listings is in need ofevaluation.

The accuracy of available information is also a con-cern. One referee urged us to delete Table 2 due to itsdependence on ‘non-peer reviewed’ material. We declined.This is the information available. The official ISSG onlinedata base too remains dependent on such material (seehttp://www.issg.org). Nonetheless, we agree that qualitycontrols, with mechanisms to correct errors and flag con-cerns, would be helpful on all such data sources – includingon peer-reviewed literature.

Much effort has focused on identifying which speciesare likely to be invasive (e.g. Binggeli 1996; Colautti andMacIsaac 2004; Ren and Zhang 2009; Andreu and Vilà2010). When an invasive species is already well estab-lished the pertinent questions are different: they reflectrather the ultimate cost of inaction versus intervention andthe range of options available. Though long recognised(e.g. MacDonald and Wilgen 2002), and despite genuineadvances (e.g. Martin and Murray 2011), there remains adearth of practical guidance on these challenges.

Alien invasions are plentiful in the tropics. Many, per-haps most, are unmonitored and undocumented. Whilesome alien species may ultimately prove harmless, othersalmost certainly will not. We need to agree clear prag-matic measures of real or potential harm or threat asaccepted thresholds to trigger control, acceptance, or addi-tional research. We call for more attention to devisingsimple and affordable decision-making procedures that canbe applied to the control and management of naturalisedalien organisms in the tropics.

AcknowledgementsWe thank many people who spotted trees; especially MeilindaWan, Miriam van Heist, Glen Mulcahy, Manuel Guariguata,Richard Dudley, Imam Basuki and members of the Bogor HashRunners. We thank Mohammad Agus Salim for developing themaps and Suhendar at the Registration Office of the Bogor BotanicGardens for providing valuable information. Erik Meijaard,Miriam van Heist and Robert Nasi kindly commented on an earlierdraft, as did several anonymous reviewers. This project was notfunded by any source but DS and MP’s time was partly coveredby support to CIFOR from the European Commission.

Notes on contributorsDouglas Sheil is a tropical forest ecologist with an interest in con-servation. He worked with CIFOR in Indonesia from 1998 to 2008and remains a CIFOR senior research associate. He is now direc-tor of the Institute for Tropical Forest Conservation in the BwindiImpenetrable World Heritage site in Uganda.

Michael Padmanaba is a forester interested in wildlife, forest ecol-ogy and biodiversity monitoring. He has worked for 8 years atCIFOR in Indonesia, where he is a research officer in the Forestsand Environment Program.

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