Received 31October 2002 Accepted 9January 2003 Publishedonline 9 April 2003

Ratesof speciesintroductio nto aremote oceanic island Kevin J.Gaston 1* ,AlexG. Jones 1†,Christine Ha¨nel 2 and Steven L.Chown 2 1Biodiversity and MacroecologyGroup, Department of and PlantSciences, University ofSheffield, Sheffield S102TN, UK 2Departmentof Zoology, University ofStellenbosch, Private Bag X1, Matieland 7602, South Africa The introductionof to areas beyondthe limits oftheir natural distributionshas amajor homogeniz- ing influence,making previously distinctbiotas more similar. The scaleof introductions has frequently beencommented on, but their rate andspatial pervasivenesshave beenless well quantified.Here, we report thefindings of a detailedstudy of pterygote insectintroductions to GoughIsland, one of the most remoteand supposedly pristine temperate oceanicislands, and estimate therate at whichintroduced specieshave successfullyestablished. Out of 99 speciesrecorded from GoughIsland, 71 are established introductions,the highest proportion documentedfor any SouthernOcean island. Estimating atotal of approximately 233 landings onGough Island sincefirst human landfall, this equatesto one successful establishmentfor every threeto four landings. Generalizations drawnfrom other areas suggestthat this may beonly one-tenthof the number of pterygote speciesthat have arrived at theisland, implying that mostlandings may lead tothe arrival ofat least onealien. Theserates ofintroduction of new species are estimatedto be two to three orders of magnitude greater than background levels for GoughIsland, an increasecomparable tothat estimatedfor global speciesextinctions (many ofwhich occuron islands)as aconsequenceof human activities. Keywords: alien species;Gough Island; homogenization; introductions;island biogeography; SouthernOcean

1. INTRODUCTION tractedperiods, often because of thenumber of routesby which introductionscan be achieved, the large magnitude The frequencyof unintentional introductions of non- ofhuman activities andthe lack ofa suitable historical domesticatedspecies to areas that lie beyondthe limits of recordof human movements. their natural distributionshas been huge. Pimentel (2001) The secondissue is the sheer spatial extentrather than estimatesthat in thepast 10 000 years more than 400 000 theoverall level ofthe areas towhich theunintentional specieshave beenmoved from oneregion oftheEarth to introductionof one or more non-domesticatedspecies has another by human agency.Movements of this magnitude occurred.Introductions have beenreported, often on an follow logically from thescale and increasing rapidity of anecdotalor case-by-casebasis, as having occurredat transport ofpeopleand goods about the planet. The often many remotelocations (e.g. Wace 1961; Wace& Dickson large numbersof alien speciesthat occurin differentcoun- 1965; Steffan1970, 1972; Wace& Holdgate 1976; tries andon different land masses,and the species traits, Bonner1984; Chapuis et al. 1994; Dingwall 1995; Ernst- ecosystemproperties andfeatures of theintroduction pro- ing et al. 1995; Chown et al. 1998, 2001; Bergstrom & cessthat lead tothe observed variation have beenmuch Chown1999; Nickoll &Disney2001). However,the debated(e.g. Drake et al.1989; Williamson 1996; numberof species introduced to isolated areas with even Lonsdale1999; Blackburn &Duncan2001 a,b).Likewise, rather lowlevels ofhuman activity has beenless well therehas been considerable discussion about theoften sig- characterized, particularly for taxonomic groups other nificant economic,management andconservation conse- than plants. quencesof introduced organisms (see,for example, In this paper, wereport theresults of an explicit investi- Vitousek et al. 1996; Williamson 1996; Perrings et al. gation ofthe probable rates andscale of unintentional 2000; Pimentel et al. 2000; Lee2001; Simberloff 2001). introductionand establishment of pterygote (‘winged’) Twoissues seem to have attracted muchless explicit insectspecies to Gough Island, one of the most remote andparticularly empirical attention.The first is the andsupposedly pristine temperate oceanicislands. relation betweenthe level ofhuman activity in anarea and thenumber of introductions that result(McKinney 2001, 2002). Although thelatter may beknown, the former is 2. METHODS typically rather difficult toquantify, particularly over pro- Gough Island(40 °179–40°229 S, 9°529–10°019 W)liesin the middleof the South Atlantic, ca.350km south– southeast of the Tristan da Cunha islandgroup. It ispart of the UKDependent *Authorfor correspondence ([email protected]). † Present address: Schoolof BiologicalSciences, University of Wales, Territoryof Tristan da Cunha,and isadministeredfrom there. Bangor, Gwynedd LL572UW, UK. The 6500hectare (ha), mountainous island formed as aresult

Proc.R. Soc.Lond. B (2003) 270, 1091–1098 1091 Ó 2003 TheRoyal Society DOI10.1098/ rspb.2003.2332 1092K. J.Gaston andothers Speciesintroduction to an oceanicisland of Neogeneorogenic volcanism (Maund et al. 1988).With the fersto the islandhave beeneither by boat or,since the 1970s, exceptionof alow-lying plateau at its southernmost end,most alsoby helicopterlandings at the meteorologicalstation. of the landrises steeply to ridgesand peaks between700 m and The invertebratesof Gough Islandhave beensubject to two 910m above sealevel. The islandhas atemperateoceanic cli- intensivesystematic studies (see Holdgate (1965)for ahistory matewith strong prevailingwesterly winds; the meanannual ofzoologicalexploration inthe Tristan daCunha islandgroup). precipitation,recorded on the southern plateau ( ca. 70 m above The first was part of the Gough IslandScientific Survey of sealevel) between 1963 and 1990,was 3154mm, and the mean 1955–1956(Holdgate 1965). This study was detailedin terms monthly temperatures,recorded during the sameperiod and at ofthe pterygote ,and achievedmuch under difficult con- the samealtitude, varied between 8.9 °CinAugust to 14.5 °C ditions,although the smallersoil (which will not con- inFebruary. These conditionssupport an altitudinalsuccession cernus here)were under-worked. A relativelydepauperate of vegetation types, fromcoastal ‘tussock grassland ’ through to invertebratefauna was documented,a pattern incommon with ‘lowlandfernbush ’, ‘wet heath’, ‘peat bog’ and ‘feldmarkand other Southern Oceanislands (Gressitt 1970). Less than half of montanerock ’ communities(Wace 1961). The relativelack of the world’sinsectorders were found to bepresent, and those human influenceon Gough Islandhas resultedin the island occurringwere shown to beimpoverished in species numbers beingdeclared a wildlifereserve under the Tristan da Cunha and exhibitedlittle, if any, radiation(for full species lists see conservationordinance of 1976,gaining scientific/strictnature Holdgate (1965)). reservestatus underIUCN category Iin1985,and beingdesig- The seconddetailed study, the Gough IslandTerrestrial nated aworldheritage site(under criteria iii, iv) in 1995. The InvertebrateSurvey (GITIS),was conductedin 1999 –2002 conservationimportance of the islandhas beenunderlined in (Chown et al. 2000; Jones et al. 2002,2003). Over a continuous the findingsof recentpriority area analyses (Chown et al. 2001). 2-yearperiod of fieldwork,a widearray ofsampling techniques Becauseof its isolation,and becausethe terrainmakes landfall (includinghand searching,Tullgren extractions and Malaise difficult,it isextremelyunlikely that prehistorichuman colonists traps) wereused at many sitesacross the islandwith the explicit arrivedat Gough Island,and thereis noevidence that they did intentionof compilinga comprehensiverecord of the island ’s so.The first recordedhuman landfallwas madein 1675(Wace terrestrialand freshwater invertebrates.The resultant material 1969).Subsequent visitorsincluded members of sporadic was sortedand identified,with the assistanceof severaltaxo- nomicspecialists (see Acknowledgements). scientific/cartographic expeditions,and commercialsealing and Basedon these surveys, we have categorizedthe free-living whaling expeditionsof the nineteenthand earlytwentieth cen- pterygote insectspecies (excluding vertebrate parasites) turies(Wace & Holdgate 1976).The numberof landingsby recordedfrom Gough Islandas: (i)indigenous; (ii) introduc- sealinggangs, scientificexpeditions and other parties before tions first recordedin native habitats in1955 –1956;(iii) intro- 1955was determinedfrom the publishedhistorical literature ductionsfirst recordedin native habitats in1999 –2002;or (iv) (e.g.Morrell 1832; Wace 1969; Bruijn et al. 1979;Busch 1985; introductionsonly ever recorded as pests inthe meteorological Headland1989) and fromunpublished information provided to station and not found to occurin native habitats (1955 –2002). C. Ha¨nelfrom several sources (see Acknowledgments). The status of speciesas indigenousor introduced was estab- Although the earliesthistorical information (from the seven- lishedfrom information on their wider distributions and biology, teenth century)is sparse (owing mostly to veryfew visits),infor- and through consultationwith appropriate experts. The indigen- mationfrom the eighteenth centuryonwards isreliable and ous floraand fauna ofGough Island(along with that ofthe other relativelycomplete owing to morecareful recording of shipping, islandsof the Tristan da Cunha group) show greater affinities sealingand scientificvisits to the region.Since the Gough Island with South Americanstocks than with those fromthe closer ScientificSurvey of 1955 –1956(Holdgate 1965), a South Africanlandmass (e.g. Brinck 1948; Frey 1954; Rand 1955; Africanmeteorological station has beenestablished on the island Holdgate 1960,1965; Kuschel 1962; Moore 1979; Jones et al. (the island’sonlystanding human construction),with asmall 2003),and prevailingwesterly winds and oceancurrents may (typically6 ± 2)annually rotated staff (seeexpedition member- favour natural colonizationfrom South Americansources ship reportedin the S.Afr. J.AntarcticRes. )that seldomtravels (reflectedin the predominantsource of vagrant birdsarriving in muchbeyond its immediatevicinity. The numberof landings these islands(Williams & Imber1982; Cooper & Ryan 1994)). onthe islandin the period1956 –2000was determinedby using the publishedliterature (e.g. Headland 1989), limited circu- lationnewsletters of the South AfricanWeather Bureauin which 3. RESULTS ANDDISCUSSION expeditionmembers usually report eventssuch as the exchange Outof the 99 speciesof pterygote insectsrecorded from of goods with fishing vessels,and unpublishedvoyage schedules GoughIsland, 28 are indigenous(six endemicto Gough of the South Africanresearch and supply vessel,the S.A. Island,10 potentially endemicto GoughIsland andeight Agulhas.Foreach visit, the likelyorigin of the vesselresponsible endemicto the Tristan daCunha island group) and71 are for transport of the visitors(in some instances the last port of introduced(table 1; Appendix A).Of thoseintroduced, 15 calldiffered from the country operating the vessel),whether the specieshave beenrecorded only from theconfines of the vesselwas involvedprimarily in fishing,sealing or research, and meteorological station,and seem unlikely tospread whether the vesselhad visitedthe nearby populated islandof further.The restof the introduced species have been Tristan daCunha was recorded.In the last case,it was assumed recordedin native habitats, andare all likely tohave suc- that fishing vessels(usually crewed by South Africansand Tris- cessfullyestablished in asmuch as they have self-sustain- tan Islanders)and the reliefvoyages (but not emergencysupply ing populations (typically being abundantand orrescue voyages) ofthe S.A. Agulhas always calledat Tristan widespread).Overall, theproportion ofpterygote da Cunha beforeproceeding on to Gough Island,because this is speciesoccurring onGough Island that has beenintro- currentlythe normalprocedure. In nocases have ships actually ducedis high compared with figures for other Southern dockedat the island,owing to the absenceof aharbour. Trans- Oceanislands (figure 1).

Proc.R. Soc.Lond. B (2003) Speciesintroduction to an oceanicisland K.J.Gaston andothers 1093

Table1. Thenumbers of indigenous and introduced speciesof pterygote insects recorded from Gough Island.

introduced

only recorded as recorded in native first recorded in native meteorological station indigenous habitatsin 1955 –1956 habitatsin 1999 –2002 pests

Blattodea 0 0 0 2 0 2 1 5 1 4 4 0 Thysanoptera 2 0 6 0 Coleoptera 6 3 3 8 Diptera 16 11 13 0 3 3 0 0 Hymenoptera 0 2 4 0 total 28 25 31 15

All ofthe indigenous pterygote insectspecies recorded region of0.2 visits per year, andhas increasedto just over from GoughIsland by the1955 –1956 survey werealso 4.1 visits per year sincethen. recordedby the1999 –2000 survey.At least 12 indigenous Assuminga total of233 landings onGoughIsland, the pterygote insectspecies recorded by the1999 –2000 survey recordednumber of introduced species equates to one werenot recorded by the1955 –1956 survey,probably half successfulestablishment with every threeto fourlandings. ofthese a resultof improved , theothers being Concentratingon those introductions that have occurred rare andnot sampled in thefirst survey.The 1999 –2002 since 1955–1956, thesehave arrived at arate ofone spec- surveyalso recordedan additional 43 introducedspecies, iesevery five toeight landings (dependingon whether with twointroduced species recorded in 1955 –1956 (both meteorological stationpests are includedor not).If the meteorological stationpests) being absentin 1999 –2000. entireflora andfauna were considered, these rates would It seemslikely that mostof these additional specieshave besubstantially higher, asspecies from several other taxo- beenintroduced since 1955 –1956. If thesame proportion nomicgroups have also beenintroduced to Gough Island. ofintroduced species was present at thetime ofthe first Cooper& Ryan (1994) list 24 introducedspecies of surveybut went unrecorded, as was the case for indigen- plants,but this includessome that wereintroduced inten- ousspecies, then 24 species(36 including meteorological tionally, andexcludes subsequent unintentional introduc- stationpests) have beenintroduced since 1955 –1956. tions(see below). Introduced species also occurin several Approximately 51 landings weremade in theperiod other invertebrate groups;for example, all ofGough 1675–1956, although this is likely tobe a somewhatcon- Island’smyriapod fauna(three species) have beenintro- servative estimate.Of theselandings, more than half were duced,as has theterrestrial isopod Porcellioscaber , and by commercial sealersoriginating primarily from North mostof the island ’smolluscs(Preece 2001). Asingle America, andonly 10 wereconcerned with research(table speciesof introduced mammal, thehouse mouse ( Mus 2). The constructionand permanent, though annually musculus),currently occurson the island. rotated,occupation of the meteorological stationhas The principal possibleexplanations for thehigh rate of meanta considerableincrease in thenumber of landings, successfulestablishment of introduced species on Gough which have also beenaugmented by exchangesof goods Island are ahigh level ofintroduction ‘effort’, low biotic andpersonnel with thevessels (mostly justtwo) that have resistanceor goodenvironmental matching. There seems beenlicensed to fish aroundGough Island since 1963. little reasonto believe that particularly high numbersof Thus,in theperiod 1957 –2000, approximately 182 land- individuals ofgiven speciesarrive at theisland, thereby ings weremade. Most of these were from vesselsthat enhancingthe likelihood ofestablishing viable popu- departedfrom SouthAfrica, ofwhich approximately half lations.Indeed, its geographical isolation andthe low fre- werefishing vesselsdelivering goodsand personnel to the quencyof landings suggestsquite the opposite, as does the island (table 2). In total, 71 visits (mostly by theSouth proportion (28%) ofestablished introduced species that African researchand supply vessel)were directly associa- are knownto be parthenogenetic ( Jones et al. 2003), a tedwith meteorological monitoring, andlimited biological figure higher than that for mostcontinental faunas andgeological researchactivities at GoughIsland (CSIRO 1991). Whereas,for insectand other species, (Cooper& Ryan 1994; Chown et al. 2000). In total, there greater levels ofintroduction effort of given specieshave have thereforebeen approximately 233 landings at Gough frequentlybeen found to be associated with an increased Island sinceits discovery.Most ( ca.78%) ofthese have likelihood ofestablishment, even very small numbersof occurredsince the establishment of the meteorological founderindividuals canbe sufficient to give rise topopu- station,which has oneregular visit eachyear, with all lations that persist(Green 1997; Memmott et al. 1998; goodsand personnel now usually being transferredby Grevstad1999; Forsyth &Duncan2001). helicopter. Overall, thefrequency of landings sincethe It has long beenheld that high rates ofestablishment first human landfall has beenin theregion of0.7 per ofintroduced species may arise onislands from thede- annum,although before1956 this wasprobably in the pauperate natureof indigenous floras or faunasand the

Proc.R. Soc.Lond. B (2003) 1094K. J.Gaston andothers Speciesintroduction to an oceanicisland

) 2.2 speciesfor every landing ontheisland. However, for their 1 +

area, islandstend to supportmore introducedspecies than s e

i 1.8 docontinents (e.g. Brown 1989; Lonsdale1999; Black- c e

p burn& Duncan2001 b).This suggeststhat thetens rule s

d 1.4 probably overestimatesthe number of introductions that e c

u have takenplace onGoughIsland. None the less, the rule d o

r perhaps provides ausefulupper boundfor anestimate of

t 1.0 n

i thenumber of introductionsthat might have takenplace.

f o

Geological, geomorphological andpalynological evi- r

e 0.6

b dencesuggests that GoughIsland emergedfrom thesea m

u ca.2–3 Myr ago, didnot experience glaciation andhas n

( 0.2 maintained arelatively stable flora for thepast 43 000 g o l years (Maund et al. 1988; Bennett et al. 1989; Hall 1990). _0.2 0.8 1.2 1.6 2.0 2.4 Assumingboth auniform colonization rate, beginning 2Myr ago, andthat indigenouscongeneric species are a log (number of indigenous species +1) consequenceof in situ cladogenesis,only 21 colonization Figure 1. Therelationship between thenumber of events,one occurring every 95 000 years,are required to indigenous and introduced speciesof pterygote insects explain theextant indigenousfree-living pterygote insect recorded from islands in theSouthern Ocean. Gough Island faunaof Gough Island. Even assuming that 95% ofall isthe filled circle, and theslope isthe line of equality. Data specieson Gough Island have goneextinct, the coloniz- from compilation in Chown et al. (1998), updated for ation rate wouldstill amountto only onespecies per mil- Gough Island byrecords from GITIS (see text). lennium.By contrast,the rate ofspecies introductions over theperiod ofhuman activity at GoughIsland (325 opportunitiesthat theseprovide (for discussionsee years),amounts either to218 per thousandyears (71 spec- Wace& Holdgate 1976; Williamson 1996; Lonsdale iesin 325 years) or,assuming that thetens rule applies 1999). Indeed,several ofthe introduced pterygote insect toislands, 2184 per thousandyears. That is,a human- specieson Gough Island, such as predatory beetles,fill mediatedcolonization rate oftwoto threeorders of mag- ecological rolesthat previously seemto have beenempty. nitudegreater than thebackground rate. Nonethe less, recent studies have foundlittle general sup- If applied toGough Island, the tens rule wouldalso port for therole ofbiotic resistancein determining geo- meanthat approximately sevenspecies of pterygote insects graphical patternsof establishment of introducedspecies, shouldhave attained peststatus there. As definedby Willi- buthave foundevidence for adominanteffect of climate amson(1996), apestspecies is onewith anegative econ- matching (Lodge1993; Chown et al. 1998; PysÏ ek 1998; omic impact. Interpretedin theusual terms, the relevance Blackburn &Duncan2001 a; Gabriel et al. 2001), ofthis onGough Island wouldbe somewhat limited. although someexperimental workhas provided evidence However,if any ofthose species that have establishedin that more specioseassemblages may bemore resistantto thefield seemedlikely to,or did,pose a significant threat invasion (e.g.McGrady-Steed et al. 1997; Levine2000; tothe integrity ofthe indigenous flora andfauna, then Kennedy et al. 2002). Certainly, thecosmopolitan but considerablesums might have tobe expended to combat more temperate origins ofmany ofthe introduced species theproblem (underthe Convention on Biological Diver- (mostof which are also established,synanthropic species sity,the UK hasan obligation to‘ preventthe introduction in theWestern Cape Province ofSouth Africa (Jones et of,control or eradicate thosealien specieswhich threaten al. 2003)) have meantthat they are well adaptedto the ecosystems,habitats or species’). Atleast onother South- environmental conditionsencountered on the island. ernOcean islands, introduced animals andplants consti- Indeed,the relatively restrictedseasonality in climate tuteone of the most significant threats toindigenous (resulting from thestrong maritime influence)may well biotas (Bonner1984; Chapuis et al. 1994; Dingwall 1995; meanthat populations have escapedthe marked Chown et al. 1998; Bergstrom &Chown1999; Chown& reductionsin numbersthat they wouldexperience as a Gaston2000; Chown et al. 2001), asa resultof direct resultof seasonally severeconditions, which might be effectssuch as predation (Ernsting et al. 1995), herbivory characteristic oftheir ranges elsewhere. (Leader-Williams et al. 1987) andcompetition (Gremmen Ratesof establishment underestimate the frequency 1997; Gremmen et al. 1998; Frenot et al. 2001), and with which propagules (in thecase of pterygote insects, indirect effectssuch as the modification oflocal nutrient presumably usually adult individuals) ofalien species cycles(Smith &Steenkamp1990). Although noexamples arrive at GoughIsland. Williamson (1996) andWilliam- have yet arisen among theintroduced invertebrates where son& Fitter (1996) argue that ausefulrule ofthumb, the eradication has beenseriously contemplated (although ‘tensrule’ , is that ca.10% ofalien speciesfound in the someof these species are among themost abundant wild becomeestablished, and that ca.10% ofthose estab- invertebrates onthe island), the accidental introduction of lishedbecome pests (where, accepting thevariance about asmall herb,procumbent pearlwort ( Sagina procumbens ), suchgeneralizations, ca.10% meansbetween 5% and didcause such a problem (Gremmen et al. 2001). 20%). Onthe basis that all ofthe introduced pterygote Indigenousto the Northern Hemisphere,this speciesis insectspecies that have beenrecorded in thefield appear well knownfor its invasive tendenciesin theSouthern tohave established,this wouldmean that 710 (355–1420) Hemisphere,where it canform large mats that greatly speciesof pterygote insectshave actually at sometime influencelocal (Gremmen& Smith 1999; beenintroduced to Gough Island, or more than three N.J.M.Gremmenand S. L.Chown,unpublished data).

Proc.R. Soc.Lond. B (2003) Speciesintroduction to an oceanicisland K.J.Gaston andothers 1095

Table2. Thenumber of landings made atGough Island during theperiods 1675 –1956 and 1957 –2000. Landings are indicated byprovenance and bythe major activityin whichthe vessels involved were participating.

1675–1956 1957–2000

provenance landings provenance landings

North America 23 South Africa 160 United Kingdom 13 United Kingdom 13 South Africa 12 Europe 3 Europe 2 unknown 3 unknown 1 South America 2 Asian 1 total 51 total 182

activity activity

sealing 27 fishing 89 research 10 research 71 fishing 2 sealing 0

Discovery ofthe species on Gough Island in 1998 in the biotic homogenization (McKinney& Lockwood1999) is vicinity ofthe meteorological stationwas followed by the taking place in twoways. Extinctions reduce the matching rapid implementation ofan eradication programme that componentsof beta diversity (e.g.Krebs 1999) that con- includedweeding, herbicide application, theuse of a spec- tribute toplace distinctiveness,whereas introductions ially imported boiler toheat thesoil andthereby reduce enhancethose that contributeto similarity. Becauseinvas- theviable seedbank, and replanting ofindigenous species ive speciesare oftenresponsible for island extinctions, in clearedareas (Gremmen et al. 2001). Although success- thesetwo routes to homogenization are closely coupled. fulin eliminating visible plants,long-term andregular Therefore,simply by reducingthe numbers of human visi- inspectionsof the area, in addition tofurther eradication torsand increasing thestringency of quarantine pro- treatmentswhere required, will beneeded to prevent the cedures,the threat ofhomogenization canbe much build-upand spread of a population. Whether complete reducedon isolated islands. eradication isa possibility remains opento debate, and thereis asignificant risk ofpropagules being transported Our research atGough Island hasbeen supported bythe Dar- (by human or other agency) toother areas ofthe island win Initiative of theUK Department for Environment, Food and Rural Affairs, withlogistic support provided bytheSouth wherecontrol measures would be much more difficult to African National Antarctic Programme (SANAP)of the implement. This meansthat although eradication pro- Department of Environmental Affairs and Tourism. We thank grammes are possible,given that they have beensuccessful theElleman Resource Centre (University of Stellenbosch, elsewhere(Veitch &Bell 1990; Newman1994; Bester et South Africa) for its support, R.Skinner and K.Booyse al. 2000; Moro2001) andhave achieved ameasureof (SANAP)for their assistancewith organization and logistics; successon Gough Island, they are unlikely tobe practical. P.Ackery, T.Anderson, D.Barraclough, R.Belshaw, M.Buff- ington, D.Carter, P.Chandler, P.Cranston, H.Disney, In summary, ourdata from GoughIsland indicatethat I.Gauld, K.Harris, P.M.Hammond, J.Ismay, C.Lienhard, evenin remotelocations, where human activities are very L.A.Mound, W.Mathis,H. Mendel, I.Miller, D.Notton, lowby global standards,high ratesof biological invasion P.Oosterbroek, D.Quicke, J.Roha´cek, O.Saether, P.Stary, are apervasive featureof the landscape. Indeed, these F.vanVeen, P.Vilkamaa,T. Wheeler, and in particular R.I. ratesare twoto three orders of magnitude greater than Vane-Wright and other members of thestaff of theNatural background levels.Globally, recentspecies-level extinc- History Museum, London, for assistancewith the identifi- cation of material; B.Baldwin, J.Glassand theTristan da tions(most of which are onislands) are also taking place Cunha administration for their support; A.J.Beintema, R.K. at arate oftwo to three orders of magnitude greater than Headland, C.Jacobs, and K.Maraisfor assistancewith the previous background levels (May et al. 1995). Theseelev- compilation of visitor frequency estimates; and S.Gaston and atedrates of introduction and extinction suggestthat three anonymous referees for comments on themanuscript.

APPENDIXA: THE FREE-LIVING PTERYGOTEINSECTS RECORDEDFROM GOUGHISLAND Insectsalien tothe island are indicatedin bold;insects only recordedon the island post-1956 are indicatedby ticks.

Blattodea Diptera Blattellagermanica Ö Bradysianocturna Deropeltis sp. Ö Bryophaenocladius sp. Ö Calliphoracroceipalpis Psocoptera Clunio cf. africanus Cerobasisannulata Coelopa cf. africana Ö

Proc.R. Soc.Lond. B (2003) 1096K. J.Gaston andothers Speciesintroduction to an oceanicisland

Psocoptera( Continued.) Diptera (Continued.) Cerobasisguestfalica Ö Dicranomyia distans Ectopsocus briggsi Dimecoenia spp. (5 1 species) Ö bostrychophila Ö Dimecoeniatristanensis Liposcelis decolor Ö Drosophila punctatonervosa Ö Liposcelis pubescens Ö Fannia canicularis Lepinotusinquilinus Fucellia tergina ramburii Ö Leptocera caenosa Ö Limnophyes minimus Ö Hemiptera Lucilia sericata Aulacorthum circumflexum Lycoriella sp. A. Ö Aulacorthum solani Lycoriella sp. B. Ö Cavariellaaegopodii Ö Megaseliarufipes Nothodelphaxatlanticus Meoneura obscurella Jacksoniapapillata Mycophila fungicola Ö ornatus Ö Ornithomyiaparva Myzus persicae Ö Phthitiaplumosula Rhopalosiphumpadi Prosopantrumflavifrons Rhopalosiphumrufiabdominalis Ö Psychodaalbipennis Pullimosinaheteroneura Ö Thysanoptera Scaptomyzaaltissima Anaphothripsobscurus Ö Scaptomyzafrustulifera Aptinothripsrufus Ö Scatella sp. Ö Frankliniella antarctica Ö Sciophilaparviareolata Ö Lissothrips sp. Ö Spelobiaparapusio Ö Merothripsbrunneus Ö Symplectaholdgatei Nesothripsinaccessiblensis Ö Symplectasp. Ö Thripshawaiiensis Ö Telmatogeton sp. near sanctipauli Thripstabaci Ö Thalassosmittia sp. near thalassophila Ö Thoracochaeta sp. Ö Coleoptera Thoracochaetabrachystoma Ö Aridius sp. Thoracochaetazosterae Cercyon depressus Cryptolestes pusilloides Ö Lepidoptera Cryptophaguspseudodentatus Agonopterixgoughi Henoticus californicus Dimorphinoctuagoughensis Hylotrupes bajulus Ö Endrosissarcitrella Hylurgus ligniperda Monopiscrocicapitella Lancetesdacunhae Peridromagoughi Liodessus involucer Peridromasaucia Notolinus hottentottus Oryzaephilus surinamensis Ö Hymenoptera Pentarthrumcarmichaeli Aphidiuscolemani Ptinellanatvigi Ö Diadegma sp. Ö Ptinustectus Ö Kleidotoma sp. Quediusmesomelinus Phaenoglyphisvillosa Ö Sepedophilusfilicornis Ö Stilpnus sp. Ö Sitophilusoryzae Ö Trichopria sp. Ö Sphaeriestessculptilis Ö Stegobiumpaniceum Ö Tristanodesscirpophilus

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