Biogeographic Breaks in Vanuatu, a Nascent Oceanic Archipelago1

Alison M. Hamilton,2,3,6 Elaine R. Klein,4,5 and Christopher C. Austin2

Abstract: The study of distinct biogeographic demarcations has played a pivotal role in our understanding processes responsible for patterns of species distribu- tions and, importantly, the role of geologic processes in promoting biotic diver- sification. Biogeographic barriers such as Wallace’s line have been shown to be the result of old geologic processes shaping ancient faunal or floral diversi- fication events. Based on distributions of birds, bats, reptiles, plants, and invertebrates we identify a distinct biogeographic disjunction in Vanuatu, a geo- logically nascent oceanic archipelago. We discuss mechanisms contributing to this concordant pattern across these disparate taxonomic groups in light of geo- logic history, ocean currents, vegetation, soil, and bioclimatic data, and propose the name Cheesman’s line to indicate the faunal and floral discontinuity be- tween the northern and southern islands of Vanuatu.

The distribution of plants and animals (Sclater 1858, Wallace 1860, 1876). Disconti- among the isolated, oceanic islands of the nuity between distinct Asian and Australian tropical Pacific has been of interest to evolu- biotas has motivated a large body of research, tionary biologists for more than 150 yr resulting in designations of the western (Huxley’s line or Wallace’s line) and eastern (Lyddeker’s line) limits of a region of biotic overlap between Asian and Australian faunas, 1 Funding for this project was provided by the Na- tional Science Foundation (DEB 0408010, DEB a region generally referred to as Wallacea 0445213, and DBI 0400797), an EPSCoR Fellowship (Wallace 1860, Huxley 1868, Lydekker and grants from Graduate Women in Science, the Amer- 1896, Weber 1902, Mayr 1944, Holloway ican Society of Ichthyologists and Herpetologists, the So- and Jardine 1968, Simpson 1977). Efforts to ciety for the Study of Amphibians and Reptiles, LSU chapter of Sigma Xi, the University of North Dakota identify the line of faunal balance within (Graduate School, Office of Research and Program De- Wallacea, west of which greater than 50% velopment and the Department of Biology), the LSU of the biota is derived from Asia and east of Museum of Natural Science, and LSU BioGrads. Re- which greater than 50% of the biota is Aus- search was conducted under LSU IACUC Protocol no. tralian (Wallace 1876, Scrivenor et al. 1943, 03-121. Manuscript accepted 9 July 2009. 2 Department of Biological Sciences and Museum of Mayr 1944, Lincoln 1975), led to detailed Natural Sciences, 119 Foster Hall, Louisiana State Uni- analysis of the biogeography of the region, versity, Baton Rouge, Louisiana 70803. enabling an appreciation of the roles of geol- 3 Current address: Department of Ecology and Evo- ogy, ecology, and evolution in shaping these lutionary Biology, 621 Charles E. Young Drive South, University of California Los Angeles, Los Angeles, Cali- biotas. fornia 90095. Within the Pacific Basin itself, the flora 4 Section of Integrative Biology and Texas Memorial and fauna of the islands of the western Pacific Museum, University of Texas, Austin, Texas 78712. are distinct from the biota of the islands oc- 5 Current address: Department of Biology, San Diego curring farther east and those in the northern State University, 5500 Campanile Drive, San Diego, CA 92182. or southern Pacific (Hedley 1899). Stoddart 6 Corresponding author (e-mail: ahamilton@ucla (1992:291) suggested that the most remark- .edu). able biotic discontinuity within the Pacific Basin occurs at the Tonga Trench (Hedley’s line) and argued that this discontinuity was Pacific Science (2010), vol. 64, no. 2:149–159 doi: 10.2984/64.2.149 biologically significant because islands on : 2010 by University of Hawai‘i Press either side of Hedley’s line differed with re- All rights reserved spect to ‘‘altitude, climate, and other envi-

149 150 PACIFICSCIENCE . April 2010 ronmental constraints,’’ rather than simply through periods of emergence and submer- geographic coordinates. gence, with the last period of emergence In addition to the broad distributional only in the last 2 myr (Carney et al. 1985, patterns in the Pacific Basin, it is important Greene et al. 1988a, Macfarlane et al. 1988). to understand biogeographic patterns on a In addition to being biologically and geologi- smaller scale, because the evolutionary his- cally nascent, the islands of Vanuatu are rela- tory of the lineages within the Pacific can tively isolated from other components of the be accurately evaluated only when distri- OMA, surrounded by the Vanuatu Trench, butional patterns—and disjunctions within the Vitiaz Trench, and the Johnson Trough them—are recognized. The Outer Melane- (Coleman 1970). The current position of sian Arc (OMA), stretching from New Guinea Vanuatu is a result of counterclockwise rota- through the Solomon Islands, Vanuatu Archi- tional movement since the Miocene (Gibbons pelago, Fijian Islands, the islands of Samoa, 1985, Greene et al. 1988a); before that rota- and southeast to Tonga (Kroenke and Rodda tion the islands of Vanuatu were located to 1984), is a major geological feature of the the north of Fiji and Tonga, adjacent to the Pacific Basin west of the Tonga Trench. Vitiaz Arc (Chase and Seekins 1988) and Components of the OMA are the result of more isolated than now. tectonic activity over the last 11.2 to 2 myr The geologically recent emergence of the (Kroenke and Rodda 1984), and the biota of Vanuatu Archipelago (2 mya) explains some this region results from speciation within the of the reptile and amphibian faunal discon- OMA (Filardi and Moyle 2005, Hamilton tinuities previously identified in Vanuatu 2008) as well as dispersal from Southeast such as the absence of frogs (Platymantis) Asia, New Guinea, Australia, and New Cale- and elapid (Ogmodon) snakes (Schmidt 1930, donia (Mayr 1965, Diamond and Mayr 1976, Zweifel and Tyler 1982). In addition, the Bauer 1988, Brown 1991, Zug 1991, Flannery absence of Ogmodon and Platymantis from 1995, 2000, Allison 1996, Austin 2000, Mayr Vanuatu—as well as the presence of Perochi- and Diamond 2001, Carvajal and Adler 2005, rus lizards in Vanuatu—has been suggested Helgen 2007, Hamilton et al. 2009). to result from the north to south counter- Historically, sharp biogeographic demar- clockwise rotational geologic movement of cations that we recognize and name as lines this archipelago (Gibbons 1985). The historic have been shown to be the result of old geo- position of Vanuatu was to the north of the logic processes shaped by ancient faunal and current location (Chase 1971); a location that floral diversification events. Wallace’s line, would explain how groups such as Ogmodon for example, identifies the suture between and Platymantis could have colonized Fiji the ancient Austral Papuan fauna of Gond- from the Solomon Islands yet not be present wanan origin with the ancient northern in Vanuatu. The distribution of Perochirus is Asian fauna derived from Laurasia. Similarly, restricted to Micronesia, with a single species Huxley’s line is concordant with the edge of (P. guentheri) occurring in Vanuatu (Bauer the continental Sunda and Sahul shelves and 1988). An older Vanuatu located north of the deep Makassa and Lombok trenches the current position of the archipelago would (Hahn and Sytsma 1999), and Lyddeker’s have permitted Perochirus to colonize Van- line occurs along the edge of the continental uatu from Micronesia (Gibbons 1985). The shelf to the west of West Irian and Australia absence of groups of vertebrates such as frogs (Simpson 1977). In contrast, our work in the and elapid snakes led to the suggestion that geologically young Vanuatu Archipelago sug- the fauna of Vanuatu may be depauperate gests that biogeographic breaks can be found (Baker 1928, 1929, Darlington 1948, Bauer in geologically young (likely less than 2 myr 1988). Recent examination of the terrestrial old) faunas. reptile diversity in Vanuatu in light of island The Vanuatu Archipelago comprises 82 is- area, isolation, and emergence history indi- lands. The oldest Vanuatu rocks are dated to cated that, in general, Vanuatu meets the 14 mya; however, the entire archipelago went expectations of diversity generated by predic- Vanuatu Biogeography . Hamilton et al. 151 tions of island biogeography theory (Hamil- mean temperature, mean diurnal range, max- ton et al. 2009). imum temperature of the warmest month, The origin and distribution of the biota of minimum temperature of the coldest month, Vanuatu have not received much attention annual precipitation, precipitation of the wet- but when discussed have generally been con- test month, and precipitation of the driest sidered to result primarily from dispersal out month. These variables were chosen to cap- of the Solomon Islands (Medway and Mar- ture major variation among islands in annual shall 1975, Gibbons 1985, Bauer 1988, Ota means and seasonality of precipitation and et al. 1998). In addition, a distinct faunal and temperature because these environmental floral break between Vanuatu and neighbor- conditions are likely to influence the distri- ing New Caledonia has been noted (Bauer bution of a small, ectothermic reptile. To 1988, Bauer and Vindum 1990, Bauer and Sad- determine whether environments were differ- lier 1993, 2000, Hamilton et al. 2009). Here, entiated across the species range limit of C. we present data from our ongoing research atropunctatus, we performed a principal com- on the reptile fauna of the Vanuatu Archipel- ponents analysis (PCA) on environmental ago on biogeographic patterns for squamate variables collected for the approximate center reptile species within this island group and of each island. note a distinct biogeographic barrier within the Vanuatu Archipelago. We focus on the scincid lizard species Caledoniscincus atropunc- results and discussion (Roux 1913) as an example, report new tatus Relationship between Vanuatu and New distributional records for C. atropunctatus, Caledonian Squamate Faunas and use this species to illustrate and discuss general biogeographic patterns within the 82 Despite the geographic proximity of the New islands of Vanuatu. The biogeographic pat- Caledonian Loyalty Islands and the southern- terns highlighted by C. atropunctatus, particu- most Vanuatu island of Aneityum (only 150 larly a biotic break between the islands of km between these island groups), the rep- southern Vanuatu (Erromango, Tanna, Fu- tilian faunas of New Caledonia and the tuna, Aniwa, and Aneityum) and the islands Vanuatu Archipelago are dissimilar (Bauer of central and northern Vanuatu, are congru- 1988, Hamilton et al. 2009). New Caledonia ent with patterns recovered from a review of has a highly endemic reptile fauna, with 86% the literature for a broad range of diverse tax- of the terrestrial reptile species of New Cale- onomic groups including vertebrates, inverte- donia restricted to those islands (Bauer and brates, and plants. Sadlier 2000). Only nine of the 71 New Cale- donian squamate species are shared with Van- uatu (Hamilton et al. 2009), and three of materials and methods those nine species (Ramphotyphlops braminus, Between 2001 and 2005 we conducted four Lepidodactylus lugubris, and Hemidactylus frena- comprehensive field expeditions that surveyed tus) are the result of human-mediated intro- 53 sites on 19 islands throughout the Vanuatu ductions into both island groups (Bauer and Archipelago and encompassed all major is- Sadlier 2000, Hamilton et al. 2009). Previous lands (Figure 1). At each site the lizard fauna authors have provided evidence for a distinct was surveyed using a variety of techniques faunal break between New Caledonia and the (Hamilton et al. 2007, 2008, Hamilton Vanuatu Archipelago (Baker 1928, 1929, 2008). Voucher specimens were collected Solem 1976, Bauer 1988), and noted that the and deposited in the herpetology collection two regions are separated by the deep (6,400 at the Louisiana State University Museum of m) Vanuatu Trench (Greene et al. 1988b). In Natural Science (lsumz). contrast to the dissimilarity with Vanuatu We obtained data for seven bioclimatic fauna, the New Caledonian squamate fauna variables from WorldClim (Hijmans et al. has affinities with Australia and New Zealand 2005), gridded to a 1 km resolution: annual (Bauer and Sadlier 1993, 2000), reflecting the Figure 1. Distribution of Caledoniscincus atropunctatus in the Vanuatu Archipelago based on fieldwork conducted at 53 sites on 19 islands over a 5 yr period is indicated by circles: survey sites at which C. atropunctatus was not observed are indicated with open circles and sites at which C. atropunctatus was collected are indicated by closed circles. The location of Cheesman’s line, a biotic break within the Vanuatu Archipelago congruent with data from several taxonomic groups including invertebrates, vertebrates, and plants is shown along with the summarized data and sources supporting this barrier. Distribution of C. atropunctatus in Vanuatu is congruent with this suggested biotic break because the species is restricted to the islands south of Cheesman’s line. Vanuatu Biogeography . Hamilton et al. 153

Gondwanan origin of New Caledonia. In ad- punctatus from the central island of Efate. dition, the long history of island emergence A.M.H. examined that specimen (amnh and isolation is likely responsible for the 42002) and determined that it is not C. atro- high rate of endemism in New Caledonia. punctatus but a species of Emoia based on the The oceanic islands of the Vanuatu Archi- presence of supranasals and fused frontopa- pelago are considerably younger than New rietals (Brown 1991). We surveyed eight dif- Caledonia (Greene et al. 1988b, Macfarlane ferent localities on and around Efate Island et al. 1988, Bauer and Sadlier 2000), and, in (Figure 1), and C. atropunctatus was never ob- general, the squamate fauna is derived from served or collected despite extensive searches overwater dispersal from New Guinea via of habitats typically occupied by this species the Solomon Islands (Bauer 1988, Allison elsewhere in Vanuatu. Based on our extensive 1996). Due to its Papuan origin, the Vanuatu field surveys and the erroneous single speci- squamate fauna is generally more similar to men record from Efate we conclude that C. those of other components of the Outer Mel- atropunctatus does not occur on Efate Island anesian Arc, such as the Fiji Archipelago and or any of the northern or central islands. the Solomon Islands (Hamilton et al. 2009). Of the six reptile species that are components Biogeographic Patterns within the Vanuatu of the native reptile fauna and have distribu- Archipelago tions that encompass both New Caledonia and the Vanuatu Archipelago, five are Papuan Caledoniscincus atropunctatus is distributed in origin, with Caledoniscincus atropunctatus throughout the southern islands of the Van- representing the sole species originating in uatu Archipelago but does not occur in the New Caledonia. This disparity may be due central and northern islands of this group. to the ocean current between Vanuatu and This distribution within Vanuatu mirrors the New Caledonian Loyalty Islands, because the distribution of other lizard species. The the southerly direction of this flow system has gecko Nactus pelagicus occurs on the same been suggested to promote colonization of five southern islands, with its congener, Nac- New Caledonia from Vanuatu or Fiji (Gib- tus multicarinatus, being found on the north- bons 1985, Treml et al. 2008). ern and central islands. The distributions of some scincid lizards in the genus Emoia also provide support for the presence of a faunal Results of Herpetofaunal Surveys in Vanuatu break between northern and central islands Fieldwork documented that Caledoniscincus (the islands of the Torres Group south to atropunctatus was common and widely distrib- Efate Island) and southern islands (Erro- uted on Erromango Island, Aneityum Island, mango Island south to Aneityum Island) and Tanna Island (Figure 1). Collections (Figure 1). Emoia sanfordi is distributed made during our fieldwork expanded the throughout the northern and central islands known range of the species in Vanuatu to in- of the Vanuatu Archipelago but does not clude the southern islands of Aniwa and Fu- occur south of Efate Island (Hamilton 2008, tuna (Figure 1). On all islands from which it Hamilton et al. 2008). Other species of Emoia was collected, C. atropunctatus was numeri- (including E. aneityumensis and E. erronan) cally common in both natural and semi- that are similar in body size, habitat use, and disturbed habitats. This species was found ecology to E. sanfordi are endemic to southern in the leaf litter and was observed basking Vanuatu (Hamilton 2008) and further cor- in patches of sunlight on the forest floor. roborate the presence of a distinct lizard Although this species was collected from fauna in the southern islands of the archipel- patches of the forest edge adjacent to gar- ago. In addition, other lizard species such as dens, villages, and overgrown coconut planta- Gekko vittatus (Gekkonidae) and Emoia nigra tions, it was more abundant in larger patches (Scincidae) have a distribution that encom- of coastal forest or interior mixed hardwood passes the Solomon Islands and the islands forests. of northern Vanuatu but no farther south Burt and Burt (1932) also recorded C. atro- (Medway and Marshall 1975, Cranbrook 154 PACIFICSCIENCE . April 2010

1981, 1985a,b, Whitaker and Whitaker 1994, connection between this southern landmass Hamilton Jennings and Austin 2002). and the northern islands of Vanuatu during This biogeographic pattern is not re- the Miocene-Pliocene allowed the invasion stricted to squamate reptiles. Distinctly from the north of the Papuan (Melanesian) similar distributional breaks have been docu- biota that is present in southern Vanuatu mented in plants (Braithwaite 1975, Chew (Cheesman 1957). The contemporary geo- 1975, Gillison 1975), bats (Medway and Mar- logical literature, however, has provided a shall 1975, Flannery 1995), birds (Doughty better understanding of the tectonic and geo- et al. 1999), freshwater gastropods (Haynes logic history of the Vanuatu Archipelago and 2000), ground beetles (Liebherr 2005), but- indicates that the islands of Vanuatu are terflies (Gross 1975, Ackery et al. 1989), and young and of oceanic origin (Lee 1975). Fur- a number of other invertebrates (Gross ther, there is no evidence for a previous con- 1975). Two general distributional patterns nection between the islands of Erromango, can be seen across a broad range of unrelated Tanna, and Aneityum, nor for a connection taxa: (1) a genus is distributed throughout between these islands and the Gondwanan the Vanuatu Archipelago, with the biota of landmass of New Caledonia (Lee 1975, northern and central islands represented by Greene et al. 1988b). The distributional pat- a different assemblage of species than the tern of the flora and fauna of Vanuatu there- southern Vanuatu islands; (2) each biotic re- fore has been the result of dispersal rather gion in Vanuatu (northern and central versus than vicariance. southern) is represented by a suite of genera, The biotic disjunction between the islands none of which bridges the biotic break be- of northern and central Vanuatu and those of tween Efate and Erromango islands. southern Vanuatu may be influenced by cli- The concordance between the Vanuatu mate as well as soil and vegetation type (Fig- distribution of C. atropunctatus and other bio- ure 2). A PCA of seven climate variables ta that dispersed from New Caledonia sug- likely to influence the distribution of organ- gests that the occurrence of this species in isms (such as ectotherms and plants) sensi- Vanuatu results from a natural dispersal event tive to environmental conditions shows clear from New Caledonia. The widespread distri- differences between the two biotic regions bution throughout the range and the high (Table 1, Figure 2). The climate data suggest abundance and frequent occurrence of this that the environmental conditions of the is- species in native, forested habitats provide lands of northern and central Vanuatu are further support for the inclusion of C. atro- wetter and less thermally variable than on punctatus as a component of the native reptile southern Vanuatu islands, as indicated by the fauna of Vanuatu. increases in mean annual precipitation and The distributional disjunction between the minimum temperature of the coldest month islands of southern Vanuatu and the northern in the northern islands, and the greater diur- and central islands of Vanuatu was previously nal temperature variation in the southern is- noted by Cheesman (1957), who suggested lands (Figure 2). Although the environments that this break represented a zoogeographic of the islands clearly vary with latitude, the line analogous to Wallace’s line but not as PCA of the WorldClim bioclimatic data illus- sharply defined. Cheesman (1957), in a pre– trates the environmental differences among plate tectonics era, suggested a vicariance islands within the Vanuatu Archipelago. model for the pattern she observed. She ar- In addition to the latitudinal cline in envi- gued that the floral and faunal similarity of ronmental conditions, soil type varies among the southern islands of Erromango, Tanna, these islands. The islands of Erromango and and Aneityum is due to a land-bridge connec- Tanna have different soils, and fewer soil tion between these islands and presumed that types, than the central islands of Efate, Epi, Erromango, Tanna, and Aneityum were pre- Espiritu Santo, Maewo, Malakula, Pentecost, viously a single landmass with a connection and the Shepherd Islands (Quantin 1975). to New Caledonia. Subsequent hypothesized Habitats and habitat diversity also vary be- Vanuatu Biogeography . Hamilton et al. 155

Figure 2. Results of a principal components analysis of seven bioclimatic variables influencing the distribution of plants, ectotherms, and invertebrates. Bioclimatic variables included in this analysis and the factor loadings for each principal component are provided in Table 1. Data for the five southern islands (south of Cheesman’s line) are indi- cated with black circles, and the data for the northern and central Vanuatu Islands (north of Cheesman’s line) are in- dicated with open circles. Northern and central islands are characterized by greater precipitation, higher minimum temperatures, and lower mean variation in daily temperature than are the islands of southern Vanuatu.

TABLE 1 tween the islands of northern and central Climate Variables for Islands in the Vanuatu Archipelago Vanuatu and the southern Vanuatu islands, and Loading for Each Variable on the First Two likely driven by clinal variation in climatic Principal Components factors and variation in island soils. Based on vegetation data collected from 54 sites in the Climate Variable PC1 PC2 Vanuatu Archipelago, Gillison (1975) recog- nized 12 major structural vegetation types in Annual precipitation 0.931 0.355 Mean diurnal range 0.929À 0.218 this island group. Ten of these 12 vegetation Precipitation of the driest monthÀ 0.918 0.305 types occur only in the northern and central Precipitation of the wettest month 0.887 À0.430 islands: fern forest, fern thicket, and eight À Minimum temperature of coldest 0.841 0.525 types of vine forest; the two others (both month types of evergreen vine forest) occur only in Annual mean temperature 0.629 0.777 Maximum temperature of warmest 0.278 0.949 the southern islands (Gillison 1975). month Differences in environmental conditions and vegetation structure within the Vanuatu Note: PC1 explains 64.85% of the variation, and PC2 explains an additional 31.89%. Climate variables used in this analysis were Archipelago in turn influence ecological fac- obtained from the WorldClim database. tors such as resource availability and habitat 156 PACIFICSCIENCE . April 2010 suitability (such as the thermal preferences of fauna of the New Hebrides. Ann. Mag. ectothermic species) and contribute to the Nat. 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