Biogeographic Breaks in Vanuatu, a Nascent Oceanic Archipelago1

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 distributions and, importantly, the role of geologic processes in promoting biotic diversification. Biogeographic barriers such as Wallace’s line have been shown to be the result of old geologic processes shaping ancient faunal or floral diversification events. Based on distributions of birds, bats, reptiles, plants, and invertebrates we identify a distinct biogeographic disjunction in Vanuatu, a geologically nascent oceanic archipelago. We discuss mechanisms contributing to this concordant pattern across these disparate taxonomic groups in light of geologic history, ocean currents, vegetation, soil, and bioclimatic data, and propose the name Cheesman’s line to indicate the faunal and floral discontinuity between 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 PACIFIC SCIENCE . 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.

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