89? 729743 Original Article COCOS ISLAND BARK BEETLES (SCOLYTINAE) L. R. KIRKENDALL and B. H. JORDAL Biological Journal of the Linnean Society, 2006, 89, 729–743. With 5 figures The bark and ambrosia beetles (Curculionidae, Scolytinae) of Cocos Island, Costa Rica and the role of mating systems in island zoogeography LAWRENCE R. KIRKENDALL1* and BJARTE H. JORDAL2 1Department of Biology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway 2Museum of Natural History and Archaeology, University of Science and Technology, N-7491 Trondheim, Norway Received 11 November 2004; accepted for publication 15 January 2006 Cocos Island is a small oceanic island midway between Costa Rica and the Galápagos Archipelago; about 2 Myr in age, it is the only tropical oceanic island in the eastern Pacific with tropical wet forest. We identified several hundred bark beetle specimens collected during recent expeditions by INBio, the National Biodiversity Institute of Costa Rica, and re-examined all specimens from earlier collections. We report 19 species in ten genera, seven or eight of which are endemic, making scolytines the largest group of beetles known from the island. We describe as new Pycnarthrum pseudoinsulare, Xyleborinus cocoensis, and Xyleborus sparsegranulosus, resurrect Xyleborus bispinatus as separate from X. ferrugineus, and report six other species as new to Cocos Island. Three-quarters of the scolytines reproduce by brother–sister mating, and we argue that inbreeders are superior island colonists because they are less affected than are outbreeders by problems of mate location and inbreeding depression. The fauna and flora of Cocos Island arrived by dispersal and human transport. We examine natural colonization patterns for the fauna, using the distributions of the relatives of island endemics: most colonization came from the Americas, but the closest relatives to some endemics are found on Caribbean or Galápagos islands. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 729–743. ADDITIONAL KEYWORDS: Baker’s rule – colonization – inbreeding – island biogeography. INTRODUCTION as evolutionarily separate entities. However, detailed analyses of island diversity and endemism are criti- Since the voyage of the Beagle, investigations of island cally dependent upon thorough sampling and sound biotas have led to key insights into major evolutionary taxonomic analyses of focal taxa. Small islands are processes behind the origin of organismal diversity. therefore excellent model systems, because these can Research on the Hawaiian, Canary, and Galápagos be sampled more thoroughly for hyperdiverse taxa Islands in particular has provided a wealth of infor- such as arthropods, resulting in more complete taxo- mation on modes of speciation, relationships between nomic lists. ecology and speciation, and the time scales over which Cocos Island (Isla del Coco) is a small island speciation processes occur (Juan et al., 2000; Gillespie (46.6 km2) belonging to Costa Rica, located midway & Roderick, 2002). Islands that are isolated over between that country and the Galápagos Archipelago, extended geological time scales ultimately foster about 500 km west of Costa Rica and 630 km north- divergent phenotypes or ecotypes, often sufficiently east of the Galápagos. The island was formed by different from their continental relatives to be named recent (2 Mya) volcanic activity; its highest peak, Cerro Iglesias, is 575 m (Castillo et al., 1988). Because of its small size, young age and isolation, this tropical *Corresponding author. island is particularly suited to the study of coloniza- E-mail: [email protected] tion and establishment of populations; among evolu- © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 729–743 729 730 L. R. KIRKENDALL and B. H. JORDAL tionary biologists, it is perhaps best known for studies logical sense to refer to those scolytines breeding in of ecological release or niche expansion, in which sin- inner bark.) Four papers have dealt with the bark bee- gle species experiencing little or no interspecific com- tle fauna of Cocos Island (Schedl, 1974; Bright, 1982; petition evolve an extraordinary degree of ecological Wood, 1982; Wood, 1986a), bringing the number of and behavioural plasticity [as in a theridiosomatid species previously known from the island to 10. spider (Eberhard, 1989) and the endemic ‘Darwin’s We describe three species new to science, report finch’ (Werner & Sherry, 1987)]. Of particular impor- another seven species as new to the island, and correct tance is the fact that Cocos Island, with its 7-m rain- previous misidentifications, resulting in a compilation fall annually, is the only tropical oceanic island in the of 19 species of Scolytinae – making bark beetles the eastern Pacific that has tropical wet (as opposed to largest group of beetles, tied with ants as the largest dry) forest, a habitat which is similar in form and family or subfamily of arthropods currently known structure (though not in species composition) to those from Cocos Island (Solomon & Mikheyev, 2005). We of most of the closest potential mainland source areas conclude with a brief discussion on general zoogeo- in Central and northern South America (Hogue & graphical trends that can be deduced from patterns of Miller, 1981). endemism for the terrestrial fauna of Cocos Island. As is typical for such a small isolated island, the flora and fauna are depauperate relative to their MATERIAL AND METHODS continental source communities, and taxonomically imbalanced (‘disharmonious’), with the island lacking Cocos Island collections examined and type material many abundant mainland taxa (Fournier, 1966; Hogue are deposited in the National Institute for Biodiver- & Miller, 1981; Brown, 1990). Among terrestrial ver- sity, Costa Rica (INBio) unless noted otherwise. Other tebrates, for example, there are only four resident bird collections examined included those from the Museum species, two reptiles, one freshwater fish, and no of Comparative Zoology, Harvard University (MCZ); amphibians or native mammals (World Conservation Natural History Museum, London (NHM); California Monitoring Centre, 1997). Fewer than 400 insect, Academy of Science (CAS); Natural History Museum arachnid, and myriapod species have been recorded of Los Angeles County (LACM); Stephen L. Wood, from the island (Hogue & Miller, 1981; Bright, 1982; Brigham Young University, property of the US Cwikla & Freytag, 1982; Johnson, 1982; Adams, 1983; National Museum (SLWC); private collection of Nickle, 1983; Bellamy, 1986; Brown, 1990; Brown, Lawrence R. Kirkendall, University of Bergen, Nor- Donahue & Miller, 1991; Woodley, 1991; Davis, 1994; way (LRKC). Material of the X. ferrugineus complex Jordal, 1998a; Brown & Miller, 1999; Anderson & Lan- was borrowed from the Zoological Museum of the Uni- teri, 2000). Wood-boring, leaf-mining, and epiphyte- versity of Copenhagen (ZMUC), the Royal Museum for dwelling species are among the more species-rich Central Africa (RMCA, in Belgium), Institute Royal insect groups on the island, supporting a key role for des Sciences Naturelles de Belgique (IRSNB), and the rafting on plant material in the colonization process Karl Schedl collection in the Natural History Museum (Hogue & Miller, 1981). of Vienna (NHMW). Comparative studies of island faunas have demon- Species determinations of problematic taxa were strated a close relationship between mating behaviour made by direct comparison with type material and the bark beetle (Curculionidae, Scolytinae) com- wherever possible. Identification of scolytines often munities of islands: species with regular brother–sis- requires meticulous observation of surface sculpture ter inbreeding are overrepresented on small islands details (e.g. Wood, 1982) which are difficult or impos- compared with neighbouring mainland forests; tropi- sible to see with normal light sources (such as fibre cal islands have a higher proportion of inbreeders optic lights); we used one or two small flourescent than do temperate islands; outbreeding species are lamps with 7-W bulbs as sources of flat light for this more likely than are inbreeders to speciate on small purpose. islands (Kirkendall, 1993; Jordal, Beaver & Kirken- Terminology follows Wood (Wood, 1986b) and mea- dall, 2001). Faunistic details for particular islands and surements were in accordance with methods described archipelagos are needed in order to explore such pat- by Jordal (1998a). Note that all measurements were terns further and to understand more fully the histor- based on perpendicular views of the dorsal aspect of a ical and ecological conditions necessary for successful specimen. Species are listed alphabetically by genus, colonization (Bright & Peck, 1998). with the order of genera following the classification by In this context, we here update the faunal composi- Wood (1982). Full synonymies and other taxonomic tion of scolytine beetles for Cocos Island, roughly dou- information for previously described species can be bling the number of species known from the island. found in Wood & Bright (1992) and are not repeated (Scolytines we refer to collectively as ‘bark beetles’, here. Complete label data are given only for type but this term is also used in a more restrictive, eco- material of new species; label data were translated © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 729–743 COCOS ISLAND BARK BEETLES (SCOLYTINAE) 731 from Spanish