The intent of this paper is to facilitate future research of the Solomon Island ant fauna and that of the larger Pacific Island region by providing the first comprehensively researched species list in over 75 years (Mann 1919; Wheeler 1935b). Reliable species lists are the foundation for biodiversity and biogeography research. This is especially true for archipelago systems such as the Solomons which serve as natural laboratories for studying the interface of geography, evolution and ecology (Diamond 1975; Diamond and Mayr 1976; Greenslade 1968; MacArthur and Wilson 1967; Mayr and Diamond 2001; Wilson 1959a; 1961). Accurate faunal lists at the archipelago level allow us to analyze biogeographic patterns at the regional scale, and faunal lists at the individual island level allow us to analyze more local scale patterns. These studies are crucial for the development of precise conservation plans that incorporate the distribution of endemic and rare taxa.

Faunal lists are also necessary for recognizing biodiversity blind spots and identifying which regions and islands are most in need of additional sampling. Increasing environmental threats such as deforestation, mining, agriculture and the spread of invasive species give urgency to surveying these poorly sampled regions. In order to assess how these threats affect native biodiversity, it is important to establish baseline inventories before local populations and endemic species are driven extinct.

The Solomon Islands is a nation in the Southwest Pacific that is composed of seven large islands, a dozen mid-sized islands and over a thousand smaller islands (Figure 1). These islands, which comprise a total land area of 27, 556 km², are situated between the latitudes 5° and 13°S, and longitudes 155° and 169°E. The major central islands include the Shortlands, Choiseul, the New Georgias, Santa Isabel, the Russells, Guadalcanal, the Nggelas (Floridas), Malaita, Makira (San Cristóbal), and Olu Malau (Three Sisters). Rennell and Bellona are southern outlying islands situated along the northern margin of the Coral Sea Basin. Northern outlying islands include Sikaiana and the Ontong Java Atoll, which are on the southwestern edge of the Ontong Java Plateau. The eastern outlying islands of the Santa Cruz group are politically part of the Solomon Islands, but are geologically linked to the islands of Vanuatu (Kroenke and Rodda 1984).

The Solomons consist of a double chain of islands separating the Pacific Plate to the north from the Australian Plate to the south (Hall 2002). The islands are believed to have been formed entirely of oceanic origin, and there is no evidence that they were ever attached to continental systems or incorporated any terrains of continental origin (Kroenke and Rodda 1984). They are, in this sense, Darwinian Islands (Gillespie and Roderick 2002). According to several geologic models (Hall 2002) the Solomon Arc formed approximately 40 Ma as part of the Melanesian Arc system. It is unclear, however, when the islands emerged above sea level.

Bougainville, which lies to the west, belongs politically to Papua New Guinea but is geographically part of the Solomon Islands. The next closest neighbor nation is Vanuatu, which lies southeast of the main archipelago and nearly due south of the Santa Cruz Is.

The climate of the Solomon Islands is characterized as humid with a mean temperature of 27 °C (80 °F) and relatively few fluctuations of temperature or weather. The cooler and drier part of the year occurs from June through August, and the warmer and wetter season occurs from September through May. The annual rainfall is approximately 3050 mm (120 in).

The first ants described from the Solomon Islands were authored by Forel (1910) in a paper on Australian ants based on the collections of W.M. Froggatt and Rowland Turner. Froggatt visited the Solomon Islands to study the insects of the coconut palms, and collected at Tulagi I. and in the Russell Group. W.M. Mann (1919) provided the first and only comprehensive revision of the Solomon Island ant fauna. He spent six months on the archipelago from 19 May to 24 November 1916, and collected on the islands Guadalcanal, Makira, Malaita, Malaupaina, New Georgia, Nggela Sule, Owaraha, Rendova, Russell Is., Santa Cruz, Santa Isabel, Tulagi and Ugi. Mann reported the occurrence of 136 currently recognized species and subspecies, of which he described 68 from his own collections. In addition to a broad discussion of the archipelago’s ant fauna, the treatise also includes keys to Melanesian species of Anochetus, Crematogaster, Cryptopone, Eurhopalothrix, Leptogenys, Myrmecina, Triglyphothrix (= Tetramorium), Turneria, and Wheeleripone (= Gnamptogenys).Additional relevant publications from Mann include descriptions of ant guests from Fiji and the Solomon Islands (Mann 1920), and accounts from his travels in the Solomon Islands (and elsewhere) in his book Ant Hill Odyssey (Mann 1948).

H. Viehmeyer (1924) described a new subspecies Euponera (Mesoponera) melanaria subsp. manni (= Pachycondyla manni) from Mann’s collections at the Museum of Comparative Zoology (MCZ). H. Donisthorpe (1941) described Nylanderia manni from a worker that was on the same pin as several workers of Camponotus loa Mann, all of which were labeled as Iridomyrmex myrmecodiae Emery (= Philidris myrmecodiae). Donisthorpe attributed the close similarity of all three species to mimicry.

W.M. Wheeler’s first contribution to the Solomon Island ant fauna was his description of Opisthopsis manni based on specimens collected by Mann from Malaupaina (Wheeler 1918). Wheeler (1934) later published on ants collected by Maurice Willows Jr. from the Santa Cruz and Danger Islands. He listed the names and collection records of 27 currently recognized taxa, including original descriptions for two species (Nylanderia dichora, Stereomyrmex dispar) and one subspecies (Polyrhachis labella brunneipes), along with the first published record of Tapinoma melanocephalum from the Solomons (Wetterer 2009). These records are combined with those of Forel and Mann in Wheeler (1935b).

William Brown treated many Solomon Island taxa in his revisions (Brown 1948; 1958a; b; 1960; 1975; 1976; 1978; 1988; 1995; Willey and Brown 1983). Gressitt (1958) reported on the pest behavior of Iridomyrmex myrmecodiae (= Philidris myrmecodiae) invading buildings in Malaita. According to Wilson (1962), the B.P. Bishop Museum, Honolulu, initiated a collecting program in the Solomons under the direction of Gressitt, and there is likely a considerable amount of ant material that remains unreported in the literature.

Research on economically important ants involved in coconut production was an active field in the Solomons from the 1930’s through the 1960’s (Leston 1973; Lever 1933; 1961; O’Conner 1949; 1950; Phillips 1940; 1956). E.S. Brown (1959) recorded over 60 species of ants (including five new country records) collected during his work among coconut plantations in Guadalcanal and Malaita.

Philip J.M. Greenslade has arguably collected more thoroughly across the Solomons than anyone since Mann. Greenslade published seven papers between 1964 and 1988 based on fieldwork he conducted in the Solomons (Greenslade 1964; 1971a; b; 1972; Greenslade and Greenslade 1970; 1971; 1977). The research focused primarily on the ecology of ants that are dominant in coconut plantations and are involved in the biological control of a coconut pest, Amblypelta cocophaga China and the premature nutfall of coconut fruit. In addition to providing valuable ecological information on the four most dominant ant species in these plantations (Anoplolepis gracilipes (Smith, F.), Oecophylla smaragdina Forel, Pheidole megacephala (Fabricius) and Philidris cordata (Smith, F.)), Greenslade also collected a broad diversity of less economically important ant species, mainly from Mt. Austen (Guadalcanal) and Kukum—the nearby Solomon Is. Department of Agriculture farm. These specimens, most of which were deposited at the ANIC, included many new species in addition to the first records of Problomyrmex (Taylor 1965) and Colobostruma (Bolton 2000) for the Solomon Islands. Interestingly, Greenslade’s (1968) work on the avifauna of the Solomon Islands was the first to apply the taxon cycle model to birds.

E.O. Wilson included many species from the Solomon Islands during his revisionary work of the Melanesian ant fauna, including species currently in the genera Amblyopone, Leptogenys, Platythyrea and Stigmatomma (1958a); Ponera, Cryptopone, Hypoponera, Pachycondyla and Rhytidoponera (1958b); Anochetus and Odontomachus (1959c); and Cerapachys (1959d). Wilson and Taylor (1967) added several new species records for the Solomons, including Ponera incerta (Wheeler) and Strumigenys karawajewi Brown (as Strumigenys dubia (Brown)). Wilson and Hunt (1967) included records for the Solomons. In addition to these taxonomic studies, Wilson also included ants from the Solomons in his influential papers on the taxon cycle hypothesis (Wilson 1959a; 1961) and the theory of island biogeography (MacArthur and Wilson 1967).

Wilson’s (1962) paper on the ants of Rennell and Bellona Islands examined specimens collected from three sources: a Danish Expedition (Wolff 1955); a British expedition (Bradley 1955), and a private collection made on Rennell and Bellona for several weeks during 1955, by Mr. E.S. Brown. Wilson recorded 25 species of ants in 17 genera from Rennell (including the first record of Dilobocondyla from the Solomons). He considered these to represent a large percentage of the actual ant diversity, but admitted that the lack of cryptobiotic ponerine and myrmicine species suggest that his list is incomplete. He concluded that the Rennell ant fauna is primarily composed of widespread Pacific natives that invaded the island relatively recently and are representative of ‘Stage-I’ species discussed in his taxon cycle hypothesis (Wilson 1959a; 1961).

Robert Taylor, in addition to describing Problomyrmex salomonis (Taylor 1965), also described Eurhopalothrix greensladei (Taylor 1968), and Stigmatomma gnoma ( = Amblyopone gnoma) (Taylor 1979) from specimens collected by P.J.M. Greenslade on or near Mt. Austen. Rudolf Kohout’s work on Polyrhachis added several new species records to the Solomons, introduced new synonyms and nomenclatural changes, and included the description of three new species (Polyrhachis greensladei, Polyrhachis setosa, Polyrhachis undulata) endemic to the Solomons (Kohout 1990; 1998; 2006). Barry Bolton described Polyrhachis nofra (Bolton 1975), from the Solomons, provided the replacement name of Tetramorium mutatum Bolton for the junior secondary homonym Triglyphothrix (= Tetramorium) pulchella Mann (Bolton 1985), and added new records of dacetines in the Solomons (Bolton 2000). Bolton(1976) also described Tetramorium vombis from specimens Mann (1919) mistakenly identified as Tetramorium obesa André. Kugler described Rogeria megastigmatica from a Greenslade collection made on Guadalcanal (Kugler 1994). Lattke included the Solomon Islands in his biogeographic analysis of Gnamptogenys in Southeast Asia (Lattke 2003) and described two new species (Gnamptogenys preciosa and Gnamptogenys solomonensis)from there (Lattke 2004). Lucky & Sarnat (2008) included Lordomyrma epinotalis Mann in their phylogenetic and biogeographic analysis of the genus. Sarnat and Moreau (2011) included Pheidole species from the Solomons in their phylogenetic and biogeographic analysis of the Fijian Pheidole and selected congeners from across the Pacific.

In total, our research suggests that the Solomon Islands support at least 237 unique ant species and subspecies. The poor sampling of many islands–some of which are quite large–and the unexamined material at the ANIC suggests that the true number is likely much greater. For example, our eight days of intensive hand collection and Winkler extractions on Makira added 67 new species records to the island (including all morphospecies) and 28 new records to the archipelago. Prior to the survey, Makira Island’s 75 species records were the second highest of the entire archipelago. Choiseul Island by comparison is approximately equal in area to Makira and closer to New Guinea, but the ant fauna of the island is virtually unknown with only eleven species recorded in the literature. There are approximately as many species known from the islands of Santa Isabel and Malaita as there are from Rennell, despite the substantially larger area of the former islands and their closer proximity to other large islands within the archipelago. The difference is that although no ant specialists have thoroughly sampled Rennell, general entomologists have collected there and the ant specimens of those surveys were the subject of several faunistic reviews (Taylor 1976; Wilson 1962). Besides Makira and Rennell Islands, the only island that has been moderately sampled–thanks to the works of Mann and Greenslade–is Guadalcanal.

Compared to Fijian islands of similar size, known species richness is generally much lower for individual islands within the Solomons, despite the fact that Fiji is much more isolated in the Pacific (Figure 2). This is likely due to relative sampling intensity of the two areas. Fiji has recently received intensive sampling efforts (Sarnat and Economo 2012), while richness differences among the Solomon Islands are still driven in large part by which islands were visited by W.M. Mann in 1916. For example, the 38 recorded species reported from the small island of Ugi (42 km²), where Mann resided and collected for several weeks, is a richness comparable with a similar-sized Fijian island. Several large islands not visited by Mann have almost no records (e.g. Choiseul 2, 966 km², 11 spp.; Kolombangara 704 km², 17 spp.). Our modest survey of Makira, where we spent approximately one week of collecting time, increased known richness from 75 to 142 species. There is no doubt that such modest collecting efforts elsewhere in the archipelago would yield similar increases.

The species list compiled from our research suggests several interesting taxonomic patterns. For example, species richness across the 51 native ant genera of the Solomons appears uneven. The 30 Polyrhachis species represent 14% of the total native species. The nine most diverse genera (Polyrhachis, Pheidole, Camponotus, Tetramorium, Vollenhovia, Pachycondyla, Strumigenys, Crematogaster, and Gnamptogenys) collectively contain over half of the total native species, while fifteen genera are represented by a single native species.

Why is Polyrhachis so strongly represented in the Solomons? These results are likely biased to some extent by idiosyncratic collecting and taxonomic study. Besides the work of Mann, and to a lesser extent Greenslade, most of the collections from the Solomons have been made by more generalist collectors, which tend to take larger, more conspicuous ants that forage on and nest in vegetation–all of which are characteristic of Polyrhachis. Furthermore, Rudolf Kohout, who has access to the considerable collection of Solomons material at the ANIC, has devoted much of his taxonomic efforts towards revising the Polyrhachis of the Indo-Australian region (Kohout 1990; 1998; 2006; 2012). Despite these apparent biases, it is somewhat remarkable that with a single exception, the eight distinct Polyrhachis lineages that colonized the Solomons (as inferred from their subgeneric classifications) were unable to colonize, or at least persist in the more eastern Pacific islands. That single exception, Polyrhachis rotumana Wilson & Taylor, is known from the island of Rotuma which belongs politically to Fiji but is quite isolated from the Fijian archipelago and shares more geological and biological affinity with the islands of Polynesia.

Pachycondyla (9 native spp.), Crematogaster (7 native spp.)and Gnamptogenys (6 native spp.) are also among the most diverse ant genera in the Solomon Islands, but are either absent from or poorly represented in more easterly archipelagos. Fiji, for example, supports a single native Gnamptogenys species (Gnamptogenys aterrima Mann), and does not support any native Pachycondyla or Crematogaster species (Sarnat and Economo 2012). The Solomons are the known eastern limit for many ant genera. Out of the 51 genera native to the Solomons, the following 19 are not known to occur in the Pacific in or east of the Fijian archipelago: Anonychomyrma, Arnoldius, Cardiocondyla, Colobostruma, Crematogaster, Cryptopone, Myopias, Myopopone, Myrmecina, Oecophylla, Opisthopsis, Pachycondyla, Podomyrma, Polyrhachis, Probolomyrmex, Rhytidoponera, Stereomyrmex, Tetraponera, Turneria.

While additional sampling may prove otherwise, the current analysis of the Solomons ant fauna does not appear to support the type of in situ single-lineage radiations that characterize much of the Fijian ant fauna to the east. Parallels to the dramatic radiations of the Pheidole roosevelti group (Economo and Sarnat 2012; Sarnat 2008), Lordomyrma (Lucky and Sarnat 2008; Sarnat 2006), and the Camponotus dentatus group (Sarnat and Economo 2012) are largely unknown from the Solomons. It is likely that the Solomons ant fauna is derived more from relatively frequent colonization events from nearby New Guinea than from sweepstakes colonists that diversified into largely unoccupied ecological niches as occurred in the more isolated Fijian archipelago. Unlike New Guinea and Fiji, the Solomons do not support any endemic ant genera.

The importance of establishing baseline faunal inventories for the entire Solomon Island archipelago and its constituent islands is especially important when considering the growing environmental impacts resource extraction, plantation agriculture and invasive species are having on native biodiversity. Perhaps the greatest threat to native ant species in the Solomons is the spread of the Little Fire Ant (Fasi 2009). The introduction of Wasmannia auropunctata into the Solomon Islands is believed to have occurred around 1974, possibly with the arrival of coconut nurseries (Fabres and Brown 1978; Ikin 1984; Wetterer 1997). Foucaud et al. (2010) determined that a single clonal queen genotype is shared between the Melanesian populations of Wasmannia auropunctata from the Solomons, Vanuatu, Papua New Guinea and Australia, and suggested that the population spread by means of traditional exchange of plants and goods among Melanesian people. Although there have been reports of the ant’s effect on vertebrates in the Solomons, such as blinding dogs and attacking hatchlings of the ground-nesting Melanesian Scrubfowl (Megapodius eremita Hartlaub) (Wetterer 1997), and also its effect on food crops and subsistence agriculture (Fasi 2009), there have yet to be any studies examining the effect of Wasmannia auropunctata on native ant diversity in the Solomons. The potential for spread of Wasmannia auropunctata across the entire archipelago is high (Fasi 2009), and it is likely a matter of years before all the major islands are infested. We hope the research presented here will help facilitate more study of the neglected Solomon Island ant fauna and aid conservation efforts before Wasmannia and other environmental threats cause irrevocable harm.