RecoRds of the WesteRn AustraliAn MuseuM 81 SUPPLEMENT Biodiversity values on selected Kimberley islands, Australia edited by L.A. Gibson, S. Yates and P. Doughty RecoRds of the WesteRn AustraliAn MuseuM 81 040–065 (2012) SUPPLEMENT Determinants of species richness and similarity of species composition of land snail communities on Kimberley islands L.A. Gibson1 and F. Köhler1,2 1 Department of Environment and Conservation, Science Division, PO Box 51, Wanneroo, Western Australia 6946, Australia. 2 Australian Museum, 6 College Street, Sydney, New South Wales 2010, Australia Email: [email protected] ABSTRACT – We investigated patterns in species richness and similarity in species composition of land snail communities in relation to biogeographic and environmental gradients on continental islands along the Kimberley coastline of Western Australia. The camaenid land snails demonstrated high overall species richness, restricted geographical ranges (mostly to a single island) and almost complete turnover of species among islands. In comparison, the overall number of the smaller non-camaenid land snail species was much lower. They also had much wider distributions and a lower level of species turnover among islands. These results are consistent with the differing dispersal abilities of the two groups, one fostering in situ speciation and the other inter-island colonisation. We also show that dispersal ability was not the only driver of species richness and composition of land snail species on islands, but that rainfall, island area and availability of habitat also played a role. Rainfall was the major environmental driver of richness and composition for both groups of land snails. Rainfall was also highly correlated with extent of rainforest cover, habitat that provides a suitable micro-climate for snails and additional ecological niches compared to more open habitats. KEYWORDS: Pulmonata, island biogeography, land snail diversity, Camaenidae, gastropod richness INTRODUCTION narrow-range endemics, with an average range size of approximately 20 km (Solem 1991), while the non- The Kimberley region of north-western Australia is camaenids generally have expansive ranges with recognised as a national biodiversity hotspot, due geographic distributions extending well outside the largely to the number of endemic plant and animal Kimberley region (Solem 1988b, 1991). The narrow species it supports (http://www.environment. range endemism observed in the camaenids has gov.au/biodiversity/hotspots/national-hotspots. been attributed to their poor dispersal capability htm). A relatively little-known, yet significant, (Solem and McKenzie 1991). component of this diversity and high endemism, Highly fragmented and isolated environments, are the terrestrial gastropods (or land snails) of the such as island archipelagos, often display high region. Surveys conducted between 1987 and 1989 levels of endemism and species diversity (Losos in the rainforests of the Kimberley (McKenzie et and Ricklefs 2010). One of the factors believed al. 1991a) uncovered a large diversity of land snail to contribute to the high diversity of camaenid species (Solem 1991). Earlier work in the Kimberley land snails in the Kimberley is the multiplicity of also showed that the snail fauna in the Kimberley is refugia distributed across the region in the form of dominated by one family, the Camaenidae (Solem rainforest patches (Solem 1991). Created by climate 1981a, 1981b, 1984, 1985, 1988a, 1991). The remaining fluctuations throughout the Quaternary and species, which represent various families of non- Mid to Late Tertiary (Bowler 1982), these patches marine gastropods, are here referred to as ‘non- essentially have been acting as habitat islands camaenids’. They account for only a comparatively whereby gene flow is restricted by intervening small proportion of the overall diversity of land unsuitable habitat, resulting in allopatric patterns snail species in the region (Solem 1988b, 1991). A of speciation and endemism in the isolated refuges further striking difference between the two groups (Cameron 1992). The high levels of endemism of of land snails is that most of the camaenids are land snails in the limestone karsts of Malaysia has RICHNESS AND COMPOSITION OF KIMBERLEY ISLANDS LAND SNAILS 41 been similarly explained (Clements et al. 2008), as contrast, we anticipated a much lower level of well as land snails occurring on islands or within turnover in the more widespread non-camaenid island archipelagos (e.g. Cowie 1995; Welter- species. The conservation implications of the results Schultes and Williams 1999; Chiba 2007; Cook 2008; are discussed, particularly with regard to issues Overton et al. 2009). involving short-range endemism. Centres of high land snail diversity in the Kimberley region largely correspond with METHODS the wetter coastal areas of the north (Solem 1991). Also within this area are extensive island STudy area archipelagos which, created by the flooding of an ancient coastline, represent much of the adjacent We selected 24 of the largest islands for sampling, mainland’s geological surfaces and vegetation targeting those with a variety of geological communities (May and McKenzie 2002). The surfaces, as well as ensuring geographic coverage rainforest survey of the late 1980s included a from north to south (see Gibson and McKenzie small number of these islands, but aside from a 2012). These islands extend from Sir Graham few opportunistic collections, the diversity of land Moore Island off the Anjo Peninsula in the north to snails on the majority of islands has remained Sunday Island off Cape Leveque in the south, and largely unknown until recently. to Adolphus Island in the Cambridge Gulf in the From 2007 to 2010, the Kimberley Island Survey east. They include islands from the Bonaparte and (KIS) of biodiversity was conducted on a selection Buccaneer Archipelagos, as well as several other of the largest islands in order to test assumptions island groups (Figure 1). of islands acting as natural refuges, particularly for Geological and detailed vegetation descriptions those species susceptible to escalating threatening of the islands are given in Gibson and McKenzie processes on the Kimberley mainland. Land (2012). Briefly, sandstone and volcanic strata snails were selected as one of the target groups, structure the island landscapes. The sandstones because they are considered to be indicators for tend to give rise to rugged, dissected terrains, while other restricted range invertebrate groups. Their the volcanics usually produce a more rounded and disjunct distributions on the mainland make them gently undulating topography. Tertiary duricrusts particularly sensitive to processes that alter their occur as mesas and dissected tablelands on some optimal habitat, such as changed fire regimes, islands, capping volcanic, sandstone or doleritic grazing and introduction of weeds. Descriptions strata. Broad vegetation communities on the islands and distributions of each camaenid and non- include tropical savannas, hummock grassland, camaenid land snail species encountered during rainforest, coastal tussock grassland, riparian this survey have been published elsewhere (Köhler paperbark woodlands and mangroves. 2010a, 2010b, 2011a, 2011b, 2011c; Köhler et al. 2012). The Kimberley experiences a tropical monsoon Briefly, a total of 104 camaenid species (83 new) in climate with two distinct seasons: a dry season 15 genera (5 new) were recorded from the islands extending from around April to October, and a wet (including a number of additional islands outside season from November to March when almost all of the survey area) with most species found to be rainfall occurs. Cyclonic activity is also a feature island endemics (Köhler 2011b). Additionally, 21 of the climate. Average annual rainfall ranges non-camaenid land snails were recorded from the from >1,500 mm at Mitchell Plateau to <800 mm islands, with four of these being freshwater species at Cape Leveque (http://www.bom.gov.au). Daily (Köhler et al. 2012). temperatures range from 23–38oC throughout the Here, we investigated patterns in species richness year, with high relative humidity peaking during and similarity in species composition of land the wet season. snails recorded during the above survey in relation to biogeographic and environmental gradients. Sampling and identification We were interested in examining if the species- area relationship held for the Kimberley island A comprehensive description of the survey snail fauna as has been found on other island implementation is given in Gibson and McKenzie archipelagos (e.g. Cowie 1995; Welter-Schultes and (2012). In summary, campsites on the islands Williams 1999; Triantis et al. 2005; Chiba 2007), were selected using information on geology and as well as the effect of island isolation on snail vegetation from maps, local knowledge, satellite richness and composition. We expected that the imagery and a reconnaissance flight. Survey teams high levels of local endemism of camaenid land (see Gibson and McKenzie 2012) needed to be snails observed on the adjacent mainland (Solem placed within walking distance of as many habitat/ 1991) would be reflected in the island fauna with vegetation types as possible. Two campsites were a high turnover of species among islands. In needed to access the environmental variation of 42 L.A. Gibson and F. Köhler FIGuRE 1 Location of Kimberley islands selected for a survey of their biodiversity assets along the north-west