Patterns in the Composition of Ground-Dwelling Spider Communities in the Pilbara Bioregion, Western Australia

DOI: 10.18195/issn.0313-122x.78(1).2010.185-204 Records of the Western Australian Museum, Supplement 78: 185–204 (2010). Patterns in the composition of ground-dwelling spider communities in the Pilbara bioregion, Western Australia Bradley J. Durrant1, Mark S. Harvey2, 4, Volker W. Framenau2, 4, Ricardo Ott2, 3 and Julianne M. Waldock2 1Department of Environment and Conservation, PO Box 51, Wanneroo, Western Australia 6946, Australia. Email: [email protected] 2Department of Terrestrial Zoology, Western Australian Museum, Locked Bag 49, Welshpool DC, Western Australia 6986, Australia. 3Museu de Ciências Naturais, Fundação Zoobotânica do Rio Grande do Sul, Rua Dr. Salvador França, 1427, 90690-000, Porto Alegre, Rio Grande do Sul, Brazil. 4School of Animal Biology, University of Western Australia, Crawley, Western Australia 6009, Australia. Abstract – Ground-dwelling spiders were sampled at 304 quadrats in 24 survey areas chosen to represent the geographical extent and diversity of terrestrial environments in the Pilbara region of Western Australia, an area of approximately 179,000 km2. Only taxa that were primarily ground-dwelling and were taxonomically robust were identifi ed to species level. A total of 375 species comprising 14 families was recorded. The families Salticidae (93 species), Zodariidae (71 species) and Oonopidae (70 species) showed marked species-level radiation. After excluding quadrats that were burnt or fl ooded during the sampling period, the distribution data of 375 species from 294 quadrats were analysed, with an average of 12.9 (SD = 6.8) species per quadrat. Singletons (species recorded at only one quadrat) constituted 26% (98 species) of the total number of species, and were found more frequently in the Roebourne and Fortescue subregions. Species assemblages were indistinct at the quadrat level, but signifi cant clustering occurred when the data were pooled into the 24 survey areas. The arrangement of the survey areas on an MDS ordination plot was analogous with their spatial positions on a map, in particular the division into a northern and southern Pilbara with the Fortescue cluster running in between. This division was also evident with a number of strong allopatric species distributions. The separation of the survey area clusters was correlated with variation in annual precipitation, latitude and dry season precipitation. Given the sporadic nature of rain events in the Pilbara bioregion and the limitations of the climatic data, however, some caution is advised in interpreting these results. INTRODUCTION environmental attributes, both climate and soil (Harvey et al. 2000, 2004; Durrant 2004; Guthrie Regional-scale approaches to surveying spiders and Waldock 2004). In contrast, other collections of have been adopted in Western Australia only ground-dwelling spiders in north-western Australia over the last 20 years, with the Kimberley rain- have been opportunistic or highly localised. forest survey in the far north (Main 1991), and Most have been confined to mining tenements the Carnarvon Basin (Harvey et al. 2000) and as part of the Western Australian government’s agricultural zone surveys (Durrant 2004; Guthrie environmental impact assessment process. and Waldock 2004; Harvey et al. 2004). All three surveys aimed to document the fauna’s composition Industrial development in the Pilbara bioregion and distribution across the relevant landscapes, of north-western Western Australia commenced but only the last two could be considered spatially when the fi rst pastoral leases were established on representative and seasonally comprehensive, with the Roebourne Plains in 1863. Within 60 years, 63 and 304 quadrats, respectively, and a sampling this industry had extended over most of the time frame that covered a 12 month period. region. The associated depletion of native grasses Both surveys revealed patterns of endemism as through grazing and burning has paved the way well as correlations between composition and for perennial buffel grass (Cenchrus ciliaris L.) to 186 B.J. Durrant, M.S. Harvey, V.W. Framenau, R. Ott, J.M. Waldock colonise much of the region, further displacing soils of the valley floors are characterised by indigenous shrub and grass communities. Even the Mulga low woodland over bunch grasses while region’s most rugged landscape components (with the skeletal soils of the ranges are dominated low pastoral value) have not escaped the collateral by Eucalyptus open low woodlands over Triodia changes in fire regimes (see McKenzie et al. hummock grasslands. Drainage is via three river 2009). Mining has also played an ever-increasing systems: the Fortescue (to the north), Ashburton role in changing Pilbara landscapes. From a few (to the south) and Robe (to the west) (Kendrick lead and copper mines opened in the late 19th 2003a). The Fortescue Plains subregion separates century, numerous areas are now dedicated to most of the Chichester from the Hamersley. In infrastructure associated with gold, iron ore and the east it comprises alluvial plains of salt marsh, salt mines. mulga-bunch grass and short grass communities Several spider families and genera have been bordering the Fortescue River. Woodlands of river revised in Australia in the past 10 years, with a gum and melaleuca occur around large permanent signifi cant proportion of the Western Australian wetlands. The plains narrow in western parts of material coming from the Carnarvon Basin survey. the subregion, eventually ending where the lower These revisions include the Lamponidae (Platnick section of the Fortescue River has incised gorges 2000), Prodidomidae (Platnick and Baehr 2006), through Chichester landscapes. The Fortescue Zodariidae (e.g. Baehr 2003a, b, 2004, 2005; Baehr subregion also marks the northern limit of mulga and Churchill 2003), Lycosidae (e.g. Framenau 2005, (Acacia aneura F. Muell.) in Western Australia 2006a, 2007; Yoo and Framenau 2006; Framenau (Kendrick 2003b). The fourth subregion, the and Baehr 2007; Langlands and Framenau 2010), Roebourne Plains, comprises the Pilbara’s alluvial Ammoxenidae (Platnick 2002), Cithaeronidae and colluvial coastal and subcoastal plains. Grass (Platnick 2002), Gallieniellidae (Platnick 2002), savannah and dwarf shrub steppes of Acacia Trochanteriidae (Platnick 2002) and Pholcidae dominate the plains, with Triodia hummock (Huber 2001). These revisions have allowed us to grasslands in the higher areas. Marine alluvial fl ats achieve a much higher level of taxonomic accuracy and river deltas support mangal, samphire and the in determining the Pilbara collection. grass Sporobolus, while eucalypt woodlands fringe the ephemeral drainage lines (Kendrick and Stanley This paper documents the composition 2003). of the ground-dwelling spider fauna of the Pilbara bioregion and explores their patterns of Two bioclimatic regions cover the Pilbara. The distribution, particularly in terms of cartographic, coast and much of the interior has a semi-desert climatic and geological attributes. tropical climate, with summer rainfall and 9–11 months of dry weather, and the remainder a desert climate with summer rain, with up to 12 METHODS dry months and higher temperatures. Beard (1990) considered a dry month to be when the amount of Study area precipitation is inadequate to sustain plant growth. The Pilbara Biological Survey covered about The annual average rainfall roughly follows an 179,000 km2 of the north-west region of Western inland to coastal and southern to northern increase. Australia. It conforms very closely with the The annual average of 290 mm ranges from extent of the four IBRA (Interim Biogeographic monthly averages of ca. 2 mm in September to ca. Regionalisation of Australia) subregions (Thackway 66 mm in February, although year-to-year variation and Cresswell 1995) overlying the Pilbara craton. is signifi cant. Cyclonic activity represents a major The study area is described in detail in McKenzie infl uence on the rainfall variation. Temperatures et al. (2009). Briefl y, the northern half is dominated range from 11.8°C min. and 25.3°C max. in July by the Chichester subregion, which comprises to 25.2°C min. and 37.8°C max. in January (Beard Archaean granite plains and basalt ranges. It is 1990). traversed by the De Grey, Oakover, Nullagine, Shaw, Yule and Sherlock river systems, all of which Field sampling strategy drain to the north. The plains support shrub steppe The survey region was divided into 24 survey of Acacia and Triodia hummock grasslands, while areas, comprising 11, 12 or 13 quadrats (Figure the ranges support Eucalyptus tree steppe (Kendrick 1). The quadrats were chosen to represent and McKenzie 2003). the geomorphic (combination of geology and The Hamersley subregion occupies most of topography) extent of the Pilbara bioregion, the southern Pilbara. It comprises Proterozoic with some pseudo-replication to provide better sedimentary ranges and plateaux, dissected by representation of the more extensive landforms gorges. The Hamersley Range reaches an elevation within each survey area. Undisturbed or least of 1250 m above sea level. The fine-textured disturbed examples of these landforms were chosen Patterns in the composition of ground-dwelling spider communities 187 Figure 1 Map of the Pilbara bioregion with the subregions, survey areas and quadrats marked. to minimise the infl uence of human-induced factors Bruce central, east and west) were sampled from such as stock grazing. Further details of quadrat July/October 2005 to August/September 2006. All selection are provided in McKenzie et al. (2009). pitfall traps were open for at least 12 months. Quadrats that were signifi cantly disturbed during The samples were sorted to taxonomic order the sampling process (predominantly from fi re or and then the groups of interest were identifi ed fl ood) were removed from the analysis, reducing to species-level. In most cases only males were the number of quadrats from 304 to 294. identifi ed. Identifi cation of females, i.e. matching Each quadrat was 50 × 50 m with a wet pitfall males and females, was often impossible at the trap at each corner and one in the centre. The pitfall species level due to lack of taxonomic resolution.

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