Herpetological assemblages of the Pilbara biogeographic region, Western Australia: ecological associations, biogeographic patterns and conservation

Abstract

The Pilbara region has one of the most diverse reptile assemblages in the world and includes many typical arid zone species as well as many saxicoline endemics. We present the results of a four-year survey of the region during which pitfall trap lines were used to sample 297 quadrats for a total of two weeks in spring and in autumn. The quadrats were located across the region and were chosen to sample the main substrate types. Environmental variables were measured to characterise the soil as well as the geomorphic, vegetative and climatic setting of each quadrat. We used the program PATN to expose patterns in species composition among quadrats, and in relation to the environmental variables. A strong signal for community structure emerged that was linked to surface type. Broad rock, clay, sand and loam categorisations seemed to explain most assemblage composition, with climatic variables showing only weak influence. However, many taxa occurred across several surface types, reflecting the close juxtaposition of habitats within the region, wide dispersion of some taxa, and perhaps the influence of recent environmental history including anthropogenic changes. Examination of biogeographic characteristics of the species groups indicated strong endemism for taxa that preferred rocky substrates. In addition, many saxicoline Pilbara taxa have ranges that extend south into rocky areas of the Gascoyne, western Murchison and adjacent areas. Sandy habitats similar to those found in the Pilbara are widespread across Australia's arid zone, and accordingly few endemic taxa showed a preference for sandy surface types, and taxa with such preferences have distributions extending far beyond the Pilbara. The sandy areas of the coastal plain in the Roebourne subregion contained many taxa from Dampierland and the Great Sandy Desert to the north and north-east, as well as some taxa from the rich sand-adapted fauna of the Carnarvon region to the south. Some endemic taxa are associated with clay in the region, although clay surfaces were not as extensive as the rocky, loamy and sandy surfaces. Species associated with the fine-textured soils of the mulga woodlands in the southern part of the region appear to be an extension of herpetofaunal communities of regions to the south. Herpetofauna composition at the regional scale appears to have persisted, and patterns discerned in our analysis appear robust, despite the changes that have accompanied European colonisation in the Pilbara, including pastoralism, changed fire regimes, weeds, introduced animals, mining and infrastructure. However, the existing reserve system does not adequately capture the region's herpetofaunal diversity, especially the endemic taxa in the Fortescue and Roebourne subregions, and there are likely to have been changes in community composition at the local level.