An arid zone awash with diversity: patterns in the distribution of aquatic invertebrates in the Pilbara region of Western Australia

Abstract

The Pilbara region of Western Australia is an arid zone with an abundance of wetlands, ranging from springs and river pools to salt marshes, claypans, rockpools and gnammas. The Pilbara is also rich in mineral resources, which are being heavily exploited, and most of the region is under pastoral lease, with both land uses potentially having an impact upon aquatic systems. In order to provide information for improved conservation planning, including environmental impact assessment and reserve system design, a survey of wetland fauna was undertaken as part of a broader biodiversity survey of the region. This paper describes patterns in the distribution of the region’s aquatic invertebrate diversity and identifies environmental correlates of those patterns. Invertebrates and environmental variables were surveyed at 100 wetlands in spring and autumn between 2003 and 2006. The Pilbara appears to have a particularly diverse fauna for an arid zone, with just over 1000 species collected, an average of 94 species per sample and a maximum sample richness of 226 species. About a fifth of the fauna is known to date only from the Pilbara. The remaining species are western endemics, occur elsewhere in northern and/or inland Australia, or have continental or wider distributions. Most species seem to be widespread in the Pilbara region but rare and/or restricted elements are present, especially in some permanently flowing springs, including those in Millstream and Karijini National Parks, and in ephemeral wetlands such as Fortescue Marsh and freshwater claypans. Faunal composition was not strongly correlated with season, drainage divides, stream order or previously recognised bioregional boundaries. A number of assemblages of species with common patterns of occurrence were identified, each showing unique associations with measured environmental variables. Flow, turbidity, salinity, sediments, macrophytes and hydrological persistence are among the environmental gradients most strongly correlated with occurrence patterns in the fauna.