Biogeographic barriers in north-western Australia: an overview and standardisation of nomenclature

Within Australia, the biodiverse monsoon tropics are of considerable biogeographical interest. However, this vast and relatively undisturbed area with its high endemicity has only recently become the focus of phylogeographic research (Bowman et al. 2010; Fujita et al. 2010; Melville et al. 2011; Toon et al. 2010). A major challenge to understand the biogeographical processes that have shaped the distribution and diversity of taxa in this region is the lack of detailed fossil and palaeoecological data (Bowman et al. 2010). Although molecular data are able to contribute to our understanding of the biogeographic history of the region (e.g. Lee and Edwards 2008; Toon et al. 2010), relatively few studies have focussed on the geologically old and topographically complex northwest region of Australia (Jennings and Edwards 2005; Fujita et al. 2010; Melville et al. 2011). It is becoming increasingly clear that the biogeographic patterns of this region and the accompanying terminology are complex (Potter et al. 2012). Here we seek to provide an historical overview of the biogeographic nomenclature used for this region in order to bring greater clarity and concordance. We also propose a standard nomenclature that could beused in future biogeographic/phylogeographic studies of north-western Australia. The tropical monsoon region of north-western Australia represents a unique biome comprising a range of habitats. Proterozoic sandstone dominates the landscape and forms ranges and dissected escarpments set within widespread savannah woodlands. Smaller areas of monsoon rainforest, gallery forest and seasonally wet grasslands also occur. The monsoonal climate is characterised by dry winters and wet summers, which drives the vegetation type and associated distribution of taxa (Bowman et al. 2010). Tropical habitats contracted north during aridification in the Miocene and late Pliocene and the monsoonal conditions commenced following the rise of the Tibetan Plateau (3.4–7.2million years ago) and the closing of the Isthmus of Panama (see Bowman et al. 2010; Fujita et al. 2010). However, it was not until the Pleistocene that the monsoonal tropics experienced fluctuating environmental changes associated with global glacial cycles that developed the more contemporary arid-adapted vegetation (reviewed in Bowman et al. 2010; Fujita et al. 2010). The less topographically complex lowlands between the disjunct sandstone outcrops have been identified as potential biogeographic barriers for a wide variety of fauna (e.g. Bowman et al. 2010). Keast (1961) regarded north-west Australia (including both the Kimberley region and Top End of the Northern Territory, Fig. 1) as a single major mesic refuge area (North-West refuge) for Australian fauna. Earlier, Keast (1958) noted that the region of the Joseph Bonaparte Gulf formed a distributional barrier for some mangrove-dependent birds, due to a lack of habitat. After an examination of the North-West refuge bird fauna, Ford (1978) identified the presence of three ‘minor biogeographic barriers’ in the region between the Kimberley and northern Northern Territory (i.e. the Top End). These were identified as ‘arid country round the Joseph Bonaparte Gulf and the Victoria River valley, a discontinuity in sandstone ranges in the region of the Daly River drainage and a stretch of coastline poor in mangroves on western side of the Joseph Bonaparte Gulf’. More specifically, the lowlands of the Daly River drainage were identified as a significant barrier for many sandstone-rangeadapted taxa (Ford 1978). The arid country around the head of the Joseph Bonaparte Gulf (in the region of the Ord and Victoria Rivers) was considered a significant barrier for monsoon and gallery forest species, aswell as formoist grassland taxa,while the break in mangrove habitat in the western Joseph Bonaparte Gulf had significantly influenced mangrove specialists (Ford 1978). This later gap in the distribution of mangroves was subsequently examined in greater detail by Ford (1982), who identified it as a significant biogeographical barrier for mangrove birds, which he termed the ‘Bonaparte Gap’. Subsequently, ‘Bonaparte Gap’ has entered the literature as a more general term