The Nullarbor Plains xeric shrublands comprises a land area of approximately 75,400 square miles, and spans portions of Western Australia and South Australia. This ecoregion is classified within the Deserts & Xeric Shrublands biome. The Nullarbor Plains are extremely level topographically, and manifest the largest single expanse of unbroken rock on Earth; moreover, this expanse of limestone is rich in notable karst formations.
This virtually treeless ecoregion of Australia is dominated by scrub of the family Chenopodiaceae. There are five threatened bird species in Nullabor Plains, along with two special status reptiles and one threatened mammal taxon. The geological formation and history of the Nullabor Plains are related to the speciation of multiple phylogenic lines in the high endemism temperate sclerophyllus regions to the east and west of the Nullabor.
A substantial fraction of this ecoregion is committed to the Nullarbor National Park, a protected area of some 578,000 hectares.
Location and general description
Vegetation in the ecoregion is dominated by low saltbush and bluebush scrub, with practically no tree species.
The fauna of the Nullarbor includes a considerable of taxa of crustaceans, spiders, and beetles adapted to the darkness of the Nullarbor Caves as well as the underground rivers and lakes that connect these cave habitats. There are a total of 301 recorded vertebrate species in the ecoregion.
Southern hairy-nosed wombat (Lasiorhinus latifrons), Australia Photograph by Department of the Premier and Cabinet, South Australia
Threatened avian species in the Nullarbor Plains include the Near Threatened Australian bustard (Ardeotis australis), the Near Threatened buff-breasted sandpiper (Tryngites subruficollis), the Vulnerable fairy tern (Sterna nereis), the Near Threatened grey falcon (Falco hypoleucos) and the Vulnerable malleefowl (Leipoa ocellata).
Special status reptiles in the ecoregion are: the Near Threatened Bardick snake (Echiopsis curta) and the Endangered woma (Aspidites ramsayi).
Special status mammals in the Nullarbor Plains ecoregion are represented by the Vulnerable plains mouse (Pseudomys australis)
Western Nullarbor Plain, Australia Photograph by Ludo Kuipers/OzOutback
The Nullarbor Plain was in much earlier geological time a shallow seabed, evinced by the presence of bryozoans, foraminifera, echinoids and red algae calcareous skeletal that comprise the limestone substrate. The region is also the location of Nullarbor limestone and it has a reputation as a significant karst region with Oligocene and Miocene cave formations.
The sequence within the limestone is comprised five formations: (1) the upper formation is the Nullarbor Limestone which dates to early middle Miocene; (2) the next layer is the paraconforming Mullamullang member which dates to about 5,000,000 years earlier; (3) Abrakurrie Limestone that was created in a central depression of the earlier formation; this member dating to late Oligocene to Early Miocene in age and does not extend to the perimeter of the plain; (4) two conforming formations; the late Eocene Toolinna Limestone lies on top of Wilsons Bluff Limestone which is mid to late Eocene in age; and (5) Toolinna Limestone, without covering the entire Nullarbor Plain, presents merely at the extreme east of the ecoregion alongside the Abrakurrie formation which lies in a depression.
Link of geological history to vicariance in lateral biomes
Flanking the Nullarbor plains are two disjunct temperate schlerophyllous biomes, each having very high plant endemism. It is probable that the emergence of the arid intervening Nullarbor was a causative factor in the major divergent speciation of these two temperate biomes. Fundamentally the calcareous limestone soils of the Nullarbor are an edaphic barrier to vegetative taxa of the flanking sclerophyll zones, which host plants that are generally highly adapted to the acidic siliceous soils of their biomes.
The vicariance engendered by geological history of the Nullabor Plains is strongly correlated with events circa 14 to 13 million years before present. (Crisp and Cook. 2007) Within the range of dating errors, this time interval correlates well with the known aridification event of circa 14 million years before present, as well as the inundation of the Eucla Basin (the name given to the prehistoric sea that covered the Nullabor Plains) circa 16 million years ago.
Furthermore, the date of 14 million years before present corresponds well to the known dating of the uplift of the essential limestone plateau, which has remained essentially exposed to the present time. This calcareaous zone thus forms an edaphic barrier to the predominant plant taxa of both temperate flanks to the Nullabor Plains, which biomes have a strong acidic siliceous soil affinity.
The conservation status of the Nullarbor Plains xeric shrublands is classified as Vulnerable; however, the ecoregion is note designated as a G200 ecoregion, meaning it has not been classed as a region deserving the highest priority of protection.
Source: Protected Planet
Types and severity of threats
Off road vehicles are an ongoing threat to the Nullarbor Plains. Overgrazing by domesticated livestock are presenting a significant threat to native vegetation. Furthermore, rabbit overpopulations threaten the stability of vegetative assemblages. Not only are the native plants at threat from both types of herbivory, but many birds and other wildlife dependent on the vegetative cover are also presented with increased risks. Additionally the introduction of alien species of weeds poses a threat to ecosystem stability.
Nullarbor Plains (semicircular tan region) along the Great Australian Bight. Source: NASA
Justification of ecoregion delineation
The Nullarbor Plains Xeric Shrubland includes the Nullarbor Interim Biogeographic Regionalisation for Australia (IBRA), and is characterized by its unusual limestone substrate and uniform, treeless vegetation, mostly widespread saltbush and bluebush shrubs (Thackway and Cresswell. 1995).
References and notes
- Michael D.Crisp and Lyn G.Cook. 2007. A congruent molecular signature of vicariance across multiple plant lineages. Molecular Phylogenics and Evolution. 43: 1106-1117. Science Direct. Elsevier.
- Jack Edmonds. 1976. Nullarbor crossing with panorama photographs by Brian Gordon. Perth. West Australian Newspapers, Periodicals Division.
- R.Thackway and I.D.Cresswell. editors. 1995. An Interim Biogeographic Regionalisation for Australia: a framework for establishing the national system of reserves, Version 4.0. Australian Nature Conservation Agency, Canberra.
- John A. Webb & Julia M. James (2006). Karst evolution of the Nullarbor Plain, Australia. In Russell S. Harmon & Carol M. Wicks (PDF). Perspectives on Karst Geomorphology, Hydrology and Geochemistry - a Tribute Volume to Derek C. Ford and William B. White. Geological Society of America Special Paper 404. pp. 65–78. ISBN 978-0-8137-2404-1.
- Portions of the Justification for ecoregion delineation section were prepared by Angas Hopkins, World Wildlife Fund