Location and General Description
This ecoregion has limited topographical relief, and is largely occupied by the Yilgarn Block and its margins. This inland plateau ranges from 280 m to 340 m with subdued dissection. There are several zones with varying degrees of stripping from the Tertiary lateritic profile. The Darling Scarp and Plateau run along the western boundary of the Yilgarn Block. The scarp was uplifted during the Tertiary and reaches elevations of 300 m to 400 m. To the south, this ecoregion is further delineated by the Whicher Range, a crescent-shaped scarp of lateritic Mesozoic sediments. Inland from the Whicher Range, the Blackwood Plateau is 100 m to 150 m above sea level, bounded to the north by the end of the Darling Range, and to the west by the Leeuwin-Naturaliste Ridge (Wardell-Johnson and Horwitz 1996). Rainfall varies from 635 mm to 1,300 mm per year, with higher rainfall received near the coast. Rainfall is strongly seasonal with a pronounced summer drought (Gioia and Pigott 2000).
Jarrah (Eucalyptus marginata) and marri (Eucalyptus calophylla) forests dominate through much of this ecoregion, with jarrah the dominant species on the Darling Range. Steep, forested valleys surround the Murray River near the Darling Scarp, leading to the more open jarrah and wandoo (Eucalyptus wandoo) woodlands further east on clayey soils. Tall blackbutt (Eucalyptus patens), jarrah, and marri forest can all be found in river valleys.
Further inland, the climate becomes drier and this ecoregion borders on the wheatbelt region of Western Australia. Vegetation here consists of open eucalypt woodlands containing wheatbelt wandoo (Eucalyptus capillosa) and powderbark (Eucalyptus accedens). Wandoo grows on lower slopes and powderbark on higher slopes and plateaus, on gravel. Isolated areas of mallee and rock sheoak (Casuarina huegeliana) also occur. Agonis spp. dominate shrublands on eluvial and alluvial deposits. In regions with Mesozoic sediments, jarrah forests are intermixed with species-rich shrublands (Thackway and Cresswell 1995).
A model and analysis of flora distributions over southwest Western Australia revealed several areas of high species richness located in this ecoregion. The Whicher Range, possibly the Blackwood Plateau, and to a lesser extent, the Helena Valley east of Perth were all predicted to have high species richness. In general, species richness decreases as one moves inland (Gioia and Pigott 2000) and areas of high species richness are often prominent upland areas (Wardell-Johnson and Horwitz 1996).
Western Australia contains some of the continent’s and, indeed, the world’s oldest rocks. The southwest region of Western Australia has undergone extensive weathering and leaching, so that low-nutrient soils characterize the region. Erosion of lateritic landforms led to a mosaic of soil types across the southwest, with greatest local diversity seen along the western side of the wheatbelt region. The high plant diversity is partially in response to variation in the soils, rapidly shifting environmental conditions in the Quatenary, and the region’s long isolation from moist eastern Australia (Hopper et al. 1996).
A number of endangered or restricted range mammals are found in this ecoregion, including the numbat (Myrmecobius fasciatus VU), chuditch (Dasyurus geoffroii VU), woylie (Bettongia penicillata), tammar wallaby (Macropus eugenii), ringtail possum (Pseudocheirus occidentalis VU), southern brown bandicoot (Isoodon obesulus), and red-tailed phascogales (Phascogale calura EN). The kwongan vegetation contains a wide range of nectar-feeding animals, including insects, honeyeaters (Meliphagidae), honey possums (Tarsipes rostratus), and pygmy possums (Cercartetus concinnus). Only one bird is endemic to the region, the western bristlebird (Dasyornis longirostris VU). A subspecies of western corella, Cacatua pastinator pastinator is also found in this ecoregion, with fewer than 2,500 mature birds present (Garnett and Crowley 2000).
The herpetofauna of southwestern Australia is characterized by a highly endemic frog fauna: 30 species are known from this region, and all but two are endemic. In this ecoregion, a number of restricted range and endemic frogs occur, including squelching froglet (Crinia subinsignifera) and yellow wart burrowing frog (Heleioporus barycragus) (Cogger 2000).
Throughout the southwest of Western Australia, 44 percent of all jarrah forest has been cleared, while nearly 90 percent of all eucalyptus woodlands has been cleared (Mittermeier et al. 1999). Logging, agriculture, and mining are all forms of land use in this ecoregion. There are a number of large protected areas in this ecoregion including Dryandra Woodland (approximately 280 km2), Lane-Poole Reserve (550 km2), and the Perup Forest Ecology Center, 520 km2 of jarrah forest and wandoo woodland. Gioia and Pigott (2000) examined conservation areas within a study region that roughly included this ecoregion and the Jarrah-Karri Forests and Shrublands ecoregion to the south and found that 70 percent of all national parks and nature reserves were smaller than 1 km2.
Fifty percent of all mammalian extinctions of the last 200 years occurred in Australia (Short and Smith 1994), and a large portion of these extinctions were concentrated in Western Australia. This ecoregion and the well-watered ecoregions along the coast have a better mammalian fauna than the more arid regions, including the wheatbelt, which is marked with a number of extinctions (Burbidge and McKenzie 1989).
Types and Severity of Threats
Threats include introduced weeds (especially grasses, Iridaceae, Asteraceae, Fabaceae, and Brassicaceae), grazing by introduced and domestic animals, and dieback (or root rot) caused by Phytophthora spp. Phytophora cinnamomi is well established in the wetter regions in southwest Western Australia and has caused the collapse of entire ecological communities (Coates and Atkins 2001). Introduced predators are also a serious threat, especially the red fox (Vulpes vulpes).
Alterations associated with agricultural land use are also of concern. Changes in hydroecology through rising water tables and salinization is a threat in the agricultural areas in the inland portion of this region (Coates and Atkins 2001). Agricultural use has also allowed galah (Cacatua roseicapella) to invade from arid regions, but galahs ringbark trees and may be contributing to woodland decline. Some bauxite mining occurs on the western edge of the Yilgarn Plateau which is disturbs native vegetation although mining companies do restore vegetation.
Justification of Ecoregion Delineation
The Southwest Australia Woodlands ecoregion has the same boundary as the ‘Jarrah Forest’ IBRA (Thackway and Cresswell 1995). The Darling Range and the Yilgarn Plateau delineate the outer boundaries of this ecoregion while inland borders are defined largely by the decrease in rainfall. This ecoregion is part of the ‘High Rainfall Zone’ while the ‘Transitional Rainfall Zone’ is located inland of this ecoregion.
Burbidge, A.A., and N.L. McKenzie. 1989. Patterns in the modern decline of Western Australia’s vertebrate fauna: causes and conservation implications. Biological Conservation 50:143 – 198.
Coates, D.J., and K.A. Atkins. 2001. Priority setting and the conservation of Western Australia’s diverse and highly endemic flora. Biological Conservation 97: 251 – 263.
Garnett, S.T., and G.M. Crowley. 2000. The Action Plan for Australian Birds 2000. Environment Australia, Department of the Environment and Heritage, Canberra, Australia. http://www.ea.gov.au/biodiversity/threatened/action/birds2000/cont.html. viewed on October 3, 2001.
Gioia, P., and J.P. Pigott. 2000. Biodiversity assessments: a case study in predicting richness from the potential distributions of plant species in the forests of south-western Australia. Journal of Biogeography 27: 1049 – 1064.
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Mittermeier, R.A., N. Myers, P.R. Gil, and C. G.Mittermeier. 1999. Hotspots: Earth’s Biologically Richest and most endangered terrestrial Ecoregions. Cemex, Conservation International, Toppan, Japan
Short, J. and A. Smith. 1994. Mammal decline and recovery in Australia. Journal of Mammalogy 75(2): 288 – 297.
Strahan, R. 1998. The mammals of Australia. Australian Museum/Reed New Holland. Syndey, Australia.
Thackway, R. 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.
Wardell-Johnson, G. and P. Horwitz. 1996. Conserving biodiversity and the recognition of heterogeneity in ancient landscapes: a case study from south-western Australia. Forest Ecology and Management 85: 219 – 238.
Prepared by: Miranda Mockrin
Reviewed by: Harry Recher, Alex George