Location and General Description
This ecoregion extends to the elevated plateau of the Yilgarn Block to the north. The extreme western tip of this ecoregion includes the Leeuwin-Naturaliste Ridge, and just to the east, the Scott River Plain as it gradually slopes down from the Blackwood Plateau. The landscape throughout much of this ecoregion is subdued, and largely consists of Proterozoic rocks, sometimes overlain with Eocene marine sediments. Prolonged leaching led to the development of the low-nutrient soils characteristic of this region. Significant granite monadnocks are located in the Walpole-Denmark region. Higher landscape diversity is seen here, with hills, ridges, steep river valleys, dune systems, and coastal cliffs (Wardell-Johnson and Horwitz 1996). In general, this southwestern corner of Australia has a cool Mediterranean climate, with high average rainfall and a marked summer drought. Rainfall can approach 1,400 mm per annum along the coast, but decreases rapidly with distance from the coast. Rainfall is less seasonal in this ecoregion than in other portions of southwest Australia (Hopper et al. 1996).
Karri and tingle forests grow to outstanding heights, interspersed by wetlands, watercourses, and granite substrates unsuitable for forest growth. These evergreen forests can be very moist in winter, but lack the abundant epiphytes, liverworts, ferns, and mosses that characterize rainforest, probably due to the dry summer season. In the karri forests, the most important families are Fabaceae, Mimosaceae, Orchidaceae, Myrtaceae, and Proteaceae. Relicts with rainforest affinities include Anthoceris sylvicola, Cephalotus follicularis, and Podocarpus drouynianus. Wetland monocotyledons are also diverse in this region, with genera from the Cyperaceaae, Xyridacease, Juncaginaceae, Restionaceae, and Orchidaeceae well-represented. As rainfall and soil nutrients decrease, jarrah and marri forests develop. Jarrah forests grow to 40 m in height and are well developed in light of their low-nutrient soils (Hopper et al. 1996).
A variety of other vegetation types are found throughout the region. Sandy coastal plains support a mixture of vegetation communities, with distribution determined by topography. Heaths grow on elevated headlands areas, giving way to low scrubs and then to woodlands. Low coastal dunes harbor a rich flora and peppermint (Agonis flexuosa) scrub grows on coastal dunes and in sheltered sites.
All of the Mediterranean-climate, southwestern region of Western Australia is characterized by a highly endemic and rich flora. The area has experienced long isolation from other temperate regions in southeast Australia, beginning with the onset of aridity in the late Tertiary. Within southwest Australia, this ecoregion is considered the most important center of endemism for conservative, high-rainfall vascular plants in Western Australia (Hopper et al. 1992 in Wardell-Johnson and Horwitz 1996). The three tingle tree species and red-flowering gum (Eucalyptus ficifolia) are endemic to this ecoregion. In total, a survey of the karri forest and adjacent vegetation communities in this ecoregion reported nearly 2,000 taxa. The species-rich Albany region is located within this ecoregion and contains many local endemics, including the monotypic Albany pitcher plant (Cephalotus follicularis) (Cephalotaceae), which belongs to one of the few families endemic to the southwest of Australia. The mild climate, readily available moisture, and closed canopy harbors a rich Gondwanan heritage, including Gondwanan flora, and small Gondwanan spiders, Moggridgea tingle and Chasmocephalon spp (Wardell-Johnson and Horwitz 1996).
These moist forests harbor a number of globally threatened animals, including the western ringtail possum (Pseudocheirus occidentalis VU). The largest numbers of chudditch (Dasyurus geoffroii VU) are found in jarrah forest, while brush-tailed phascogales (Phascogale tapoatafa) are arboreal forest-dwellers (CALM 1997), and the quokka (Setonix brachyurus VU) inhabits dense moist vegetation, often near swamps (Strahan 1998). A number of other species are largely restricted to the mesic southwest and found in this ecoregion: the yellow-footed antechinus (Antechinus flavipes leucogaster), southern brown bandicoot (Isoodon obesulus), and the woylie (Bettongia penicillata ogilbyi) (Strahan 1998, Burbridge and McKenzie 1989).
Avian richness and endemism are low in comparison to structurally similar forests in southeastern Australia. Diversity is low partially due to the generally low ecosystem productivity but also in response to extinctions during periods of extreme Pleistocene aridity when moist vegetation and forest contracted. Most of the avifauna found in southwest Australia are either habitat generalists or tend to be widespread. A number of restricted range birds are found in this ecoregion, including the red-eared firetail (Stagonopleura oculata) which inhabits karri/tingle forest and creeklines and the white-breasted robin (Eopsaltria georgiana) which can also be found in karri and tingle forest (CALM 1997, Stattersfield et al. 1998). Several threatened species are found in the heath vegetation of this ecoregion, including the western bristlebird (Dasyornis longirostris VU), western whipbird (Psophodes nigrogularis nigrogularis), and the western ground parrot (Pezoporus wallicus flaviventris), while the noisy scrub-bird (Atrichornis clamosus VU) lives in densely vegetated, eucalypt-dominated gullies (Garnett and Crowley 2000, Hilton-Taylor 2000). The western subspecies of the rufous bristlebird (Dasyornis broadbenti litoralis) was once found in the Leeuwin-Naturaliste region, but has not been sighted since 1906 (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 Geocrinia vitellina, G. alba, and Spicospina flammocaerulea, a newly discovered monotypic species which is restricted to peat swamps (Cogger 2000)
The freshwater stream fauna in this ecoregion is known for its high endemism, but this fauna is still depauperate in comparison to biota from southeastern Australia. Aquatic invertebrates display high endemism and species richness. Freshwater ecosystems harbor a number of Gondwanan relicts, including endemic freshwater worms from the family Phreodrilidae, and freshwater crayfish from the genera Cherax and Engaewa (Wardell-Johnson and Horwitz 1996).
While the southwestern corner of Western Australia is recognized for its highly endemic and rich biota, it is also the portion of Western Australia most modified and reduced by human interactions. Agricultural modification and urbanization have caused the most significant habitat reduction. This ecoregion contains forests prized for their timber, and state managed logging continues today.
The protected area system in this ecoregion includes a number of large, well-known national parks, including Shannon National Park (535 km2), D’Entrecasteaux National Park, Mt. Frankland National Park (308 km2), and Walpole-Nornalup National Park (200 km2). However, Gioia and Pigott (2000) report that nearly 70 percent of conservation areas (national parks and nature reserves) were smaller than 1 km2, over a total study area that roughly encompassed this ecoregion and the Southwest Australia Woodlands ecoregion to the north.
Australia has experienced nearly 50 percent of the world’s mammalian extinctions in the past 200 years, concentrated in medium-sized, ground-dwelling species (Short and Smith 1994). In Western Australia, 17 species are extinct on the mainland and 24 have reduced ranges. However, the well-watered southwest, including this ecoregion, has served as a refuge, and many of these species have persisted only in the wetter parts of their ranges (Burbidge and McKenzie 1989).
Types and Severity of Threats
Fire, logging, and associated disturbances are the most serious threats in the Jarrah-Karri Forests and Shrublands ecoregion. Logging of open forest jarrah communities began soon after European arrival. Tall-open karri forests are also harvested (and often clearfelled), while forests with marri trees as major components are also logged. Greater utilization has lead to an increasing intensity of logging in jarrah forests. Fire management is an integral part of silviculture practice in karri and jarrah forests. Repeated burns lead to some vegetation types being burnt more frequently than others and may result in decreased diversity in vegetation communities. Burning may be incompatible with the Gondwanan elements found in forests here.
Increased infrastructure, such as road building may alter water runoff pattterns and spread pests and disease (Wardell-Johnson and Horwitz 1996). Introduced mammals, including cats (Felis catus), foxes, and rats (Rattus rattus and R. novegicus) are threats in this ecoregion (Burbidge and McKenzie 1989). Western Shield, a State sponsored fox control program, has achieved some success in restoring previously threatened and declining bird and mammal populations by reducing red fox (Vulpes vulpes) numbers. As a result, several species, including the woylie have been removed from threatened species lists, although they remain conservation dependent. Dieback disease caused by Phytophthora spp., in particular P. cinnamomi, is perhaps the most significant threat facing this ecoregion (Coates and Atkins 2001, Hopper et al. 1996).
Justification of Ecoregion Delineation
Primary Sources and Justification for Ecoregion Delineation: The Jarrah-Karri Forests and Shrubland ecoregion has the same boundary as the ‘Warren’ IBRA (Thackway and Cresswell 1995).
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Prepared by: Miranda Mockrin
Reviewed by: Harry F. Recher