Location and General Description
South of Perth down to Cape Naturaliste, the Swan Coastal Plain forms a low-lying belt 25 km to 30 km wide, bordered to the east by the Darling Scarp and the Precambrian Yilgarn Block. The Swan Coastal Plain is covered by shoreline and associated dune deposits from the Pleistocene and Holocene that overlie Paleozoic and Neogene deposits of the Perth Basin. The Plain also includes large microtidal estuarine systems, such as the Swan-Canning Estuary and a number of lakes cut off from the sea by barrier dunes. The Swan Coastal Plain is transected by rivers flowing west from the Darling Plateau, and interspersed by wetlands (Hopper et al. 1996).
Progressing inland from the coast, coastal dunes with scrub-heath communities give way to woodlands. Kwongan (an Aboriginal term for heath-like vegetation), eucalypt, and Banksia low woodlands occur on the soils of the coastal dunes. Generally, kwongan scrub heath grows on sandy plains and swales and on lateritic upland soils while mallee eucalyptus woodland grows on duplex soils of upper valley slopes. Open heath favors drier areas, while a better watertable allows the development of eucalypt shrubs. Scrub-heath species include Banksia attenuata and B. menziesii with Casuarina spp. and E. todtiana. Christmas trees (Nuytsia floribunda) become locally dominant in drier areas. Shrubs are present, largely in wetter areas, with Acacia, Adenanthos, Banksia, Casuarina, Dryandra, Grevillea, and Hakea species found here (Beadle 1981).
Eucalypt woodlands can be found on the sandy loams, loams, and heavy loams on lower slopes and valley floors (Yates et al. 2000). Tall tuart (Eucalyptus gomphocephala) forests grow at the southern end of the Swan Coastal Plain (Hopper et al. 1996). Tuart usually occurs in monospecific stands, but can also be found with E. decipiens and/or E. cornuta. In the south, pure stands grow with peppermint (Agonis flexuosa), while in the north, peppermint is replaced by small Banksia trees and Casuarina fraserana (Beadle 1981). Now largely cleared, marri (Eucalyptus calophylla) – kingia (Kingia australis) woodlands grow on heavy soils of the Swan Coastal Plain, and marri – grass tree (Xanthorrhoea preissii) woodlands and shrublands also grow on the Swan Coastal Plain.
Western Australia contains approximately 12,000 angiosperms (half of Australia’s total flora), 75 percent of which are endemic to the southwestern region. The southwestern mediterranean climate regions are estimated to contain 8,000 species, with three quarters endemic. Finally, the Swan Coastal Plain is estimated to contain more than 2,000 species. Several large genera, such as Banksia, Caladenia, and Leucopogon, have the centers of their distribution in the mediterranean climate, southwestern region. Other genera are wholly restricted to the southwestern region, including Dryandra and Synaphea. Endemism is highest at the species level, and there are few endemic plant families (Hopper et al. 1996).
Mediterranean-climate vegetation shows a variety of adaptations to the poor soils, summer drought, and fires. The Christmas tree is a root parasite, the only member of the Mistletoe family to grow as a tree. Three quarters of the world’s carnivorous Drosera species are found in Western Australia, many of them in the kwongan and sandplain heaths, obtaining nitrogen and phosphorus from the insects they capture (White 1994). Also noteworthy, approximately 15 percent of southwestern wildflowers are vertebrate pollinated. Vertebrate pollinators include the western pygmy possum (Cercartetus concinnus) and honey possums (Tarsipes rostratus), the only mammal apart from some bats to subsist entirely on nectar and pollen.
Other mammals found in this ecoregion include tammar wallabies (Macropus eugenii), which were widely distributed in this ecoregion prior to European settlement, but remain offshore on Garden Island. In Western Australia the tammar wallaby lives among bushes of Gastrolobium bilobum, a fire-dependent, nitrogen-fixing legume that allows the growth of grasses, which are rare in the southwestern forested regions. Tammar wallabies feed on these grasses (White 1994). Rottnest Island lies 18 km offshore of Perth and harbors a stable population of quokkas (Setonix brachyurus VU), probably because there are no foxes (Hilton-Taylor 2000, Johnson and Thomson 1996). Heathland and wildflower habitats contain a rich avifauna, especially among the honeyeaters. Other birds found in this ecoregion include the western thornbill (Acanthiza inornata), and the short-billed black-cockatoo (Calyptorhynchus latirostris EN) which inhabits sandplain woodlands and mallee, and the long-billed black-cockatoo (Calyptorhynchus baudinii) which is found in open eucalypt woodland and farmland.
Seasonal wetlands (dry in the summer and wet in winter) are the most diverse habitat on the Swan Coastal Plain, and 16 out of 30 major floristic communities occur in wetlands. Coastal and near-coastal areas the next richest, containing 7 major communities (Hopper et al. 1996). The junction between the Swan Coastal Plain and the Darling Scarp is biologically diverse with relatively fertile soils, granite outcrops, and unusual clay-based ephemeral swamps. These swamps provide habitat for the critically endangered western swamp tortoise (Pseudemydura umbrina) which is found only in this ecoregion and may be Australia’s rarest reptile. It appears to be restricted to shallow, ephemeral swamps over the clay soils of the Swan River Valley (Cogger et al. 1993). The region around Perth is known for a diverse herpetofauna, including 16 frogs, two freshwater turtles, 51 lizards, and 24 snakes (Hopper et al. 1996).
Nearly 80 percent of the Swan Coastal Plain has been cleared (Beard 1995). Sandplain heathlands near Perth have almost entirely been converted as the urban center expands. Urban expansion reaches as far east as the Darling Scarp and has spread in a north-south direction as well (White 1994).
A number of Swan Coastal Plain woodland communities are listed as Endangered under the Commonwealth Environment Protection and Biodiversity Conservation Act: marri (Eucalyptus calophylla) – kingia (Kingia australis) woodlands on heavy soils, marri (Eucalyptus calophylla) – grass tree (Xanthorrhoea preissii) woodlands and shrublands, shrublands and woodlands of the eastern Swan Coastal Plain, shrublands and woodlands on Muchea Limestone of the Swan Coastal Plain, and shrublands and woodlands on Perth to Gingin ironstone (Environment Australia 2001).
Protected areas in this ecoregion include the Hills Forest conservation area which comprises five national parks (John Forrest, Gooseberry Hill, Greenmount, Kalamunda, and Lesmurdie Falls) and the Mt Dale Conservation Park. This ecoregion has a lower percentage of land in protected areas than the Jarrah-Karri Forests and Shrublands and Southwest Australia woodlands ecoregions (Thackway and Cresswell 1995).
Types and Severity of Threats
Continuing urban expansion and human use are severe threats. Proteaceae are susceptible to dieback disease caused by Phytophthora spp, and its spread through the wildflower heathlands is a serious concern. Too frequent fires may significantly alter plant species distributions. Fires poses a different threat on the coastal plain than they do in forests; on the coastal plain they are commonly the agent for woodland invasion by weeds (Hopper et al. 1996).
Justification of Ecoregion Delineation
This ecoregion occupies the Swan Coastal Plain, bounded by the ocean to the west and by the Yilgarn Block to the east. It has the same boundary as the ‘Swan Coastal Plain’ IBRA (Thackway and Cresswell 1995).
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Prepared by: Miranda Mockrin
Reviewed by: Harry Recher, Alex George