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EnviroNorth > VRD-Sturt > Plants and Animals

Plants and animals

by John Woinarski, Parks and Wildlife Commission of the NT

Crossroads for biodiversity

The Victoria River District (VRD) and Sturt Plateau form crossroads for biodiversity. Along a latitudinal gradient, the fauna and flora typical of the higher rainfall forests gradually gives way to that of the arid and semi-arid woodlands, shrublands and grasslands. Longitudinally, the VRD forms the eastern frontier for the range of many characteristically Kimberley species. It is very much a region of interchange and range limits, with relatively few distinctive endemic species.

Lancewood (Acacia shirleyi ) grows in almost mono-specific stands across extensive areas of the NT and central Qld
Lancewood ( Acacia shirleyi ) grows in almost mono-specific stands across extensive areas of the NT and central Qld, and forms a distinct habitat from eucalypt woodlands which often abut it. Typically there is far less grass in lancewood forests, and this open understorey favours a range of animals. Photo: John Woinarski 

Sturt Plateau

The Sturt Plateau marks part of the divide between eastward (to the Gulf of Carpentaria) and westward (to the Joseph Bonaparte Gulf) flowing watersheds. It is a relatively featureless, topographically simple elevated plain derived mostly from Tertiary laterite. Its main environments are eucalypt forests and woodlands (mostly dominated by bloodwoods) and tall shrublands or woodlands of lancewood Acacia shirleyi and bullwaddy Macropteranthes kekwickii .

These two main environments support quite different biota. The lancewood and bullwaddy habitats may include an inland component of rainforest-affiliated plants, due in part to the relatively closed canopy in parts of this habitat and its relatively low frequency of fire.

Effect of fire

Fire appears to be a major controller of the boundary between eucalypt woodlands and lancewood/bullwaddy patches. The relatively grass-free understorey of lancewood/bullwaddy vegetation supports a different suite of animals to that of the eucalypt woodlands. Common species include grey-crowned babbler, apostlebird and hooded robin, which all forage in the open ground or litter; and the spectacled hare-wallaby and northern nailtail wallaby, which commonly shelter under bullwaddy trees during the day.

Naitail and hare-wallabies have had a generally bad time of it over the last 200 years. Of the three species of nailtail wallabies alive at the time of European colonisation, the one occurring in central Australia has become extinct and the one in eastern Australia is now restricted to a few properties in central Queensland. In contrast, the northern nailtail wallaby remains locally common in both the VRD and Sturt Plateau, suggesting that the environment there remains in reasonably good shape. Of the four species of hare-wallabies ( Lagorchestes spp.) present 200 years ago, two are extinct (one from eastern Australia and one from central Australia), and another has become extinct across its former extensive continental range but persists now on a few islands off Western Australia. Only the northern species has persisted reasonably well, again with a stronghold in the VRD-Sturt Plateau region.

Victoria River District

Purple-crowned fairy wren
Purple-crowned fairy-wren 

The Victoria River District is far more diverse than the Sturt Plateau. The most distinctive main environments are the riparian strips of the Victoria River itself (and other main rivers in the region), the sandstone ranges, tussock grasslands on black soil (basaltic)and eucalypt woodlands on limestone (loam) soils. Riparian environments vary substantially depending upon stream order and topography. For much of its length, the Victoria River channel supports dense river-side vegetation characteristically including Pandanus aquaticus, Eucalyptus camaldulensis , Terminalia platyphylla , Melaleuca spp., Lophostemon lactifluus , Ficus spp., Nauclea orientalis and a diverse range of other often rainforest-associated species. This dense strip provides the major habitat for a range of birds such as the channel-billed cuckoo, koel, shining flycatcher, dollarbird, little shrike-thrush, crimson finch, lemon-bellied flycatcher , and such species extend into relatively low rainfall areas only or typically along these relatively rush riparian strips. Dense stands of "cane grass" Mnesithea  rottboellioides occur alongside some stretches of the river, and these support the restricted purple-crowned fairy-wren, a species considered  susceptible to habitat degradation through bad pastoral management. (NB Click on the species to see a list of research findings).

Sandstone ranges

The sandstone ranges and outcrops of the Victoria River District are neither as extensive nor rugged as those of the north Kimberley and western Arnhem Land, and hence tend to support a smaller set of sandstone biota, although this is still generally notably different from that of the VRD lowlands. Characteristic vertebrate species include the ningbing antechinus, splendid tree frog, white-quilled rock-pigeon (all at the eastern edge of their predominantly Kimberley distribution), nabarlek, short-eared rock-wallaby and sandstone shrike-thrush.

Tussock grasslands

Tussock grasslands on black soil occur in the lower Keep River-Spirit Hills-Legune area, in patches of the West and East Baines valleys, and more extensively in the south of the VRD. Mitchell grasses ( Astrebla spp.) dominate in some of these grasslands, especially in the southern VRD, and this environment has many common features with that of the far more extensive Barkly Tablelands. Many animal and plant species are shared between the VRD and Barkly, although more "grassland" species typically occur on the Barkly.

Woodlands

The woodlands and forests of the Sturt Plateau interrupt these grasslands and have provided a barrier preventing some Barkly Mitchell Grassland species (e.g. the large goanna Varanus spenceri ) from (re)colonising the VRD grasslands. Common animal species in the grasslands of the VRD include the bustard ("bush turkey"), singing bushlark, red-backed fairy-wren, long-tailed planigale and a lizard Ctenotus rimacola first described as recently as 1998. However, the faunal diversity in these grasslands is generally low compared with the other main environments in this region.

Eucalypt woodlands, mostly dominated by bloodwoods and silver-leaved box Eucalyptus pruinosa , dominate the most extensive areas of the VRD, occurring on a range of soil types but particularly on loams. These environments support a relatively diverse range of fauna, with nectarivorous birds (honeyeaters, friarbirds and lorikeets) extremely abundant when the dominant trees flower.

Environmental challenges

There are some substantial conservation and management issues in the VRD and Sturt Plateau. Much of the region's native mammal fauna appears to be in bad shape, with local extinctions and regional declines of possums, bandicoots and quolls. Fire regimes appear to have undergone major changes in the last 150 years, with contemporary regimes leading to more destructive fires in the sandstone ranges and fewer fires (and hence vegetation change probably including increase in woody shrubs) in the prime pastoral lands (the blacksoil plains). Some environments are subjected to high grazing pressure and feral animals (especially donkeys) can reach extremely high denisties. Many weed species are common and some environments (notably riparian strips) are now grossly modified by weed invasion.