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.
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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
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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
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Purple-crowned fairy-wren
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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.
