Victoria Bonaparte
Bioregional Description
Phanerozoic strata of the Bonaparte Basin in the north-western part are mantled by Quaternary marine sediments supporting samphire - Sporobolus grasslands and mangal, and by red earth plains and black soil plains with an open savanna of high grasses. Outcrops of Devonian limestone karst in the west support tree steppe and vine thicket. Plateaux and abrupt ranges of Proterozoic sandstone, known as the Victoria Plateau, occur in the south and east, and are partially mantled by skeletal sandy soils with low tree savannas and hummock grasslands. In the south east are limited areas of gently undulating terrain on a variety of sedimentary rocks supporting low Snappy Gum over hummock grasslands and also of gently sloping floodplains supporting Melaleuca minutifolia low woodland over annual sorghums. Dry hot tropical, semi-arid summer rainfall.
The bioregion is divided into 3 subregions, reflecting major geomorphological differences.
Special values
Much of this bioregion is very sparsely populated, and comprises rugged ranges and gorges supporting a rich biota which is largely a subset of that of the even more dissected north Kimberley. In marked contrast, the bioregion also includes substantial areas of lowland valleys, floodplains and significant wetlands. Threatened species present include the gouldian finch, two species of freshwater sharks and the Angalarri grunter, an endemic fish.
- Summary of overall condition and trend
Most of this bioregion is in good condition, largely spared from intensive land use. However, some pastoral areas have suffered some degradation, and feral animals (pig, donkey, horse, water buffalo and cattle) are widespread and abundant. Fire regimes have changed substantially over the last century, generally to now comprise a higher frequency of extensive hot late dry season fires. Weeds are becoming more abundant and widespread, particularly in riparian areas and wetlands, and some exotic pasture grasses are still being deliberately spread. One of the bioregions major rivers, the Ord, has been substantially modified by impoundment and water use. Large-scale expansion of the Ord horticulture area has been proposed recently, and this would substantially affect the Keep River and its floodplain area. Reflecting the disparity in land-use intensity across this bioregion, the three subregions have been allocated three different continental stress classes: 3,4 and 5. - Summary of priority management/conservation priorities
The existing reserve system is limited and biased towards more rugged areas. Further expansion would be required to achieve a CAR regional reserve network. There are two other main NRM priorities - the extensive integrated management of feral animals, fire and weeds across the less developed bulk of the bioregion, and the establishment of sustainable land-use practices in those sections of the bioregion (to be) used for intensive horticulture. - Wetlands
- Nationally important wetlands
The bioregion includes all or part of five nationally significant wetlands: Legune wetlands (NT030: wetland types B6, B10, B13 and C2), the Ord estuary system (WA099: wetland types A6, A7, A8 and A9), Parry floodplain (WA100: wetland types B4, B1, B2, B6, B10, B14 and B17), and the artificial wetlands of Lake Argyle (WA097: wetland type C1) and Lake Kununurra (WA098: wetland type C1, B1 and B9). These are generally in good condition, although livestock and feral animals affect at least parts of the Legune, Parry and Ord estuary sites, weeds are expanding on most of these wetlands, and the Ord and Parry wetlands are affected by water use and horticultural practices.
- Other wetlands of subregional significance
There are many other wetlands of significance in the bioregion, although there has been no detailed assessment of their values. The bioregion includes some spectacular "wilderness" rivers, most notably the Fitzmaurice River. The lower reaches of the Victoria and Keep Rivers support significant waterfowl breeding colonies and feeding/roosting sites for migratory shorebirds. There are unusually extensive areas of rice-grass Xerochloa floodplain grasslands in patches of the Victoria, Ord and Keep systems.
- Nationally important wetlands
- Riparian zones
Major river systems in the bioregion include the Fitzmaurice, Victoria (including its tributary Baines, Angalarri and Ikymbon Rivers), Keep, Ord, Pentecost and Chamberlain. The condition of the riparian zones is variable, depending upon land use. Those rivers flowing mainly through non-pastoral lands (such as the Fitzmaurice) are mostly in near pristine condition, albeit affected somewhat by weeds and feral animals. Riparian areas of some other systems have been degraded by exposure to unrestricted access by livestock, although this situation is improving as riparian areas are exclosure-fenced and there is a greater reliance on artificial water sources. Riparian areas in the lower Ord system have been substantially modified by impoundment and water use: in some areas the reduced seasonal variability of water flow has benefitted some components of riparian vegetation, leading to more extensive patches of rainforest-type vegetation. Heavy infestations of weeds (such as noogoora burr, parkinsonia, and castor-oil plant) are a feature of most riparian systems. - Ecosystems at risk
There has been no formal assessment of the threatened status of ecosystems across most of the bioregion. In the Western Australian portion of the bioregion the following communities may be at risk: wild rice Oryza australiensis grasslands on alluvial flats of the Ord (threatened by clearing and changed fire regimes); an invertebrate community at Zebedee Springs, El Questro station (through impacts of tourism); vine thickets of limestone Ningbing ranges (by changed fire regimes), and rainforests and paperbark forests associated with lowland and mound springs (by grazing pressure, changed fire regimes and weeds). Across the bioregion there is also concern for the savanna communities generally (with broad-scale decrease of fire-sensitive plants such as Callitris intratropica), rainforests, wetlands, heathlands on sandstone and sandplains, and riparian areas, due to the pervasive impacts of changed fire regimes, weeds, livestock and feral animals. - Species at risk
23 species occurring in this bioregion are listed as threatened at national or NT/WA level. Note that there is a major disparity in the status assigned to many plant species, due to a very recent revision of the status of all NT plant species. As all of the plant species listed are NT endemics, this revised assessment is likely to be applied also at the national scale.
Number of taxa in the Victoria-Bonaparte bioregion listed as threatened at national and/or NT level.
(nb this table includes only species definitely recorded from the bioregion (rather than putative occurrences based on modelling) and presumed to be still extant in the bioregion).
| taxa | National | Northern Territory | ||
|---|---|---|---|---|
| endangered | vulnerable | endangered | vulnerable | |
| plants | 0 | 0 | 1 | 5 |
| fish | 0 | 2 | 0 | 1 |
| reptiles | 2 | 4 | 0 | 1 |
| birds | 1 | 5 | 1 | 3 |
| mammals | 0 | 2 | 0 | 1 |
- Other flora values for eucalypts and acacias
- Endemism
There are no endemic eucalypt species and one endemic acacia species in this bioregion: Acacia richardsii.
- Richness
Species richness is high for the largest and best-known subregion 1, with 83 species of Acacia and 60 species of Eucalyptus, but low for the two other subregions (5-18 species for Acacia and 8-21 species for Eucalyptus).
- Endemism
- Birds
Over the period between the two bird Atlases, there was an increase in the reporting rates for most granivorous birds and some waterfowl. This change may reflect higher rainfall periods during the second bird Atlas, and some increased abundance of birds favoured by horticultural expansion. - Mammals
Of 55 mammal species recorded from this bioregion, 4 are now extinct, 1 has declined and 50 are stable.
Management Responses
- Reserve consolidation
Partly associated with proposals for increased horticultural developments, some large areas have been added to, or foreshadowed for, the reserve system. These would contribute substantially to improving the comprehensiveness of the existing network, although the representation of lowland fertile areas is still limited. - Off park conservation for species and ecosystem recovery
There is one formal off-reserve conservation agreement established in the bioregion, which excludes livestock from an important breeding colony of the endangered gouldian finch. At another breeding site for this species (Mt Thymanen on the military training area of Bradshaw Station), the Department of Defence is developing a management plan to institute appropriate burning regimes and to limit other disturbances. - Integrated NRM
There is a priority for enhanced integrated (across tenures and jurisdictions) broad-scale management of weeds, feral animals and fire across the entire bioregion. In those parts of the bioregion affected by intensive horticulture, the management of water resources is a major issue, and a water allocation plan is critical. In pastoral areas, exclosure fencing of riparian areas would reduce impacts.
NRM issues in this bioregion are being considered systematically through the recently established Ord-Bonaparte Project.
Further Information and Gaps
- Major data gaps and research priorities for bioregion
An assessment of the amount of biodiversity data available in this bioregion has been completed recently, indicating large areas with little information. There has been no systematic biodiversity sampling across the bioregion as a whole. The available information is also limited by some cross-jurisdictional problems in data access and integration. Management would benefit from more detailed environmental mapping, in particular vegetation mapping at the scale of 1:100,000, or better (1:50,000) for those areas subject to development pressure.
Currently in this bioregion, there is no substantial monitoring program which includes as a major goal the assessment of trends in biodiversity conservation (although there are well-established pastoral monitoring programs in both the NT and WA portions of the bioregion). Such a program should be a management priority. - Other information