Ord-Victoria Plains
Bioregional Description
The Ord-Victoria Plains bioregion comprises three main components: (1) abrupt Proterozoic and Phanerozoic ranges and scattered hills and mesas mantled by shallow sand and loam soils supporting Triodia hummock grasslands with sparse low trees; (2) extensive plains based on Cambrian volcanics and limestones, with short grass (Enneapogon spp.) on dry calcareous soils and medium-height grassland communities (Astrebla and Dichanthium) on cracking clays. (3) in the south-west, Phanerozoic strata expressed as often lateritised upland sandplains with sparse trees. This component recurs as the Sturt Plateau Region in central Northern Territory.
Four subregions have been described for the region.
Special values
Some isolated ranges within this bioregion (notably the Bungle Bungle Ranges) support a distinctive biota with some limited endemism, and highly disjunct occurrences of species more widespread elsewhere. The biota includes a blend or overlap of arid-adapted and species typical of higher rainfall areas. As foxes are generally absent, the bioregion continues to support some mammal species which have declined or become regionally extinct across much of the rest of their range (e.g. bilby, spectacled hare-wallaby). The endangered gouldian finch is present, along with a very high diversity and abundance of granivorous birds generally. The bioregion includes large sections of some major river systems, notably the Ord and Victoria Rivers.
- Summary of overall condition and trend
Overall condition varies substantially across the bioregion. Over the first half of the twentieth century, many riparian areas and valley floors were highly degraded by high stocking levels, poorly regulated pastoral practices and high densities of feral animals (especially donkeys). In these extensive areas, plant species composition changed markedly and soil erosion became a major problem. These changes included broad-scale declines in preferred or susceptible plant species and concomitant at least local losses of some bird species (notably the riparian purple-crowned fairy-wren and white-browed robin). To some extent, this degradation has been checked and, in some cases, reversed, by stock exclosure, control of feral animals and more sustainable pastoral practices. However, there remain major problems with weeds, feral animals and changed fire regimes, and pastoralism probably continues to modify most lowland environments. In a somewhat generous assessment, the subregions have been scored continental stress classes of 4,5,5 and 6. - Summary of priority management/conservation priorities
The bioregion includes all or parts of some major reserves (notably Purnululu and Gregory NPs), but the overall regional reserve system is heavily biased, with little representation of the more fertile lowland plains. Enhancement of this system is needed to achieve a regional CAR network.
However, probably more effective for the management of this bioregions conservation values, is the continuation and enhancement of integrated (cross-tenure and cross-jurisdiction) control measures for feral animals, weeds and fire.
There is also some scope for enhancement of conservation actions on pastoral lands, notably stock exclosure for riparian areas and wetlands, and the retention of some water-remote lightly-grazed areas in environments favoured by grazing. - Wetlands
- Nationally important wetlands
The Ord-Victoria Plains bioregions includes two nationally significant wetlands, both on pastoral properties: Birrindudu waterhole and floodplain (NT015: wetland types B6, B4, B10, B13 and B14) and Nongra Lake (NT016: wetland types B6 and B14). Both may be under some pressure from cattle grazing on the fringes, but there has been some recent initiatives to fence Birrindudu wetlands. The artificial Lake Argyle (WA097: wetland type C1) occurs across the border between this bioregion and Victoria-Bonaparte bioregion.
- Other wetlands of subregional significance
Other important wetlands in the bioregion include the major river systems, and gorges within the Bungle Bungle and Osmand Ranges.
- Nationally important wetlands
- Riparian zones
The bioregion includes large stretches of the Ord and Victoria Rivers, and their tributaries including the Negri, West Baines, Camfield, Armstrong, East Baines, Wickham and Humbert Rivers. It also includes parts of the headwaters of the Fitzroy River (Christmas and Cherrabun Creeks), and part of the inland-draining Sturt Creek system. Riparian condition varies from poor to good condition, partly depending upon land tenure and management. Many weeds are widespread in riparian areas (e.g. noogoora burr, castor-oil plant, bellyache bush, parkinsonia). Many other riparian areas suffer from trampling or fouling by livestock and feral animals, and/or from the consequences of widespread increased run-off. However, the condition of many riparian areas has improved over the last 30 years due to some stock exclosure, and increased use of artificial water-points. - Ecosystems at risk
No ecosystem in this bioregion has been formally assessed for risk status. However, wetland and riparian habitats have been degraded across most of their range in this bioregion. There has been widespread structural and floristic change in some lowland vegetation types exposed to heavy grazing pressure. - Species at risk
11 species occurring in this bioregion are listed as threatened at federal or State/Territory level. Of these, the purple-crowned fairy-wren may be increasing due to some recovery in the extent and condition of riparian areas.
Number of taxa in the Ord-Victoria Plains bioregion listed as threatened at national and/or NT/WA 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 | 0 | 4 |
| birds | 1 | 3 | 1 | 3 |
| mammals | 0 | 3 | 0 | 2 |
There is also some evidence that there is broad scale decline affecting at least some groups of mammals and birds in this bioregion, in addition to those species currently listed as threatened.
- Other flora values for eucalypts and acacias
- Endemism
This bioregion contains two endemic acacia and two endemic eucalypt species: Acacia zatrichota from OVP1, Eucalyptus sp. (Kalkarindji D76647) and E. sp. (Montejinni A2171) from OVP2, and A. armentifera from OVP3. There is no information available about the status of these taxa, nor their threats, nor trends in their condition.
- Richness
Richness Subregions 1 and 2 have relatively high diversity of Acacia (54 and 71 species respectively) and Eucalyptus (45 and 56 species respectively). Subregions 3 and 4 have far lower diversity (35 and 20 species respectively for Acacia and 27 and 10 species for Eucalyptus), reflecting their smaller size and more uniform environmental composition.
- Endemism
- Birds
There is some indication of increases in many bird guilds over the period between the two bird Atlases, but any interpretation of such changes needs to be treated with some caution due to generally more favourable conditions during the second Atlas. - Mammals
Of the 53 species recorded from this bioregion, 3 are now regionally extinct, 5 have declined and 45 remain stable.
Management Responses
- Reserve consolidation
The bioregion contains some large and important reserves (notably Purnululu and part of Gregory NPs), which include good representation of the areas more rugged upland elements. However, a more comprehensive reserve network should include additions to represent more of the fertile plains environments. Given the pastoral quality of such areas, such addition may be both expensive and difficult to achieve. - Off park conservation for species and ecosystem recovery
There has been increasing use of exclosure fencing of riparian areas and wetlands across parts of the bioregion, for habitat protection and for increased efficiency in stock management. Some large properties in the Ord catchment have been de-stocked for several decades, to reduce sedimentation loads entering Lake Argyle. These exclosures are leading to some recoveries in previously degraded systems. - Integrated NRM
As with many other bioregions, the main priorities for NRM are the integration, across tenures and jurisdictions, of management actions to combat weeds, feral animals and changed fire regimes. For such implementation to be successful, more resources and capacity-building are required. Fire management may be a complex issue to resolve, as some pastoralists are reluctant to use fire as a management tool: however, there is evidence that prime pastoral grasslands are being invaded by native woody "weeds" (mostly Terminalia volucris), at least partly because of lack of fire. Exclosure fencing of riparian areas has been established on some pastoral properties, and there is a strong management case to extend this practice further. Both jurisdictions in this bioregion include some relatively comprehensive and substantial monitoring of vegetation on pastoral lands. There is also a monitoring program for the impacts of fire on vegetation in Gregory NP. The occurrence of fire is being monitored across the bioregion, and satellite imagery is being used for broad-scale monitoring of pastoral land condition.
Further Information and Gaps
- Major data gaps and research priorities for bioregion
Management would benefit from more detailed environmental mapping, in particular vegetation mapping at the scale of 1:250,000 or better.
A recent analysis of data bases for flora and fauna in this bioregion demonstrated substantial biases and shortcomings in collection effort. A more comprehensive system of biodiversity sampling is required, with more integration of data bases and conservation assessment across jurisdictions.
The existing vegetation monitoring on pastoral and conservation lands could be enhanced by inclusion of some fauna components; and there needs to be more substantial monitoring of threatened species, weeds, feral animals and the response of these to management. - Other information