Biodiversity Assessment - Mitchell Grass Downs
Mitchell Grass Downs
Introduction
The bioregion is characterised by undulating downs on shales and limestones with Mitchell grass (Astrebla spp) grasslands and Acacia low woodlands. The soils are predominantly deep heavy grey and brown cracking clays often with self-mulching and sometimes stony surfaces. The plains are interspersed with drainage lines, supporting open grasslands, herblands or eucalypt woodlands and isolated remnant plateaus supporting a variety of hummock grasslands and shrubland vegetation.
There are 8 subregions within the Mitchell Grass Downs.
Summary of overall condition and trend
The Mitchell Grass Downs bioregion is in fair to good condition with most ecosystems stable.
The bioregion is subject to intensive grazing due to value of downs for pastoral use and the expansion of available watering points. As some species may be intolerant of grazing (Landsburg 1999) this wide geographical spread of grazing is a significant threat to biodiversity. The relatively smaller property size of downs 'blocks' in Queensland versus the Barkly Tableland means that this threat is potentially greater. Perennial Mitchell grass pastures are relatively resilient to grazing pressure however a decline in perennial grass species and an increase in annual, herbaceous species is associated with heavy total grazing pressure. Areas such as riparian frontages, alluvial plains and wetlands and waterholes are often heavily impacted by total grazing pressure as evidenced by extensive areas of bare ground and trampling.
Some vegetation clearing has occurred in the bioregion associated with the introduction of exotic pasture species. Exotic weed species are having a significant impact on the bioregion.
Five of the subregions have a landscape health class of 3, and three subregions have a landscape health class of 6.
Summary of conservation priorities
The main priorities for reservation in the Mitchell Grass Downs bioregion include wetlands and chenopod swamplands and various wooded Mitchell grasslands including A. peuce and boree (A.tephrina). Such reserves should also aim to capture the variation within grasslands across their geographic range to fully represent floristic diversity. To complement a representative reserve system, it is a priority to secure ungrazed or low intensity grazed areas through incentives, fencing and covenants on privately held lands.
Natural values
The alluvial areas within the Mitchell Grass Downs support a high diversity of arid zone fauna. Species of flora and fauna of the Mitchell Grass Downs commonly show some level of specialisation to allow them to cope with the high temperatures and deep cracking nature of the soils. Threatened fauna species that occur in the bioregion include the endangered bilby, the night parrot and vulnerable plains wanderer. The bioregion also supports populations of endemic freshwater fish associated with mound springs.
Ephemeral wetlands, swamps and lakes provide important habitat for waterbirds included the rare freckled duck. These wetlands act as a refuge and stop over point for migratory species. Areas of bluebush in the Northern Territory provide core habitat for the vulnerable yellow chat.
Click here to link to a table of natural values within each subregion
Wetlands
There are nine wetlands of national significance within this bioregion that include shallow drainage lines and artesian mound springs. These wetlands are of value for their uniqueness and rare ecosystems. While these wetlands have been considered threatened by grazing, changes in hydrology and invasion by exotic weeds, they are being actively managed to minimise these threats and the condition is improving.
There are 137 other wetlands of regional significance. In general these are in good condition although declining. The major threat to these systems is uncontrolled access for grazing
Nationally important wetlands
Map: IBRA map showing DIWA locations, towns, subregions, major roads and reserves and most common threatening processes.
Click here to link to a table of Australia's Important Wetlands (Directory of Important Wetlands of Australia): their type, condition, trend and threatening processes within each subregion.
Regionally important wetlands
Click here to link to a table of provisional identification of wetlands of regional significance: their type and special values within each subregion. The reliability of the overall subregional assessment is indicated.
Click here to link to a table of provisional identification of wetlands of regional significance: their condition, trend and threatening processes within each subregion.
Riparian Zones
The Mitchell Grass Downs includes the upper catchments of the Diamantina, Georgina and Flinders River systems. Six of the subregions have riparian zones that are in either good or fair condition, and are static in trend. Two subregions have riparian zones that are in fair or good condition, but declining in trend. Major threats to the riparian zones are grazing pressure, feral animals and exotic weeds.
Map: Riparian threatening processes.
Click here to link to a table of riparian zones: their average condition, trend and threatening processes for each subregion. The reliability of this overall assessment is indicated.
Ecosystems at risk
Within the Mitchell Grass Downs, there is one endangered and 31 vulnerable regional ecosystems. This equates to approximately 0.6 and 20 percent respectively of the total number of ecosystems occurring in the bioregion. Grazing pressure is the most significant threatening process within the bioregion. Other threatening processes include exotic weeds, changed fire regimes and fragmentation of remnants.
Ecosystems under threat tend to occur on alluvium associated with drainage lines and levees. Over 70% of the threatened ecosystems within the Mitchell Grass Downs are described as outliers, with their greatest distribution in the bioregions adjacent to this bioregion. The most common threatened ecosystems are the arid eucalypt low open woodlands with tussock grass (31% of all threatened ecosystems) followed by arid acacia low open woodlands and shrublands with chenopods (16% of all threatened ecosystems). The condition of the majority of the threatened ecosystems is declining.
Map: IBRA map showing frequency of threatening processes for ecosystems.
Click here to link to a table of provisional list of threatened ecosystems in Australia: their broad vegetation type (National Vegetation Information System - Major Vegetation Subgroup), recommended status, current legislative protection as a threatened ecosystem, trend and bioregional distribution. These ecosystems are arranged in the bioregion of their principal occurrence. The reliability of the recommended status is indicated.
Click here to link to a table of provisional list of threatened ecosystems in each subregion: their threatening processes.
Click here to link to a table of provisional list of threatened ecosystems in each subregion: their recommended recovery actions
Species at risk
There are 32 species listed as threatened in the bioregion, one that is considered extinct, eight endangered and 24 vulnerable. The general trend of populations of the threatened species is declining, although the trend is unknown for the Edgbaston goby, Elizabeth Springs goby, kowari. Recovery of the majority of the threatened species will not occur in the medium term and requires significant intervention. The key threatening processes are related to predation by feral animals, grazing pressure and invasion by exotic species particularly prickly acacia and parkinsonia. Concern is increasingly being expressed at the invasion of Mitchell grass by buffel grass. Significant pressure is exerted from broad scale vegetation clearing of wooded areas and the fragmentation and loss of remnants. Species such as the dusky hopping-mouse are rapidly declining in the bioregion.
Table: Threatened species within each taxon occurring within the bioregion.
| Taxon | State and Territory Listing | Commonwealth Listing | ||||
|---|---|---|---|---|---|---|
| Endangered | Vulnerable | Extinct | Endangered | Vulnerable | Extinct | |
| Amphibians | 0 | 0 | ||||
| Birds | 2 | 7 | 1 | |||
| Insects | 0 | 0 | ||||
| Fish | 1 | 2 | ||||
| Mammals | 2 | 4 | ||||
| Plants | 2 | 15 | 2 | 9 | ||
| Reptiles | 1 | 0 | ||||
| Total | 8 | 28 | 2 | 10 | ||
NB: This table includes threatened taxa listed by both the Northern Territory and Queensland. Where a species is listed on one side of the border and not the other, it has been included. Where a species has different listings on either side of the border, it has been assigned the status that refers to the majority of the bioregion.
Map: IBRA map showing frequency of threatening processes for species.
Click here to link to a table of species at risk in each subregion: their status, trend and subregional distribution. The reliability of the assessment of trend is indicated and whether recovery plans have been prepared.
Click here to link to a table of species at risk in each subregion: their threatening processes.
Click here to link to a table of species at risk in each subregion: their status recommended recovery actions.
Eucalypts and Acacias
No endemic eucalypts or acacias were identified in the Mitchell Grass Downs subregion.
Birds
This huge bioregion, which has a greater longitudinal spread than any other, has a relatively low level of avian biodiversity and few limited range or threatened taxa. There are also few introduced taxa. A number of species characteristic of the Mitchell Grass Downs, such as Flock Bronzewing, Letter-winged Kite and Yellow Chat have also been recorded in at least 10 other bioregions. However, as the kite was not recorded at all in the bioregion during the second Atlas period, and the reporting rate of the other two declined, the status of these species should be assessed. Grassland birds as a group, however, did not have a significantly lower reporting rate, contrary to the national trend.
Status: Low overall diversity, and among guilds.
Rare and threatened: No major populations.
Increasers: Australian Raven.
Indicators: Emu, Banded Lapwing, Flock Bronzewing, Yellow Chat, Horsefield's Bushlark.
Trend: No major trends detected, higher rainfall possibly masking changes.
Scenario: Probable continued decline of grassland species.
Actions: Ensure adequate representative areas are protected from grazing by introduced herbivores.
Click hereto download a summary report including the physical characteristics of the bioregion, a species list, and summary statistics [Excel file]. The file may open on your screen. To save it to your system 'Save as' under the File menu.
Mammals
Number of species and status
There are 34 mammal species within this bioregion. (The maximum number of species recorded in a bioregion is 86 and the minimum is 25).
Click here to link to a table of number of species in each status class for this bioregion.
Click here to link to a list of mammal species and their status for this bioregion.
Critical weight range
The critical weight range (35 - 5500 g) of mammals is the size range of Australian mammals that have been most affected by environmental changes following European settlement. In this bioregion, the proportion of mammal fauna within the critical weight range is .353. (The maximum proportion of fauna within the critical weight range recorded in a bioregion is 0.632 and the minimum is 0.222).
Faunal Attrition Index
Faunal attrition is a measure of contraction or loss of species richness with a region. A high index value means many species have declined or are extinct in the bioregion. The index can be used to compare the status of mammal fauna to regional attributes such as changes since European settlement and average annual rainfall. The Faunal Attrition Index for mammals in this bioregion is .13. (The maximum faunal attrition index value recorded in a bioregion is 0.66 and the minimum is 0).
Click here to link to a table of Faunal Attrition Index for groups of mammals shows the contributions of each group to overall patterns of faunal decline.
Faunal Contraction Index
A range contraction index is a measure of the extent to which the range inhabited by a particular species has contracted. A high index value means that many of the species comprising the region's original mammal fauna have contracted from a high proportion of the regions they originally occurred in. The faunal contraction index for the mammal fauna in this bioregion is .19. (The maximum faunal contraction index value recorded in a bioregion is 0.51 and the minimum is 0.07).
Faunal Endemism Index
Endemic species are those restricted to certain regions. Regions containing endemic species are considered to have high biodiversity conservation values because opportunities to conserve those species do not exist elsewhere. A high index value means that the species comprising the original mammal fauna typically occurred in few bioregions. The faunal endemism index value for the mammal fauna in this bioregion is .6. (The maximum faunal endemism index value recorded in a bioregion is 0.79 and the minimum is 0.52).
New Endemism Index
Extant (still surviving) species that have undergone major range contractions can be considered 'new endemics'. Bioregions that contain new endemic species are often important refugia for threatened species. The new endemism index for the mammal fauna in this bioregion is .66. (The maximum new endemism index value recorded in a bioregion is 0.93 and the minimum is 0.5).
Table: Translocated Species
There is no data available for this table within the bioregion.
Exotic Mammals
The number of introduced exotic mammal species that occur within this bioregion is 13. (The maximum number of introduced exotic mammal species in a bioregion is 16 and the minimum is 5).
Click here to link to a list of introduced exotic mammal species for this bioregion.
Extinct mammal species
The number of extinct mammal species that previously occurred within this bioregion is 2. (The maximum number of extinct mammal species in a bioregion is 29 and the minimum is 0).
Click here to link to a list of extinct mammal species for this bioregion.
Management responses
Reserve consolidation
The Mitchell Grass Downs has 1.1% of the total area of the bioregion under reserve tenure in Queensland and the Northern Territory. Approximately 36% of all regional ecosystems are included in the reserve estate. Protected areas are dominated by ecosystems located on undulating plains and alluvial areas.
The major reservation priorities consist of fringing alluvial ecosystems of coolabah, river red gum and bluebush. Gidgee, brigalow, mound springs and Acacia peuce communities are also a priority. The priority communities include 4.3.11 - coolabah ? river red gum on alluvium, billabongs and permanent waterholes, 4.3.21 -Acacia peuce on alluvium, 4.3.2 - river red gum ? coolabah on drainage lines, 4.3.1 -river red gum ? Melaleuca spp. on drainage lines, 4.3.13 - Eragrostis setifolia and Marsilea drummondii ? Chenopodium auricomum in drainage depressions, 4.9.11 - gidgee with scattered shrubs such as false sandalwood and wilga on fresh Cretaceous sediments, 4.9.15 - Brigalow and scattered emergent whitewood ? Eucalyptus spp. and 4.9.7 - Astrebla spp. with A. tephrina on Cretaceous sediments, and 4.3.2 - river red gum ? coolabah on drainage lines.
Priority ecosystems identified by the Northern Territory include A. georginae (gidyea) low open-woodland with open-grassland understorey, Chenopodium auricomum (bluebush) low open-shrubland with ephemeral grassland understorey, Astrebla pectinata (barley Mitchell grass) grassland, seasonal grassland with Muehlenbeckia cunninghamii (lignum) low sparse-shrubland overstorey, E. microtheca (coolibah) low open-woodland with Chenopodium auricomum (bluebush) sparse-shrubland understorey, E. microtheca (coolibah) low-open woodland with Eulalia aurea (silky browntop), Astrebla (Mitchell grass) grassland understorey.
The Mitchell Grass Downs are a priority for increased reservation as the representation of communities across their range is low. The main threat to these ecosystems is grazing pressure. However, the biodiversity values in this bioregion may be able to be sustained for a greater period under existing conditions compared to bioregions with other threats such as broad scale tree clearing.
The priority subregions for increased reservation include Southern Wooded Downs (where the main threat is broad scale tree clearing) and Georgina Limestone.
Click here to link to a table of comprehensiveness, adequacy and representativeness (CAR) of the National Reserve System in terms of ecosystems and area sampled and a ranking of reserve management. The bioregional priority for consolidating the National Reserve System is based on this CAR analysis and threat.
Click here to link to a table of bioregional and subregional priorities and ecosystem priorities to consolidate the National Reserve System and associated ecosystem constraints.
Off-park conservation for species and ecosystem recovery
Brigalow, gidgee and blackwood communities, gidgee communities, arid zone mammals, woodland fauna, reptiles, and grassland fauna are identified as the priority groups within the bioregion. Inventory and life history information is required for the fauna groups. Ecological understanding of the relationship between fire, grazing and biodiversity values and the implementation of adaptive management techniques are required for recovery actions for the ecological communities. An understanding by the community of the biodiversity values of the area is needed to support the implementation of recovery actions.
Integrated NRM
Within most of the bioregion, the capacity to integrate conservation values has been identified. There are however, significant constraints to the integration of conservation within the Southern Wooded Downs subregion (MGD8). This subregion has the highest clearing rates in the bioregion. The main community groups involved in natural resource management within the bioregion are the Lake Eyre Catchment Co-ordinating Group made up of the Cooper Creek Catchment Committee and the Georgina/Diamantina Catchment Committee. A number of Landcare groups also operate within the bioregion.
Natural resource management projects that are being implemented in the bioregion include coral cactus control, monitoring the extent and impact of dryland salinity, weed management, bore drain replacement and grazing management and a devolved grant for advancing on-ground nature conservation. The implementation of strategies to conserve ungrazed areas on large grazing properties should be seen as an essential part of sustainable management practices.
Map: IBRA map showing frequency of recovery actions (species).
Map: IBRA map showing frequency of recovery actions (ecosystems).
Map: IBRA map showing existing projects part of NRM.
Click here to link to a table of contribution of integrated Natural Resource Management to the protection of biodiversity in each subregion: existing measures and effectiveness.
Click here to link to a table of contribution of integrated Natural Resource Management to the protection of biodiversity in each subregion: feasible opportunities and comments.
Further Information & Gaps
Data gaps and research priorities
Ecological and life history data, and systematic fauna surveys are the major information gaps for the Mitchell Grass Downs
References
Environment Australia 2000. Revision of the Interim Biogeographic Regionalisation of Australia (IBRA) and the Development of Version 5.1. - Summary Report. Department of Environment and Heritage, Canberra.
A complete list of references is available by clicking here.
Further information
View the Landscape Health in Australia report.
View the Terrestrial Biodiversity Assessment 2002 report.
Download the Terrestrial Biodiversity Assessment 2002 Database - Biodiversity Audit Data Entry System (BADES), and specifications
Click here to link to a table of some major data gaps in each subregion in terms of protecting biodiversity.
PDF files
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