Arnhem Plateau
Bioregional Description
This bioregion is centred on the extensive and highly dissected Proterozoic sandstone massif of western Arnhem Land, which forms the headwaters of many of the major river systems of the Top End. It supports an unusually diverse biota, including very many relictual and endemic plant and animal species. The major vegetation types include sandstone heathlands, rainforests (characteristically dominated by the endemic tree Allosyncarpia ternata), hummock grasslands and eucalypt open woodlands (with a range of dominants including Eucalyptus phoenicea, E. kombolgiensis, E. miniata and E. dichromophloia). Most of the bioregion is Aboriginal land, including a major part of Kakadu National Park. The bioregion includes two subregions.
Special values
This bioregion has outstanding values for biodiversity conservation. It has very high levels of endemism, including at least 40 plant species (and probably far more), three birds (including the white-throated grass-wren Amytornis woodwardi), 12 reptiles (such as the extremely large python Morelia oenpelliensis), 5 mammals (such as the black wallaroo Macropus bernardus), 3 fish, 1 frog and very many invertebrates. Many species are relictual (such as the plants Drummondita, Hildergardia and Podocarpus) and many species are known only from one or few populations within a range of <1-10 km2. Some groups have radiated extensively (e.g. Stylidium, Micraira), resulting in this bioregion being a major centre of biodiversity. 28 species are listed as threatened at the Territory or national level. The rugged nature of this bioregion affords its biota some protection from threatening processes which have affected biodiversity elsewhere. The bioregion includes some highly distinctive environments, most notably sandstone rainforests dominated by the tall endemic Allosyncarpia ternata, and floristically rich sandstone heathlands.
- Summary of overall condition and trend
Compared to much of Australia this bioregion is in good condition, at least partly because of its low level of development, relative inaccessibility, and deeply dissected rugged substrate. However, this condition is declining in the face of altered fire regimes (to more frequent extensive hot fires), increased abundance and distribution of feral animals (principally pigs and buffalo) and spread of weeds. The two subregions are scored as continental stress classes 5 and 6. - Summary of priority management/conservation priorities
The major management priority is to defend the conservation attributes against ongoing landscape-wide threatening processes (weeds, feral animals and altered fire regimes). To some extent, these processes are under some management control within the large reserved portion of this bioregion (Kakadu National Park), but this needs to be complemented with comparable and integrated management in the unreserved eastern portion of the bioregion. Such extension is rendered especially difficult by its relative inaccessibility. - Wetlands
- Nationally important wetlands
Most of the reserved part of this bioregion is as a Ramsar wetland of international significance, in recognition of the extraordinary diversity of relatively pristine wetland habitats, including upland springs, gorges, waterfalls, networks of billabongs and major rivers, tidal channels and seasonally-inundated floodplains (although most of the latter are in adjacent bioregions). These support a particularly diverse and abundant wetland biota.
The bioregion includes parts of three DIWA wetlands: NT017 (Kakadu NP: wetland types A1, A2, A5, A6, A7, A8, A9, B1, B2, B3, B9, B10, B14, B17 and C1), NT018 (Katherine River Gorge: wetland type B1), and NT028 (Murgenella-Cooper floodplain system: wetland types B4, A8, A6, A9, A7, B1, B2 and B10).
- Other wetlands of subregional significance
- Nationally important wetlands
- Riparian zones
The bioregion includes the sources of several of the largest river systems in northern Australia, including the Katherine/Daly, Mary, South Alligator, East Alligator, Mann, Goomadeer, Cadell, Blyth and Roper Rivers.
As with other environments in this bioregions, riparian areas are generally in very good condition, although this is being destabilised over large areas by increasing abundance and distribution of some feral animals (particularly pigs and buffalo) and weeds. Unfavourable fire regimes are also degrading some riparian areas, resulting in shrinkage of the dense riparian strips. There is also some localised degradation of riparian vegetation and pollution associated with intensive visitor use to some sites within Kakadu National Park, and some localised impacts upon water quality of chemical pollution from current and abandoned mines. - Ecosystems at risk
Although not formally listed as a threatened community, sandstone heathlands appear to meet the federal criteria for listing. The prevailing fire regime is leading to broad-scale loss of obligate re-seeder plants, which provide much of the ecological fabric of this community. To maintain the extent, diversity and intactness of this ecosystem, management needs to reduce the frequency of extensive hot late dry season fires.
The prevailing fire regime is also degrading and diminishing the monsoon rainforest network, and particularly those rainforests dominated by Allosyncarpia ternata. This ecosystem appears to be in retreat over a large portion of its range. Monsoon rainforests are also being degraded by feral animals (particularly pigs) and weeds.
Even within the apparently more resilient eucalypt forests and savanna woodlands, there is substantial evidence of recent vegetation change, and most particularly the widespread decline or loss of fire-sensitive species such as Callitris intratropica. This change is being exacerbated by rapid proliferation of exotic pasture grasses (especially gamba grass Andropogon gayanus and mission grass Pennisetum pedicillatum), which dominate the understorey and build up extremely high fuel loads. - Species at risk
29 listed threatened species occur in the bioregion. These include many localised plant species, and species associated with sandstone heathlands. The bioregion continues to support relatively abundant populations of many threatened vertebrates which have suffered major declines elsewhere (e.g. gouldian finch Erythrura gouldiae, partridge pigeon Geophaps smithii, red goshawk Erythrotriorchis radiatus), however, there is evidence of recent decline of some threatened species within this bioregion (e.g. golden-backed tree-rat Mesembriomys macrurus, brush-tailed rabbit-rat Conilurus penicillatus).
Number of taxa in the Arnhem Plateau bioregion listed as threatened at national and/or NT level.
| taxa | National | Northern Territory | ||
|---|---|---|---|---|
| endangered | vulnerable | endangered | vulnerable | |
| plants | 0 | 3 | 4 | 13 |
| invertebrates | 0 | 0 | 1 | 0 |
| reptiles | 0 | 0 | 0 | 1 |
| birds | 1 | 3 | 1 | 3 |
| mammals | 0 | 3 | 1 | 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
Six species of eucalypts and acacias are endemic to the two Arnhem Plateau subregions: Acacia elsherana, A. rigescens, A. amanda, A. sp. (Jim Jim Falls D19390), Eucalyptus kakaduensis and E. koolpinensis, all of which are endemic to ARP1. The Acacia species may be disadvantaged by increased incidence of hot late dry season fires. There are no major threats to the Eucalyptus spp. There is limited monitoring of most species, associated with Kakadu fire monitoring programs, but no clear information yet about trends.
- Richness
This bioregion has a relatively high diversity of Acacia (41 and 73 species per subregion) and Eucalyptus (33 and 43 species per subregion).
- Endemism
- Birds
Sampling effort in this bioregion was insufficient to determine changes in status for the period between the two bird Atlases. - Mammals
The mammal fauna of this bioregion has been relatively resilient. Of 59 species recorded, 1 is extinct, 2 have suffered severe decline, 1 has suffered decline and the remaining 55 species are stable.
Management Responses
- Reserve consolidation
This bioregion includes part of the large Kakadu National Park, and a small section of the adjacent Nitmiluk (Katherine Gorge) NP. Together these provide a reasonably comprehensive representation of the bioregions range of environments. Notwithstanding the relatively large proportion of reserved area in this bioregion, there is a compelling biodiversity conservation case to enhance the existing reservation extent, in order to include the entire western Arnhem Land massif. - Off park conservation for species and ecosystem recovery
Kakadu National Park has implemented a management plan for all federally-listed threatened species present. Some federally listed threatened species are also covered by their own specific recovery plans (e.g. gouldian finch). The Kakadu wetlands are also managed according to Ramsar management guidelines. - Integrated NRM
The major threats to the biodiversity of this bioregion are extensive - indeed almost pervasive. Priorities for natural resource management are to monitor these threats and their effects, and to reduce the impacts. Currently, Kakadu NP has a well-established fire monitoring program (which maps the distribution of fires and assesses their impact upon vegetation) and management plans for fire. This has been somewhat successful in maintaining more favourable fire regimes within the reserved area, but there is little comparable effort devoted to the unreserved portion of this bioregion. Likewise, there is a major management program for weeds and feral animals within Kakadu NP, but this is unmatched elsewhere in the bioregion.
There is also some intensive management effort associated with more localised threats, such as visitor use and the impacts of current and historic mines.
Further Information and Gaps
- Major data gaps and research priorities for bioregion
The major biodiversity gap for this bioregion is inventory of the western Arnhem Land massif outside Kakadu NP. Other research priorities include the assessment of relationships between biodiversity and threatening processes, leading to better developed adaptive management, and ongoing monitoring of the biota and its threatening processes.
Management would also benefit from more detailed environmental mapping, in particular vegetation mapping at the scale of 1:100,000 or better. - Other information
