Biodiversity Assessment - Tiwi Cobourg
Tiwi Cobourg
Introduction
This coastal region comprises Australia's second and fifth largest islands (Melville and Barthurst Island in the Tiwi island group), Croker Island and the adjacent Cobourg Peninsula. Coastal vegetation includes some mangroves and saline flats, although this bioregion lacks the large rivers which influence vegetation patterning in other coastal regions. Most of this bioregion is covered by tall eucalypt open forests, typically dominated by Darwin woollybutt (Eucalyptus miniata), Darwin stringybark (E. tetrodonta) and Melville Island bloodwood (E. nesophila), but often with northern cypress-pine Callitris intratropica and the tall palm Gronophyllum ramsayi co-dominant. The Tiwi Islands support a relatively high density and total area of monsoon rainforest patches, with distinctive species composition. There are also substantial areas there of a distinctive treeless plain vegetation. This bioregion is of low relief, with laterite and Cretaceous sandstone the dominant substrates. The bioregion is entirely Aboriginal land. It includes two subregions.
Summary of overall condition and trend
Much of the bioregion contains extensive superficially intact environments, in remote areas with little human impact: indeed, some of the most pristine environments in the nation, providing refuge for many species which have declined or disappeared from much of the rest of their range beyond this bioregion. However, a range of threatening factors has begun to undermine this condition. Notably, these include feral animals (cats, pigs, water buffalo, cattle, banteng, and horses), weeds and changed fire regimes. Additionally, Melville Island may be subject to a more acute threat of broad-scale vegetation clearance (30,000 to 100,000 ha for plantations of exotic timber). The subregions are continental landscape stress classes 5 and 6 as assessed by the Landscape Health report (1 is most stressed, 6 is least stressed).
Summary of conservation priorities
The main conservation priorities for this bioregion are the linked issues of capacity building (and resource provision) for Aboriginal landholders and broad-scale management of feral animals, weeds and fire. Currently, the resources available for this management are very limited: inadequate to prevent ongoing degradation, and insufficient to attract the engagement of most landholders. Especially in the event of the development of an exotic plantation industry, there is a need for some formal protection of significant and representative vegetation on the Tiwi Islands. There is currently no conservation security for most of the region's important colonial seabird rookeries and nesting sites for marine turtles. These should be better protected, at least through management agreements with their Aboriginal landowners.
Natural values
The Tiwi Islands support about 20 endemic plant and vertebrate animal taxa, and about 44 taxa considered threatened at Territory or national level (including 14 taxa listed under the EPBCA). The bioregion contains important breeding sites for marine turtles and colonial seabirds, and a Ramsar wetland on the Cobourg Peninsula. It contains the highest density of rainforest patches in the Northern Territory, and two rainforest types not known elsewhere. Eucalypt forests are better developed (highest basal area, canopy height) in this bioregion than elsewhere in northern Australia. The Tiwi Islands contains treeless plain vegetation, without parallel on the NT mainland. The bioregion is particularly significant because the isolation of its islands and mainland peninsula offer some refugial protection for its biota from threatening processes.
Click here to link to a table of natural values within each subregion
Wetlands
All wetlands of the Cobourg Peninsula (including swamplands, mangroves and coasts) are included within a Ramsar site, and recognised as the nationally important wetland complex NT023 (wetland types A6, A7, A8, A9, B14 and A10).
The bioregion includes seasonally inundated floodplain on Croker Island and seasonal swamp at Andranagoo Creek on Melville Island.
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
There are few major watercourses in this bioregion. The best developed are on Melville and Bathurst Islands, notably the Andranagoo Creek, Jessie and Johnston Rivers. These are generally in very good condition, except that those on Bathurst are subject to damage by feral pigs, and those on Melville by water buffalo. Proposed broad-scale clearing on Melville Island may reduce aquatic and riparian habitat quality.
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
Rainforest patches in this bioregion suffer impact from feral animals, fire and weeds, and (although not yet formally designated) meet the criteria for vulnerable. The treeless plains of the Tiwi Islands have been used for timber plantations over the last 20 years, and their area substantially reduced. The tall eucalypt open forests of Melville Island are likely to be substantially reduced by proposed clearing for plantation forestry.
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
Forty-four taxa from this bioregion are listed (or meet criteria for listing) as threatened (endangered or vulnerable) at Commonwealth or NT level. These include 7 reptiles, 6 birds, 5 mammals and 26 plants. For many of these species, the Tiwi-Cobourg bioregion is the only site, or one of the most important sites. Many of the plant species are restricted to 10 or fewer rainforest patches. Most of the other species are threatened by broad-scale factors, including changed fire regime, feral animals and weeds.
Note that the major disparity in Commonwealth and Territory listings of threatened species in this bioregion is because the Territory listings includes re-consideration of status, which has not yet been incorporated in national assessments, but as all the species considered are NT endemics (many with highly localised ranges), it is likely that these recent revisions will flow on to Commonwealth re-allocation of conservation status.
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.
Number of taxa in the Tiwi-Cobourg 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 | Commonwealth | Northern Territory | ||
|---|---|---|---|---|
| Endangered | Vulnerable | Endangered | Vulnerable | |
| plants | 1 | 0 | 8 | 11 |
| reptiles | 2 | 4 | 0 | 1 |
| birds | 0 | 4 | 1 | 1 |
| mammals | 0 | 2 | 0 | 2 |
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
Endemism
No endemic Eucalypt and Acacia species.
Richness
Relatively low species richness of Eucalypt and Acacia species (subregional tallies of 12-22 spp.).
Birds
The Tiwi Islands have been isolated from the mainland for long enough for phenotypic differentiation among at least some bird species. Two of the island subspecies, the Endangered Masked Owl and the Vulnerable Hooded Robin are seen rarely enough to be considered threatened. The failure of recent surveys to find any Hooded Robins suggests the status of the subspecies should be Endangered or Critically Endangered. No introduced species have been recorded. There were too few 10-minute surveys or area searches to determine trends.
Status: Moderate diversity of tropical woodland species, with some endemism below species level.
Rare and threatened: Masked Owl, Hooded Robin.
Increasers: No information.
Indicators: Partridge Pigeon, Varied Lorikeet, Black-tailed Treecreeper, Jacky Winter.
Trend: Possible decline in taxa needing fine-scale fire mosaic.
Scenario: Probable declines as a result of land clearing and change in fire pattern.
Actions: Ensure representative areas managed in a manner that preserves avian diversity, including maintenance of a fine-scale fire mosaic.
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 41 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 .463. (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 0. (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 .09. (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 .68. (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 unknown. (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 9. (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 0. (The maximum number of extinct mammal species in a bioregion is 29 and the minimum is 0).
Table: Extinct Species
There is no data available for this table within the bioregion.
Management responses
Reserve consolidation
Especially if proposed plantation development proceeds on Melville Island, there is a need for greater conservation security for rainforest patches and treeless plains, and for retention of large areas of the dominant eucalypt tall open forests. Significant breeding sites for seabirds and marine turtles merit greater protection.
A single large reserve, Garig Gunak Barlu National Park (formerly Gurig National Park and Cobourg Marine Park), includes a high proportion of this bioregion, but does not provide especially good comprehensiveness for the bioregion as a whole. The reserve has substantial problems with the management of feral animals, and with some competing interests between conservation and the maintenance of an abundance of feral animals to support safari hunting as revenue for the reserve owners.
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
Priorities are for protection of the many threatened and endemic taxa, especially on the Tiwi Islands. There is no overall recovery plan for these taxa. Many taxa can be managed together effectively, especially those plant species restricted to the monsoon rainforest network.
Integrated NRM
The fundamental conservation management requirement is for resources and capacity building to encourage and allow Aboriginal landowners to manage their country to reduce the impacts of weeds, feral animals and detrimental fire regimes. A conservation plan has been prepared recently for the Tiwi Islands, and is now being revised following further field survey of Bathurst Island.
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
Systematic wildlife survey of Croker Island and the small islands to its east. Monitoring and assessment of impacts of major threats (especially ferals, weeds and changed fire regimes). Assessment of the invertebrates of the Tiwi Islands. Assessment of the feasibility of some sustainable utilisation options to encourage Aboriginal landowners to enhance conservation management.
Management would also benefit from more detailed environmental mapping, in particular vegetation mapping at the scale of 1:100,000 or better.
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.
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