Tiwi-Cobourg

Bioregional Description

This coastal region comprises Australia's second and fifth largest islands (Melville and Bathurst 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.

Special 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.

• 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 scored as continental stress classes 5 and 6.
• Summary of priority management/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 regions important colonial seabird rookeries and nesting sites for marine turtles. These should be better protected, at least through management agreements with their Aboriginal landowners.
• Wetlands
  • Nationally important 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).
     
  • Other wetlands of subregional significance
    
    The bioregion includes seasonally inundated floodplain on Croker Island and seasonal swamp at Andranagoo Creek on Melville Island.
     
• 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.
• 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.
• Species at risk
  
  44 taxa from this bioregion are listed (or meet criteria for listing) as threatened (endangered or vulnerable) at national 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 federal and Territory listings of threatened species in this bioregion is because the Territory listings includes very recent re-consideration of status, which has not yet been incorporated in federal 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 national re-allocation of conservation status.
  
  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	National		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

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
  1. Endemism
     
     No endemic Eucalypt and Acacia species.
      
  2. Richness
     
     Richness Relatively low species richness of Eucalypt and Acacia species (subregional tallies of 12-22 spp.).
• Birds
  
  Too few records to determine trends from Australian Bird Atlas, but other information suggests rapid recent decline of endemic subspecies of hooded robin, and possible regional losses of a few other species.
• Mammals
  
  One of the few bioregions in Australia showing no evidence of mammal decline (of 41 native mammal species present, all are regarded as stable).

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.
• 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.

Further Information and Gaps

• Major data gaps and research priorities for bioregion
  
  

  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.

• Other information

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