Australian Natural Resources Atlas

Natural Resource Topics

Biodiversity Assessment - Darwin Coastal

Darwin Coastal

Location Map Corroboree Lagoon, 80km east of Darwin, is typical of many of the coastal floodplains in the bioregion, with large open floodplains, stands of melaleuca, and large numbers of birds. Photo: Martin Armstrong

Introduction

The Darwin Coastal bioregion comprises gently undulating plains on lateritised Cretaceous sandstones and siltstones; sandy and loamy red and yellow earths and siliceous sands from near the mouth of the Victoria River to just west of Cobourg Peninsula. The most notable vegetation feature is the extensive and diverse floodplain environment associated with the lower reaches of the many large river systems. There are also substantial areas of mangroves, and rainforest and other riparian vegetation fringing the rivers. Inland from the coast, the dominant vegetation type is eucalypt tall open forest, typically dominated by Darwin woollybutt (Eucalyptus miniata) and Darwin stringybark (E. tetrodonta). The Darwin Coast bioregion has no subregions.

Summary of overall condition and trend

Most of the bioregion is in reasonably good condition. However there has been extensive clearing (about 200 kmē) associated with urban development and horticulture in the coastal plains near Darwin, very large areas (800 kmē) of floodplain have been transformed by the woody weed Mimosa pigra, and saltwater intrusion has affected the lower reaches, billabongs and floodplains of several major river systems (e.g. 240 kmē of the Mary River system alone has been substantially degraded). Changed fire regimes, feral animals and livestock, and other weeds have had more subtle impacts more pervasively across the bioregion. The continental landscape stress class is 3 as assessed by the Landscape Health report (1 is most stressed, 6 is least stressed).

Summary of conservation priorities

A relatively high proportion (almost 30%) of the bioregion is reserved, and additional reservation in this bioregion is not a high priority. However, the existing reserve system is geographically highly biased with little representation in that half of the bioregion south-west of Darwin.

Although not necessarily forming part of the formal reserve system, there is a need to retain more bushland fragments and corridors within the burgeoning horticultural and rural lands around and to the east of Darwin. There is a current local government regulation to maintain 50% of native vegetation in Litchfield Shire, which encompasses most of this horticultural area.

An integrated catchment plan and recently completed conservation plan for the Mary River catchment provides a good basis for the broad-scale management of weeds, feral animals and fire for this large segment of this bioregion. It also provides management guidelines for mitigation of saltwater intrusion, and for a range of other issues including tourism, recreational fishing and pastoralism.

There are far fewer resources or planning mechanisms directed at the south-western part of this bioregion, which includes a high proportion of Aboriginal lands. However, many of the threatening processes are the same as for elsewhere in the bioregion, so the management issue is largely about access to management resources and capacity building.

Natural values

This bioregion contains some of the most extensive and rich floodplain systems in northern Australia, extensive and diverse mangrove forests, and significant rainforest and riparian vegetation. The bioregion is the most important in the Northern Territory for colonially breeding waterfowl. It contains parts of two Ramsar wetlands and several other significant wetlands. It also includes a relatively high diversity of threatened species (33 listed at either Territory or national level).

Click here to link to a table of natural values within each subregion

Wetlands

This bioregion includes all or part of several nationally important wetlands. In addition to portions of the Ramsar listed wetlands of Kakadu Stages 1 and 2 (notably the South, East and West Alligator Rivers systems: NT017, with wetland types A1, A2, A5, A6, A7, A8, A9, B1, B2, B3, B9, B10, B14, B17 and C1) and of Cobourg Peninsula (NT023: wetland types A6, A7, A8, A9, B14 and A10), this bioregion includes 7 nationally listed wetlands: the Adelaide River floodplain system (NT020: wetland types B4, A6, A8, A9, A7, B1, B6, B9, B10, B14 and C1), the Daly-Reynolds floodplain-estuary system (NT024: wetland types B4, A6, A8, A9, A7, B1, B2, B9, B10 and B14), the Finniss floodplain and Fog Bay system (NT025: wetland types B4, A7, B2, B9, B10, B14, A6, A8 and A9), the Mary floodplain system (NT026: wetland types B4, A8, A7, A6, A9, B1, B9, B10 and B14), the Moyle floodplain and Hyland Bay system (NT027: wetland types B4, A7, B1, B2, B10, B14, A6 and A9), the Murgenella-Cooper floodplain system (NT028: wetland types B4, A8, A6, A9, A7, B1, B2 and B10), and Port Darwin (NT029: wetland types A1, A6, A7, A9, A3 and A2).

Nationally important wetlands

Map: IBRA map showing DIWA locations, towns, subregions, major roads and reserves and most common threatening processes.

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

This bioregion includes significant lower sections of many major rivers, including the Moyle, Daly, Mary, Finniss, Adelaide, South Alligator and East Alligator Rivers. Most riparian zones in this bioregion are in good condition, however this is generally declining because of damage associated with proliferation of feral pigs and buffalo, invasion by weed species, and (in some cases) unrestricted access by livestock. Some lower reaches of rivers have been severely impacted by saltwater intrusion, resulting in extensive damage to riparian vegetation (in some cases including broad-scale death of the dominant Melaleuca and almost all other vegetation).

Map: Riparian threatening processes.

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

Although not formally listed as threatened, several ecosystems are at risk in this bioregion. Seasonally moist sandsheets supporting a heathland or wet herbfield are highly restricted and a high proportion of this habitat has been cleared around Darwin for sandmining, rural development and horticulture. Proposed developments will further substantially reduce the area of this environment. The floodplains across this bioregion face a range of serious threats from weed incursion (notably by Mimosa pigra, which completely re-models their composition and structure, but also by olive hymenachne Hymenachne amplexicaulis and para grass Brachiaria mutica), from saltwater intrusion and elevated sea levels generally, and from the impacts of feral animals and livestock. The floodplain environments may also be undergoing floristic change associated with altered fire regimes (in this case by reduced frequency of fine-scale fire). The monsoon rainforest patch network is being degraded by feral animals (principally pigs), incursion of weeds and the impacts of more frequent hot late dry season fires. The extensive eucalypt forests are broadly affected by feral animals and livestock, but especially by the rapid spread to dominance in the understorey of exotic pasture grasses (principally gamba grass Andropogon gayanus and mission grass Pennisetum penicillatum), and the spread of these exacerbates the detrimental impacts of an increasing frequency of extensive hot late dry season fires. The eucalypt open forests are also the principal environment targetted for clearing for horticultural development and rural expansion, with rapidly accelerating rates of clearance.

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

With 33 listed species, this bioregion is unusually rich in threatened species. This listing includes several marine turtles, several of which nest in the bioregion. Many of the threatened species are restricted to very few known localities, and occur only or mostly in this bioregion: these include the endangered palm Ptychosperma macarthurii, and endangered floodplain subspecies of the yellow chat Epthianura crocea tunneyi, as well as the gecko Diplodactylus occultus, and the plants Acacia praetermissa, Monochoria hastata, Malaxis marsupichila, Endiandra limnophila, Grevillea dunlopii, Grevillea longicuspis, Cycas canalis canalis, Luisia teretifolia, Habenaria rumphii, Typhonium taylori, Utricularia dunstaniae, Utricularia subulata and Zeuxine oblonga. In part, the very restricted range of these taxa reflects dependence some distinctive local environments within this bioregion (notably seasonally moist sandsheets just east of Darwin). The threatened status of these taxa recognises the encroachment of sand mining, horticulture and other developments upon these limited but distinctive areas. Note that the major disparity in Commonwealth and Territory listings of threatened species in this bioregion is because the Territory listings includes a 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 Darwin Coastal 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 6 10
Fish 0 2 0 0
Reptiles 2 4 0 2
Birds 2 3 1 2
Mammals 0 1 0 2

Map: IBRA map showing frequency of threatening processes for species.

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

This bioregion contains no known endemic acacia or eucalypt species.

Richness

This bioregion supports moderately high richness of Acacia (47 species) and Eucalyptus (44 species).

Birds

The coastal plains and forests in this bioregion, which is situated east and south of Darwin, support a high diversity of taxa, including small populations of many of the Top End endemics. One threatened taxon is endemic, the Endangered subspecies of Yellow Chat, which occurs on the grassy floodplains plains of the northern rivers. There are few introduced taxa, with all records except those of the Rock Dove being brief appearances of vagrants that do not seem to have persisted. While there has been considerable development around Darwin over the last two decades, and there are problems with weed invasion of the floodplain, the declines in reporting rate suggested by the analyses seem excessive. It is more likely that they are related to the behaviour of the birdwatchers, the avid listers of the first Atlas period having been succeeded by more sombre biologists who have sampled the landscape more methodically, but less exhaustively.

Status: Moderately diverse tropical woodland avifauna, with most Top End endemics being recorded.

Rare and threatened: Yellow Chat on coastal floodplains.

Increasers: Rock Dove.

Indicators: Emu, Partridge PigeonPartridge Pigeon, Varied Lorikeet, Black-tailed Treecreeper, Yellow ChatYellow Chat, Hooded Robin.

Trend: Possible declines in reporting rate of grassland, ground and hollow nesting and ground-feeding insectivorous birds but these may be sampling artefacts.

Scenario: Eventual decline in woodland birds as clearance continues.

Actions: Secure representative areas against clearing. Maintain integrity of floodplains.

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 58 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 .483. (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 .02. (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 .07. (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 .7. (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

The bioregion includes significant portions of several major conservation reserves, most notably including Litchfield National Park, Mary River National Park and Kakadu National Park, which together (and with other smaller reserves) provide a reasonably good representation of the region's environmental variation. There is some priority for limited enhancement of the formal reserve system around Darwin to provide better representation of the narrowly endemic and threatened seasonally damp sandsheet environments, and some desirability for increasing the geographic representation of the bioregion's reserve system, by including additional areas in the Moyle-lower Daly system south-west of Darwin.

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

Management plans are needed primarily for several of the highly localised threatened plant species within the Darwin rural area.

Integrated NRM

There is a broad range of existing natural resource management mechanisms and actions within this bioregion. The most significant of these are the integrated catchment plan and conservation plan for the Mary River catchment, land-use plans and concepts for the Darwin area, a conservation plan now in preparation for the Litchfield Shire, draft guidelines for vegetation clearance, and a landholder conservation agreement scheme (Land for Wildlife) recently established in Litchfield Shire. There are also well-established plans of management for the main conservation reserves in the region, most notably including parts of Litchfield and Kakadu National Parks. Additionally, there is a fire management strategy across the bioregion. The main NRM priorities remaining for the bioregion are to implement plans which aim to ensure the retention of biodiversity within a framework of increasing horticultural development (around Darwin and to the east); to secure resources and capacity building to allow Aboriginal owners to improve management of their lands (to the southwest of Darwin); and to implement a more integrated management of weeds and feral animals across the bioregion.

Map: IBRA map showing frequency of recovery actions (species).

IBRA map showing frequency of recovery actions (species).

Map: IBRA map showing frequency of recovery actions (ecosystems).

IBRA map showing frequency of recovery actions (ecosystems).

Map: IBRA map showing existing projects part of NRM.

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

Conservation planning and management would benefit from more detailed environmental mapping, in particular vegetation mapping at the scale of 1:100,000 or better.

There is also some need for systematic wildlife inventory in the southwestern portion of the bioregion, and for broad-scale monitoring of the occurrence, impact and responses to management of threats (weeds, feral animals and fire).

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.

Before you download

Most publications are downloadable as PDF files. Adobe Acrobat Reader  is required to view PDF files.

If you are unable to access a publication, please contact us to organise a suitable alternative format.

Key

   Links to an another web site
   Opens a pop-up window