Darwin Coastal
Bioregional Description
The Daly 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 is not divided into subregions.
Special 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).
- Summary of overall condition and trend
Most of the bioregion is in reasonably good condition. However there has been extensive clearing (about 200 km2) associated with urban development and horticulture in the coastal plains near Darwin, very large areas (>800 km2) 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 km2 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 stress class is scored as 3. - Summary of priority management/conservation priorities
A relatively high proportion (almost 30%) of the bioregion is reserved, and additional reservation in this bioregion vis-`-vis other bioregions 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. - Wetlands
- Nationally important 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).
- Other wetlands of subregional significance
- Nationally important wetlands
- 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). - 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. - 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 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 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).
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
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).
- Endemism
- Birds
There is some evidence of declines affecting many birds in this bioregion over the period between the two bird Atlases. However, this apparent decline may be artefactual: "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" (Garnett & Crowley). - Mammals
The mammal fauna of this bioregion has proven to be unusually resilient. Of 58 species recorded, 1 has declined and 57 are considered to be stable.
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 regions 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. - 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.
Further Information and Gaps
| taxa | National | 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 |
- Major data gaps and research priorities for bioregion
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). - Other information