Plants and animals

By John Woinarski, Parks & Wildlife Commission of the NT

Landscape and environmental variation

Kakadu/Arnhem Land escarpment provides an extremely important refuge area for biota  Photo © Martin Armstrong PWCNT

The Darwin-Kakadu region is a major centre of biodiversity largely because of its relatively high rainfall and environmental variability. The "stone country" (the sandstone plateau and escarpment) of western Arnhem Land, including parts of Kakadu, is probably the most important refuge area for biota in northern Australia, sheltering many plant and animal species which occur nowhere else on earth.

Its very complex topography, including deep and extensive gorges and very large areas of sandstone platforms and rock piles, provides a diverse array of microclimates, and this has allowed many species to persist here through times when the regional climate has been generally unfavourable. The rugged gorges and sheer escarpments have also provided some protection from fire, thereby providing some shelter for fire-sensitive plants and animals.

Kakadu sandstone biota

Some of the Kakadu sandstone biota is shared with other rocky areas in northern Australia, and in particular with the north Kimberley, the other main rugged stony environment in northern Australia. Examples include (click on them to see a list of research findings):

• short-eared rock-wallaby Petrogale brachyotis  
• nabarlek rock-wallaby Petrogale concinna  
• rock ringtail possum Petropseudes dahlii  
• white-lined honeyeater Meliphaga albilineata  
• carpenter frog Megistolotis lignarius  
• long-tailed rock monitors Varanus glebopalma and V. glauerti .

Other taxa have diverged between the Kimberley and Kakadu, and are now represented by closely-related pairs of species in the two areas. Examples include:

• white-quilled and chestnut-quilled rock pigeons Petrophassa albipennis and C. rufipennis  
• the black and white-throated grass-wrens Amytornis housei and A.woodwardi  
• Kimberley and Arnhem Land rock-rats Zyzomys woodwardi and Z.maini.

Yet other taxa, including the black wallaroo Macropus bernardus , the Oenpelli Python Morelia oenpelliensis , and numerous skinks, are restricted only to the stone country of western Arnhem Land. The stone country also offers a keyhole for viewing the environment of tens of thousands of years ago, as old rock art depicts some animals, such as thylacines and Tasmanian devils, which are now extinct or occur only far from here.

Eucalypt open forest and grasslands

Some of the tallest eucalypt forests in northern Australia occur in this region, most notably on the Tiwi Islands and Cobourg Peninsula. The fauna of this habitat usually occupies very extensive areas, and includes such characteristic species as (click on these to see a list of research findings):

• frilled lizard Chlamydosaurus kingii , the tree monitors Varanus tristis and V. scalaris , 
• fawn antechinus Antechinus bellus  
• black-footed tree-rat Mesembriomys gouldii , antilopine wallaroo Macropus antilopinus  
• red-tailed black-cockatoo Calyptorhynchus banksii  
• northern rosella Platycercus venustus  
• partridge pigeon Geophaps smithii  
• black-tailed treecreeper Climacteris melanura .

Darwin woollybutt in flower during the mid-dry season Photo: Sam Setterfield

A feature of these open forests is the mass flowering of several species, most notably the Darwin woollybutt Eucalyptus miniata , during the mid-dry season. This profuse resource supports hundreds of thousands of lorikeets, honeyeaters and flying-foxes. Like the human tourists, several other species of birds use these forests in the dry season as an escape from the winter of southern Australia.

Floodplains and lowland wetlands

The Darwin-Kakadu region is also notable for its floodplains and lowland wetland environments. The Daly, Mary, Adelaide, Wildman, South and East Alligator Rivers provide some of the most significant wetlands in Australia, and some sites of international conservation significance. The floodplains are mostly of recent origin (less than 10,000 years old) and are unusually fertile compared with most other environments in northern Australia.

For animals which can cope with their extreme seasonal variability, the floodplains provide the richest habitat available. Examples include the water python and dusky rat, whose floodplain densities may surpass levels of 700 pythons per km ² and 150 rats per hectare (or about 1 tonne/km ² ) - abundances and biomasses which are unparalleled in Australia and perhaps the world.

The water python Liasis fuscus very abundant in season with a keen appetite for goose eggs Photo: Peter Whitehead

The pythons and rats survive the seasonality by migrating, shifting to upland open forests and river margins during the dry season. The floodplains are also major breeding grounds for waterfowl, most notably supporting hundreds of thousands of magpie geese in the wet season.

The floodplains also provide an important habitat for many other species, including:

• death adders Acanthophis praelongus
• the large goanna Varanus panoptes  
• crocodiles 
• long-necked turtles 
• many frog species (notably Dahl's aquatic frog Litoria dahlii ) 
• common planigale Planigale maculata  
• brolga Grus rubicundus  
• pratincoles 
• many waders (shorebirds).

Many fish species also rely on the floodplains as part of a complex cycle knitting together estuarine areas, billabongs, floodplains and main rivers.

Coastal fringe

Downstream of the flooplains, the coastal fringe of the Darwin-Kakadu area is an important site for many animal species. The mangroves of this coast are among the best developed and most diverse in Australia, and support a distinctive range of birds. These include:

• collared kingfisher Todirhamphus chloris
• mangrove robin Eopsaltria pulverulenta
• white-breasted whistler Pachycephala lanioides
• mangrove golden whistler P. melanura
• yellow white-eye Zosterops lutea
• chestnut rail Eulabeornis castaneoventris
• reptiles (notably the mangrove monitor Varanus indicus )
• mammals (including the grassland melomys Melomys burtonis and false water-rat Xeromys myoides ).

Monsoon rainforests

The pied imperial pigeon also known as the Torres Strait pigeon Photo: Martin Armstrong

Although small in extent when compared to the open forests, monsoon rainforests are a highly significant environment for wildlife in this region, supporting a distinctive fauna including:

• pied imperial-pigeon Ducula bicolor
• rose-crowned fruit-dove Ptilonopus regina
• rainbow pitta Pitta iris
• rufous fantail Rhipidura rufifrons
• grey whistler Pachycephala simplex
• green-backed gerygone Gerygone chloronata .

Especially in the wet season, the rainforests provide concentrations of fruit far greater than those across the rest of the landscape, and these attract flying-foxes and fruit-eating pigeons, orioles, cuckoos and figbirds. Many of these animals move between rainforest patches and the surrounding open forest, and the conservation of this rainforest environment is probably dependent upon the maintenance of as many patches as possible and extensive areas of their surrounding habitats.

Conservation threats in this region include many of those that are widespread across northern Australia generally, such as the imposition of detrimental fire regimes, high densities of feral animals (notably pigs and buffalo), and invasion by weeds (most notably mimosa and a set of introduced grasses; gamba grass, para grass and mission grass). There are also some more unique threats, including saltwater intrusion into the floodplain environments, and levels of clearing and habitat modification which are relatively high for northern Australia. To some extent balancing these threats, the proportion of land within conservation reserves is relatively high, and includes some of Australia's most significant National Parks.