Landscapes

Plants of the Stone Country

Plants growing in the stone country and on the outliers must survive extremely hot, waterless conditions for many months each year. Among the best examples of plants well adapted to these harsh conditions are the resurrection grasses, which dehydrate in the absence of moisture and spring back to life within 24 hours of rain.

Monsoon forests often develop in the cool, moist gorges that dissect the stone country. They are generally dominated by Allosyncarpia, a large, spreading, shady tree restricted to the Kakadu and Arnhem Land region.

More widespread and easily recognisable stone country and outlier plants are spinifex and the sandstone pandanus.

The dominant sandstone escarpment of the Arnhem Land Plateau ranges in height from 30 metres to 300 metres, and includes the rock shelters and outliers of Ubirr and Nourlangie. In some areas the escarpment is eroding by up to one metre every thousand years.

Tidal flats and coast

Kakadu is made up of almost 500 square kilometres of coastal and estuarine areas, mostly lined with mangrove forests. The estuaries and tidal flats are home to an array of plants and animals adapted to living in the oxygen-deficient saline mud. They form important nurseries for many fish including barramundi.

Further inland, these areas are dominated by mangrove swamps and samphire flats, with pockets of monsoon rainforest forming in areas such as Manngarre, where freshwater springs occur.

The shape of the estuaries and tidal flats vary between the wet and dry seasons. During the dry season tidal action deposits silt along the river beds and banks. During the wet season the river beds are eroded by the floodwaters and large quantities of fresh and saline water flow out across the tidal flats, where silt is deposited. Large silt loads are also carried out to sea, with some of the silt being deposited as a nutrient-rich layer on the sea floor, contributing to the muddy waters that characterise Kakadu's coastline.

The dominant habitats are mangrove swamps and samphire flats. Where freshwater springs occur along the coasts and river banks, isolated pockets of coastal monsoon rainforests form, as at Manngarre.

Estuaries and tidal flats can be seen at West Alligator Head and in the lower sections of the South and East Alligator Rivers.

Wildlife of the tidal flats

Many birds live in the estuarine and tidal areas of the park including the chestnut rail, the collared (or mangrove) kingfisher, the broad-billed flycatcher, the black butcherbird, the mangrove gerygone (or warbler), and the red-headed honeyeater. During the wet season egrets, ibis’, herons and cormorants nest in large colonies in the mangrove tree tops.

Mangroves are also used by the black and little red flying foxes which congregate in colonies or camps. Mangrove monitors and estuarine crocodiles often wait below the camps to eat fallen flying foxes.

On the sandy beaches of West Alligator Head and Field Island, flat-back turtles haul themselves out of the water to lay their eggs in darkness. Other animals that are often found in the coastal area are the white-bellied mangrove snake, Darwin and Hardwick's sea snakes, and the dugong (or sea cow), which feeds on seagrass beds.

The shady coastal monsoon pockets offer plenty of vegetation that orange-footed scrubfowl can rake up into large mounds. The birds lay their eggs in the mounds, using the heat from the rotting vegetation to incubate the eggs. The mounds are used by succeeding generations—some mounds in the park are well over 100 years old.

Also living on the forest floor are the colourful rainbow pittas, which can sometimes be heard scratching through the leaf litter for insects. They are also often seen on the walking track to Maguk.

Fruit from the fig trees that grow in the forest provides food for the rarely seen rose-crowned fruit dove. The figbird, oriole and migratory Torres Strait pigeon also eat these figs.

Plants of the tidal flats

Mangroves are common along the banks of tidal creeks and rivers. Some 39 of the 47 Northern Territory species of mangrove occur in Kakadu. Mangroves are important for stabilising the coastline and serve as feeding and breeding grounds for many animals, including fish such as barramundi.

Like other plants growing in estuaries and tidal flats, mangroves must be able to cope with oxygen-deficient soils and periodic inundation by salt water. Mangroves use a range of mechanisms to cope with these conditions. Some species, such as the grey mangrove, have roots projecting through the soil (pneumatophores). Others, such as the spider mangrove, have an amazing raised root system-like the legs of a spider-to help with oxygen intake. A number of mangroves are able to exclude salt through specialised filters in their roots or through salt glands in their leaves.

On the tidal flats behind the mangroves, hardy succulents (samphire), grasses and sedges grow. Isolated pockets of monsoon forest grow along the coast and river banks. These forests contain several impressive trees including the banyan fig (recognisable by its large, spreading aerial roots) and the kapok tree, which has a spiny trunk, large, waxy red flowers and pods full of cotton-like material.

Flood plains and Wetlands

Floodplains undergo dramatic seasonal changes. Following wet season rains, a sea of shallow freshwater spreads over the plains for hundreds of square kilometres. As the floodplains start to dry, waterbirds and crocodiles seek refuge in the remaining wet areas such as Yellow Water.

Nutrient-rich soils carried in the floodwaters add nutrients to the floodplains, which, along with an abundance of water and sunlight, make the floodplains an area of prolific plant and animal life. During the dry season the water recedes into rivers, creeks and isolated waterholes or billabongs. 

Wildlife of the Wetlands

The floodplains and wetlands of Kakadu are important refuges and feeding grounds for many Australian waterbirds, especially during the dry season. The populations of some waterbirds that are abundant in Kakadu are largely restricted in Australia to a narrow band along the northern coastline. Typical of these birds are the magpie goose, the green pygmy goose, the Burdekin duck, and the wandering whistling duck.

Other commonly seen and more widespread waterbirds are the jabiru, the comb-crested jacana (or lotus bird), cormorants, darters, egrets, ibises and herons.

Kakadu's wetlands are visited each wet season by about 30 species of migratory birds, such as the little curlew, the snipe and the godwit. The birds' breeding grounds are in the Northern Hemisphere, in places such as Siberia, China and Japan. Each year they leave their breeding grounds at the end of the northern summer to fly south to warmer climates. Most migratory birds visit Kakadu during the wet season. As noted, the wetlands of Kakadu are on the list of Wetlands of International Importance and Australia has entered into agreements with the governments of China and Japan to protect the breeding and summer grounds of migratory birds.

A number of reptiles live on Kakadu's floodplains. Northern snake-necked turtles are perhaps the most frequently seen turtles. They bury themselves in mud as the water dries up at the end of the dry season.

The larger pig-nosed turtles are more secretive. It was Aboriginal rock art paintings of the pig-nosed turtle that first suggested to scientists that these animals occurred in the area. The Arafura file snake is abundant but rarely seen because it lives in billabongs among the roots of the river pandanus. Freshwater and Macleay's water snakes are sometimes seen swimming gracefully through the water. King brown snakes and water pythons are common on the floodplains.

Estuarine or saltwater crocodiles are found in both freshwater and salt water. They are often seen on the Yellow Water and East Alligator River boat cruises. Their nests are usually mounds of mud and rotting vegetation next to permanent water. Nesting occurs during the wet season (between December and April), and about 80 per cent of mature females nest each year, laying about 50 eggs. Generally, at least 75 per cent of the eggs laid fail to hatch because the nest becomes flooded.

The paperbark forests that fringe the floodplains provide ideal nesting sites for wetland birds such as the jabiru, the white-bellied sea eagle, the whistling kite and the green pygmy goose. Paperbark forests are also home to the brush cuckoo, the lemon-bellied flycatcher, the rufous-banded honeyeater, and the restless flycatcher.

When they are in flower paperbarks provide food for nectar-feeding birds such as honeyeaters and lorikeets. Kingfishers such as the blue-winged kookaburra and the forest kingfisher, with its white wing patches, are often seen darting through the paperbark forest. The rainbow bee-eater and several species of flycatcher are also often seen. The mistletoe bird feeds on the berries of the parasitic mistletoe plant on the paperbarks, spreading the plant's seed to other parts of the forest.

Paperbark forests also provide a safe haven for agile wallabies, who must leave the safety of treed areas to drink and graze at the water's edge.

Plants of the Wetlands

Where floodplains are inundated for two to six months a year grasses and sedges such as spike rush occur. Clumps of freshwater mangroves (itchy tree), pandanus and paperbarks are found on slightly higher ground. Herbaceous swamp vegetation dominates areas covered by water for six to nine months a year. A variety of waterlilies, such as the blue, red, yellow and white snowflake, are commonly found in these areas.

The outliers

The outliers are essentially pieces of the Arnhem land plateau that have, through past erosive action, become separated from the plateau complex. They were islands in the ancient seas that once covered much of Kakadu. Nourlangie Rock and Ubirr are good examples of outliers.