Kakadu supports more than 10,000 species of insect including grasshoppers, beetles, flies, termites, butterflies and moths, bees, wasps, ants, dragonflies and damselflies, caddis flies, non-biting midges and mayflies. The great variety of insects is a result of the varied habitats and relatively high temperatures throughout the year.
Perhaps the most striking insect-created features in the Park are the termite mounds. The mounds in the southern part of the Park are particularly large and impressive. Leichhardt’s grasshopper, in colours of orange, blue and black, is perhaps the most spectacular insect found in Kakadu. It is also found on the Arnhem Land plateau and in Gregory National Park.
Kakadu has seen several invasive species threaten the native habitat, particularly in recent decades. Introduced fauna including the Water Buffalo, wild pig and more recently, the Cane Toad have had major effects on habitat. Invasive weeds include "Mimosa pigra", which covers 800 km² of the Top End, including vast areas of Kakadu, invasive paragrass displaces the native food of much of Kakadu's birdlife. "Salvinia molesta" has infested the Magela floodplain. Brumbies also inhabit areas of the National Park, including Yellow Water.
There are six main landforms in Kakadu National Park:
Each landform has its own range of habitats. Kakadu’s varied landscapes and the habitats they contain are features that contributed to its listing as a World Heritage Area.
Most of Kakadu was under a shallow sea about 140 million years ago, with the escarpment wall formed from sea cliffs and Arnhem Land from a flat plateau above the sea. Currently the escarpment rises 330 metres above the plateau and stretches 500 kilometres along the East edge of the park and on into Arnhem Land. The escarpment varies from near vertical cliffs in the Jim Jim Falls area to isolated outliers and stepped cliffs in the North.
Chasms and gorges form a network that cut through the rocky platforms on the plateau. The plateau top is a harsh, dry environment where water drains away quickly and top soil is scarce in most areas. Sparse pockets of open forest and woodlands have developed in these areas. However creeks have carved deep gorges in the escarpment in which tall monsoon forests grow. These areas form microclimates for plants and animals and often serve as a refuge during the dry season. Allosyncarpia ternata, a large shady tree found only in the Kakadu and Arnhem Land, is the main plant species.
The outliers are pieces of the Arnhem Land plateau that have become separated from the plateau by erosion. They were islands in the ancient seas that once covered much of Kakadu. The lowland plains stretch over much of the Top End and they make up nearly 70% of the Park. The soils are shallow and often are above large sheets of laterite (ironstone) and a thick profile of strongly leached rocks.
During the wet season, water carried down from the Arnhem Land plateau often overflows from creeks and rivers onto nearby floodplains. Alluvial soils carried in the floodwaters add nutrients to the floodplains. Nutrient-rich soils 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 drains into rivers, creeks and isolated waterholes or billabongs. Kakadu’s wetlands are listed under the Convention on Wetlands of International Importance (the Ramsar Convention) for their outstanding ecological, botanical, zoological and hydrological features.
The southern hills and basins cover a large area in the south of the Park, including the headwaters of the South Alligator River. Rocks here have been exposed from beneath the retreating Arnhem escarpment; they are of volcanic origin and are extremely old (2500 million years). This landform is characterised by rugged strike ridges separated by alluvial flats.
Kakadu’s coast and the creeks and river systems under tidal influence (extending about 100 kilometres inland) make up this landform. The shape of the estuaries and tidal flats varies considerably from the dry season to the wet season. 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, 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 estuaries and tidal flats are home to many plants and animals adapted to living in the oxygen-deficient saline mud. 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.
The art sites of Ubirr, Nourlangie Rock and Nanguluwur are world famous as examples of Aboriginal rock art. These sites are found among the rocks that have given shelter to the Aborigines for thousands of years. The painting in these rock shelters were done for different reasons:
Ubirr is a group of rock outcrops in the northest of the Park, on the edge of the Nadab floodplain. There are several large rock overhangs that would have provided excellent shelter to Aboriginal people over thousands of years. Because it is close to the East Alligator River and Nadab floodplains, it means that there would have been plenty of food. This is shown in much of the rock art here. Animals shown in the main gallery include barramundi, catfish, mullet, goanna, snake-necked turtle, pig-nosed turtle, Rock-haunting Ringtail Possum, and wallaby and thylacine (Tasmanian Tiger).
There are also images of the Rainbow Serpent said to have created much of the landscape as well as mischievous Mimi spirits and the story of the Namarrgarn Sisters. Many stories connected to Aboriginal rock are highly complex and linked to other stories. Often the true meanings have been lost, but they all have a purpose which is usually to serve as a lesson or a warning to the young or to those passing through the area.
Nourlangie is located in an outlying formation of the Arnhem Land Escarpment. There are a number of shelters in this large outcrop linked by paths and stairways. The shelters contain several impressive paintings that deal with the Dreamtime. The stories connected to these artworks are known only to certain Aboriginal people and remain secret.
Anbangbang Billabong lies in the shadow of Nourlangie Rock and is home to a wide range of wildlife which would have sustained traditional Aboriginal people well.
Nanguluwur, a small art site near Nourlangie, has several rock art styles. These include hand stencils, dynamic figures in large head-dresses carrying spears and boomerangs, Namandi spirits and mythical figures, including Alkajko, a female spirit with four arms and horns. There is also an interesting example of ‘contact art’ showing a two-masted sailing ship with anchor chain and a dinghy trailing behind.
Humans have made major changes in the area during the 19th and 20th century. Introduction of water buffalo from Southeast Asia caused damage to the fragile floodplains and wetlands. Since then, buffalo have largely been removed from the area so the land is now recovering. Crocodile hunting which has been banned since 1972 made a huge impact on crocodile numbers. Since they have been protected, the crocodiles have recovered so successfully that some think there are now too many.