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Savanna Explorer > Arnhem Land > Plants and Animals > Refuge for plants and animals

Refuge for plants and animals

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Jim Jim creek in Kakadu National Park is on the western edge of the plateau

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Boronia viridiflora habitat

While the human story of the Arnhem Plateau is an extraordinarily long one, according to western science, this is just the latest chapter in an older story of how this plateau became one of the world’s great refuges for distinctive assemblages of plants and animals.

An island of stability

If you could see a movie that looked down on this part of the world in which the last 100 million years was compressed into a few hours, you would see the Arnhem Land Plateau as a fuzzy but solid presence persisting amidst a chaos of change in the surrounding lowlands. The flatter country would be blurred by rising and falling sea levels and changing climates — one minute a sea, the next a desert — and fires would sweep endlessly across these flatlands like luminous ripples.

This relative stability of the great sandstone formations that make up the plateau — and the protection it has offered from fire and flood — has allowed the plants and animals that live here to evolve in relative isolation into specialized forms exquisitely adapted to their surroundings.1

There are many plants and animals like Boronia viridiflora on the plateau that have small populations with limited distributions and this points to another feature of the region: the complex topography with thousands of small valleys, crevices and caves can easily isolate populations of plants, and to a lesser extent animals, allowing them to evolve into separate species.

There are now at least 170 species of plants that are only found in the Arnhem Land Plateau and they are joined by five mammal, three bird, 12 reptile, one frog and three fish species found nowhere else.

The animal species include the black wallaroo, the Arnhem rock-rat, the chestnut-quilled rock pigeon, and the Oenpelli python which can reach 4m in length. Added to these species is a range of endemic invertebrates that includes a family of shrimps and a whole genus of isopods (slaters). 1,3

The rich biodiversity of the plateau is shown in the maps below of the numbers of co-occurring species of vertebrates (left) and plants (right) that are only found in the Northern Territory (endemic NT species). Green/yellow shades show lower numbers and orange/red shades show higher numbers with Kakadu National Park shown in outline. Note how the Arnhem Land Plateau has the highest levels of co-occurring endemic species in the Northern Territory4.

  Endfauna Endplants

 Co-occurence of endemic vertebrates (left) and plants (right) in the NT - Kakadu National Park shown in outline.

 

Rainforests

WALFA & rainforests

Rainforest patches in the Northern Territory (NT parks and Conservation Masterplan)

The long-term stability of the plateau environment and the protection it offers from fire has allowed ancient lineages of rainforest plants to survive. One of the characteristic trees of the rainforests of the plateau is the majestic large evergreen Allosyncarpia ternate – a member of a group of plants that was more widespread in ancient times before the dominance of the eucalypts.

In the map below, green indicates rainforest patches between 10 and 100 hectares and red indicates patches over 100 hectares in size. Note that the West Arnhem Plateau area covers the major concentration of large rainforest patches in Australia’s Northern Territory.

 

 

 

Heathlands

Much of the plant diversity in the Arnhem Plateau occurs in the “heath” communities of low shrubs that cover much of the open sandstone sheets. These plant communities can survive on very low nutrient, thin soils and typically are composed of many different species from groups like Boronia, Callytrix, Grevillea and Banksia.

It was the explorer William Dampier who in 1699, on seeing plant communities in Western Australia, named them ‘Heath’ as they looked like to the heaths in England. The Australian heaths, however, have quite a different mix of species to the northern hemisphere heaths. The Arnhem Land heathlands share characteristics with heathlands in places like south west Western Australia, such as having many species that can only sprout from seed.

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Arnhem Land heathlands

In the fire-prone lowland savannas, many plants re-sprout from buds protected from fire beneath thick bark, or on stems or roots. If plants have to re-sprout from seed they have to spend quite a few years as a small seedling with little protection from fire. Such plants, known as obligate seeders, will struggle to survive if they are subject to frequent fires – and this is now posing a problem for the Arnhem Land heath communities (see section on Callitris intertropica, page .. ).1,5



 

 

 

International conservation significance

Because of all the factors outlined above, the Arnhem Land Plateau has been identified as having international conservation significance.4 The map below shows that significant areas of high conservation occur in the WALFA project area (outlined).

WALFA & Biodiversity sm

Areas of international conservation significance in the 
NT - WALFA area shown in outline (NT Parks and Conservation Masterplan)



 

 

 

 

 

 

 

 

 

 

References

1. Woinarski, J., Mackey, B., Nix, H., and Traill, B. (2007) The Nature of Northern Australia. ANU E Pres

2. Woinarski, J. (2008). Biodiversity and Fire in Western Arnhem Land. In Managing fire regimes in north Australian savannas - ecology, culture, economy (eds J Russell-Smith, PJ Whitehead, P Cooke). CSIRO Publications, Melbourne. (in preparation)

3. Woinarski, J.C.Z., Hempel, C., Cowie, I., Brennan, K., Kerrigan, R., Leach, G., and J. Russell-Smith (2006) Distributional pattern of plant species endemic to the Northern Territory, Australia Australian Journal of Botany, 54, 627–640

4. NT Department of Natural Resources, Environment and the Arts (2005). Northern Territory Parks and Conservation Masterplan.  http://www.nt.gov.au/nreta/parks/masterplan/index.html

5. Dyer, R., Jacklyn, P., Partridge, I., Russell-Smith, J., Williams, R.J. (eds) 2002, Savanna burning: Understanding and using fire in northern Australia, Tropical Savannas Management Cooperative Research Centre, Darwin.