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Jim Jim creek in Kakadu National Park is on the western edge of the
plateau
Boronia viridiflora habitat
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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.
Co-occurence of endemic vertebrates
(left) and plants (right) in the NT - Kakadu National Park shown in
outline.
Rainforests
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Rainforest patches in the Northern Territory (NT
parks and Conservation Masterplan)
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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
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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).
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Areas of international conservation significance in the
NT - WALFA area shown in outline (NT Parks and
Conservation Masterplan)
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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.
