Location and General Description
This ecoregion is characterized by a markedly seasonal, monsoonal climate, with a short wet season from October to March, and a long, nearly rain-free dry season for the rest of the year. Average rainfall declines from the coastal north to the inland south, so that approximately 1,400 mm fall on Mitchell Plateau in northernmost Kimberley, decreasing to 600 mm around Broome in the southwest. Temperatures are high throughout the year and monthly average maxima range from 25oC to 35oC.
Vegetation distribution in the ecoregion is strongly associated with soil, geological factors, and rainfall. The most extensive vegetation, especially on sand and loam soils, are bloodwood eucalypts (Corymbia spp.) or, on deeper soils in higher rainfall areas, eucalypt-dominated woodlands with Darwin stringybark (Eucalyptus tetrodonta) and Darwin woollybutt (E. miniata). Typically, canopy height varies from 5 m to 15 m, and understories are dominated by tall grass species, especially Sorghum, Heteropogon, Themeda, Chrysopogon, Aristida, and Eriachne spp. Parts of the North Kimberley are notable in having a relatively dense subcanopy of sand palms (Livistona eastonii) (Western Australian Museum 1981).
Pindan is a distinctive vegetation type associated with the red sandy soils of much of the southwestern corner of this ecoregion on the Dampier Peninsula. Pindan is dominated by a range of wattle species over a sparse grass layer. Species represented include Acacia eriopoda, A. tumida, A. platycarpa, and A. colei (Kenneally et al. 1996). Canopy height is typically low (3 m to 8 m), although occasional bloodwoods emerge above the wattle layer. Dahl (1926) described the pindan as a "low crippled forest" in response to the apparent uniformity and dwarf stature of the vegetation.
Smaller lowland areas of finer-textured soils (clays and clay-loams) support open woodlands dominated by Terminalia and Bauhinia species, or grasslands of Chrysopogon, Aristida, Dicanthium and (in wetter floodplain area) Xerochloa spp. These have been the focus of more intensive agricultural developments, notably in the lower Ord catchment.
Riparian strips are another distinctive localised vegetation feature, with vegetation associations varying according to river order, flooding regime, and substrate, but typically including river red gum (Eucalyptus camaldulensis), Terminalia platyphylla, Ficus spp., Melaleuca spp., Nauclea orientalis, and/or Pandanus spp. (Woinarski et al. 2000). Several large rivers cross the ecoregion, including the Fitzroy, Ord, Victoria, and Daly Rivers.
The ecoregion’s extensive coastline is highly variable, but dominated by the extraordinary daily tidal range, typically 5 m to 10 m. Mangrove vegetation and saline grasslands or herblands are well developed in flatter coastal areas and support a rich distinctive bird fauna (Johnstone 1990), but much of the coastline in the north Kimberley comprises almost sheer cliff faces. Many of these support rainforest thickets, probably because they offer protection from fires. Small rainforest patches also occur widely across the ecoregion (McKenzie et al. 1991, Russell-Smith 1991), typically in association with rugged topography, in areas with unusually persistent water availability or in riparian strips. These rainforests support high levels of endemism for some groups, notably land snails. Many of these species are known from only one or few patches of rainforest (McKenzie et al.1991).
The north Kimberley coast is dotted with many uninhabited islands (Burbidge and McKenzie 1978), some of which are important for conservation, because they protect species that have declined on the adjacent mainland, and because they support significant breeding sites for seabirds, waterbirds, and marine turtles.
The vertebrate fauna of much of the Western Australian portion of this ecoregion is relatively well known through a series of important inventory studies conducted largely by the Western Australian Department of Conservation and Land Management (Miles and Burbidge 1975, Kabay and Burbidge 1977, Burbidge and McKenzie 1978, McKenzie 1981a, 1981b, 1983, Western Australian Museum 1981, Burbidge et al. 1991, McKenzie et al. 1991). The Northern Territory portion is less well known, with the exception of a recent study in the Daly Basin in the far northeast of the ecoregion (Price et al. 2001). The vegetation of the Western Australian portion is also unusually well known (Wheeler 1992, Kenneally et al. 1996).
These studies have helped to document a highly distinctive biota, most closely related to the geologically and topographically similar western Arnhem Land sandstone massif. About 230 plant species are considered to be endemic to this ecoregion (Wheeler 1992). There is highly marked endemism in some invertebrate groups, although most are poorly known (McKenzie et al. 1991). There has been no previous summary of the endemicity of the vertebrate fauna, but it is known to include at least 16 fish species, 10 frog species, 31 reptile species, two bird species, and 6 mammal species.
The endemic frogs include Limnodynastes depressus, Litoria cavernicola, L. splendida, Uperoleia aspera, U. minima, U. crassa, U. marmorata, U. mjobergi, U. talpa and U. variegata. The endemic reptile species consist of the geckoes Diplodactylus mcmillani, Gehyra occidentalis, G. xenopus, Oedura filicipoda, O. gracilis, O. obscura and Pseudothecadactylus cavaticus, the agamid lizards Diporiphora convergens, D. pindan, D. superba, and Pogona microlepidota, the skinks Carlia johnstonei, Ctenotus burbidgei, C. mastigura, C. tantillus, C. yampiensis, Egernia douglasi, Glaphyromorphus brongersmai, Lerista apoda, L. borealis, L. kalumburu, L. separanda, L. walkeri and Omolepida maxima, and the snakes Ramphotyphlops howi, R. micromma, R. troglodytes, R. yampiensis, Morelia carinata, Denisonia ordensis and Vermicella minima. Among the birds, the black grass-wren (Amytornis housei) and white-quilled rock-pigeon (Petrophassa albipennis) are endemic. The 6 mammal species include the ningbing pseudantechinus (Pseudantechinus ningbing), scaly-tailed possum (Wyulda squamicaudata), monjon (Petrogale burbidgei), yellow-lipped bat (Vespadelus douglasorum), Kimberley mouse (Pseudomys laborifex), and Kimberley rock-rat (Zyzomys woodwardi).
Most, but by no means all of these species are associated with the rugged sandstone and higher rainfall areas of the north Kimberley, illustrating the important refugial function of this area. Among the most notable of these species: the python Morelia carinata was described as recently as 1981 and is still known only from a handful of records; the splendid tree-frog (Litoria splendida) is an exceptionally large green frog with white spotting; the scaly-tailed possum is an unusual possum in a monotypic genus; and the black grass-wren is confined to hummock grasslands in especially rugged sandstone environments.
There are many threatened species in this ecoregion for several reasons. Some endemic species have extremely restricted known ranges, some have taken refuge in the sandstone complex after declining in the rest of their range, and within the ecoregion, some species are in decline, especially in the flatter lands around the ecoregion’s margins.
The ecoregion has some important wetlands, most notably in the Ord and Daly River systems (Chatto and Whitehead 1996, Lane et al. 1996). Lakes Argyle and Kununurra are man-made lakes in the southeast Kimberley, formed by the damming of the Ord River. They serve as important dry season refuges for waterbirds, with up to 200,000 waterbirds counted at one time. Over 200 aquatic and terrestrial bird species are estimated to use the lakes, and the most numerous species include the glossy ibis (Plegadis falcinellus) and the magpie goose (Anseranas semipalmata). The lakes also contain large numbers of freshwater crocodiles (Crocodylus johnstoni) and are listed as a Ramsar site (Frazier 1999).
Downstream of the artificial lakes, the estuary and lowland floodplains of the Ord (and the adjacent Parry floodplains) are also listed as Ramsar sites, and support a broad range of wetland types and high numbers of migratory shorebirds. Migratory shorebirds are also a feature of the southwestern Kimberley coast, notably the Eighty-mile Beach, Roebuck Bay (both Ramsar sites), and Roebuck Plains. Roebuck Bay is considered "one of the most important stop-over areas for shorebirds in Australia and globally" (Lane et al. 1996).
A series of gorges, mostly within limestone ranges, are also a notable feature of the central Kimberley. Windjana, Tunnel Creek, and Geikie Gorges support abundant bat colonies in extensive cave systems and are rich in Devonian reef fossils. Some of the islands and mainland coast of this ecoregion are important as colonial breeding sites for seabirds, waterfowl (herons, cormorants, etc.), and marine turtles (Burbidge and McKenzie 1978, Prince 1998, Chatto 2000, 2001).
The environments of this region are largely unaffected by broad-scale clearing, except for the Daly Basin where about 1,500 km2 has been cleared for horticulture and improved pastures and the Ord River Irrigation Area where about 1,000 km2 has been cleared for horticulture and another 1,000 km2 inundated by water impoundment. Much of the ecoregion is devoted to extensive pastoralism. Grazing by livestock and feral stock, principally cattle (Bos taurus and B. indicus) and donkeys (Equus asinus), affects almost all areas of the ecoregion. Feral stock densities may be very high, and in many areas are greater than 10 individuals/km2 (Freeland and Choquenot 1990).
Conservation reserves comprise about 18,840 km2 or 5.4 percent of the ecoregion, with larger reserves in the gazetting process in the north Kimberley and far northwest of the Northern Territory. The most significant existing reserves include Prince Regent River, Drysdale, Gregory, and Keep River. The protected areas system is biased in favor of rugged rocky areas, with little representation of lowland fertile flats of high agricultural potential.
This ecoregion continues to harbour good populations of many species which have declined or been lost from their range elsewhere, including the endangered gouldian finch (Erythrura gouldiae) (Hilton-Taylor 2000).
Notwithstanding the apparent security offered by the rugged sandstone environments, components of the biodiversity of this ecoregion are being lost or degraded. McKenzie et al. (1991) reported widespread damage of rainforest patches by feral animals (principally feral cattle and feral pigs) and altered fire regimes. Within the more extensive eucalypt forests, fire-sensitive species, such as the northern cypress-pine (Callitris intratropica), are declining over very extensive areas. Of 28 mammal species (other than bats) known from the southwest Kimberley before European colonization, seven have become extinct there (McKenzie 1981b). These changes have been profound. For example, Dahl (1897) reported that, for the burrowing bettong (Bettongia lesueur), a small kangaroo
"the ground was nearly everywhere and in all directions excavated by the burrows of this little Macropod … all the scrubs, and especially the slopes … are inhabited by countless numbers";
the golden bandicoot (Isoodon auratus) was
"very numerous in the coast country around Roebuck Bay … great numbers being brought to me";
and for the golden-backed tree-rat (Mesembriomys macrurus)
"the houses of settlers … are always tenanted by (this species)".
None of these species now occurs in the southwest Kimberley, although the golden-backed tree-rat and golden bandicoot are still present in the rugged north Kimberley.
The threatened purple-crowned fairy-wren (Malurus coronatus) has been lost from much of the Ord and Fitzroy River systems, mostly because of trampling and overgrazing of riparian vegetation (Smith and Johnstone 1977, Rowley 1993), but persists in the ecoregion in less disturbed areas.
Types and Severity of Threats
The main threats to this region are widespread and far-reaching, rather than localized and acute. The early years of pastoralism, from 1880 to 1910, were accompanied by major environmental degradation, especially around natural water sources. Degradation continued in many of the more susceptible environments, especially alluvial flats, with denudation of vegetation leading to the formation of deeply eroded gullies and/or extensive sheet erosion in the Ord and Fitzroy systems.
Several mammal species, including bilby (Macrotis lagotis VU), northern quoll (Dasyurus hallucatus), pale field-rat (Rattus tunneyi), golden-backed tree-rat, and golden bandicoot have declined, especially in the lower rainfall lowland portions of this ecoregion (McKenzie 1981b). The burrowing bettong, which never occurred in the more rugged, higher rainfall areas, has become extinct in the ecoregion. These losses may be due to grazing impacts, but may also be related to the spread of feral cats and altered fire regimes.
Across most of the ecoregion, traditional Aboriginal fire regimes (relatively fine-scale, frequent burning particularly in the early dry season) have largely been replaced by attempted fire exclusion. As a result, the incidence of uncontrolled and extensive late dry season fires has increased. This management change has resulted in some pronounced changes in vegetation patterning and the decline of fire-sensitive species and environments.
Many exotic plant species have become pastoral and environmental weeds in this ecoregion. These include noogoora burr (Xanthium strumarium), parkinsonia (Parkinsonia aculeata), bellyache bush (Jatropha gossypifolia), and castor oil plant (Ricinus communis).
In contrast to these generally wide-ranging threats, the relatively small pockets of fertile lowland soils have been exposed to localized, intensive horticultural developments. There are proposals to further intensify use in most of the remaining fertile areas, including proposals for major impoundment and irrigation schemes.
The conservation values of this ecoregion largely lie in the remoteness and relative inaccessibility of the gorges, escarpments, and deeply dissected sandstone plateaus of the north Kimberley. This impunity is being gradually eroded by rapidly increasing and poorly managed tourism, and may be further jeopardized by ongoing proposals for mining developments.
Justification of Ecoregion Delineation
The Kimberley Tropical Savanna ecoregion includes the five IBRAs Dampierland, North Kimberley, Central Kimberley, Victoria Bonaparte and Daly Basin (Thackway and Cresswell 1995).
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Prepared by: John Woinarski