Kimberley tropical savanna

• Category: Grassland, Tundra, and Human Biomes.
• Geographic Location: Australia.
• Summary: The tropical heat of the region and varying rainfall regimes have resulted in unique flora and fauna, which are threatened by land degradation and introduced species.

The Kimberley region of Australia is located in the northern part of the state of Western Australia; it covers some 163,521 square miles (423,517 square kilometers), an expanse slightly larger than the country of Japan. The area comprises the rugged sandstone and limestone ranges of northwestern Australia, the more arid lowlands of Dampier Land in the southwest, and the Daly Basin to the northeast.

The Kimberley is one of the hottest parts of Australia, with an average annual mean temperature of 81 degrees F (27 degrees C). About 90 percent of the region’s rainfall occurs during the wet season of November to April, when cyclones are common (especially around Broome) and the rivers flood.

The annual rainfall is highest in the northwest, where Kalumburu and the Mitchell Plateau average 50 inches (1,270 millimeters) per year. Precipitation is lowest in the southeast, at about 20 inches (520 millimeters), and semiarid conditions are particularly harsh during the May-to-October dry season.

Flora

This region is mainly covered by open savanna woodland with boab trees (Adansonia gregorii)—known for their hardiness and longevity—in the drier areas, and Darwin stringybark (Eucalyptus tetrodonta) and Darwin woollybutt eucalypts (E. miniata) in other parts of the Kimberley.

Canopy heights are usually 16–49 feet (5–15 meters). Understories are dominated by such tall grasses as sorghum, heteropogon, themeda, chrysopogon, aristida, and family Eriachne. Northern parts of the Kimberley region are known for their dense subcanopy of sand palms (Livistona eastonii).

Various tree species and stands of tropical dry broadleaf forests are located along the banks of the Ord and Fitzroy Rivers here. Other patches of tropical dry broadleaf forest, called monsoon forests, along with deciduous vine forest or vine thicket, are located in sheltered gorges up north where there is a lot of rain.

Vegetation in the ecoregion is linked to soil, geological factors, and rainfall. The red sandy soil of the Dampier Peninsula is known for its pindan wooded grassland, while the more fertile areas such as the Ord Valley are grasslands consisting of Chrysopogon, Aristida, Dicanthium, and Xerochloa (rice grass) in the wetter valleys. Plant varieties are greatest along the river banks.

Many introduced plant species, including the castor oil plant (Ricinus communis), cocklebur (Xanthium strumarium), and parkinsonia (Parkinsonia aculeata), are most commonly found in areas around the catchments of major rivers.

Fauna

Typical of Australia, the animal population includes marsupials. The Kimberley is home to bilbies (Macrotis lagotis), also known as rabbit-eared bandicoots. The bird life in the Kimberley region includes emus, parrots, cockatoos, various migratory species, and raptors.

Reptiles here include the saltwater crocodile (Crocodylus poroxus)—the largest of all living reptiles, growing as long as 18 feet (5.5 meters) and weighing up to 2,200 pounds (1,000 kilograms)—and the freshwater crocodile (Crocodylus johnstoni), which reaches a maximum length of 10 feet (3 meters) and weights of up to 200 pounds (90 kilograms).

The area also includes goannas (monitor lizards), which grow up to 8 feet (2.5 meters) long, as well as blue-tongue lizards and other reptiles that have adapted to survive in the dry environment and drink very little water.

The Kimberley is host to the egg-laying function of a range of marine turtles, particularly the green turtle (Chelonis mydas), loggerhead turtle (Caretta caretta), flatback turtle (Chelonia depressa), and hawksbill turtle (Eretmochelys imbricata). The Kimberley Tropical Savanna biome and nearby regions host some 25 species of tree frogs and 51 species of ground frogs—one-third of the known Australian frogs. In addition, the Kimberly has vast numbers of insects, especially ants.

Local fauna includes species introduced to the area over the past two centuries, such as foxes, rabbits, cats, dogs, and bristly-back razorback pigs. These animals are present throughout Australia, much to the detriment of indigenous creatures. Not only do these introduced species compete with native animals for food and water, some have become predators.

Environmental Conservation and Threats

The rich diversity of the Kimberley’s flora and fauna has led to the establishment of six national parks—Drysdale River, Geikle Gorge, Hidden Valley, Mitchell River, Purnululu (Bungle Bingle), and Tunnel Creek—as well as the Prince Regent Nature Reserve. Conservation largely resides in the area’s remoteness and difficulty accessing the gorges, escarpments, and deeply dissected sandstone plateaus of the northern reaches. Accessibility is increasing, however, due mostly to poorly managed tourism and ongoing proposals for mining developments.

Climate change has resulted in rising water levels in coastal areas. Rainfall has increased by as much as 10 inches (250 millimeters) per year since 1967, but recent studies suggest Asian-sourced particulate air pollution—and not global warming—is the main culprit.

The major threats in modern times result from human activity such as land clearance, mining, and introduced species. The largest area of cleared land occurred in the Ord Valley, but the expansion of sheep grazing in the catchment of the Fitzroy River also has resulted in land degradation, with topsoil loss. By 1976, nearly a third of the catchment area had degraded to poor or very poor condition. Widespread and intensive grazing remains a problem in the region, from both ranched and feral livestock. Grazing also increases soil erosion within the area.

Bibliography

Andersen, Alan N. "Faunal Responses to Fire in Australian Tropical Savannas: Insights from Field Experiments and Their Lessons for Conservation Management." Diversity and Distributions, vol. 27, no. 5, 2021, pp. 828-843. DOI: 10.1111/ddi.13198. Accessed 12 Nov. 2024.

Beard, J. S., K. A. Clayton, and K. F. Kenneally. “Notes on the Vegetation of the Bougainville Peninsula, Osborn and Institut Islands, North Kimberley District, Australia.” Vegetation 57, no. 1 (1984).

Burt, Jocelyn. The Kimberley: Australia’s Unique North-West. Nedlands, Australia: Tuart House, 1999.

Edwards, Hugh. Kimberley: Dreaming to Diamonds. Swanbourne, Western Australia: Hugh Edwards, 1991.

Lehmann, Caroline E. R., Lynda D. Prior, and David M. J., S. Bowman. “Fire Controls Population Structure in Four Dominant Tree Species in a Tropical Savanna.” Oecologia 161, no. 3 (2009).

McGonical, David. The Kimberley. Terrey Hills, New South Wales: Australian Geographic, 1990.

Tyler, Ian. Geology and Landforms of the Kimberley. Kensington, Western Australia: Department of Conservation of Land Management, 2005.

Winter, W. H. “Australia’s Northern Savannas: A Time for Change in Management Philosophy.” Journal of Biogeography 17, nos. 4–5 (1990).