Plants and animals

by Alaric Fisher, Parks and Wildlife Commission of the NT

Landscapes

The Mitchell Grasslands are the most extensive tussock grasslands of Australia, stretching almost uniformly over more than 300,000 km² from south-eastern Queensland to the mid-north of the Northern Territory, with smaller scattered patches extending further west to the East Kimberley and south to northern South Australia.

The tiny, and endangered, Julia Creek dunnart Photo: Greg Calvert

They form one of the most distinctive environments of northern Australia, characterised by a general lack of tree and shrub cover, cracking clay soils and extensive cover of relatively short grasses. These features dictate many of the characteristics of the fauna. The relatively simple environment ensures that vertebrate species richness is generally remarkably low, with an absence of most arboreal birds, and relatively little turnover in species composition across the entire area.

Wildlife

Notwithstanding this general impoverishment of the fauna, there are some highly distinctive features of the wildlife of the Mitchell Grasslands. Several of the region's animals have an extraordinary boom-bust population cycle, with periods of "plagues" followed by years of very low density. The most notable of these are the long-haired rat Rattus villosissimus, flock bronzewing Phaps histrionica and letter-winged kite Elanus scriptus (nb: also locusts). These population fluctuations appear to be related to rainfall patterns, but even so their causes are only very sketchily known.

The flock bronzewing suffered a long period of decline over much of this century, and until recent decades, it was considered to be heading towards extinction. However numbers appear to have built up in many areas and flocks of hundreds or even thousands are still reported, and are one of the most striking features of this grassland region.

Ground fauna

The cracking clay soils support a distinctive ground fauna, notable for a very high diversity of large elapid snakes, several endemic reptile species (such as the very large Spencer's Monitor Varanus spenceri, the speckled brown snake Pseudonaja guttata and Ingram's brown snake P. ingrami), very high densities of the several grassland birds (such as the singing bushlark Mirafra javanica, brown songlark Cincloramphus cruralis, and several quail) and the smallest marsupials ( long-tailed planigale Planigale ingrami), and the localised occurrence of the endangered dasyurid mammal, the Julia Creek dunnart Sminthopsis douglasi.

Most of this fauna shelters within the cracking soil over the course of the dry season. The rains of the wet season waterlog the soils, and close these cracks. The wet season heralds high densities of some burrowing frogs, and filling of the depressions in the otherwise generally typically flat terrain. Some of these depressions can form very extensive swamps, typically fringed by bluebush. In many years these are nationally and internationally significant for breeding waterbirds, such as pelicans, ibis, herons, terns and ducks. The grasslands themselves are also a major summering ground for some migrant birds, such as the little curlew and oriental pratincole, whose movements are intercontinental.

Environmental challenges

There are some major conservation challenges for the fauna of the Mitchell grasslands. Until the recent dedication of several large National Parks in Queensland, almost all of the Mitchell grasslands was devoted to pastoralism. This is still the case in the Northern Territory, with the notable exception of one single reserve, Connell's Lagoon. Several species are known to be adversely affected by grazing, and have undoubtedly declined substantially across this region. Conversely, the extensive provision of artificial water sources across much of the region has undoubtedly benefited many species, such as some macropods, crested pigeons Ocyphaps lophotes and cockatoos.

This map of Barkly Tablelands properties shows areas at various distances from watering points. Areas far from watering points tend to be less exposed to heavy cattle grazing, and consequently act as refuges for many native animals.

Effect of fire

Pastoral land management may have limited the range of fire regimes occurring in this environment, possibly to the detriment of some wildlife. Feral cats are now widespread, and especially common around bores and other water sources, and are probably having some at least localised major impacts. In parts of western Queensland, there have been major invasions of Mitchell Grasslands by woody weeds, notably Acacia nilotica: the biodiversity costs of this change are not known.