Unlocking the secrets of an inland river

Friday 20 June, 2008

Carissa Free, PhD student at the University of Queensland, writes of her three-year study of the Field River which flows through Ethabuka Reserve.

The dry riverbed of the Field River, Ethabuka Reserve, Qld. Photo Wayne Lawler / EcoPix.

The dry riverbed of the Field River, Ethabuka Reserve, Qld. Photo Wayne Lawler / EcoPix.

Australia’s inland rivers have the most variable flows of any rivers in the world. Many remain dry for several years, or even decades. But when they do flow, some, like the Cooper Creek in central Queensland, can create floodplains covering 100 000 square kilometres.

There have been very few studies of these ephemeral dryland rivers and how they affect local biodiversity, particularly the types and quantity of vertebrate fauna that live along them. It was with this question in mind that I set out on a three-year study of the Field River in the Simpson Desert.

Desert rivers such as the Field River, which runs through Bush Heritage’s Ethabuka Reserve in far-western Queensland, are refuges rich in nutrients and moisture in an otherwise dry landscape.

Mature coolibahs arch over the dry riverbed. Photos Wayne Lawler / EcoPix.

Mature coolibahs arch over the dry riverbed. Photo Wayne Lawler / EcoPix.

Large trees, grasses and shrubs grow along them, creating corridors that provide a habitat for species that are less tolerant of desert conditions. My study aimed to quantify the differences in habitat and diversity of fauna along the river and on the adjacent dunes.

Lastly, I wanted to establish if and how the abundance of small vertebrate populations was affected by the river.

To monitor changes in the populations of small mammals and reptiles over the duration of the study, I established lines of pitfall traps at three sites along the river.

Pitfall traps are created by burying PVC pipe vertically in the ground, the top of the pipe level with the ground surface. Each pipe is 600 mm long and 300 mm in diameter and is fitted with a base of wire mesh so that animals can't dig their way out.

The pipe is closed with a PVC cap when not in use. The traps are positioned along a drift fence up to 20 metres long, which guides small animals towards the pits.

The animals trapped were identified, measured, weighed and then released unharmed.

I caught and released 47 species and over 1,500 individual animals from March 2006 to May 2008. Although there was no significant difference in richness of species between the dune and riverine habitats, several species – including the desert tree frog, excitable delma (a small legless lizard), long-nosed dragon, desert burrowing frog, Burton’s legless lizard and western hooded scaly-foot (a large legless lizard) – were found exclusively along the river.

Low grassy woodland along the Field River. Photo Wayne Lawler / EcoPix.

 Low grassy woodland along the Field River. Photo Wayne Lawler / EcoPix.

Many of these species spend a large amount of time either in the trees or foraging in the leaf litter below. I caught 76 introduced house mice along the river, whereas only ten were caught in the dunes. This suggests that the river and the associated floodplain may be a refuge for this species.

I also measured changes in the climate, habitat complexity, plant cover and availability of food over the study period. The riverine corridor had significantly more grass, leaf litter, tree cover and soil moisture than the dune habitats.

Although my research is still ongoing, it's suggesting that inland rivers provide a unique habitat within the surrounding desert environments, offering shelter and foraging opportunities to species that depend on trees.

This is despite the fact that rivers in arid areas have traditionally been overgrazed or diverted for irrigation and to provide water for livestock.

I hope this research will add to the information already available about inland rivers, and enable better management strategies to be developed for their conservation.

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