PhD student Annemarie van Doorn has studied one of northern Australia's endemic bird species—the Purple-crowned Fairy-wren—along the Victoria River for the past five years.

Her findings will underpin a new managment plan for the bird in the Northern Territory, and one that coincides with good land management practice: controlling erosion and weeds in riverside areas.

The western sub-species of the Purple-crowned Fairy-wren (Malurus c. coronatus) lives in riverside vegetation in northern Australia from the Kimberley in Western Australia east to the Victoria River District in the Northern Territory.
However, its presence is patchy, and it is currently listed as a vulnerable species. This patchy distribution has been attributed to both habitat degradation and alteration (Garnett & Crowley 2000, Rowley & Russell 1993).

As the wren is restricted to riparian habitat it is a good candidate for studying the impacts of some of the primary threats to biodiversity in northern Australia. There has already been one previous study, however, in habitat decidedly different from the one it occupies along the Victoria River (Rowley & Russell 2003).

Purple-crowned Fairy-wren habitat

In the Victoria River District the wren occupies river grass (Chionachne cyathopoda) sometimes in conjunction with northern cane grass (Mnesithea rottboelioides).

River grass forms dense stands along the Victoria River and its tributaries and is often referred to as cane grass, however this term has been used for a multitude of riverine grasses causing some confusion in the past. The Purple-crowned Fairy-wren depends heavily on this grass for breeding, foraging and cover; it was not found in areas without river grass coverage.

Because so many areas along the Victoria River are difficult to access, in the past the extent of river grass coverage could only be estimated. However, a helicopter survey conducted during this study found that river grass does not occur in a continuous stretch but is highly fragmented consisting of multiple patches of varied quality.

This poses a major problem to the long-term viability of the wren, as it has limited abilities to disperse. However, the largest and healthiest patch of river grass occurs within Gregory National Park where park staff are working to preserve this species and its habitat.

Threats to the population

The primary threats to populations of wrens include grazing, weeds, erosion and fire. These threats are confounding and often have cumulative effects. In particular, intense grazing at unfenced sites was identified as the primary threat to the population. The effects of grazing were most noticeable during the end of the dry season when cattle tended to congregate along the riparian corridor and caused significant damage to river grass stands. During Annemarie’s study where two previously ungrazed sites were subjected to intense grazing, there was at least a 50% reduction in the abundance of Purple-crowned Fairy-wrens. This decrease can be attributed to a lack of cover which ultimately reduced forage and breeding opportunities as well as increasing predation rates.

Weeds were abundant in the river grass habitat, in particular Noogoorra burr (Xanthium strumarium) and castor oil plant (Ricinus communis) were present at all research sites. Although the Purple-crowned Fairy-wren did forage among these species, in particular Noogoorra burr, it only did so during the short period that the plant was green (at the beginning of the dry season). However, once this species dries it provides no cover or foraging opportunities and at no time of the year was it an adequate nesting substrate.

Both these weeds are more prolific in open and disturbed areas and especially in areas where intense grazing has led to an increase in the percentage of bare ground.

In addition to weeds, erosion is widespread in the Victoria River District and has no doubt been exacerbated by the large floods in recent years. River grass regenerated quickly after flooding, however once the root base has disappeared re-colonisation­—based on seed propagation alone—takes much longer.

Only one fire was witnessed during this project which resulted in low adult mortality, mainly due to the small area affected. River grass regenerated very quickly after the fire and the area was re-colonised within one breeding season. For the majority of the year, river grass does not burn easily but a fire at the end of the dry season can result in a widespread burn, particularly in an area of dense river grass with high connectivity such as in Gregory National Park.

Future outlook

Conservation of river grass will have widespread positive outcomes in the Victoria River District in addition to conserving Purple-crowned Fairy-wrens and providing habitat for other species, such as the Yellow-rumped Mannakin (Lonchura flaviprymna). In addition to species-specific effects, river grass can also help stabilise riverbanks as well as reduce the amount of bare ground available for weeds to establish.

Greening Australia is currently investigating propagation techniques for river grass that will further enhance any future rehabilitation efforts.

Changing land practices (i.e. fencing and erosion control) in combination with an increased interest in this species on properties in the district indicate that there is a possibility of reducing and possibly reversing some of the threatening processes. A comprehensive and multi-faceted management plan is being developed using the findings of this study. This management plan will provide valuable management options to conserve River Grass and Purple-crowned Fairy-wren habitat in the Victoria River District.
 

References

Garnett, S.T. & Crowley, G.M. 2000, ‘The Action Plan for Australian Birds’, Environment Australia, Canberra.
Rowley, I. & Russell, E. 1993, ‘The Purple-crowned Fairy-wren Malurus coronatus. I. History, distribution and present status’, Emu, 93:220–234.

Project Details

This collaborative project was funded by the Victoria River District Conservation Assoc­iation, TS–CRC, and the School for Environmental Research, CDU. Additional assistance by Gregory National Park and Biodiversity Conservation Unit of NT Dept. Natural Resources, Environment and The Arts. Research was conducted in partial fulfilment of the PhD through the University of Florida (Supervisors: Dr Patricia Werner, Dr John Woinarski and Dr Barry Brook).