Cattle graziers would like to think that if they look after their pastures and keep weeds and pest animals under control, the rest of the environment can look after itself. To some extent this makes sense; particularly in the rangelands where paddocks still contain reasonable tree cover, and where maintaining the native grass species is as important to cattle as it is to wildlife.
Achievements of the Pigeonhole Biodiversity project
- Cooperation between pastoralists and conservation managers
- Demonstrating the resilience of black soil flora and fauna
- Identifying grazing-sensitive species with special conservation requirements
- Developing grazing guidelines for maintaining biodiversity in tropical savanna rangelands
But do we actually know that this is the case? Finding out is no easy task; even on a single property there are hundreds of plant and animal species to consider, and the natural variation in the climate means that their numbers can increase or decrease even where no cattle are present. In order to establish the large number of paddocks needed to test different levels of grazing pressure, a vast tract of land is needed, along with the flexibility to graze cattle at a range of densities that reflects real grazing practices. Then you need to find a large team of dedicated staff willing to spend hundreds of hours of measuring, trapping, identifying and recording the plants and animals in each paddock. And most importantly, to ensure the local pastoralists have confidence in the study's conclusions, the project needs their cooperation from the start. Meeting the requirements is a massive task. That is why the Pigeonhole project in the Northern Territory's Victoria River District is so remarkable.
The Pigeonhole project, which ran from 2003 to 2007, was a one of the largest grazing trials ever undertaken in Australia, and the only one to consider the impact of different grazing levels on biodiversity in such fine detail. Central to the project was the Heytesbury Beef pastoral company, which donated a 308 km2 parcel of land for the five years of the study. The other partners in the study were Meat and Livestock Australia, CSIRO, the University of Queensland, the Tropical Savannas Cooperative Research Centre, and the Northern Territory Departments of Primary Industries, Fisheries and Mines (DPIFM) and Natural Resources, Environment and the Arts (NRETA).
One of the imperatives for undertaking this study was the push to develop northern Australian pasturelands to turn-off more stock each year, by increasing both the number of cattle each station carries and the infrastructure, such as watering points and fencing to allow this development. Most of the partners worked on various aspects of pastoral production, moving and monitoring the condition of the 3,000 to 5,400 cattle used in the trial. They found that 20-25% utilisation of annual pasture growth was likely to be sustainable, and that evenness of use (and therefore production) could be maximised by having small (30-40 km2) paddocks with two watering points. Rotational grazing systems that spelled pasture during the wets season were also a viable option that was likely to improve pasture condition.
These findings were presented to the pastoralists at the Pigeonhole Field Day held in 2005 and 2007. Because the field days were attended by pastoralists from as far afield as the Kimberley to north Queensland, the presenters stressed that these recommendations had been developed for black soils plains, and that more moderate grazing regimes apply to poorer country, such as on red soils.
Meanwhile, staff of NRETA's Division of Biodiversity Conservation had been studying the impact of the grazing regimes on the property's wildlife. Each year, they visited the property for fortnight-long field trips, once early in the year when the plants were fresh and the animals active, and once late in the dry season, when the grasses were most grazed, and animals more likely to be restricted close to waters.
The surveys revealed a suite of 240 plant species, 80 birds, 8 small mammals, 25 reptiles and 63 ants in the study area. Alaric Fisher, the biodiversity project's leader, explains that measuring condition of biodiversity is not just about numbers. "There are some species that decline in numbers in response to disturbances such as grazing," he says. "These are called decreaser species. There are also increaser species that flourish, while other species don't seem to respond at all. So while there may be plenty of plants and animals in a paddock, it is important to know to which group they belong."
One examples of an increaser species is Annual Sorghum Sarga timorense, which replaces the more productive perennial grasses when these are overgrazed. These perennial grasses are important for the health of wildlife, especially that of seed-eating birds. Other increaser species include common and widespread birds such the Crested Pigeon and Galah, which are benefited by additional water and forage in areas of bare ground.
Species found to be sensitive to grazing included the VRD Black Soil Ctenotus Ctenotus rimacola. This skink is more abundant away from watering points, where grazing pressures is low. The Long-tailed Planigale Planigale ingrami, which lives in cracks in the black soil, is less common where the soil has been compacted by overgrazing. Bird species that prefer dense grasses, such as Red-backed Fairy-Wren and Golden-headed Cisticola, are also decreasers.
Biodiversity was also monitored in a number of large fenced areas that were set up within the trial paddocks, and where grazing was excluded. After five years, differences in structure and composition of the ground layer within these exclosures was becoming obvious, with an increase in the cover, frequency and basal area of the palatable perennial grasses (such as Mitchell Grasses Astrebla and Ribbon Grass Chrysopogon) that are heavily grazed in the trial paddocks. The exclosures contained high densities of decreaser bird species such as Singing Bushlark and Golden-headed Cisticola, and were strongly favoured by the Northern Nailtail Wallaby. “These exclosures demonstrate the potential of the pasture to recover if it is spelled” Alaric said, “although it clearly takes a number of years for the perennial grasses to come back. It has also been very interesting how the soil surface becomes much less compacted and more friable after several years without stocking”.
One of the most significant findings was the large number of plants and animals on Pigeonhole, both increasers and decreasers, that seemed insensitive to the grazing levels used in the trials, even to the highest utilisation rates of 40%. No-one was more surprised at this than the pastoralists themselves. "When we announced this at the final Pigeonhole Field Day," says Alaric, "the pastoralists were sure that if we could keep the trials going a few more years, greater differences would become apparent." And they are probably right, black soils being among the most resilient grazing lands in northern Australia, they are likely to take a long time to degrade from overgrazing. Even so, the utilisation rates of 20-25% recommended for maximising pasture condition and animal production were well below the levels damaging to biodiversity.
The project has also contributed to the development of a number of best practice principles for combining pastoral production with biodiversity conservation in northern Australia. Maintaining native grass cover and controlling weeds and pest animals are indeed among them, as are the spelling practices recommended from Pigeonhole's pastoral studies. Additional landscape-wide practices recommended include retaining the natural tree cover, along with the fallen timber and leaf-litter to provide a variety of habitats. Also recommend is excluding cattle from small sections of each property, particularly from sensitive areas, such as natural water holes. As wildlife depends on a wider range of native grasses and ground cover plants than do cattle, and can be disadvantaged by many of the introduced pasture species, land managers need to monitor the composition of their pastures and use spelling and fire to restore the natural diversity. Pastoralists also need to be aware of the plants and animals found on their property, so that any changes can be identified early, and to develop a property plan so that new developments on the property are not undertaken without considering the needs of biodiversity.
"Managing pasturelands well is extremely important for conserving biodiversity in northern Australia," says Alaric. "These areas are the home to many species found nowhere else." The Pigeonhole grazing trials are leading the way in showing how pastoral land managers can make a living while ensuring the long term health of north Australia's pastoral resources. The Pigeonhole Biodiversity trials demonstrate that with a little extra care, pastoral producers can also make a significant contribution to conservation of its wildlife.