The history of Australia’s rangelands includes regional episodes of land and pasture degradation. Many of these episodes follow a sequence of increasing animal numbers (domestic stock and other herbivores), in response to several years of favourable rainfall, and changing economic conditions. The subsequent return to ‘normal’ and/or drought conditions resulted in severe over-grazing. Knowledge of the causes of climatic variation, for example the Inter-decadal Pacific Oscillation, might help to prevent the repetition of these episodes.

1. Introduction

The history of grazing of sheep and cattle in Australia’s rangelands over the last hundred years is the story of successful property development in a harsh environment. Grazing enterprises have been developed over time against a background of both high rainfall and commodity price variability occurring at annual and decadal time scales.

History also tells a story of degradation episodes often associated with extreme drought. Many of these have led to major government inquiries and intervention (Table 1). Sometimes these degradation episodes have been followed by recovery periods in which favourable seasons (and a pulse of mineralised nitrogen) resulted in luxurious plant and animal growth. Sometimes, however, the productivity of land for grazing purposes did not return, and scalds and woody weeds have remained.

There are a number of paradoxes resulting from the interaction of land management and climate variability:

• Our culture has taken the stoic courage of the grazing families who battled the physical elements into the soul of our nation. The word ‘drought’ still stirs an emotional supportive response in the wider community and is regarded as synonymous with ‘natural disaster’; but, in contrast, the episodic horror of dying animals and the apparent destruction of the natural resource (soil and plants) have only infrequently captured the concern of the community.
• Governments have sponsored surveys and inquiries that attributed some of the causes of degradation to too many stock at the wrong time leading to the loss of desirable perennial pastures. Through legislation and/or extension, governments and grazier groups have sought to implement regional solutions to prevent further degradation, and aid recovery. However, in some cases government policies (e.g. inappropriate closer settlement) have also put in place the conditions that were likely to cause the next regional degradation episode.

The German philosopher Hegel commented in the early 19^th century that ‘we can learn from history that we do not learn from history’. In fact it may be that ‘history’ does not lend itself to teaching unless we can understand how the processes of climate, ecology, management and policy interacted. In an attempt to overcome this human dysfunction, a group of idealistic scientists, supported by some visionary funding, have set out to build a drought/degradation alert system, ‘Aussie GRASS’, that would objectively assess and communicate the year-to-year change in degradation risk (Carter et al. 2000).

Carter et al. (2000) described eight major historical degradation episodes in Australia’s rangelands. Seven of the eight episodes involved severe drought periods (Table 1) lasting two to six years. Land and pasture degradation processes documented at the time of these episodes included loss of desirable perennial vegetation, dust storms, drifting sands, loss of surface soil, reduced infiltration capacity, water-driven soil erosion and woody weed infestation. Other features of these episodes were increased animal and human mortalities and financial failure of enterprises.

Most of the episodes were preceded by a buildup of stock numbers (and other herbivores such as rabbits) in response to periods of good seasons and substantial price variation. In eastern Australia, good seasons occurred in the early 1890s, late 1910s, early 1920s, mid 1950s, early 1970s and late 1990s. Because of the work of Power et al. (1999), we now suspect that quasi-decadal changes in the Pacific Ocean sea surface temperatures (the Inter-decadal Pacific Oscillation or IPO) may result in the amplification of the effects of La Niña conditions on rainfall in eastern Australia (IPO negative phase). The retention of high stock numbers through subsequent drought periods resulted in the regional degradation episodes described in Table 1. Thus the impact of current favourable seasons in eastern Australia on stock numbers should be closely monitored as an early indicator of increasing risk of degradation, especially when the mode of Pacific Ocean behaviour returns to conditions that appear to make ‘big wets’ unlikely (i.e. positive IPO condition). Historical evidence indicates that where conservative stocking policies have been adopted, or when rapid reduction in stock numbers occurred in response to the onset of drought, degradation appears to have been minimal.

Other more specific causes of degradation were:

1. over-utilisation in pre-drought period resulting in loss of desirable perennial species as evidenced by the rapid collapse in stock numbers with the onset of drought;
2. extreme utilisation in first years of drought by retained stock causing the loss of perennial species and cover, and exacerbation of the effects of drought in subsequent years;
3. retention of stock through a long drought period, accumulating damage to the resource and delaying recovery; and
4. retention of high stock numbers at the breaking of the drought period that resulted in over-utilisation of recovering vegetation.

1. Conclusion

Although the IPO is not yet fully understood, its behaviour appears to have had a strong effect on historical degradation and recovery episodes. Paradoxically, wet periods, associated with negative IPO, have contributed to degradation episodes by: (a) raising expectations of the land’s carrying capacity which proved too high to be sustained through subsequent dry and drought periods; and (b) providing the conditions for woody weed establishment which, in the absence of fire, have caused the loss of desirable perennial grasses and reduction in carrying capacity. However, the wet periods have also provided the opportunity for some recovery of vegetation following historical degradation episodes through regeneration of desirable perennial species. Thus this aspect of the Pacific Ocean behaviour (La Niña and negative IPO conditions) seems to be a very important component of the resilience of eastern Australia’s rangelands. The uncertain impact of future global warming on this aspect of Pacific Ocean behaviour indicates that we should not take this climatic component of resource resilience for granted.

The funding from AGO and CVAP (LWRRDC) is very gratefully acknowledged.

References

Table 1. Regional degradation episodes in Australia’s rangelands.