Modelling the resilience of Australian savanna systems to grazing impacts

Savannas occur across all of northern Australia and are extensively used as rangelands. A recent surge in live cattle exports to Southeast Asia has ca used excessive grazing impacts in some areas, especially near watering poin ts. An important ecological and management question is "how resilient are s avanna ecosystems to grazing disturbances?" Resilience refers to the abilit y of an ecosystem to remain in its current state (resist change) and return to this state (recover) if disturbed. Resilience responses can be measured using field data. These responses can then be modelled to predict the like ly resistance and recovery of savannas to grazing impacts occurring under d ifferent climatic conditions. Two approaches were used to model resilience responses. First, a relatively simple mathematical model based on a sigmoid response function was used. This model proved useful for comparing the rel ative resilience of different savanna ecosystems, but was limited to ecosys tems and conditions for which data were available. Second, a complex proces s model, SAVANNA, was parameterised to simulate the structure and function of Australian savannas. Simulations were run for 50 years at two levels of grazing to evaluate resistance and then for another 50 years with no grazin g to evaluate recovery. These runs predicted that savanna grasslands were m ore resistant to grazing (changed less) than red-loam woodlands, which reco vered relatively slowly from grazing impacts. The SAVANNA model also predic ted that these woodlands would recover slightly slower under the climate ch ange scenario projected for northern Australia. (C) 2001 Elsevier Science L td. All rights reserved.