Simulating kangaroo farming: extending GRASP to include kangaroo population density simulation

Abstract

It has been argued that converting some production in semi-arid rangelands from beef and wool to kangaroo commodities would have beneficial ecological and environmental consequences (Wilson and Edwards, 2008). The kangaroos ability to react to changes in weather could be used to mitigate the affect of droughts and post-drought recovery in marginal areas. Income from kangaroo harvesting during and after a drought could financially counter restocking costs of domestic animals, providing an incentive for landholders to consider diversifying into kangaroos.A model was constructed to explore changes in kangaroo levels over time for a landholder farming kangaroos. The model developed uses a physiological structured population model (PSPM) in conjunction with GRASP to assess kangaroo population densities and harvest quantities. GRASP simulates the effect of weather, soil condition, and stocking rates of either cattle or sheep and the associated production of beef or wool. It does this diurnally through estimating the total standing dry matter (tsdm). GRASP has been validated for large sections of semi-arid rangelands in New South Wales and Queensland (Littleboy and McKeon, 2005).Historical and simulated weather was used in simulations to analyse the model. The model was run using the previously included species of cattle, sheep as well as kangaroos. Comparisons were made between forage utilisation of the different species used for each commodity produced, beef, kangaroo meat, and wool. The comparisons were made using dry sheep equivalents (dse). The results in this study are in agreement with the most recent analysis on kangaroo dse (Munn et al., 2009). It is noted that the current kangaroo dse figure, with which this study concurs, is approximately half the figure that landholders often use (Grigg, 2002;Munn et al., 2009). The simulated commodity production resulted in greater production from cattle in semi-arid rangelands. Implying that any financial analysis may favour cattle, unless the commodities from sheep and kangaroo have a much larger per unit value. This study highlights the impact of high juvenile mortality of kangaroos (Dawson, 1995) and endeavours to make harvesting a commercially viable option for landholders.