Population Dynamics of Red Kangaroos (Macropus rufus) in Relation to Rainfall in the South Australian Pastoral Zone

Abstract

1. Populations of red kangaroos (Macropus rufus) in the 282,000 km2 pastoral zone of South Australia were assessed by aerial surveys conducted each year in winter for the period 1978-88, inclusive. The results of these surveys are used in a kangaroo management programme, the basis of which is the control of kangaroo numbers through regulated culling. 2. Rainfall in the pastoral zone is generally low and unreliable. Comparatively good seasons, with rainfall near to, or above average prevailed from 1978 to 1981. During this period, red kangaroo numbers increased throughout the pastoral zone. A drought occurred between 1982 and 1983. Associated with this drought was a dramatic decline in red kangaroo numbers; to the lowest level since 1978. Following the drought, the populations recovered, with steady increases in numbers from 1984 onwards. 3. The changes in red kangaroo numbers, in the form of the yearly exponential rate of population increase (r) were found to correlate best with intervals of rainfall at short time-lags from the second of any two successive aerial surveys used to determine r. 4. The numerical response of the red kangaroo populations to rainfall was investigated. Asymptotic models in the form of the negative exponential Mitscherlich equation were fitted to data for the period 1978-84. The rainfall input into these models was that for the calendar summer-autumn period between successive aerial surveys (January-June). 5. The response to summer-autumn rainfall in the Western region of the pastoral zone was different to that in the Central & Eastern region. Separate numerical response models were derived for each of these two regions. These models implied positive rates of exponential population growth (r>O) when summer-autumn rainfall exceeded 74 mm in the Western region and 107 mm in the Central & Eastern region. They also implied maximum levels of r of 0.92 in the Western region and 0.38 in the Central & Eastern region. 6. Predictions by the numerical response models of the changes in red kangaroo numbers in relation to summer-autumn rainfall for the period 1984-88 proved to be rather poor, particularly for the years immediately following the drought. It is suggested that the reason for this was that the age structures and sex ratios of the post-drought populations were different from those of the populations used to develop the models. Drought is thought both to truncate the age structures of kangaroo populations and to bias the sex ratios towards the females. A result of this could be an increase in the numerical response to rainfall above that of a population near to the carrying capacity of its environment and with a near-stable age structure.