Trends in Kangaroo Numbers in Western New South Wales and their relation to Rainfall

Abstract

Annual aerial counts of kangaroos within randomly selected blocks of the western plains of New South Wales showed that the numbers of kangaroos doubled between 1975-76 and 1982, and that the widespread drought of 1982 reduced the populations on average by 43%. Localized reductions of similar magnitude occurred after regional droughts in 1977 and 1980 within parts of the monitored area. The observed trends in kangaroo numbers, with eastern and western blocks treated separately, were correlated with annual rainfall with a time lag of 6 months in the response. The relationships show that kangaroos reach their maximum rate of increase following rainfall 100 mm above the annual average in the east and approximately 50 mm above the annual average in the west. At average annual rainfall kangaroos increase at 25% (greys) and 35% (reds) per annum in the east and at 25% (greys) and 30% (reds) per annum in the west. Rate of increase is zero when rainfall is 100 mm below average in the east and approximately 60 mm below average in the west. When rainfall is below these values, kangaroo numbers decline.