The Population Dynamics of Red and Western Grey Kangaroos in Arid New South Wales, Australia. II. The Numerical Response Function

Abstract

Two numerical response models were fitted to the rate of increase and lagged rainfall data for red [Macropus rufus] and western gray [M. fuliginosus] kangaroos inside the Park: these were the Michaelis-Menten and the Ramp. Both models produced similar estimates of maximum rate of increase (rm) for each species. For red kangaroos these were 0.53 per annum [p.a.] (Michaelis-Menten) and 0.58 p.a. (Ramp); and that for western gray kangaroos, 0.60 p.a. (Michaelis-Menten) and 0.52 p.a. (Ramp). Both models produced markedly different estimates of maximum rate of decrease (a) for each species. For red kangaroos these were -1.50 p.a. (Michaelis-Menten) and -0.30 p.a. (Ramp); and that for western gray kangaroos, -1.50 p.a. (Michaelis-Menten) and -0.48 p.a. (Ramp). The level of rainfall were r equals zero in both models coincides with the annual average rainfall isohyets that mark the western distribution and abundance patterns found for both species in eastern Australia. The models indicate that the rm of a population may be influenced by its current age distribution and sex ratio. The demographic efficiency and rm of both species were not significantly different within the variability of the data. A clear examination of the independent effects of kangaroo density and rainfall on rates of increase could not be made from this study.