The Functional Response of Kangaroos, Sheep and Rabbits in an Arid Grazing System

Abstract

The functional responses of red kangaroos, merino sheep and rabbits were compared from an intensive grazing trial in the zone of western New South Wales, Australia. Animals grazed a chenopod shrubland composed of low halophytic bushes of Maireana pyramidata interspered with a herb layer of annual forbs and grasses. The maximum food intakes of rabbits, kangaroos and sheep during the trial were 68, 62 and 61 g kg-0.75 day-1, respectively. These intakes are similar despite the 30% difference in basal metabolic rates between eutherians and marsupials. All species avoided Maireana but when food was scarce sheep switched to it more strongly than did the other herbivore species. Sheep consumed 20% of their intake as Maireana at a pasture biomass of 250 kg ha-1 compared to only 10% by the other 2 spp. at this biomass. Maireana reached 20% of the intake of rabbits and kangaroos at a pasture biomass of 100 kg ha-1. Harvesting efficiences (i.e. the ability to maintain food intake at low vegetation biomasses) of kangaroos, sheep and rabbits were considerably higher than that of sheep grazing sown pastures in higher rainfall areas. The 3 herbivore species appeared equally capable of reducing the pasture to an ungrazable residuum of less than 20 kg ha-1. The relationship between food intake and food availability for each species of herbivore was an asymptotic function, suggesting that this, rather than a linear function, is the typical pattern for most mammalian herbivores.