Dynamics of Large Herbivores in Deserts: Kangaroos and Caribou

Abstract

The dynamics of desert plant-herbivore systems are similar whether the deserts are hot or cold, an assertion we support with information on the dynamics of red kangaroos and caribou. The essence of these systems is structural simplicity combined with high year-to-year variation in weather. We argue that such conditions simplify study of the mechanisms underlying changes in rate of increase of herbivores. Except where the herbivore is intrinsically regulated by spacing behaviour, the systems are best summarised by the functional and numerical responses of the herbivores to available forage, coupled with the growth response of the plants. A graphing of rate of increase against density provides little insight even though these systems are density-dependent in the formal sense. We show that long-term aperiodic fluctuations characteristic of such systems are, at least for kangaroos, a mathematical consequence of unpredictable short-term fluctuations in weather. Even when the weather has no time trend there will be marked trends in numbers of herbivores, either up or down, on a time scale of one or two decades. Such trends are intrinsic to the system and do not necessarily reflect special and persistent causes. Similar fluctuations of caribou numbers may be, at least in part, of similar origin.