Oscillations in large mammal populations: are they related to predation or rainfall?

Cyclic population dynamics is relatively common among populations of small mammals in high latitudes but is not yet established among African savanna ungulates. However, oscillations may be expected in large mammal populations subject to quasi-periodic oscillations in regional rainfall. We evaluated evidence for environmentally entrained oscillations in a large-mammal predator-prey system in Kruger National Park (KNP), South Africa, where rainfall exhibits quasi-periodic oscillations. The evaluation is based on analysis of comparative changes in the abundance of twelve ungulate species throughout South Africa's KNP using population counts over the period 1965-1996. We present evidence suggesting that (i) twelve ungulate populations display cyclic variability with half-periods ranging between 10 and 18 years, (ii) this variability was associated with lagged rainfall between 3 and 10 years back in the past for different ungulate species, and (iii) the ungulate species respond in contrasting ways to rainfall, with some reaching highest abundance during periods of low rainfall and others under conditions of high rainfall. These findings are not consistent with the response pattern we would expect if the population oscillations were driven directly by the rainfall influence on food availability. Instead they seem to be an outcome of predator-prey interactions, which are entrained by the effect of rainfall on habitat conditions affecting the relative susceptibility of the different ungulate species to predation.