THE EFFECT OF DROUGHT ON THE LARGE MAMMAL POPULATIONS OF ZAMBEZI RIVERINE WOODLANDS

Large mammals were counted in Zambezi alluvial woodlands and on the adjacent ecotone in the north of Mana Pools National Park, Zimbabwe, from April 1981 until October 1989. Densities of all large herbivores on the alluvium in the daytime increased during the dry season. Year-to-year variation was studied by comparing end-of-dry-season densities. Flooding of the Zambezi River, controlled by Kariba Dam 100 km upstream, ceased in 1981 and the densities of all species on the alluvium declined in 1982. Rainfall was low in 1982 and very low in 1983 and 1984. The density of buffalo in the study area declined, owing to high mortality and a change in habitat utilization (buffalo herds spent more time in the south of the park). Densities of other species were high during the drought, but the densities of waterbuck, kudu, eland, bushbuck and warthog declined in 1985. Zebra density declined on the ecotone, but not on the alluvium. The densities of eland and kudu increased in 1986 and 1988, respectively, but the densities of the other species were still low in 1989. Impala density on the alluvium in October was related to rainfall during the preceding wet season and was high in drought years. Elephant density declined after a management cull, Rhinoceros density decreased by > 90% during the 1985/6 wet season, as a result of poaching and captures. The long-term decline in the densities of grazers was probably due primarily to the cessation of flooding by the Zambezi River, rather than to low local rainfall in some years. During the drought, the large grazers (e.g. buffalo) died before the smaller grazers (e.g. warthog). Waterbuck density declined less than the density of other grazers, because waterbuck could cross to vegetated sandbanks in the Zambezi River.