Patterns of Use of a Flood Plain by Zambian Mammals

Abstract

Every year the Kafue River, one of the main tributaries of the Zambezi River in Zambia, floods a broad level area, the Kafue Flats, to a depth of up to 5 m for several months. The flats are 235 km long and up to 40 km wide. The life of the flats is conditioned primarily by the alternating rainy and dry seasons and by the floods. About 80 cm of rain falls from November to April. The Kafue rises slowly during the rains, is highest in May, and falls during the latter part of the dry season. Vegetation is composed primarily of grasses, especially Oryza barthii. Because of the abundant water the primary productivity of the flats is much greater than that of the surrounding woodlands. Secondary productivity also is high. Every year there is an alternation of aquatic and terrestrial faunas. During the floods fish move onto the flats from the Kafue, and most spawning takes place there. Terrestrial species are driven off, but as the floods recede they reoccupy the flood plain and use what is by far the best grazing in the region. Large mammals find shelter in tall stands of grass, small mammals in the thick mat of vegetation that covers much of the ground or in the deeply cracked soil. There is a gradient in use of the flood plain: some species (hippopotamus, otter) always stay near the river at low water, others (lechwe, zebra, wildebeest) go for varying distances onto it, and more than half of the mammals (kudu, squirrel, vervet, aardvark) go onto it little if at all. Failure to use the flood plain seems to be due to absence of suitable habitats or food, rather than exclusion by the floods. The most abundant large mammal on the flood plain is the lechwe, the only terrestrial ungulate that sometimes feeds in the floodwaters. Several shrews and mice, especially Mastomys natalensis, are common on the food plain and breed there during the rains. During the floods they leave the flood plain or take refuge on natural levees along the Kafue. Large populations of water birds are common. Crocodile and monitor live near the water's edge and move in and out with the floods like the hippopotamus. Some snakes are common on the flats, but turtles and frogs are not. Although ants and termites are abundant in the surrounding region, they are largely excluded from the flats by the floods.