HERBIVORE POPULATION CRASHES AND WOODLAND STRUCTURE IN EAST-AFRICA

1. Between 1985 and 1991, bush encroachment was serious in Lake Manyar a National Park, northern Tanzania. Shrub cover increased by c. 20%. T he increase was independent of initial (1985) shrub cover. 2. Since 19 87 there has been a steep decline in the number of African elephant in the Park due to poaching. Elephant density decreased from about 6 km- 2 to about 1 km-2. However, shrub establishment, as determined from co unting tree-rings, preceded poaching. 3. Shrub establishment in two ar eas of the Park coincided with anthrax epidemics that drastically redu ced the impala population. In the northern section of the Park this wa s in 1984, in the southern section in 1977. 4. The diameter increment of Acacia tortilis was 5-24 mm year-1, irrespective of the size of the trees. Size measurements indicated an even-aged stand of Acacia estab lished in 1961, which coincided with another anthrax outbreak among im pala. 5. Size measurements of old Acacia tortilis trees indicated anot her even-aged stand established at the end of the 1880s. The size of t rees of this stand was not significantly different from a stand in Tar angire National Park, nor from a stand near Ndutu (on the boundary bet ween Serengeti National Park and Ngorongoro Conservation Area), also n orthern Tanzania. All three stands are likely to have originated from bush establishment caused by the rinderpest pandemic at the end of the 1880s. 6. It is suggested that seedling establishment of Acacia is a rare event under the prevailing conditions of high browsing pressures by ungulates such as impala. Punctuated disturbances by epidemics amon g these ungulates create narrow windows for seedling establishment, wh ich may explain the occurrence of even-aged stands.