Herbivore population crashes and woodland structure in East Africa

1. Between 1985 and 1991, bush encroachment was serious in Lake Manyara National Park, northern Tanzania. Shrub cover increased by c. 20%. The increase was independent of initial (1985) shrub cover.2. Since 1987 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 counting tree-rings, preceded poaching.3. Shrub establishment in two areas of the Park coincided with anthrax epidemics that drastically reduced the impala population. In the northern section of the Park this was 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 established in 1961, which coincided with another anthrax outbreak among impala.5. Size measurements of old Acacia tortilis trees indicated another even-aged stand established at the end of the 1880s. The size of trees of this stand was not significantly different from a stand in Tarangire National Park, nor from a stand near Ndutu (on the boundary between Serengeti National Park and Ngorongoro Conservation Area), also northern 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 among these ungulates create narrow windows for seedling establishment, which may explain the occurrence of even-aged stands.