Ecoscience: Ecosystem Stability in the Serengeti

In this, the third of four columns on the Serengeti ecosystem, the authors examine factors affecting regional ecosystem stability.

| May/June 1985

  • ecosystem stability - Serengeti dependencies flow chart
    Note: Minus signs indicate thjat an increase in the element in the preceding oval leads to a decrease in the element in the following oval. Plus signs show where an increase leads to an increase.
  • ecosystem stability - Anne and Paul Ehrlich
    Ecologists Anne and Paul Ehrlich consider the factors that govern ecosystem stability.

  • ecosystem stability - Serengeti dependencies flow chart
  • ecosystem stability - Anne and Paul Ehrlich

In The Serengeti Ecosystem and Predators of the Serengeti, we examined some of the intricacies of the complex Serengeti ecosystem. But what can be said of the region’s ecosystem stability? If it's disturbed, does it return to its previous state? Two large-scale disruptions of the system have been observed historically. The first was a great epidemic of rinderpest, a ruminant-attacking viral disease that's native to the steppes of Asia. The origin of the epidemic may have been viruses introduced with cattle brought to Africa by the British from Russia in 1884, during their unsuccessful attempt to relieve General "Chinese" Gordon at the battle of Khartoum. Or it may have been introduced around 1889 with zebu cattle brought from India to Abyssinia to feed Italian troops.

Whatever its origin, the disease first attacked the cattle of the Masai and other herding tribes. This led to disastrous famines between the 1890's and 1920, in which at least two-thirds of the Masai perished. By 1890 the disease had moved into native ruminants, and the buffalo, wildebeests, and giraffes were disappearing.

The decimation of the herbivores caused the predators to starve, and some lions switched to eating people. The man-eating lions of Tsavo became famous in 1898, and in 1920 there was an outbreak in Uganda in which a single lion was reported to have devoured 84 people.

The appearance of the man-eaters led to the abandonment of land by cultivators, and this, combined with the disappearance of the herders and the native grazing and browsing animals, opened the door to a reinvasion of plains areas by woodland flora. Then, by 1910 or so, the wild ruminants began to become resistant to the disease, and the resultant increased population of grazers nourished tsetse flies, which expanded their range into the new brushy areas. The sleeping sickness carried by the tsetse fly further reduced the region's human population.

In the 1930's, brush-control programs gradually brought the tsetse flies under control, and vaccination began to reduce the impact of rinderpest on the cattle population. The native ruminants' resistance to the rinderpest virus slowly became stronger until the disease killed only yearlings that had not yet acquired immunity and then, finally, it disappeared altogether in the early 1960's.

The results were dramatic. The survival of wildebeest yearlings increased from 25% to 50% and the total population of these ruminants shot up from a quarter million in 1961 to a half million in 1967. Furthermore, buffalo increased from 30,000 to 50,000 during the same period. Interestingly, the nonruminant zebras, immune to rinderpest, showed no such population changes.

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