The Gnu's World by Estes Richard D.;

Author:Estes, Richard D.;
Language: eng
Format: epub
Publisher: University of California Press

Top-Down and Bottom-Up Population Control

Species whose abundance is limited by predation are said to be under top-down control. Probably most of the small and cover-dependent ungulates fall into this category. Species whose abundance is limited not by predation but by food supply are said to be subject to bottom-up control. These two different types of population control act simultaneously on the diverse Serengeti herbivore community.3,4,11 Wildebeest, topi, buffalo, giraffe, hippo, rhino, and elephant are subject to bottom-up control. In the gnu’s case, predation by about eight thousand lions and hyenas has hardly any impact on the immense migratory population. But predation may limit the relatively small resident populations.12 One would suppose that a quarter million zebras would be subject only to bottom-up regulation, but their failure to increase in over four decades has caused Serengeti ecologists to suspect top-down control by predation,13 possibly on foals.8,14 If so, pack-hunting hyenas would be the prime suspects,15 because the zebra’s group defense by the family stallion and his mares must be an effective antipredator strategy—how could it persist otherwise?6 Absent observational evidence of widespread foal predation, this explanation is suspect.

However, Sinclair16 offers a more plausible, though still circumstantial, explanation for top-down control of the zebra. He points out that predation accounts for 60–74 percent of annual adult zebra mortality, compared to only 25–30 percent of wildebeest and buffalo mortality. Moreover, half the zebra population does not migrate with the wildebeest but remains scattered throughout the woodlands, where they are in effect resident prey and are killed by lions three times more frequently than would be predicted. Thus zebra are more available to predators throughout the year than are migratory wildebeest.

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