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Serengeti wildebeest population dynamics : regulation, limitation and implications for harvesting Mduma, Simon Abia Raphael


Principal challenges in managing large mammals include methods for estimating abundance, understanding the interaction between environmental conditions, density, and reproductive success and survival, and estimation and regulation of harvest. Here I present the population dynamics of the Serengeti wildebeest (Connochaetes taurinus) as a case study in exploiting large mammals. The Serengeti wildebeest population increased six fold between 1960-1977 from 0.25 to 1.4 million; thereafter it remained constant at 1.3 million. Previous studies have suggested that density dependent mortality regulates the population through food shortage during the dry season. However, little was learned about which stage(s) in the life cycle were involved in the regulation process, or of the extent of human-induced mortality. I present recent demographic data on wildebeest reproduction, recruitment, and adult mortality. I combine my results with those of previous studies to construct a life table, and use key-factor analysis to investigate the influence of density dependence, predation and food limitation on the wildebeest population dynamics. I use poaching data to estimate illegal harvesting, which has been presumed to be extensive. I develop a population dynamics model to test for the consistency of the estimated demographic components. Life table analysis suggests that the Serengeti wildebeest population is regulated through density dependent adult mortality. Fertility loss acted in a density dependent fashion but was weak. Available evidence supports the food limitation hypothesis through dry season calf and adult mortality. Dry season calf mortality was the "key factor" stage causing population fluctuations. While neonatal mortality appeared to contribute the greatest to annual reduction, its pattern was not well understood. Limitation by predators and human offtake appeared to play a minor role. Synthesis of available data suggests that the Serengeti wildebeest population has reached its environmental carrying capacity under the present rainfall regime. Changes in rainfall greatly influence population size, and the reproductive surplus available for harvesting is almost directly related to the amount of rainfall in the dry season of the year. Population dynamics models indicate that in wet years additional animals could be taken in a controlled harvesting program, but elimination of the illegal harvest is essential.

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