Stability in a multi-species assemblage of large herbivores in East Africa: an alternative interpretation

Prins and Douglas-Hamilton (1990) analyzed data based on nine census counts of large herbivore species in Lake Manyara National Park, northern Tanzania, over the period 1959-1984. Their major conclusion was that even if individual species-showed large fluctuations in numbers, the different species compensated the fluctuations of the other species in a way resulting in a constancy of total herbivore biomass, constancy of plant biomass consumption, and "overall stability of the system under natural conditions". The authors believed that they had found a support for this view by calculating the "stability index" based on correlations between numbers of large herbivores. In this paper I show that Prins and Douglas-Hamilton's calculation of the stability index was not justified. Grazing and browsing pressure by large herbivores in Lake Manyara National Park seems to be remarkably constant. However, available information does not allow any rigorous conclusions about the stability of this community. We need more data from other systems dominated by large herbivores to be able to make comparisons and to be able to say which systems are more stable and in what sense.