Grazing as an Optimization Process: Grass-Ungulate Relationships in the Serengeti

Abstract

A substantial literature is reviewed which indicates that compensatory growth upon tissue damage by herbivory is a major component of plant adaptation to herbivores. Experiments in Tanzania's Serengeti National Park showed that net above-ground primary productivity of grasslands was strongly regulated by grazing intensity in wet-season concentration areas of the large ungulate fauna. Moderate grazing stimulated productivity up to twice the levels in ungrazed control plots, depending upon soil moisture availability. Productivity was maintained at control values even under very intense grazing, suggesting that conventional definitions of overgrazing may be inapplicable to these native plant-herbivore systems. A laboratory clipping experiment with a sedge abundant in one of the most intensely utilized regions resulted in a maximum net above-ground productivity of 11.6 g/m2 · day when clipped daily at a height of 4 cm. Few plant species have been reported with the ability to maintain a significant level of productivity under such intense clipping. This suggests that the high grazing load of the Serengeti ecosystem has constituted strong selection on the plants for compensatory growth upon defoliation.