Relationship between grass species richness and ecosystem stability in Kruger National Park, South Africa

The relationship between grass species richness and ecosystem stability was investigated in the Kruger National Park. A total of 135 489 individual grasses were identified from 189 sites spread across 19 485 km(2) of savanna biome, making this one of the largest studies of its kind. At each site, grass percentage abundance and standing crop were measured at one year intervals to provide an index of ecosystem function. A severe drought that affected the region between 1991 and 1993 provided a convenient perturbation. At the height of the drought, mean species richness declined by 12.7% while standing crop declined by 38.1%, from 3199 to 1979 kg ha(-1). Percentage grass abundance declined to 87.5% of its pre-drought value. After the drought had passed species richness, standing crop and percentage abundance recovered to 92.1%, 113.8% and 92.8% of their pre-perturbation values, respectively. Statistical analysis of these responses revealed that grass assemblages of low species richness were more resistant to drought than assemblages of high species richness. Species-poor sites also showed better recovery from perturbation after the drought had passed. These findings suggest that ecosystem stability may be negatively related to grass species richness in South African savanna grasslands.