Grazing depletes forb species diversity in the mesic grasslands of KwaZulu-Natal, South Africa

Abstract

Forbs constitute over 80% of the species richness of mesic grassland but their response to grazing is largely unknown. The influence of grazing on the forb composition, richness and diversity of two species-rich grasslands in the coastal hinterland and midlands of KwaZulu-Natal, South Africa was examined in plots subject to grazing intensities ranging from lightly grazed nature reserves through to heavily stocked communal areas and commercial ranches. Grazing altered forb species composition and considerably reduced forb richness and Shannon diversity in both grasslands. Mean forb richness declined by 64–84%, from 50–55 species per 100 m2 in lightly grazed areas to fewer than 10–20 species in the most heavily grazed plots. Many long-lived perennial indigenous forbs were extirpated or markedly reduced in abundance by overgrazing and replaced by a few, mostly exotic, hardy species, suggesting these grasslands did not evolve under heavy grazing. Indigenous forb richness was non-linearly related to, and thus not reliably predicted by, range condition score and the abundance of two dominant decreaser and increaser grasses, but the poorest grassland was the most depauperate and good condition grassland was invariably diverse. Judicious grazing management with light stocking is thus required to maintain the diversity of mesic grassland.