Relationships between biomass and plant species richness in arid-zone grazing lands.
The relationship between biomass and species richness in temperate vegetation has been described as a hump-back response model. The hump-back model predicts that herbaceous species richness is highest at an intermediate level of biomass. However, this has not been investigated in arid-zone grazing lands. We tested the hump-back prediction in an arid tropical grazing region in northern Kenya where a seasonal grazing exclosure system is practised. We compared vegetation structure, species richness and composition on an open range and exclosures at 5 sites to elucidate the potential mechanisms behind variation in species richness. More biomass was accumulated within seasonal exclosures than in continuously grazed areas. Species richness in exclosure plots varied from 5.3 to 8.3 species m-2, while that in open plots varied from 5.1 to 7.5 species m-2. A pair-wise test showed no difference in 2 of the 5 sites with respect to both total and herbaceous species richness. The primary floristic gradient illuminated through ordination was related to biomass, while the secondary gradient was related to species richness. The exclosure plots had more abundant species, especially compared with open plots, which had more rare and occasional species. A total of 37 herbaceous species was recorded; 22% were indifferent to grazing, 30% grazing intolerant and 48% promoted by grazing. The relationship between biomass and herbaceous species richness showed (i) no trend within the exclosures (maximum biomass 800 g m-2); (ii) a positive trend in the open grazing land (maximum of 500 g m-2); and a hump-back pattern when (i) and (ii) were analysed together. Optimum richness corresponded to a biomass level of 400-500 g m-2. Species richness declined with increase in age of exclosures. We confirmed that species richness will decline when biomass exceeds 500 g m-2, as predicted by the hump-back model, even in arid grazing lands. Seasonal grazing exclosures may increase species richness to a certain level, but the decline in species richness with age of exclosures indicates that long-term exclusion of grazing may not necessarily increase species richness in arid-zone grazing lands.