Herbaceous layer production and utilization by herbivores under different ecological conditions in an arid savanna of Kenya

Abstract

A three-year study was done to determine herbaceous biomass production and its utilization by herbivores in an arid zone inhabited by a largely nomadic population in northern Kenya. The indicator selected for study was aboveground live standing biomass of grasses, forbs, dwarf shrubs and the total sum biomass of these vegetation categories (total herbaceous layer). Sampling was done along grazing gradients in order to estimate the utilization levels in the arid zone 7 and the semi-arid zone 6. Mean peak standing biomass for grasses, forbs, dwarf shrubs and total herbaceous layer under non-grazed conditions was 184.4, 374.2, 1094.4 and 1504.0 kg/ha in eco-zone 7, respectively. In the zone 6, mean peak standing biomass was 55.3, 98.8, 4259.1 and 4320.1 kg/ha under non-grazed conditions, respectively. Results indicated graminoid removal to be respectively 57.1–99.8% and 24.2–87.2% of mean peak standing aboveground live biomass in the zone 7 and 6. Forb utilization in zone 7 was estimated to be 51.5–99.3%. Mean peak forb standing biomass however, showed a general increase on grazed sites compared to the non-grazed plot in zone 6. The corresponding utilization of the dwarf shrub layer was estimated to be in the range 40.5–80.0% and 76.6–92.3% in zone 7 and 6 respectively. Total herbaceous layer consumption of 39.3–85.3% and 74.0–90% was estimated for zone 7 and 6 respectively. These findings suggest that herbivores exert considerable control over biomass dynamics of the herbaceous layer in this zone and contribute to degradation in heavily utilised areas. Efforts to understand and tackle the desertification problem in this area must therefore take this fact into account.