Effects of Grazing Pressure on Plant Species Composition and Diversity in the Semi-Arid Rangelands of Mbulu District, Tanzania

The study was carried out in the semi-arid rangelands of Mbulu District that have been used for livestock grazing since the 18th C. The study assessed the effects grazing pressure on plant species composition, diversity and distribution. It was found that, grazing pressure negatively affected the species composition, diversity and resulted into uneven distribution of the common semi-arid plant species in the rangelands and land degradation and erosion were the consequences. However, similarities in plant species composition among the study sites ranged between 0 and 0.566 with the highest coefficient of similarity between Dongobesh and Maretadu since they were in close proximity, hence suffered similar level of effects of grazing pressure. Species diversity ranged between 4.067 0.217 (from Dongobesh) with moderate grazing pressure and 1.07 0.0586 (from Tlawi) with the maximum level of grazing pressure. DCA ordination showed the highest effects of grazing pressure on plant species distribution where as at the first two axes (axis1 and axis 2) 64.7% counted for the samples of species variation. DCA resulted into the three clusters showing the different levels of grazing pressure (the highest intensity of grazing, moderate grazing and the lowest) with increasing gradient in an ordination space. Plant species such as Themeda triandra, Hyperhenia filipendula and Panicum maximum were absent in overgrazed areas and survived in areas with the lowest level of grazed due to being palatable to livestock. On the other hand, the better performing vegetation which was essentially composed of Hypoestes forskalii, Tragus beternonianus, Ageratum conyzoides and Solanum incanum existed in overgrazed areas as they were not palatable to domestic livestock.