How pastoralism changes savanna vegetation: impact of old pastoral settlements on plant diversity and abundance in south-western Kenya
For centuries, pastoralists have influenced savanna ecology through their construction of settlements, traditional movement patterns in search of forage, water and safety for their livestock. Construction of settlements initiates localised changes in the vegetation due to clearance of vegetation at construction and collection of construction materials. During the occupation period a lot of dung is deposited in and around settlements. When pastoral families abandon settlements and move away, they leave behind mud huts and livestock corrals surrounded by fences. These sites become nutrient-rich patches supporting a different abundance and diversity of plants and large mammals compared to the rest of the savanna. This study aimed to broaden our understanding of how pastoral land use influences plant diversity in East African savannas. Past work on the effects of settlements has been done in dry places (<600 mm rainfall) with relatively poor soils. To complement and extend these studies, we selected a contrasting site with high rainfall and rich soils in the Mara ecosystem of south-western Kenya. We sampled the occurrence of different plant species along transects radiating from 28 settlements abandoned by pastoralists and ranging in age from 2 months to 48 years on two different soil types. Mean plant species richness and abundance peaked at intermediate distances, 12.5 and 25 m from the edges of settlements. We recorded a total of 210 plant species during the study, 65 of which occurred only within the impact ring of the abandoned settlements. The effects of settlements on plants were stronger on shallow sandy soils than on deep clay soils. Our findings show that abandoned settlements were key sites for regeneration and replenishment of shrubs harvested by the Maasai, but support few other plants of biodiversity value. These unique habitat patches in the savanna ecosystem are under threat as pastoral Maasai become increasingly sedentary and as wildlife conservancies are established, thus reducing the number of abandoned settlements. In essence, the Maasai, by changing their traditional lifestyle, will reduce these nutrient hotspots and hence landscape heterogeneity and biodiversity.