Macrophyte species composition,distribution and diversity in relation to some physicochemical factors in the littoral zone of Lake Ziway,Ethiopia

The species composition, distribution and diversity of macrophytes in Lake Ziway were determined at the end of dry and wet seasons along nine littoral sites during 2010 using a belt transect method. Some physicochemical parameters were also measured. Fourteen macrophyte species were identified with low species diversity for the lake (H' value of 1.805). The macrophyte species composition of the lake has undergone many changes during the last few decades. Arundo donax, which was never reported earlier, attained the highest relative density (30.7%) and frequency (30.5%) in this study, followed by Echinochloa colona, Potamogeton schweinfurthii, Cyperus articulatus, Typha latifolia and Cyperus papyrus. The average densities of A. donax, C. articulatus, E. colona and Pistia stratiotes were positively correlated with nutrients, whereas the density of the other species had negative correlation with physicochemical variables (CCA). Generally, the present macrophyte composition and abundance indicates healthy ecological condition of the lake. But if nutrient levels continue to increase and water levels continue to decline, we expect further changes in macrophyte composition and especially a shift towards invasive floating species.     

Diversity, regeneration status and socio-economic importance of the vegetation in the islands of Lake Ziway, south-central Ethiopia

This study was conducted in the islands of Lake Ziway located in the main Ethiopian Rift Valley in south-central Ethiopia. Its main objective was to investigate the diversity, regeneration status, socio-economic importance and the factors that cause degradation of the plant resources in the islands. A total of 73 relevés, measuring 20×20 m, were established along line transects laid from the central part of each island to the lake edge in eight aspects. Herbaceous species were sampled by using a 2×2 m sub-relevé laid within each relevé. In each relevé, the cover/abundance of each species was estimated, woody species were counted, diameter at breast height and height of trees and shrubs and the environmental variables altitude, slope, aspect and position were measured. Participatory Rural Appraisal was employed to generate the socio-economic information. A total of 141 species belonging to 113 genera and 55 families were identified, of which 18.4% were trees, 21.3% trees/shrubs, 12.1% shrubs, 3.5% lianas, 4.3% herbaceous climbers and 40.4% herbs. The overall Shannon diversity and evenness of woody species in the islands were 2.60 and 0.62, respectively. The number of species on the islands correlated with island's size, habitat diversity and degree of human disturbance. The similarity in species composition between the islands was low, indicating that each island has its own unique flora. Nine plant communities were recognized and described. The total density and basal area of the vegetation were 2979 individuals ha⁻¹ and 191 m² ha⁻¹, respectively. The importance value index and the diameter and height class distribution revealed that some species are threatened and need the highest priority for conservation. The socio-economic survey showed that the island communities are highly dependent on the natural vegetation for various purposes. On the other hand, the islanders have maintained the soil and vegetation resources for centuries through their indigenous resource management systems and practices. Human population growth coupled with farmland expansion, soil erosion, free grazing and cutting trees for various purposes are the major threats to the vegetation resources in the islands. The implications of the results are discussed and recommendations are suggested for conservation, management and sustainable utilization of the fragile island ecosystems.