Assessment of woody plants species diversity in Rusinga island, Homa bay county, Kenya.

Abstract

Land degradation is a major environmental problem as manifested in global decline in woody vegetation and loss of biodiversity. Understanding the dynamics of plant communities is imperative in developing appropriate conservation and management plans for sustainable development. This requires analysis of the plant species composition, diversity, density and cover as a prerequisite for planning and sustainable management of ecosystems for sustainable development. A random sampling design was used to select 150 households from six sub-locations found in Rusinga Island. Household interviews were conducted with the use of a semi-structured questionnaire complemented by four (4) Focus Group Discussions (FGDs) and thirty (30) Key Informant Interviews (KIIs). Ecological data were collected using purposive sampling technique where three hills (Ligongo, Kabade and Wanyama) were selected as reference points to capture the variations in land use gradient from the lowlands through to the hilltops. The slopes of the hills were demarcated into three study zones: lower, middle and upper zone differentiated by the slope gradient and land use. Line transect in combination with sampling plot methods were used during data collection. This involved laying down four-line transects on each hill following the four compass directions, running from the top of the hill to the shores of the lake and cutting across the three zones. Trees were sampled within plots measuring 20 m x20 m (400m2) placed at an interval of 200m, while shrubs and lianas were sampled in sub-plots of 10m by 10m nested in main plots in each of the three study zones along the predetermined line transects. The cover of woody plant species on the three hills at the three different study zones was determined using a line intercept method where a 20m measuring tape was systematically placed along the predetermined transect at an interval of 200m. Any woody plants species intercepted by the tape were identified and intercept distance recorded for determination of cover. The collected data on Shannon’s evenness and diversity index, species richness, number of individuals, density, frequency and cover of woody plants were subjected to one-way analysis of variance (ANOVA) to determine the differences among the three study zones. Results from household interviews show that most (86%) of the respondents had observed changes in vegetation composition and abundance in the study area. The changes were attributed to deforestation, high human population, overgrazing, inadequate rainfall, and soil erosion. The observed changes were reported in the forests/hills (68%), in the entire Island (15.3%) and in the homesteads (2.7 %). The community proposed tree planting, protection of existing trees, use of alternative sources of fuel, increased awareness creation on environmental conservation and controlled livestock grazing as the best strategies to reduce vegetation degradation. In total, 63 woody plant species from 51 genera and 32 plant families were encountered in the study area, 42 (66.7%) being trees, 20 (31.7%) shrubs and 1(1.6%) liana. The upper zones had significantly higher woody species diversity, species richness, and abundance than the middle and lower zones. The lower zones of the hills had lower plant species diversity (1.87) compared to the middle zones (2.23) and upper zones (2.60). This was attributed to the various activities carried out in the lower zones particularly clearing of the woodland to give way for crop production and settlements. A statistically significant difference was observed in both the density of trees and shrubs (P<0.001) among the three study zones. However, tree species recorded the lowest density in comparison to shrubs. The dominance of shrubs was attributed to intensive and selective logging of trees for different uses. Majority of the people in Rusinga Island are aware of the decline in woody vegetation resources in their area and the possible driving factors affecting the vegetation dynamics. For woody plant species to be effectively conserved and managed, the local people must be actively involved in the management and rehabilitation efforts. The lower species diversity, abundance and species richness in the lowlands and around settlements calls for promotion of agroforestry practices through planting of multipurpose tree species for enhanced ecosystem services. In addition, awareness creation on the consequences of deforestation and the significance of environmental conservation is imperative for sustainable environmental management.