THE STATUS OF BUSH ENCROACHMENT AND ITS IMPACT ON GRASS BIODIVERSITY AND ECOSYSTEM SUSTAINABILITY: THECASE OF NECH SAR NATIONAL PARK (SNNPR, ETHIOPIACASE OF NECH SAR NATIONAL PARK (SNNPR, ETHIOPIA

Abstract

This study was conducted in Nech Sar National Park , Gama Gofa zone, Southern Ethiopia with the objective of determining the major encroaching woody plant species in the park, their seed distribution pattern, their impact on grass biodiversity as well as the possible causes of encroachment in the area. Systematic sampling method was used to collect vegetation data. A total of 80 quadrats each with 400 m 2 (20 m X 20 m) were established along four line transects for woody species data collection. Grass species data was collected from 1 m 2 sub plots at the 4 corners and center of the 20 main plots. To know the pattern of seed distribution for dominant species soil samples were taken from the center and four corners of 15 selected plots. Data on species cover abundance, presence of a species, height, ' altitude and geographical coordinates were recorded from each quadrat. Density, frequency, dominance and abundance were used to describe vegetation structure. Alpha, Gamma and Beta diversity and also Shannon-Wiener diversity and evenness index were used to assess the levels of diversity. Correlation analysis was conducted to investigate the impact of woody species on grass diversity. A total of 24 woody species belonging to 15 genera and 9 families were recorded in the park. Species such as Cyathula orthacantha, Abutilon figarianum, Dichrostachys cinerea, Barleria acanthoides, Solanum incanum, Acacia oerfota and A. mellifera were found to be the major encroaching species. The plant community analysis revealed the existence of four community types. Density of woody species in the park found to be 4402 ha- 1. Overgrazing, fire suppression, climate change in terms of erratic rainfall and drought are considered to be among the main factors that caused woody plant encroachment. Expansion of bush cover is confirmed to have a negative impact on grass diversity. Since, diversity of grass species negatively correlated with woody density (r=-0:943, p<0.10). it is . believed that bush encroachment has already started to affect sustainability of the park particularly in the form of habitat change. Grazing animalsare assumed to be the primary victims of the current dynamics. Therefore, implementation of holistic approach that involves the whole community and experts to control expansion of bush encroachment and decrease the contribution of over grazing and settlement to bush encroachment are recommended as possible solutions.