Demography and population dynamics of mountain nyala (Tragelaphus buxtoni) before its population crash in 1991 in the bale mountains national park, Ethiopia

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International Journal of Development Research

Demography and population dynamics of mountain nyala (Tragelaphus buxtoni) before its population crash in 1991 in the bale mountains national park, Ethiopia

Abstract: 

This study was conducted to assess demography and dynamics of mountain nyala (Tragelaphus buxtoni) in relation to habitats/vegetation, slope, seasons, altitude. The data were collected in 1984 eight years before population crash of T. buxtoni in 1991 in Bale Mountains National Park. Sample total count method was used to survey the population in three sites where the species known to be abundant in the northern part of the Park. The sample total count of mountain nyala numbers were 10,862 (including 634 of unidentified individuals in to age and sex categories) of which 8,679, 1,349 and 834 were observed from Gaysay, Adelle/Amacho and Dinsho/Gojera study sites respectively. The total population estimate of the species including counts outside the three sites were 2400 individuals. The total number of observed among the sites were significantly different (df = 2, F = 4.886, P <0.01), with the highest number of observation was made in Gaysay grassland site (8,679). The overall male to female sex ratio of the species was 1:2.7. Sex ratios among different habitats/vegetation types were significantly different (df = 9, F = 2.112, P < 0.05). Higher proportions of females per male were observed in montane grassland (15.0) followed by Helichrysum herb (13.0) habitat types; and the least was observed in Hagenia woodland (3.0) habitat. The overall mean group size or numbers of the species was 8.33, and mean group sizes among the sites were significantly different (df= 2, F = 4.962, P < 0.001), Thus, the grassland habitat had the biggest group sizes of mountain nyala than the forested habitats. Similarly, group size among different vegetation/habitat types were significantly different (df = 9, F=5.373, P>0.001), with higher group sizes were recorded in Open Grassland (11.52) followed by Helichrisum herb (10.37); while the lowest was recorded in Juniperus Woodland (5.00). The average age composition of the species was dominated by adults with a proportion of adults (41%), sub-adults (26%), juveniles (25%) and calves (8%). High proportion of adults in the present study could indicate higher degree of survival of sub-adults. Age compositions across the study sites were not significantly different except for calves (df = 2, F = 2.191, P<0.05). But, the proportions for all age groups were significantly different across the vegetation/habitat types (df = 9, P<0.05). Calves productivity was 0.16 calves per adult female (or 16 calves per 100 adult females; while juvenile mean productivity was 0.54 (or 54 juveniles per 100 adult females). Productivity of both calves and juveniles were significantly different across months (calves: df = 11, F=7.173, P > 0.001; juveniles: df = 11, F=5.098, P>0.001), with the highest calves and juveniles productivities were recorded in the months of November and March respectively Total number of mountain nyala observed among different altitudes showed no significance differences (df = 26, F = 1.339, P > 0.05). More than 95% of the species observed were within the narrow altitudinal ranges between 3100 and 3200 in their habitat. The species number showed significance difference among the slope categories (df= 2, F= 13.312, P<0.001). Mountain nyala population reduced more than by half in the past three decades, and thus needs focused conservation attention to reverse the trend. Findings of this study could serve as yardsticks to make comparison against future research findings on the species.

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