Bird species composition and diversity along an ecological gradient on Mount Kasigau in Taita-Taveta County, South East kenya

Abstract

Interactions of bird species with their biotic and abiotic environment influence their occurrence and spatial distribution. This is because physical and biological conditions vary along an ecological gradient. Elevation gradient, vegetation mosaic and climate variations can influence avifaunal diversity and assemblages. Birds being very good indicators of the state of the environment can be used to demonstrate the impact of these changes on species communities especially in an Afromontane environment in face of unprecedented habitat loss and vagaries of climate change. This study assessed trends in avian diversity, distribution and composition along an elevation gradient and different vegetation mosaic at Mount Kasigau. Birds were captured at various elevations using mist-nets set on a 300-meter-long transect per each ringing ecotone from 850 meters up to 1,550 meters above sea level. The captured birds were identified and ringed and their sex and ages determined. A series of biometrics such as body mass, wing, and tarsus, and molting. The habitats were mapped, vegetation cover, canopy height and dominant plant species along transects were determined. Four habitat related to altitude were used: bushland (850-1104m), woodland (1105-1327m), evergreen forest (1328-1546m), and montane cloud forest (1547-1640m). Data were analyzed using both descriptive statistics and inferential statistics Plant species recorded on the mountain were 84 belonging to 36 families. Plants relative abundance ranged between 1.2 to 11.9 and the common families were Euphorbiaceae (11.9), Fabaceae (11.9) Rubiaceae (8.3), and Apocynaceae (6).Plant species composition decreased with increase in altitude from 48 species and 23 families in the bushland; 27 species and 15 families in the woodland; 18 species and 17 families in the evergreen forest; and 15 species and 13 families in the montane cloud forest. Bird species richness recorded on the mountain was 53 species belonging to 31 families and the relative abundance ranged between 1.9 and 11.3. The highly represented families were Muscicapidae (11.3), and Pycnonotidae (9.4). Relative abundance of bird species ranged from 0.24 to 10.78. The species with the highest relative abundance were Black-backed puffback (Tchagra jamesi), Red-capped Robin Chat (Cossypha natalensis) and collared sunbird (Hedydipina collaris) all with a relative abundance of 0.167. Bird species diversity (H’) decreased with increase in altitude; the bushland area had (H’= -2.264) which rated as high; the woodland had (H’= -2.144); evergreen forest (H’= - 1.748); and montane cloud forest (H’= -1.471), which classified as low. There were temporal differences in bird species diversity over the seven (7) year period of the study (2011-2019) and these differences were statistically (p < 0.05) significant. There were no statistical (p > 0.05) significant correlation (r) between bird species diversity and the mean annual rainfall distribution. Bird species distribution were related to differences in altitudinal physical (abiotic) conditions and biotic (vegetation differences). To guide the conservation of bird diversity the conservation interventions should be geared at safeguarding the integrity of these valuable ecosystems. Conservation managers can use these results to detect, measure and accurately predict range shifts to promptly mitigate impacts on biodiversity.