Abstract
Avian study in the southern Gulf of Lake Tana and head of the Blue Nile River, Ethiopia was conducted from June 2016 to April 2017 during the wet and dry seasons to examine species composition, relative abundance, and habitat association of birds. The study area was classified based on its vegetation cover and composition and point count method was used to gather information about the occurrence of avian species and their relative abundances. A total of 131 avian species belonging to 15 orders and 53 families were identified. The wet and dry seasons data indicated that 122 and 108 avian species, respectively, were recorded.Agapornis tranta and Oriolus monachal are endemic birds of Ethiopia and Eritrea that are occurring in the study area. Moreover, two Intra-African migrant, Threskiornis aethiopicus and Milves migrans, and two Palearctic migrant, Delichon urbicum andMotacilla flava were identified in the area. The highest avian diversity was recorded in the Debremariam wetland (H’=3.96), while the lowest was at the adjacent areas of Lake Tana (H’=3.22). The relative abundance score showed that 54.8% of the avian species were uncommon and there was a significant difference in relative abundance of birds between the wet and dry seasons (p<0.05). Different anthropogenic activities and urbanization are the main threats to the conservation of birds in the southern Gulf of Lake Tana. Therefore, appropriate management actions should be designed and implemented to ensure the conservation of birds in the Lake and its adjacent habitats.
Keywords : Blue Nile, endemic, Lake Tana, species diversity, point count
1. INTRODUCTION
Ethiopia is known for its avifauna and the diverse avian species of the country is mainly due to its different habitat types. Over 863 species of birds are identified from Ethiopia representing approximately 9.5% of birds in the world and 39% in Africa. It is known that nineteen avian species are endemic to Ethiopia, three are rare or accidental to the country, while fourteen species are shared with Eritrea and thirty one are globally threatened (Lepage, 2021). According to Ethiopian Wildlife and Natural History Society (EWNHS) (1996) forests, wetlands, and riverine ecosystems are sites for wintering or migrant birds in Ethiopia. As a result, there are 214 Pale-arctic migrant avian species, and a large number of these birds have breeding populations in the country (Pol, 2006).
In Ethiopia, most avian species are found in Important Bird Areas (IBAs), but these areas cover only 5% of the country’s total area and wetlands comprise 41% of the IBAs (Ayenalem & Bekele, 2008). Habitat variables of wetlands differ in water depth, water level fluctuation, salinity, topography, and vegetation (Colwell & Taft, 2000). Wetland birds are categorized into wetland specialist and wetland generalist (Airinatwe, 1999). Wetland specialists are completely dependent on aquatic habitats, while wetland generalists are frequently found in wetlands and they also use other habitats (Wondafrash, 2003). Wetland birds are placed into four categories as resident, sporadic, summer visitor, and vagrant (Clout & Hay, 1989).
The sizes of wetland habitats influence species richness and relative abundance of water birds (Froneman et al., 2001). Larger wetlands, which have high habitat heterogeneity can support a greater diversity of water birds than smaller ones (Colwell &Taft, 2000). Water birds foraging close to the shores persist in both large and small ponds and are considered as area-independent species, while those species foraging in open and deep-water habitats are considered as area-dependent species and are restricted to relatively large ponds (Paracuellos, 2006). The distribution of water bird species had a nested pattern among wetlands of different sizes (Paracuellos & Telleria, 2004). The kinds and amounts of resources available for breeding and foraging activities can also affect avian communities (Lee & Rotenberry, 2005). Thus, larger wetlands are of greater conservation value than smaller ones in supporting diverse water bird species (Paracuellos, 2006).
A number of environmental factors including food availability, temperature and the presence or absence of competitors and predators have been found to influence bird species diversity and their relative abundance. According to Robert & Kathleen (1992) habitat characteristics at different spatial scales, habitat modification or fragmentation, urbanization and the surrounding landscape composition can influence avian species diversity and abundance. Habitat loss and fragmentation are widely regarded as major factors contributing to the decline of avian populations (Raman, 2006). One in eight of the world’s avian species face extinction in the next millennium because of habitat destruction (Delannoy, 2010). Therefore, the main objective of this study is to explore species composition, relative abundance, and habitat association of birds in the southern gulf of Lake Tana and head of the Blue Nile River to design appropriate conservation strategies and ensure conservation of birds in the Lake and its adjacent habitats.
2. MATERIALS AND METHODS
2.1. Description of the study area
Study on avian diversity and relative abundance was conducted in the southern gulf of Lake Tana and head of the Blue Nile River, Ethiopia. Lake Tana has a total drainage area of approximately 15,000km2 and the catchment of the lake is one of the global top 250 lake regions most important for biological diversity conservation (Barker, 2004). It has varied topography, and the altitude ranges from 1784m a.s.l. in the south of the lakeshore to 3712m a.s.l. in the northeast end. Lake Tana is the largest lake in Ethiopia with a total area of 3600 km2 and is the source of the Blue Nile River.
Lake Tana is located at 12o00’N latitude and 37o15’E longitude and it is 565km far from the capital Addis Ababa. It has approximately 84km long and 66 km wide, with an average depth of about 9m (Dejen, 2003) and has more than 30 islands (EWNHS, 1996). It has more than 40 tributary rivers, but the major rivers feeding the Lake are Gilgel Abay from the South, Ribb and Gumara from the East, and Megetch from the North (Fig. 1).
Blue Nile River is the largest river in terms of volume of discharge and the second largest in terms of area in Ethiopia, which comprises of 176,000km2 (17%) of the area of Ethiopia (Conway, 2000). There is a significant waterfall at Tis-Isat, roughly 25km from Lake Tana, where the river drops 50m into the Blue Nile gorge. Most of Lake Tana’s catchment area is characterized by croplands, while only few limited areas of highlands are forest patches. Mean monthly maximum and minimum temperature recorded at Bahir Dar are 30.3oC and 7.82oC, respectively. The 10-year (2006 to 2016) rainfall data of the area shows unimodal distribution. Small rains occur sporadically during the months of April and May and heavy rainfall is recorded from June to August.