Discussion
The 35 reservoirs investigated present a mixture of contrasting age gradient and structural heterogeneity of habitat in terms of differences in size, elevation and liminological characteristics, among others. I show here that these reservoirs are an important repositories for birds including species designated as Near Threatened (NT), Vulnerable (VU), Critically Endangered (CR) under the International Union for Conservation of Nature (IUCN) conservation status (IUCN 2019) and endemic birds. In the face of an unprecedented habitat loss and degradation of natural wetlands, it can be argued that such artificial waterbodies can play an important role for waterbird conservation. These reservoirs and its associated wetlands, have the capacity to maintain high avian diversities. In line with results presented here, other studies have likewise indicated that urban reservoirs and artificial ponds harbor waterbird communities and act as refugia for waterbirds (Hale et al. 2019; Almeida et al. 2020; Tatiane et al. 2020).
Studies elsewhere indicated that wetland area and habitat heterogeneity as being the most important features that affect aquatic bird species richness and abundance (Roth 1976; Hoyer & Canfield 1994; Scheffer et al. 2006; Kortz & Magurran 2019). According to the continental species area relationship (SAR-hypothesis, (MacArthur & Wilson 1967), large reservoirs are expected to contain more species (i.e. higher taxonomic richness). In line with the long established theory of island biogeography (MacArthur & Wilson 1967; Losos & Ricklefs 2009), a significant difference in species richness between large and small reservoirs investigated is observed in this study. Community ecology and population genetic studies have indicated that spatial variation in species/clonal composition is the result of environmental sorting, dispersal limitation and historical factors at different temporal scales (Rossetto et al. 2008; Orsini et al. 2013; Haileselasie et al. 2016). Here, I have reported that highly significant variation in bird species richness attributed to environmental variables and a small but significant effect of age of reservoir is also observed. This result is in agreement with previous research that showed that pattern of beta diversity are sensitive to environmental and historical factors (Haileselasie et al. 2016) where high Daphnia clonal richness was observed in an age gradient of ponds/lakes created by glacial retreat in Greenland. The small but significant effect of age of reservoirs on bird species richness is also in line with long established meta-population theory which argue with an increase in age there will be an increase in the heterogeneity of habitat (Hanski 1998). Thus, providing different resources for nesting, foraging and roosting habitats for different bird species (MacArthur 1964; Brown & Dinsmore 1986; Hoyer & Canfield 1994; Edwards & Otis 1999; Almeida et al. 2020).
Despite the fact the fact that positive correlations between habitat cover, habitat area and species richness have long been established (Murphy, Kessel & Vining 1984; Murphy & Dinsmore 2018); what drives patterns of species composition in a landscape, however, remained debatable. For example, Merckx, Miranda & Pereira (2019) argued that habitat amount not patch size drives species richness against the long established island biogeography (MacArthur & Wilson 1967; Losos & Ricklefs 2009). In addition, to the contrary of the theory of island biogeography some studies have suggested that small habitat size and isolation promotes species richness in shallow lakes and ponds (Scheffer et al. 2006). While others have argued that larger habitats (or islands) accumulate a higher number of species (MacArthur & Wilson 1967; Losos & Ricklefs 2009). In favor of this argument, studies have indicated that wetland area, vegetation cover, and structural heterogeneity of habitat are the most important features that affected wetland bird richness and abundance (Guevara et al. 2016; Dinesen et al. 2019). The result of this study provides an additional evidence in favor of the later argument. However, the mere size is not significant but possible presentation of large sized habitat accompanied by an increase in habitat heterogeneity does supporting greater species richness. In contrast to a previous positive relationship between bird richness and lake depth (Guevara et al. 2016) this study did not support the positive relationship between reservoirs’ depth and bird species richness. This could be due to the effect of reservoir depth being species specific (Guevara et al. 2021).
With regard to the distribution of the waterbirds two patterns are apparent in this study. Some species are widely distributed water specialist birds while others are restricted in their distribution but also non-specialist or opportunist. Such pattern of distribution has been reported previously elsewhere (Hoyer & Canfield 1990). Some of the bird species, recorded in this study, occurred in almost all the reservoirs thus are generalists. While others are reported from single reservoir with specific habitat features such as presence of settlements, large wetland forest downstream or in the near proximity of the reservoir. Some reservoirs are very close to residential areas (example Bokoro and Mai Sessela) or have very large wetlands downstream (example Meskebet reservoir). This might be responsible for the presence of Eastern Grey Plantain-eater (Crinifer zonurus ), Long-crested Eagle (Lophaetus occipitalis ) and Rose-ringed Parakeet (Psittacula krameri ) in Meskebet reservoir. The presence of these birds in Bokoro, Mai Seyie, and Meskebet reservoirs could likely be due the presence of housing and plantation very close to the reservoirs and also the positive influence of shrub density on species richness via the provision of nesting, foraging and roosting habitats for different bird species for example Kites, Eagles and Vulture. Generally, bird abundance has been reported to increase in response to increase in degree of urbanization (Chace & Walsh 2006) and this increase has been attributed to the availability of food subsidies and the reduction of predation pressure. Nonetheless, the presence of these birds should be interpreted with caution and only in terms of conditions stated in this study. There could be a possibility of seeing these birds outside the study area covered here. This calls for further investigation into larger areas and/or more reservoirs than studied here. For example Long-crested Eagle and Rose-ringed Parakeet are recorded only from Meskebet reservoir not in any of the reservoirs investigated here. However, Rose-ringed Parakeet has been recorded from Mereb river irrigation farms which is 10 km west of Mihtsab Azmati reservoir (Haileselasie & Teferi 2012). Another surprising finding is the absence of Cape Shoveler (Anas smithii ). A preliminary study by Asmelash et al . (2007) reported the presence of Cape Shoveler in 10 reservoir. Unfortunately, I have not seen any Cape Shoveler during the field campaigns in the 35 reservoirs including reservoirs where it was reported to have been seen previously. This could possibly be a typical example of local extinction of the species. Previous studies have suggested that human interference could lead to locally extinct (Hassall & Anderson 2015). Besides, globally, 14% of the known bird species are threatened with extinction (IUCN, 2019). This clearly indicates a worrying development and there might even be a higher risk of loss of biodiversity of aquatic birds in particular because aquatic habitats are highly sensitive ecosystems that are also threatened by global trend of climate change, agricultural expansion and intensifications (Johnson et al. 2013; Newbold et al. 2015).
In conclusion, the patterns of distribution in birds depended on the level of limnological characteristic, age and biotic components in the studied reservoirs. In this study bird species richness was only significantly influenced by area which is in line with previous studies elsewhere (Murphy & Dinsmore 2018). Nevertheless, the strength of these correlations was only moderate, indicating there are other habitat variables important to species richness that were not measured in this study.
Waterbirds play key functional roles in many aquatic ecosystems and can be effective bio-indicators of change in aquatic ecosystems (Green & Elmberg 2014). There is a clear consensus that waterbirds are good biological indicator but when using waterbirds as indicators, a thorough knowledge of their ecology is essential monitoring program. It is my strong believe there is dire need for more research into the status and ecology of these essential ecosystems and their role towards strengthening our knowledge on aquatic bird ecology and natural history of African Eurasian Migratory waterbirds. These reservoirs investigated forms a gradient of many ecological variables (Teferi et al. 2014) and different degree of eutrophication which vary with season (Asmelash et al. 2007; Asmelash 2009) and could be an excellent experimental macrocosms to study many ecological questions.