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.