Discussion
Species composition, rareness, and
distribution
The total species count recorded was comparable with the FishBase
overall estimate of 278 described fish species in Uganda (Froese &
Pauly 2019). Also, the species accumulation curve approached the
asymptote, suggesting that most of the species were considered in the
analysis (Gotelli & Colwell 2001). Large lakes had higher species
richness and more rare species compared with rivers and small lakes.
Differences in species richness and rarity among water bodies may be
attributed to geomorphological, abiotic, and biotic factors (Brown et
al. 2007). Also, isolated inland water bodies favor rapid allopatric
speciation and adaptive radiation (Basiita et al. 2018), which exposes
the species to different evolutionary pressures. For example, along the
Victoria Nile, Murchison Falls hinders fish migration to Lake Kyoga from
Albert (Basiita et al. 2018). The falls along River Semiliki prevents
fish passage to Lake Albert from Edward (Acere & Mwene-Beyanga 1990).
Similarly, between Lake Victoria and Kyoga have been separated by the
Owen and Bujagali falls (Basiita et al. 2018), and a sandbar between
Lake Nabugabo and Victoria (Stager et al. 2005). These biogeographical
barriers may have led to allopatric speciation; for instance, Lake
Nabugabo which was once connected to Lake Victoria had five endemic
species (Ogutu-Ohwayo 1993; van Alphen et al. 2004). At the species
level, L. niloticus from lakes Kyoga and Victoria were
genetically different (Basiita et al., 2018) possibly due to manmade
barriers between the two lakes, which has impeded gene flow (Basiita et
al. 2018).
High species richness and rarity are expected in habitats of long
geological age, geographically isolated, and where species are prone to
speciation (Strayer 2013). Also, according to the biogeographical
principle, large areas have more species (Rosenzweig 1995). In this
study, the high species richness and rareness observed in Lake Victoria
is not surprising, given its surface area, explosive speciation and
adaptive radiation of haplochromine cichlids because of hybridization,
allopatric, microallopatric and sympatric mechanisms (van Alphen et al.
2004; Meier et al. 2017). A similar trend of high species richness with
increased lake area was observed (Amarasinghe & Welcomme 2002). In
large water bodies, species can shift to favorable habitats, for
example, Squalius lucumonis and Telestes muticellus that
migrated upstream in the Mediterranean Rivers due to climate warming
(Carosi et al. 2019). The level of rarity of recently described species
could not be determined in the available data. For example, the
distributions of H. akika in Lake George (Lippitsch 2003) andH. katonga in River Katonga (Schraml & Tichy 2010) are not well
understood except where the specimen was obtained. Also, haplochromine
cichlids are mostly lumped as haplochromines (Marshall 2018), which may
have accounted for the absence of certain species in particular water
bodies.
For rivers, habitat degradations and manmade obstructions mostly damming
have affected fish species migration, feeding, and recruitment patterns
(FAO 2001). For instance, only 70 of the 177 large rivers in the world
are free from damning (WWF, 2020). In Uganda, along the Upper Victoria
Nile, three dams were constructed. These obstructions affected species
richness and gene flow along the river (Basiita et al., 2018). For
example, the stocks of H. simotes and L. victorianus have
been affected by dams along the Upper Victoria Nile (Sayer et al. 2018).
Similar effects of damming have been reported along Yangtze and Mekong
(Dugan et al. 2010; Yi et al. 2010).
Conservation Priority Index (CPIw)
for inland water bodies
Biological metrics including indices, indicators, or targets have been
used by managers and decision-makers to prioritize areas for
conservation (Tognelli et al. 2019; Linke et al. 2011). These metrics
may include prioritizing areas with threatened species (Kirkpatrick
1983), threatened species affected by climate change (Tognelli et al.
2019), or fixed percentages of an area (Linke et al. 2011). However,
some indices such as species richness do not incorporate the component
of complementarity of the areas in question, thus, highly-ranking areas
with the same species (Kirkpatrick, 1983). Shannon Weaver diversity
index does not distinguish habitats with the same species evenness and
richness (Omayio & Mzungu, 2019). Most priority indices are computed
for terrestrial ecosystems or specific for a particular region, thus,
cannot be extrapolated to other systems (Brum et al. 2017). For example,
the Forest Conservation Priority index (FCPI) (de Mello et al. (2016)
used the area and shape of the forest without considering the species
conservation status. The Cave Conservation Priority Index (CCPi) (Souza
Silva et al. 2014) did not consider the species or their conservation
status but species richness, distribution, and impact weights. In the
species conservation importance index, the species conservation status
and rarity were considered (Halmy & Salem, 2015), but the index applied
to terrestrial plants but not an aquatic ecosystem. Thus, these priority
indices cannot be used to rank water bodies for site-based priority
conservation.
In most instances, biodiversity measures such as species richness are
usually higher for large water bodies, which would mean they are
prioritized for conservation. Indeed, this study also showed higher
species richness for large lakes compared to small ones. However, the
novel conservation priority index (CPIw) was significantly higher for
small lakes compared with large lakes. This observation is consistent
with expectation, given the low ecological substitutability for the
species and higher levels of exposure to human-induced threats in small
water bodies compared to large systems. For a system such as Lake
Victoria, with a vast habitat heterogeneity, fish species can easily
seek refugia in other habitats (Seehausen et al. 1997; Chapman et al.
2003), which may not be possible in a small water body. The index showed
that, for example, Lake Gigate with a surface area of 1.7
km2, but with 4 critically engendered haplochromine
cichlids (H. latifasciatus, H. obesus, H. parvidens, and H.
argenteus ) could be prioritized for conservation ahead of a large
system with higher species richness. The likelihood of the species
getting extinct in Lake Gigate is eminent if a similar magnitude of
stress is applied to both lakes. However, the index does not imply that
other water bodies with low CPI should not be monitored, but it may
allow a conservation manager or decision makers to rank water bodies for
urgent intervention, especially if the resources are limiting. Because
conservation interventions should address social needs (Linke et al.,
2011), large waterbodies, which are usually productive would be
difficult to fully conserve. The index should, therefore, be adopted as
a rapid metric measure to rank water bodies to enable prioritizing them
for conservation. Further, if the size and species in habitats in the
waterbody are known, the index can be downscaled to a habitat level.
However, index could not be applied on rivers because most of them are
dammed or obstructed, creating distinctive habitats along the river.
Supporting
Information
The map large lakes in Uganda (Appendix S1), Waterbody species richness
and IUCN status (Appendix S2), Rare species (Appendix S3), Ordination
plot for waterbodies (Appendix S4). Ordination plot for species
(Appendix S5), Simper analysis plot (Appendix S6), Waterbody
Conservation priority index values (Appendix S7)