4. DISCUSSION
Although
the food resources of O. niloticus were quite specific and
similar (algae, diatoms, zooplankton, and detritus), but algae and
diatoms were the most important items in the diet of native C.
rendalli species. Both species in the size class <50 and
51-100 mm fed mainly on the similar food resources. The principal
difference between the diets of exotic Nile tilapia and the native
species was the size class 101-150 mm in which particular food items
came to be more important, since the native utilized algae as a broader
food category in different size classes. Our results also indicated thatO. niloticus were more generalist with respect to diet
composition and the abundance of food items, and relied more heavily on
zooplankton and chlorophytes in the 101-150 mm size class than C.
rendalli . This more generalist foraging on microphages, macrophages,
microfauna, and detritus probably explains the high abundance ofO. niloticus relative to native species in the river.
Contrary to our main hypothesis, the stomach content analysis results
give congruent feeding preferences for both in different size classes in
the upper Kabompo River. For both species, similarity in feeding item
was evident in the size classes <50 mm, except for size class
51-100 mm which showed a partial niche segregation. Oreochromis
niloticus evidently had a more diverse and broader food resource niche
than native C. rendalli species. This was observed by dominating
fish communities of O. niloticus having been observed to have a
wide trophic niche feeding zooplankton and detritus (Kırankaya &
Ekmekçi, 2013; Werner & Gilliam, 1984; Winemiller, 1989; Mason et al.,
2008; Zengeya et al., 2015). This was evident in our study as O.
niloticus was seen have wider feeding spectrum than native C.
rendalli . However, competitive interactions and dietary overlap with
natives such as O. macro chir may force O. niloticus to
occupy a wider trophic niche (Kenzo & Mazingaliwa, 2002; Marshall,
2011; Zengeya et al., 2015). Hence, the observed narrow niche use of
native species may partly result from increased competition for food
resources following the invasion of O. niloticus . In Lake
Chivero, exotic O. niloticus and natives presented a low dietary
overlap and wide niche areas, while omnivores had a high trophic niche
overlap where food sources were limited (Junor, 1969; Marshall, 2011).
However, O. niloticus feeds on larger quantities of food than
native species in Lake Victoria (Marshall, 2011; Trewavas, 1983; Zengeya
et al., 2015). Inclusion of more detritus and the algaeMicrocystis , Gonium , and Phacus in the diet ofO. niloticus may also indicate differences in foraging tactics
relative to native species.
Our results from SCA suggested that the wide dietary niche of O.
niloticus is largely due to marked size class differences in niche use.
Some size classes of O. niloticus had exceptionally wide or
narrow trophic niches, indicating low or high levels of interspecific
competition in diets, respectively, and high individual variation in
feeding habitats (Werner & Gilliam, 1984; Zengeya et al., 2015). The
small number but high quantity of food items consumed by O.
niloticus in the 101−150 mm size class is probably due to specialized
foraging on abundant food sources in certain habitats, whereas the
relative importance of food items consumed by most native species and
also some O. niloticus in the <101 mm size classes
probably indicates a diet dominated by algae chlorophytes and diatoms.
The selection of these food items at size class 151–302 mm of the
species is likely associated with their high abundance in the upper
Kabompo River, as reported for a nearby section of the river outside the
sampled area (Bok & Bills, 2012; AES, 2014).
Previous studies suggest that dietary niche overlap between species can
be interpreted as evidence of multiple shared food sources (Helfman et
al., 1997; Pilger et al., 2010) and therefore as an indicator of
potential competition (Eloranta et al., 2015). The exotic speciesO. niloticus and the native species C. rendalli exhibited
high trophic niche overlap in <50 mm size class. However,
their wider trophic niche area suggests that the O. niloticus are
opportunistic and can change to overcome possible competition in trophic
position or diet composition, and may explain the coexistence of these 2
species (Lowe-McConnell, 1987; Winemiller, 1991, Winemiller &
Kelso-Winemiller, 2003; Agostinho et al., 2015). In contrast, all native
species in our study had narrower dietary niches. For example, although
cichlids might include many species, individual species demonstrate a
narrow dietary niche area, suggesting that these species are
specialists. Orochromis macrochir and Coptodon rendalliwhere caught in relatively similar abundant of the aforementioned size
class during the study. The species are of similar size classes, feeding
mainly on filamentous algae, benthic invertebrates, and diatoms (Kenzo
& Mazingaliwa 2002). Thus, the dietary niche of these species may
overlap with that of O. niloticus at the base of the trophic web.
Nowadays this native species is among the less abundant species in the
upper Kabompo River (DoF 2018). In contrast, the O. niloticuspopulation has greatly increased in the last decade, and highly abundant
as recorded during this study. Oreochromis niloticus is a native
pelagic herbivore which feeds mainly on detritus, zooplanktons, small
invertebrates (in size classes 51-100 and 101-150 mm), and algae (in
size class <50 mm), suggested a potential dietary niche
overlap with C. rendalli (Kenzo & Mazingaliwa 2002, Marshall
2011, Zengeya et al., 2015).
Niche differentiation can explain how species co-exist in an ecosystem,
but empirical support has been sparse (Mason et al., 2008; Zengeya et
al., 2015). However, niche specialization has been regarded as a common
phenomenon in fish community assemblages (Pilger et al., 2010; Eloranta
et al., 2015). In the upper Kabompo River, there seems to be dietary
niche specialization among native species, supported by their relatively
low trophic niche overlap. The only substantial overlaps among native
species were observed in <50 mm size class for C.
rendalli. Oreochromis niloticus and Coptodon rendalli in
the 51−100 mm size class showed partial segregation in their diets
(Minshull, 1969; Junor, 1969; Kenzo & Mazingaliwa, 2002; Zengeya et
al., 2015). In contrast, the high dietary niche overlap was observed for
both species in <50 mm size class suggests potential
competition for food resources (Brendonck et al., 2003; Marshall, 2011;
Zengeya et al., 2015). The presence of O. niloticus can also
affect populations of native species indirectly. In many studies,
invasion by O. niloticus has been related to cascading effects in
the numbers of aquatic plants and phytoplankton biomass, and a reduction
in macroinvertebrate and macrophyte populations (Moriarty & Moriarty,
1973; Marshall, 2011). For example, in the Olifants River in the Western
Cape Province of South Africa, Nile tilapia became the dominant species
within a few years of introduction and eliminated macrophytes within a
few years of establishment, coinciding with significant declines in
previously abundant native species within the community. As a result,
natives are now confined to smaller tributaries and headwaters (Zengeya
et al., 2015). Furthermore, the presence of O. niloticus can lead
to further trophic cascading effect on ecosystem functioning (Marshall,
2005; AES, 2014; Zengeya et al., 2020).
The narrow dietary niche of native species observed in 51-100 mm size
class suggests that a small change in food resource abundance could
generate a big shift in dietary intraspecific competition of native
species in the population sizes. This narrow dietary niche could
contribute to population decreases of native species,
particularly, C. rendalli , which showed a relatively high
degree of dietary overlap with O. niloticus . This may be because
the system has been changing rapidly over the last few years due to
anthropogenic activities in the surrounding area, as predicted by a 2010
biological monitoring study (Bok & Bills, 2012; DoF, 2018). Mining
activities may affect fish habitat condition and affect populations of
native fishes. Another factor is dietary niche overlap among natives,
suggesting shared resource use and potential competition. However, as
indicated by wide dietary niches in <50 mm class size,O. niloticus species is able to switch food sources depending on
the abundance of these sources, while native species that feed only on a
small range of food sources are more vulnerable to changes in resource
abundance. Such vulnerably contributed by presence of exotic species may
create intraspecific competition within size classes of the species and
interspecific competition of the species, leading to habitat
displacement and possible extinction of native species.
In conclusion, partial dietary segregation observed in size class ofO. niloticus and C. rendalli confirm the coexistence and
interspecific competition between them in the upper Kabombo River.
Further studies are required to investigate food web structure that may
help to explain population decreases of some native species due to
invasion in the upper Kabompo River and other rivers across African
where the species coexist.