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Ecology 6e Chapter 17 Online Supplementary Materials
Concept: Exotic predators can collapse and simplify the structure of food webs.
Introduced Fish That Simplify Aquatic Food Webs
Humans have moved all sorts of species around the planet, but some of the most
commonly introduced organisms are fish. Unfortunately, introduced fish are often the
source of unintended ecological change. For instance, introduced fishes have devastated
the native fishes of Lake Atitlan and Gatun Lake in Central America. In both these cases,
introduced predaceous fish completely reshaped the lake food web, producing a less
diverse community. The dramatic impact of these exotic predators may result from their
being outside the evolutionary experience of the prey populations in the local community.
Today we are witnessing what may be the greatest devastation ever wrought by an
introduced predator. That predator is the Nile perch, Lates nilotica, and the aquatic
system is Lake Victoria, one of the great lakes of East Africa. Lake Victoria is
approximately 69,000 km2 and lies right on the equator. Despite its large surface area, the
lake is relatively shallow. The deepest point is approximately 100 m, and most of the lake
is less than 60 m deep.
Lake Victoria harbored one of the greatest concentrations of fish species in the world,
and the Nile perch may be producing the greatest extinction of vertebrate animals to
occur in modern times. The fish fauna of the lake, which included more than 400 species,
is being rapidly reduced to a community dominated by a handful of species. Just three
species now dominate the fish catches around Lake Victoria: the introduced Nile perch,
the introduced Nile tilapia, Oreochromis niloticus, and a single native species, the omena,
Rastrineobola argentea, a planktivorous minnow of the open waters of the lake.
Hundreds of fish species appear on their way to extinction because humans
introduced one more fish species into a lake already containing over 400 species. The
Nile perch is a predaceous fish native to East Africa, where it attains a length of nearly 2
m and a weight of 60 kg. It may have eliminated species from other East African lakes in
the distant past, since lakes with Nile perch have fewer species of fish in the family
Cichlidae than do lakes without it. Nile perch were introduced to Lake Victoria around
1954, along with several other fish species from surrounding rivers and lakes. However,
Nile perch remained a minor component of the fish fauna for nearly two decades, and
then in the early 1980s, its population exploded.
As the population of Nile perch grew, the populations of nearly all other fish species
in the lake declined. The only native fish that increased during this period was the
planktivorous omena. W. Ligtvoet and F. Witte (1991) used food webs to represent the
changes in the Lake Victoria fish community. As you can see in figure 1, the original
food web consisted of a complex mixture of fishes belonging to several families and
orders. In contrast, three species of fish dominate the present food web. Notice that Nile
perch appear twice in the present food web, as a top predator and as a middle level
predator. In other words, there is a substantial amount of cannibalism in the community,
with small Nile perch serving as a trophic link between insect larvae and large Nile perch.
As impressive as the contrast shown in figure 1 may be, keep in mind that it is a gross
simplification and shorthand for the trophic interactions of over 400 species of fish. For a
comparison, look back at Winemiller’s food webs for Caño Volcán, a food web with only
10 species (see text, fig. 17.3). The original Lake Victoria food web contained 40 times
the number of fish species in the Caño Volcán food web.
Figure 1 Influence of an exotic predator, Nile perch, on the food web of Lake Victoria
(data from Ligtvoet and Witte 1991).
The Nile perch has had a major effect on the food web of Lake Victoria and is
contributing to a massive extinction. In addition, the Nile perch population explosion was
accompanied by major changes in the Lake Victoria ecosystem. Les Kaufman (1992)
points out that the changes in the Lake Victoria fish community coincide with other
changes in the ecosystem. For instance, concentrations of dissolved oxygen have declined
significantly. Lake Victoria water was once oxygenated from top to bottom. Before 1978,
organisms requiring oxygen lived in the deepest portions of the lake, which experienced
periods of reduced oxygen concentration but were not anoxic. Now, however, the water
below about 30 m is often essentially devoid of oxygen.
As discussed in chapter 18, predaceous fish like Nile perch can produce changes in
lake ecosystems that may in turn strongly influence populations and communities.
Though we look for single causes of complex phenomena, ecological systems are
affected by a complex interplay between biotic and abiotic factors. For instance, Nile
perch may be impacting the native fishes of Lake Victoria in two ways—directly through
its predation on native fishes and indirectly through its influences on the lake ecosystem.
Concept Review
1. Why, compared to native predators, might exotic predators have a greater impact
on native prey populations?
2. How might the Nile perch’s reduction of native fish populations, most of which are
herbivores, contribute to reduced oxygen concentrations in Lake Victoria?