Environmental Problems, Their Causes, and Sustainability 1

13 SustainingAquatic BiodiversityBiodiversityCASE STUDYA Biological Roller CoasterRide in Lake VictoriaLake Victoria, shared by Kenya, Tanzania, and Ugandain East Africa, is the world’s second largest freshwaterlake (Figure 13-1, left). It has been in ecological troublefor more than two decades.Until the early 1980s, Lake Victoria had more than500 species of fish found nowhere else. About 80% ofthem were small algae-eating fishes known as cichlids(pronounced “SIK-lids”), each with a slightly differentecological niche.Since 1980 some 200 of the cichlid species havebecome extinct, and some of those that remain are introuble.Four factors caused this dramatic loss of aquaticbiodiversity. First, there was a large increase in the populationof the Nile perch (Figure 13-1, right). This fishwas deliberately introduced into the lake during the1950s and 1960s to stimulate local economies and thefishing industry, which exports large amounts of thefish to several European countries. The population ofthis large, prolific, and ravenous fish exploded by displacingthe cichlids and by 1985 had wiped out manyof them.Also, the native people who depended on thecichlids for protein cannot afford the perch, and themechanized fishing industry has put most small-scalefishers and fish vendors out of business. This has increasedpoverty and protein malnutrition.Second, in the 1980s the lake began experiencingfrequent algal blooms because of nutrient runofffrom surrounding farms, deforested land, untreatedsewage, and declines in the populations of the algaeeatingcichlids. This greatly decreased oxygen levelsin the lower depths of the lake and drove remainingnative cichlids and other fish species to shallowerwaters, where they are more vulnerable to fishingnets. The turbid water caused by eutrophication alsomade it hard for female cichlids to select mates bycolor and can lead to the extinction of some species.Third, since 1987 the nutrient-rich lake has beeninvaded by the water hyacinth (Figure 12-9, p. 235).This rapidly growing plant carpeted large areas ofthe lake, blocked sunlight, deprived fish and planktonof oxygen, and reduced the diversity of importantaquatic plant species. Good news. The population ofwater hyacinths has been reduced sharply by introducingtwo weevils for biological control and mechanicalremoval at strategic locations.Fourth, the Nile perch now shows signs of beingoverfished. This may allow a gradual return of someof the remaining cichlids.This ecological story shows the dynamics of largeaquatic systems and illustrates that we can never dojust one thing when we intrude into an ecosystem ofconnected species. There are always unintendedconsequences.AFRICASUDANKENYALAKEVICTORIAETHIOPIAFigure 13-1 Althoughthe Nile perch (right) is afine food fish, it hasplayed a key role in amajor loss of biodiversityin East Africa’s LakeVictoria (left).BURUNDIZAIRETANZANIAINDIAN OCEAN