One of the key features of the modern Lake Victoria ecosystem is the narrow range of species of fish. The lake, which only some 14,000 years ago was home to some 400 species of cichlids (small indigenous species of fish), is now dominated by just three species: the dominant, introduced Nile perch, the small sardine-like Omena (Rastrineobola argentea), the only member of the native fauna apparently favoured by the new condiitons, and the introduced Ngege or Tilapia (Oreochromis niloticus).
To understand the biota of Lake Victoria, one must see both the lake and its fauna in the context of its giant East African neighbours, Lake Malawi and Lake Tanganyika as well as the context of the regional zoogeography. The faunas of all three lakes exhibit the products of rapid speciation from very few ancestors. Best known is the fish family Cichlidae, of which more than 90% of the species in each lake are endemic (Greenwood 1984).
Catfish, mormyrids (elephant-nosed electric fishes), carps, gastropod and bivalve molluscs, insects, and crustaceans also have produced clusters of endemic species, but, with the possible exception of the Lake Tanganyika gastropods, these radiations are much less diverse and morphologically varied than those of the cichlids. Closely related but less species-rich cichlid flocks also occur in the nearby, smaller lakes Edward, George, Kivu, Kyoga, and Nabugabo (Greenwood 1974, 1981).
The cichlid faunas in the three great lakes in East Africa are strikingly similar and often cited as examples of evolutionary parallelism (Eccles and Trewavas 1989, Fryer and Iles 1972, Witte 1984). Three apparent lineages are present in the lakes: haplochromines, tilapiines, and lamprologines. The haplochromines are a species-rich and geographically widespread lineage. The tilapiines are species poor but also widespread. The lamprologines are species rich, but in the great lakes occur only in Lake Tanganyika.
The life history strategies of cichlids in general entail small broods and extended parental care so that as a group, the cichlids contrast with the Nile perch and the Omena, both of which invest heavily in high fecundity and little in parental care. Most East African haplochromine and tilapiine cichlids brood relatively few (5 to 100) large eggs and develop their young in the mouth, but the substratum-spawning lamprologines and tilapiines place large clutches(hundreds to thousands) of small eggs in nests on the lake bottom. All defend their young until they are self-sufficient. They have limited dispersal (except for seasonal inshore movements to spawn or release young) and strong site attachment- characteristics that should in theory make them highly vulnerable to extinction (Jablonski 1986, Gaston and Lawton 1990).
Curiously, this view is sharply at odds with certain other of the cichlids' attributes. They are aggressive, behaviorally and physiologically adaptable, phenotypically plastic, and prone to extraordinary evolutionary diversification (Avise 1990, Fryer and Iles 1972, Liem 1974). Much of this versatility has been attributed to a fundamental reorganization of the pharyngeal jaws that cichlids share with several closely allied fish families (Kaufman and Liem 1992).
Many cichlids can individually alter tooth and skull morphology in response to a change in diet (Greenwood 1965, Kaufman 1989, Meyer 1990, Sackley 1991, Witte et al. 1990). One cichlid, the Victorian snail-crusher Astatoreochromis alluaudi, was introduced to West Africa to help control the snail vectors for bilharzia, but these cichlids fed on insects instead of snails and stopped producing the massive dentition and musculature necessary to crush snail shells (Sloot-weg 1987).
Cichlids have escaped from tropical fish hatcheries into the canals and Everglades of south Florida, where they reproduce more successfully than do native sunfishes (Centrarchidae; Courtenay and Robins 1973, Hogg 1976, Taylor et al. 1984). Introduced all over the world as a ready source of home-grown protein for developing nations, tilapiine cichlids may have affected hundreds of native fish communities around the globe.
In short, cichlids have adapted to an incredibly wide range of conditions. One might expect such adaptability and ecological versatility to offer some measure of protection against extinction. That it has not done so in Lake Victoria may be one indication of the magnitude of change that has taken place.
A fishery that once drew on hundreds of species, mostly endemic, now rests on three: a native pelagic minnow called the Omena (Rastrineobola argentea) or Dagaa in Tanzania; the introduced Nile perch (Lates niloticus), known as Mbuta; and the introduced Nile tilapia, Orechromis niloticus.
Although there are many features of the biota of Lake Victoria that are of intense interest to biologists, it is fish that have received the most attention. Most of the fish species now in the lake also lived in the preceding, west-flowing rivers, but the cichlids, in particular, had a remarkable burst of speciation in response to the change from river to lake conditions. Similar things happened in other great lakes, but in Lake Victoria it happened much more recently, more rapidly, and with at first sight, less opportunities for ecological isolation in different types of habitat. The cichlids are capable of rapid genetic change, and more prone to speciation than other groups of Africa fish. There are more than 200 endemic species and 4 endemic genera of cichlids in Lake Victoria, more than 150 species of which are of the genus Haplochromis. Another major lineage is the tilapiines. From the primitive insect-eating types, mouths and pharynxes have evolved to allow feeding on plants, molluscs, fish, and even the eggs and young larvae carried in mouth of brooding females of most cichlid species.
The non-cichlid fishes have also changed, and there are at least 50 species, of which 29 are endemic, with one endemic genus. The non-cichlids in Lake Victoria show much less divergence from the original riverine stock than is the case with similar non-cichlid fish in Lake Tanganyika since this has had a much longer geological history for them to diversify. While most of the species remain year round in the lake, there are a number of (13 species) anadromous (migratory) fish, especially cyprinids, characids and siluroids, which swim up the rivers when they are in flood, breed in a suitable place, and return to the lake as the level drops.
The Historical Context of the Ecosystem Changes: Next Section
Page Author: Dr Rick T Leah, Univ of L'pool - Contents last reviewed15/12/2005