Similarities and Differences Among the African Lakes and their Fishes
Written by Russell McAndrews
East Africa is punctuated from the north to the south by the Great Rift Valley. There are three Great Lakes associated with the Great Rift Valley; Lake Victoria, a relatively shallow saucer shaped lake, and the two older, deeper Rift Lakes, L. Malawi and L. Tanganyika. The Rift Lakes are so called because they actually formed by interrupted Zambezi and Zaire rivers flooding the newly formed cracks in the Earth. This geologic upheaval is also responsible for the formation of Victoria as westward flowing rivers began backing up to fill the broad basin in a process called ponding. All three lakes are noted for their similarities. All three are located in tropical E. Africa. All three were colonized predominantly by cichlids. All three experienced varying degrees of isolation, and a colorful history of environmental perturbations. For instance, lake level fluctuations have caused population fragmentation and recombination. To a point, this reputation of similarities is that they are similarly unpredictable. Shared isolation equates to hared opportunity, and manifests itself as phenomenally explosive adaptive radiation.
Here the differences become apparent as each ecosystem evolves in its own way. While some inter-lake developments (parallel or convergent evolution), may seem similar, the overall pattern is one of divergence.
In its extreme youth, effectively less that 15,000 years L. Victoria has shown us, where older ecosystems have not, that what we think of as a single species may have developed independently at many locations simultaneously. Several startling observations have resulted from working with these fished, primarily thanks to the Dutch/Tanzanian effort, the Haplochromis Ecology Survey Team (H.E.S.T.). A new twist which has been gleaned from the on-going Victorian disaster (Kaufman pers. com.), is the simultaneous, independent development of a common trait at multiple locations. This is an important revelation which adds many facets to the accepted beliefs that speciation is an occasional event and that new species displace old species. It now appears that co-evolution may play a more important role in the process of speciation.
Lake Victoria serves as the best example of the three Great Lakes in that it is in a very early and very interesting stage of development. Granted, the L. Victorian Haplochromini are an extreme and specific example, however, observations could be applied, at least in part, to other isolated pockets of Haplochromini. Much, but not all, of the data observed is local in nature and as such may be inapplicable to other ecosystems. For instance, the situation in L. Victoria involves two, mutually exclusive extinction mechanisms delineated by a very severe oxycline. Below there is not enough oxygen to survive and above awaits the introduced predator, the Nile perch.
One of the two most telling observations made recently in the lake (Kaufman, Sackley, ‘90), was the discovery that the native shrimp (Caridina nilotica), is a functional anaerobe and penetrates and thrives where there is no measurable oxygen. This is important for two reasons, because of the nutrient transport from the anoxic benthic zone and as a refuge from the perch for at least one species; ultimately this could be the only stable food for the perch. The other was the verification of a thin layer of fish concentrating at the oxycline, their lesser oxygen requirement allowing them to go just that much deeper than the Lates. A shift in the wind is all that’s necessary to cause severe tilting of the ox cline which has surprised and killed entire schools, in this way creating the observed layer of dead fish hanging in mid water.
Understanding the mechanisms and their motions from such an example as this may be our best means of avoiding further disasters. It serves the same end as conservation to want to understand the workings of such a process. The extinction is unique in many respects. The relative youth of the species flocks, the fishes themselves, the lake, not to mention the people and the date. Never in modern history have we had such an unknown on which our instruments could focus. Occasionally, when analyzed, the shear volume of observations reveals an underlying patter or structure which could be applied to other similar ecosystems. Additionally, this particular set of extinction dynamics can teach us a great deal.