Diversity, disturbance and succession in the fish community of Lake Kariba, Zimbabwe, from 1960 to 2001

In 1969 Eugene Odum presented his ‘Strategy of Ecosystem Development’ and suggested a series of ecological attributes for measuring ecological succession and stability. The work has now become classical and widely cited but unfortunately rarely tested empirically. Man-made Lake Kariba in Southern Africa is a grand-scale ecological laboratory where the development in the fish community has been monitored continuously for more than 40 years through nearly weekly experimental gillnet catches from a permanent station. The changes in fish species diversity over the four-decade period the lake has existed are described, and related to biotic and abiotic factors to understand the mechanisms behind the dynamics. The results show that fish species succession in Lake Kariba took approximately three decades to stabilize. Overall fish diversity has steadily increased, but inter-annual variations are significantly negatively correlated to mean annual lake level changes and to the abundance of the main teleost predator, the tigerfish (Hydrocynus vittatus). These two factors, one abiotic bottom-up and one biotic top-down, can be regarded as key disturbances that play a regulatory role. From the data it was possible to test eight of Odum’s indicators for ecological succession, such as standing biomass, P/B ratio, net production, size of organisms, diversity and resistance. All eight tested confirmed his predictions. The changes in these attributes indicate that Lake Kariba, although fluctuating, is becoming increasingly mature and stable. In agreement with the intermediate disturbance hypothesis there is a negative relationship between fish productivity and diversity, and nutrient inputs (indexed by flushing rates) appear to be the most important factor for regulating this