Ecosystem analysis and simulation of the Mkomazi Reserve, Tanzania

Summary The Mkomazi Reserve of northeastern Tanzania encompasses about 3300 km 2 of semi‐arid steppe between Tsavo National Park on the north and the Usambara Mountains on the south. Analyses of the various components of the ecosystem range from the measurement, synthesis and interpretation of climatic data to indices of large herbivore diversity. An analysis of the whole system and two simulation models are included. Of overriding importance to understanding the ecology of the Mkomazi is the recognition of many east‐west gradients ranging from elevation, solar input and rainfall to physical and biological soil properties. Since the biological communities are greatly affected by these they also reflect gradients but they, in turn, effect changes back on the soils and other variables such as the intensity of fire. With rainfall ranging from 35 cm year 1 in the east central region to about 60 cm in the west, the central section was found to be about 50% more arid than the west. The soils are all quite fertile with significant differences in cation‐exchange‐capacity, organic carbon, and nitrogen as a function of both depth and position on the slope. Vegetation consists of four major associations: (i) Acacia‐Commiphora bushland; (ii) wooded grassland; (iii) open grassland; and (iv) dry montane forest. Standing crop values range from about 200 g m ‐2 in the central region to around 600 in the west. Cumulative seasonal accretion values on denuded plots levelled off at about 150 and 300 g m ‐2 respectively. The maximum measured rate‐of‐change was 6 g m ‐2 day ‐1 while the wet season mean was 1 ‐9 g m ‐2 day ‐1 . There is a marked gradient of animal numbers from an annual average of only one or two animals knr 1 in the east to an annual average of twelve in the northwest. Double the annual average occurs around the permanent water during the dry season. Average annual biomass values range from about 5500 g km ‐2 in the west to about 750 g km ‐2 in the west central and 100 g km ‐2 or less in the east central. In addition to the east‐west gradient there is another gradient from very low numbers near the southern boundary settlements to higher numbers along the Tsavo Park boundary. Most of the area is more of a wet season liberation area than a dry season refugium. Even though species richness is high, the actual species diversity is low. For example, in the northwest section three species constitute about 90% of the numbers, 93% of the biomass and 90% of the energy transfer at any particular time or place. Elephants ( Loxodonta africana Blumenbach) alone account for 82% of the entire reserve's wet season biomass. Approximately 20% of the net primary production is estimated to be consumed by the large herbivores. The ratio of large herbivore poundage to large carnivore poundage is about 333: 1 indicating a reasonably slow energy turnover rate. Despite the large animal biomass densities in the savanna ecosystem the energy flow rates are probably not that much greater than other systems because of the large mean body sizes. Water and nutrient cycling are briefly discussed. Simulation results from two different computer models suggest that under existing land use and human population growth conditions elephants will continue to be a habitat degradation threat for several decades. Intermittent cropping seems desirable and humane.