First approximation of the effects of rainfall on the ecology and energetics of a Namib Desert dune ecosystem

Abstract

Ecological surveys, conducted after a prolonged dry period (mean annual precipitation—14 mm) and after a high rainfall event (118 mm), showed that the Namib dunes respond dramatically to rain. Prior to rain the biomass of plants measured 2·6 g m−2, of detritus 0·4 g m−2 and of animals 0·01 gm−2. These figures represent the lowest biomass data reported for any terrestrial ecosystem. After rain the potential energy contained in the plant biomass increased ninefold while the potential energy in the detritus and the animal biomass increased sevenfold and sixfold, respectively. This sudden increase in biomass was largely due to the rapid growth of ephemeral grasses. Rain decreased the extreme patchiness of the biota by causing niche overlap, expansion of the home ranges of animals and a change in distribution patterns of plants. The various habitats within the dune ecosystem differ markedly with respect to both productivity and stability. The dune slope is the most productive and the least stable, while the interdune valley is the most stable and the second most productive, in terms of contribution to the total biomass of the ecosystem. However, when the individual habitats are evaluated separately, the slipface supports the highest biomass of vegetation and animals, the former being entirely in the form of detritus. The most important limiting factors identified were water, and the nitrogen and phosphorus content of the dune sand as well as the physical stability of the sandy substrate in certain habitats. The wide trophic ratios established between animal biomass and the biomass of plants and detritus (1: 249) suggest that available energy does not limit the size of herbivore and omnivore populations. In contrast, the low protein content of the vegetation during dry periods (3·1 per cent) is considered to be an important limiting factor. During both dry and wet periods, available energy may, however, limit carnivore populations. Unique features of the ecosystem are to be found in the absence of a significant microbiological decomposition loop, the degradation of large amounts of accumulated detritus by tenebrionid beetles, the dependence of the biota on fog and the physiological and behavioural characteristics of the fauna. These include fog imbibition, fog basking and water storage which enhances their opportunism in exploiting the erratic resources of this unusual ecosystem.