VARIABILITY IN THE INVERTEBRATE FAUNA OF AN INTERMITTENT STREAM OVER THREE CONSECUTIVE MIDSUMMER PERIODS

Abstract

Non-perennial rivers in semi-arid regions are exposed to unpredictable and highly variable rainfall and hydrology which can have considerable effects on faunal organisation at different scales. Spatial and temporal dynamics in the aquatic invertebrate fauna of an aseasonal intermittent river in the Eastern Cape, South Africa, were studied over three consecutive midsummer periods. Taxonomic composition varied widely from month to month and between years. Faunal abundances were generally low, and spatial distribution was highly heterogenous. Most of the abiotic variables measured remained within a narrow range over the study period, and only current speed in individual biotopes could be linked to biotic organisation. Faunal similarities were greatest between samples collected during similar hydrological conditions (rather than during similar times of the year). These findings suggest that rainfall and hydrology are major determinants of biotic organisation in this section of the river. The erratic nature of both these factors was seen to disrupt or inhibit linear or cyclical development of juvenile fauna and climax communities, indicating that the concept of faunal succession in such environments should be examined. Although pattern and coherence in the biota was lacking at many scales, two aspects of the invertebrate assemblage showed some consistency over time: i) taxonomic evenness remained consistently high (demonstrating a lack of dominance by individual taxa) and ii) the taxonomic distinctness of the invertebrate fauna (i.e. the degree to which individuals present were related taxonomically) fluctuated minimally. These elements of unity within the greater picture of disorganisation suggest that some thought should be given to the application of mathematical models such as Chaos theory to the study and management of temporary rivers.