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Effenberger, Michael (2009): Influence of flood disturbances and biotic interactions on the microdistribution of stream invertebrates. Dissertation, LMU München: Faculty of Biology
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Abstract

Most living communities form a temporally shifting patchwork of irregularly distributed organisms. Besides many habitat-specific biotic and abiotic environmental conditions, two key drivers are known to shape community structure: abiotic disturbance and biotic interactions (most notably competition and predation). Few other ecosystems possess either the frequency or intensity of disturbances observed in running waters. Therefore, disturbance (mainly in the form of floods) is discussed to be the dominant organizing factor in streams and rivers. The aim of my thesis was to investigate the interplay between flood disturbances and biotic interactions in determining the small-scale distribution of benthic invertebrate communities in streams. Especially during small and mid-sized floods, the high shear forces that move and rearrange parts of the stream bed result in a complex mosaic of small (≤ 1 m2) bed patches that experience scour, sediment deposition or remain undisturbed (“local disturbance history”). In my thesis, I found that local disturbance history patterns caused by natural floods (Chapter 1) or created experimentally (Chapters 2, 3 and 5) played an important role for the distribution of mobile invertebrates. Further, stable bed patches seemed to act as invertebrate refugia during and shortly after floods and, in the longer term, several common invertebrate taxa preferably colonized depositional or scour patches. Various habitat parameters such as current velocity, substratum size or food resources were also partly responsible for the heterogeneous distribution of stream invertebrates (Chapters 1, 2 and 5). The combined findings of my manipulative experiments described in Chapters 2 and 5 suggest that immediate, 'direct' effects of local disturbance on the invertebrates (mostly negative, i.e. density reductions in disturbed bed patches) are often in the longer term (several weeks after a flood) replaced by 'indirect' effects mediated via disturbance-induced changes in habitat parameters such as current velocity, substratum size and resource availability. Previous studies indicate that biotic interactions such as competition, grazing and predation can also be important determinants of the distribution of stream biota. However, although most streams are subject to considerable discharge variations, almost all of these earlier studies were performed in streams or artificial channels with permanently stable flow, or during long periods of stable flow in periodically disturbed streams. To date it is still unclear if biotic interactions are also important in frequently disturbed streams. To begin closing this knowledge gap, I conducted three experiments that examined the interactive effects of physical disturbance and interspecific competition on benthic stream invertebrates and algae. Singular (Chapter 3) and repeated (Chapter 4) local disturbances were combined with frequent manual removals of the most common invertebrate taxa. Disturbance played an important role for the microdistribution of invertebrates in all experiments. By contrast, competition was only found to be an important driver in shaping community composition in a stable stream (Chapter 4). In both experiments conducted in frequently disturbed streams, I found no evidence that competition influenced the invertebrate community (Chapters 3 and 4). Moreover, there were hardly any interactions between disturbance and competition treatments. Collectively, the results from previous research conducted in stable streams and my own experiments support the hypothesis that the importance of competition in shaping aquatic communities should decrease with increasing frequency or intensity of disturbance. In my last experiment (Chapter 5), I examined the separate and interactive effects of patchy bed disturbance and fish predation on benthic invertebrates and algae. While experimental disturbance had strong and lasting effects on the benthic community, effects of local fish exclusion were weaker. Moreover, effects of fish predation on invertebrate and algal densities were generally present or absent regardless of the disturbance history of the studied patches of stream bed. These results emphasize the pervasive importance of patchy bed disturbances for the microdistribution of stream organisms and also indicate a notable, but less prevalent, influence of fish exclusion at the patch scale on this microdistribution. Collectively, my findings on the interplay between disturbance and competition or predation confirm the key role of local disturbance history for the small-scale distribution of stream invertebrates both in stable and in frequently disturbed streams (Chapters 3, 4 and 5). Furthermore, local habitat parameters such as current velocity or food resources may define suitable bed patches for stream invertebrates, but several of these parameters themselves seem to be influenced by local disturbance history, as well. Finally, the frequency and/or intensity of such disturbances may determine whether populations become so dense that competition or predation can strongly influence the structure of the benthic stream community.