L. Forró szerk.: Miscellanea Zoologica Hungarica 13. 2000 (Budapest, 2000)

Spolwind, R., Schludermann, C. , Schuster, A.; Waidbacher, H.: Comparison of fish and amphibian communities in a floodplain system of the rivers Traisen and Danube westwards of Vienna

Discussion Intensive investigations on the impact of fish on amphibians were performed by Breuer (1992). Spolwind & Pintar (1997a, b) and Pintar & Spolwind (1998) show results of comparable Danubian alluvial systems. Fish prédation seems to be an important factor for the distribution of several amphibian species (mainly brown frogs, Triturus vulgaris, Hyla arborea) (Breuer 1992, Clausnitzer 1983a, b, Fog 1988, Pintar & Spolwind 1998, Spolwind & Pintar 1997a, b), while other species, like Bufo bufo, are not negatively influenced by fish prédation (Gossling et al. 1981). In his field experiments Breuer (1992) showed that only Leuciscus cephalus and Perca fluviatilis feed on Bufo larvae at higher rates, similar results are published by Clausnitzer (1983a, b), Filoda (1981), Hehmann & Zucchi (1985), and Kwet (1996). Clausnitzer (1983a, b) and Pintar & Spolwind (1998) point out the importance of structured habitats with shallow areas that cannot be reached easily by adult fish individuals. In our study fish and amphibians were investigated on a holistic level because co-existence and interspecific interactions between these two groups can be used to evaluate the status of complex aquatic systems and can function as an indicator of habitat diversity (Pintar & Spolwind 1998, Spolwind 1999). Our results clearly show that high abundance of fish does not necessarily limit the occurrence of amphibians. In the riverine forests of the Danube-Traisen area, comparatively high density of suitable spawning sites lead to higher densities of individuals and species of both amphibians and fish. Fish cluster 4.3 (stagnant backwaters) and amphibian cluster 2.4, representing a large backwater system near the Danube show high fish and amphibian species numbers and also high biomass of both groups. This backwater system is a good example of habitat diversion due to lateral and vertical segregation. The rather devastated riverine forest area of the Traisen shows nearly no stable fish-coenoses in the standing water bodies, and rather limited numbers of a few ubiquitous amphibian species. Only fish cluster 1.2 (stagnant backwaters) with its limnophilic fish association shows a stable, balanced fish coenoses. The impact of species as M. fossilis on amphibians is regarded as low (Pintar & Spolwind 1998), comparatively high species numbers of amphibians do coexist in here (amphibian cluster 4). This does not mean that there are no negative effects of fish on the amphibian coenoses in the Traisen-Danube area. But the more balanced ecosystem, even if only in a seminatural situation, allows co-existence of the two animal-groups on a higher unit of scale. The main reason for this should be the higher availability of aquatic habitats, the less influenced total area and the higher degree of structural richness e.g. temporary shallow waters and dense vegetation that allow the described co-existence (see also fish cluster 2.3 in stagnant habitats in comparison with amphibian cluster 2.5). The area of the Traisen system, a heavily human-influenced section, with nearly no habitat diversity, showed the occurrence of generalist amphibian species (R. dalmatina, amphibian cluster 3.2) or a lack of amphibian populations. The lack of vertically structured habitats leads to an exclusion of fish and amphibians in most habitats in this area. The Traisen region with its mainly small, shadowed backwaters or running water systems (fish cluster 2 and 3 in flow water systems) can be opposed to the Danube-Traisen part with its comparatively low human influence and various backwater types. Co-existence of stable coenoses of both groups could only be found here. Species richness of fish communities is an excellent indicator of hydrological dynamics here, fish cluster 4 represents connected or partly connected systems, fish species numbers increase especially significantly.