In order to determine whether gravel mining in the creeks Gseng and Weißenbachl had a negative impact on the epigean arthropod fauna (walking beetle, short-winged beetle, spiders, centipedes, asseln, weavers), 72 collections were carried out between May and September 2012 in 10 test areas and two reference areas. The selection of the test areas followed a before-after-control impact (BACI) design structure.
Within each sample area, seven sample points were located randomly along the creek trench, which were processed in random order. A total of 235 beetles, 795 spiders, 75 centipede, 5 thousand feet, 29 Landassel and 2 Weberknecht individuals were identified. The most frequent and steady was the wolf spider Pardosa saturatior. Other biotope-typical and characteristic species of the trenches were the Bembidion longipes, the spring spider Sitticus atricapillus and the short-winged Stenus asphaltinus, but these were much less common. For the zönotic analysis, the barrel beetle and spider societies were compared with previous collections that had taken place in the mouth. For multivariate ordination, correspondence analysis (DCA) as well as nonmetric multidimensional scaling (NMDS) based on the Renkonen Similarity Index were also used for additional validation. For both spiders and barrel beetles, these techniques showed significant differences between the zenoses of the drying brook trenches and the zenoses of the river banks. The two reference collections in the water-bearing lower course of the Weißenbachlgraben fell into the clusters of the Enns and Johnsbach shore sites.
A DCA analysis of all 70 incremental samples (plus the two reference samples) showed that the Zönotic variation increases significantly downstream. Natural and gravel degradation-influenced creek trenches differed significantly in terms of their zönotic development trajectories. An analysis of the distribution of individual species showed that Pardosa saturatior is more common in the Haindlkar than in all other creeks. The relatively euryöke barrel beetle Bembidion cruciatum and the short-winged Stenus longipes prefer the gravel removal-influenced test areas. A quality assessment based on biodiversity measures number of individuals of endangered species and proportion of individuals of endangered species showed on average higher values for the natural creek trenches, but the differences were not statistically significant. Overall, the drying creeks represent an ecologically and zoologically independent biotope type, which has so far hardly been researched and requires special management. Gravel mining in these areas has been shown to have caused changes to the arthropod census, but the extent of which is not excessively profound and appears reversible when the trenches are properly recultivated.
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