Our understanding of functional roles of aquatic invertebrate taxa is still limited even for common species, although being crucial for explanations of patterns observed in natural communities. As only recently shown, the common native European amphipod Gammarus fossarum, traditionally treated as a shredder of leaf litter, shows predatory behaviour which may influence the composition of invertebrate assemblages. However, the evidence for the predation effect of G. fossarum on natural assemblages at the within-site scale is still lacking. Therefore, we collected 50 quantitative samples of macroinvertebrates along with the important environmental variables within a heterogeneous calcareous spring fen. Using linear regression, we explored the relationships between the abundance of G. fossarum (separately adult and juvenile) and the abundance and number of taxa for two groups of invertebrates differing in their susceptibility to predation, (a) hard-bodied taxa with protective body structures, such as shells and calcified cuticles, and (b) soft-bodied taxa without those protections. We separated the effect of G. fossarum from that of environmental conditions using variation partitioning. Our results showed that only the abundance of soft-bodied invertebrates was negatively correlated with the abundance of adult G. fossarum. However, the proportion of variation explained purely by predation (5.5%) was much lower than the one explained by the environment (33.8%). Both G. fossarum and soft-bodied invertebrates were positively associated with organic matter. Although hard-bodied invertebrates consisted of only a few taxa, they were more numerous than soft-bodied invertebrates, and only environmental control was confirmed for them. Despite the limitations of the used correlative approach, we conclude that G. fossarum can significantly control the abundance of vulnerable taxa in natural assemblages. Its predatory effects, however, may be relatively low and easily confounded by the effect of environmental control.
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