The spatial heterogeneity of a river bottom: a key factor determining macroinvertebrate communities

We examined the relationships between mesohabitat heterogeneity and benthic macroinvertebrate assemblages in a fourth-order stream (Mortagne, North-Eastern France). The heterogeneity of the mesohabitat mosaic was described around each sampling point for eight different surfaces from 0.5 to 4 m radius. Environmental descriptors integrated both the composition and configuration heterogeneity of the substrate mosaic. Faunal data were analysed in terms of biocenotic indices and taxonomic composition. The maximum number of significant Spearman correlations was obtained for a mosaic with a diameter of 2 m around the sampling point and significant correlations were both the highest and the most numerous for this surface. Relationships were established between community structure and substrate heterogeneity. The faunal richness was higher in a heterogeneous environment composed of numerous substrates, an elevated patchiness and with high perimeters. Such a mosaic potentially offers a great number of niches for invertebrates. A reduced distance between two types of substrate favours exchange of species. At the opposite, a very homogeneous mosaic offers a low variety of niches and shelters fewer taxa. Furthermore, in a homogeneous environment we observed that one or two particular taxa dominated the community, probably because competition with taxa coming from neighbouring patches was reduced. The community composition mainly depended on the characteristics of the mesohabitat sampled. Our results showed that the neighbouring environment around such mesohabitats also had a significant influence. Further research must be conducted to specify the influence of the mosaic heterogeneity on biological and ecological traits of invertebrates.