Poor species diversity in rivers is often assumed to be due to harsh environmental conditions, particularly when the latter are caused by human impacts, such as removal of riparian vegetation that causes bank erosion and sedimentation of the bed. However, species may fail to colonise these locations because resources of food and living space are lacking rather than because the physical environment is unsuitable.
We tested this hypothesis with an experiment, carried out in six rivers, in which resources of terrestrial plant detritus were boosted at manipulation sites by increasing channel retention using pairs of wooden stakes. Other sites acted as controls. All rivers had intact terrestrial vegetation upstream of experimental sites but only sparse riparian vegetation otherwise. Benthic invertebrates, standing stocks of detritus and retentiveness were quantified prior to manipulation and re-sampled 9 months later.
Detrital standing stocks and taxon richness increased strongly at manipulation sites in three rivers with weaker effects in the other three rivers. The strongest effect sizes occurred in rivers that had the poorest, pre-existing capacity to retain drifting detritus. Furthermore, faunal composition at manipulation sites changed significantly in all six rivers. Our results demonstrate that species can disperse into and colonise seemingly degraded environments if scarce resources are boosted, and these findings have significant implications for river restoration.