Anthropogenic litter is a novel habitat for aquatic macroinvertebrates in urban rivers

Anthropogenic litter (solid manufactured waste) is an understudied but pervasive element of river systems worldwide. Its physical structure generally differs from natural substrates, such as gravel and cobbles (hereafter rocks). Consequently, anthropogenic litter could influence ecological communities in urban rivers by providing novel habitats. This study compares the macroinvertebrates recorded on anthropogenic litter with those on rocks to test whether the different substrates support distinct communities. Macroinvertebrates were collected from individual rocks and anthropogenic litter, predominantly plastic, metal, and glass, in three U.K. rivers. Macroinvertebrate communities on anthropogenic litter were consistently more diverse than those found on rocks, reflecting its greater surface complexity, but the density of macroinvertebrates was similar among substrates. The community composition also varied between substrates, with five taxa only recorded on anthropogenic litter. Community differences largely reflected greater abundances of common taxa on anthropogenic litter, which were relatively insensitive to environmental quality. Plastic and fabric anthropogenic litter communities were the most dissimilar to those on rocks, probably due to their flexibility, which could replicate the physical structure of aquatic macrophytes. Our findings indicate that anthropogenic litter supports a distinct and diverse community of macroinvertebrates in urban rivers, which are otherwise relatively homogenous in habitat structure. Removal of anthropogenic litter from urban rivers may not be beneficial for local biodiversity. Understanding the functional habitats provided by anthropogenic litter could help better manage urban rivers to replace habitat lost through urbanisation.