Can river litter support more diverse invertebrate communities than rocks?

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Human-made litter—solid manufactured waste—is widespread in rivers worldwide but has been understudied. Its physical structure generally differs from natural substrates such as gravel and cobbles, called rocks here. By providing novel habitats, litter could influence ecological communities in urban rivers.

In a comparative field study, researchers compared macroinvertebrates recorded on litter and rocks to test whether the substrates supported distinct communities. They collected macroinvertebrates from individual rocks and pieces of litter, predominantly plastic, metal and glass, in three U.K. rivers.

Communities on litter were consistently more diverse than those on rocks, reflecting the litter’s greater surface complexity, but macroinvertebrate density was similar across substrates. Community composition also differed: five taxa were recorded only on litter. The differences largely reflected greater abundances of common taxa on litter that were relatively insensitive to environmental quality. Communities on plastic and fabric litter were most dissimilar to those on rocks, probably because their flexibility could replicate the physical structure of aquatic macrophytes.

The findings indicate that litter supports a distinct, diverse macroinvertebrate community in urban rivers, whose habitat structure is otherwise relatively homogeneous. Removing litter from urban rivers may not benefit local biodiversity. Understanding the functional habitats litter provides could help improve urban river management to replace habitat lost through urbanisation.