Macroplastic litter colonization by stream macroinvertebrates relative to that of plant litter: A meta-analysis

Abstract

Environmental pollution by anthropogenic litter is a global concern, but studies specifically addressing the interaction between macroplastics and macroinvertebrates in streams are scarce. However, several studies on plant litter decomposition in streams have also used plastic strips as a methodological approach to assess if macroinvertebrates colonize plant litter mostly as a substrate or a food resource. Looking at these studies from the plastic strips perspective may provide useful information on the interaction between macroplastics and macroinvertebrates in streams. I carried out a meta-analysis of 18 studies that have compared macroinvertebrate colonization of macroplastic litter and plant litter in streams to estimate the overall macroinvertebrate colonization of macroplastic litter relative to plant litter, and identify moderators of this difference. Macroinvertebrate colonization of macroplastic litter was overall lower (by ∼ 40%) compared with plant litter. However, differences in macroinvertebrate colonization between macroplastic litter and plant litter were observed when considering leaf litter but not wood litter, which may be a poorer substrate and food resource for macroinvertebrates. Also, differences in macroinvertebrate colonization between macroplastic litter and leaf litter were observed for shredders, collectors and predators, but not for grazers that may feed on the biofilm developed on macroplastics. Macroplastic litter supported lower macroinvertebrate density, biomass, abundance, and richness, but higher macroinvertebrate diversity than leaf litter. Higher macroinvertebrate diversity on macroplastic litter may have occurred when macroplastics represented more heterogeneous substrates (e.g., mixture of plastic types) than leaf litter (e.g., needles). Differences in macroinvertebrate abundance between macroplastic litter and leaf litter were not significantly affected by plastic type, mesh opening size, plant functional group or plant identity. By testing previously untested hypotheses, this meta-analysis guides future empirical studies. Future studies should also consider the geographical areas most affected by macroplastic pollution and the plastic types most often found in the streams.