Aggregation among resource patches can promote coexistence in stream-living shredders

1. We conducted a new single-site study and a meta-analysis of pre-existing studies from multiple streams to assess whether intraspecific aggregation among leaf packs promotes coexistence among leaf-eating invertebrates (shredders) in Swedish streams. 2. In the single-site study, 48 standardised leaf bags were exposed for 1 month for shredder colonisation in a homogeneous glide of a forested stream. Current velocity, water depth and substratum composition were additionally assessed to investigate how these factors affected shredder distributions. 3. The meta-analysis included information on shredder colonisation of leaf packs from seven other studies of detritus decomposition to assess patterns of aggregation. Intra- and interspecific aggregation and their relative strength were assessed using indices (J, C and A respectively) originally developed for terrestrial insects also dependent on patchy and ephemeral resources. 4. In both parts of the study, intraspecific aggregation was much stronger than interspecific aggregation, which was weak overall, indicating that the conditions under which aggregation is expected to facilitate coexistence were fulfilled in our shredder assemblages. 5. In the single-site study, shredder abundances were weakly associated with environmental variables suggesting that habitat heterogeneity only partly explains aggregation patterns. 6. Our results strongly suggest that shredder diversity in streams, particularly during periods of leaf limitation (such as might occur in spring), is promoted by the aggregation of individual species among patches of resource.