Contributions of source populations, habitat suitability and trait overlap to benthic invertebrate community assembly in restored urban streams

Abstract Background Understanding the temporal development of community assembly processes is essential for assessing the recovery of degraded ecosystems after restoration. Community development in restored streams is often slow or absent, due to inadequate restoration, catchment-scale pressures, and/or colonisation barriers. Recovery processes involve three key filters: dispersal, environmental conditions and biotic interactions. Dispersal is critical for initial colonisation, while environmental conditions influence successful population establishment. Lastly, as available niches fill, biotic interactions, such as competition, gain importance. Despite the presence of many theories on how these three filters interact during community assembly, they have rarely been investigated simultaneously. Our detailed species- and site-specific approach allowed us to analyse the three filters in a hierarchical analysis. We assessed the effect of the three filters, by examining benthic invertebrate communities at 20 sites in the Boye catchment (Western Germany). The Boye and most of its tributaries were used as open sewers for a century, i.e. they were concrete channels transporting untreated sewage before gradual restoration was started in the 1990s. The bank reinforcements and concrete beds were removed, while riparian vegetation was left to natural succession. Accordingly, the sites were grouped as 'unimpacted', 'recently restored' (< 4 years), and 'mature restored' (> 10 years). An additional 28 sites provided information on distances to source populations, while the species’ habitat suitability assessed environmental filtering. Biotic (interaction) filtering was evaluated through trait overlap analysis. Results Communities at recently restored sites differed from mature and unimpacted sites, while mature sites resembled unimpacted ones. Taxa at recently restored sites had nearer source populations, while those at mature and unimpacted sites better matched present habitats. Trait overlap did not differ between present and absent taxa. Conclusions Our results indicate that dispersal was essential in early recovery stages, with mass effects from upstream sources supporting taxa found at recently restored sites despite low habitat suitability. Over time, habitat suitability became more influential, shaping mature communities. Competition appeared relatively unimportant, yet competitive exclusion may explain small proportions of absent taxa at mature and recently restored sites. Hence, to effectively support stream recovery, it is essential to consider how different filtering processes operate at various stages of the recovery process. For example, mature communities could further develop if habitat availability increases, while the connectivity to source populations would only play a minor role.