Catchment-scale urbanization diminishes effects of habitat complexity on instream macroinvertebrate assemblages

While provision of in-stream habitat complexity remains common practice in efforts to restore streams, the evidence of positive effects on in-stream communities is inconsistent. In streams of urban catchments, where both reach-scale habitat manipulation and catchment-scale actions to ameliorate the disturbance regime of urban stormwater runoff are common management responses, clearer understanding of the effects of habitat complexity under different degrees of urban impact are needed. We experimentally assessed the effect of increased surface complexity in wood, the dominant hard substrate in our 18 study reaches on 14 small streams, on in-stream macroinvertebrate assemblages across a range of urban impact. Increased surface complexity increased abundance of most taxa, but this effect was less pronounced in urban streams, partly because of the reduced species pool tolerant of urban stormwater impacts, and partly because of a lesser response of some species to increased complexity in more urban streams. Collectively these taxon-specific effects resulted in small, uncertain increases in taxon richness with increased complexity in rural streams, and no change in richness of the less diverse assemblages of urban streams. Increased abundances suggest increased availability of refugia or resources with increased surface complexity, while the reduced effect of complexity in urban streams suggests that any refuge or resource provided by greater surface complexity is less effective in more disturbed environments receiving urban stormwater runoff. The reduced abundance of sensitive taxa in more urban streams, and the resultant reduced richness, confirms that urban stormwater runoff acts as a strong environmental filter, limiting the species pool available for community assembly. Restoration of habitat complexity in streams without catchment-scale drivers of degradation is likely to have positive benefits to in-stream biotic assemblages, but the efficacy of such approaches in catchments subject to urban stormwater runoff will be greatly diminished. In such cases, restoration activities should first be aimed at controlling the larger-scale problem.