River invertebrate biodiversity benefits from upstream urban woodland

Abstract

In urban environments, invertebrate communities are subjected to a broad mixture of impacts, including diffuse pollution. Pollutant mixtures and habitat degradation can combine to apply stress on community diversity. Water quality is influenced by the assemblage and mosaic of catchment land cover. Amongst a wider suite of Nature-Based Solutions, the value of urban woodland is increasingly recognized as having potential to support a range of ecosystem services. Despite an increasing focus on establishing urban woodland for aquatic conservation, its actual influence is yet to be manifested. Therefore, we explored trees’ location in riparian and upstream catchment, within and outside of the urban area. We conducted a combination of systematic literature review and statistical analysis to better understand the woodland influence. Despite the wide range of bioindicators studied and broad worldwide spectrum of geo-climatic regimes covered, literature evidence for benefits were found in at least half the cases. With a focus on the overall family richness and the sensitive orders Ephemeroptera, Plecoptera and Trichoptera family richness as bioindicators, the statistical analysis comprised a national study in England covering 143 sites with substantial urban cover, totaling 4226 invertebrate community observations over 30 years. Two satellite-derived land cover maps were used to enable discrimination between urban and extra-urban woodland. The analysis supported the literature evidence that impervious land had negative effects and woodland positive effects. In the urban and upstream catchment, woodland was more important than pasture or cropland. There was some evidence of those woodland effects being more advantageous when trees are located within the urban area itself. Benefits attributable to woodland were distinctly apparent against a backdrop of improving macroinvertebrate diversity found to be synchronous with long-term reductions in urban pollution signatures. The presence of sparse land, even in small amounts, was detrimental to macroinvertebrate diversity. These areas of low vegetative cover might be detrimental due to high sediment input and legacy industrial contamination. Given the increasing accessibility of land cover data, the approach adopted in this case study is applicable elsewhere wherever macroinvertebrate community data are also available.