Urbanization results in highly dynamic, degraded benthic macroinvertebrate communities in North Carolina streams

Abstract Benthic macroinvertebrates are frequently used as bioindicators of stream health. We compared macroinvertebrate communities from 20 urban stream sites with a range of development and impervious surface cover to those from 12 highly forested stream sites to determine how urban land cover impacts in-stream communities. We also assessed the temporal change in species compositions in the urban streams from 2001 to 2021 to test whether increasing development in watersheds that are already highly urbanized led to further community degradation. Urban streams and forested streams were alternate community states, with dispersed, non-overlapping clustering in a nonmetric multidimensional scaling analysis. Urban streams were degraded by all metrics (richness, diversity, pollution tolerance) relative to the forested streams. In the urban streams, species compositions differed significantly over 20 years, with pollution-tolerant species replacing other tolerant species. However, these changes were not related to changes in land cover, and most community-level metrics (richness, diversity, pollution tolerance) remained stable. A decline in gamma diversity suggests urban macroinvertebrate communities may gradually become more homogenized. Overall, we found that land cover changes associated with urbanization, particularly increases in impervious surface cover, result in persistently degraded, dynamic macroinvertebrate communities containing a few highly tolerant species.