The Role of Connectivity and Source Populations in Stream Restoration—If You Build It, Will They Come?
Mark Southerland
TetraTech
Ownings Mills, MD
The history of biological response to stream restoration includes some notable successes but many instances of little or no improvement in the resident biological communities. While failure of restoration designs to include adequate habitat; continuing, unresolved water quality problems, and time lags are often invoked as reasons why biological uplift has not occurred, relatively little attention has been paid to the need for source populations for restorations to succeed. Jarod Lyon et al. (2019) recently conducted a study of population size of fish in a lowland river in Australia, following restoration of structural habitat, and concluded, “Successful restoration of poor quality “sink” habitats for target species relies on connectivity with high-quality “source” habitat.” Previously, Sundermann et al., (2011) concluded from an analysis of 24 German restoration projects and 1,231 nearby stream sites that, while local habitat failed to explain variation in invertebrate assemblage structure, proximity to an intact regional species pool did explain the variation. In a study funded by the Chesapeake Bay Trust, Southerland and Swan (2017) compiled biological monitoring data from 18 stream restoration sites in Anne Arundel, Baltimore, Frederick, Howard, and Montgomery Counties. 625 sites from the MBSS and countywide biological monitoring programs in adjacent stream networks were included as reference sites. The hypothesis was that biological uplift (using the benthic macroinvertebrate Index of Biotic Integrity, B-IBI) at comparable stream restoration sites would be lower in stream networks with poorer biological conditions. Specifically, we identified reference sites with B-IBI of 2.75 or greater within a 15-km radius of each restored site, calculated the shortest along-stream-network (typological) distance between restored and reference sites, calculated the difference in B-IBI scores, and modeled the degree of biological uplift on typological distance. 12 sites with at least 2 years of monitoring post-construction and 3 or more reference sites were analyzed. Only 4 of 12 sites displayed significant uplift, but the overall multiple regression showed significant effects of typological distance and difference in time of sampling; the interaction was not significant, nor was the effect of drainage area. In a separate study of the available source pools of benthic macroinvertebrate taxa for Prince George’s County, Maryland’s Quincy Run, Tetra Tech characterized the relative abundance of tolerant, moderately tolerant, and sensitive taxa that would be candidates to recolonize restored stream reaches. Percent occurrence of particular taxa across the Anacostia watershed was quantified from countywide stream monitoring data. Even where reaching biological integrity scores comparable to reference may not be attained in the short run, incremental increases in occurrence of moderately tolerant or sensitive taxa can serve as intermediate signs of biological uplift. We recommend that the potential effect of source populations inform expectations for biological uplift and that practitioners consider incorporating good streams as “stepping stones” to enhance recolonization.
About Mark Southerland
Mark Southerland has a Ph.D. in ecology and has spent the last 30 years working as a consultant to monitor, assess, and restore ecosystems in the Chesapeake Bay watershed. He was the primary author of the EPA national program guidance on biological criteria and has been the lead consultant on the Maryland Biological Stream Survey for the Maryland Department of Natural Resources (DNR) since its inception in 1993. He has also supported the impaired waters, stressor identification, and TMDL programs for the Maryland Department of the Environment (MDE). Since 1996, he has helped 16 counties and cities in Maryland, Virginia, Delaware, and South Carolina develop stormwater programs and comply with the Chesapeake Bay and local TMDLs. Mark was previously chair of the Maryland Water Monitoring Council Board and Howard County Environmental Sustainability Board. He is also a member of the Science Council of the Maryland Academy of Sciences, Safe Skies Maryland, Patapsco Heritage Greenway, and Howard County Conservancy Board.