The impact of stream restoration on macroinvertebrates

The enhancement of in-stream habitat has emerged as a major river restoration tool with over $1 billion spent over the last ten years. These projects often involve increasing the physical complexity in the stream by adding habitat features such as large woody debris and boulders or reconfiguring the channel.

For all that money spent, the question arises whether restoration projects are consistently benefiting aquatic organisms by increasing diversity, density, and biomass and what factors explain success. To answer the question, Scott Miller and fellow researchers from Utah State University conducted a meta-analysis of published studies on the impacts of in-stream restoration projects on macroinvertebrate species.

Macroinvertebrates, which include mollusks, insects, worms, and many other organisms, play a critical role in aquatic food webs but receive little attention when it comes to restoration planning or monitoring ecological responses. The study which represents the first ever meta-analysis of macroinvertebrate response to in-stream restoration looked at total of 24 studies.

The researchers found that treatments aimed at increasing habitat heterogeneity had a significantly positive impact on species richness. Of the treatments, the addition of large woody debris had a more consistently positive impact than either adding boulders or reconfiguring the channel.

The overall impact on macroinvertebrate density was also positive but not statistically significant. This was because the results were highly variable especially when it came to adding boulders and channel reconfiguration. The study found that restoration of forested reaches had the most consistently, positive impacts compared to restoration within agricultural or urban areas.

The study authors warn that given the paucity of studies that met their criteria for analysis, the results should be interpreted with caution. The biggest take home message from their meta-analysis is that scientists are doing way too few rigorous studies on this topic. This is part of an even bigger problem that few stream restoration projects conduct rigorous, long-term monitoring of the ecological response. The authors write,

"With the completion of over 6,000 in-stream habitat restoration projects over the last decade at a cost exceeding $1 billion there is a clear need for post-project effectiveness monitoring...The low number of published macroinvertebrate studies, as well as data of inconsistent quality highlights the current state of stream restoration science and reaffirms the need to implement more rigorous study designs and improve data reporting. Specifically, the failure to sample a control reach or to collect and/or report quantitative density and richness estimates excluded the majority of published studies."

--Reviewed by Rob Goldstein

Miller, S., Budy, P., & Schmidt, J. (2009). Quantifying Macroinvertebrate Responses to In-Stream Habitat Restoration: Applications of Meta-Analysis to River Restoration Restoration Ecology DOI: 10.1111/j.1526-100X.2009.00605.x