Meandering streams do not benefit biodiversity as expected

Since the 1990s, the restoration of streams, which for decades were straightened and deepened to make room for agricultural land, has increasingly captured the attention of politicians.

The goal is to return nature to its original baseline so that plants, fish, and small animals can thrive in the streams, and flora and fauna can flourish in the river valleys.

As a result, many of the country's streams have had their natural curves recreated and gravel laid out on their beds.

Now, a new research project from Aarhus University shows that the restoration of these stressed aquatic environments does not have the intended effect on stream biodiversity and environmental status as targeted by EU environmental goals. While small animals get better conditions, the effect on water plants and overall biodiversity is far more limited.

"Our research shows that the effect of stream restoration, where streams are remeandered and coarse material is added, depends on which organism group you look at," says Professor of Biology Annette Baattrup-Pedersen. She is a co-author of the study, which has been published in the scientific journal Restoration Ecology.

"While there is a clear positive effect on the ecological status of small animals, such as mayflies, stoneflies, and caddisflies, water plants are struggling. Here, only 39 percent reached a good ecological status. Looking at biodiversity, a more complex picture also emerges. The restored streams generally become more uniform in their species composition, and the restoration does not lead to more rare and declining species, such as long-leafed pondweed and shining pondweed," the professor explains.

Comparing streams
The research team's conclusions were reached by examining the composition of water plants and small animals in 18 restored streams in central Jutland and comparing them with 17 near-natural and 24 degraded streams in the same region, respectively. All of this was done to analyze how efforts to restore natural conditions in the streams affect plant and animal life.

"The result is significant because it shows that the restoration measures we use today do not necessarily work equally well for all organism groups, nor do they necessarily restore biodiversity," says Annette Baattrup-Pedersen.

"Current measures particularly seem to create habitats that benefit small animals associated with coarse substrate and fast-flowing water, while species associated with other types of habitats do not always get better conditions. For water plants, we do not see the same positive effect. This may be because the restoration fails to recreate the processes and habitats that the plants need."

"However, this also underlines the need to understand the cause of the failure to achieve targets before choosing restoration measures. If the problem is primarily weed cutting or nitrogen runoff, for example, physical remeandering or gravel placement is not inherently the solution," Annette Baattrup-Pedersen points out.

Integrating biodiversity
She hopes that the research results can contribute to a more targeted restoration effort that, among other things, takes into account the physical structure of each water body—including variations in water depth and bed conditions—as well as water volume and flow conditions in the streams.

"We see a need to increasingly base efforts on natural hydromorphological processes and local variation, creating multiple different types of habitats that characterize natural and well-functioning streams—not just coarse substrate and fast-flowing water, but also calm areas, variations in depth and substrate, and a natural transition between land and water," says Annette Baattrup-Pedersen, elaborating:

“More broadly, we also hope the project can clarify that good ecological status is not necessarily the same thing as high biodiversity. Therefore, biodiversity should be integrated more directly both when we choose restoration efforts and when we subsequently assess whether the restoration has been a success. This is particularly important because rare species may be missing even at sites with high ecological status.”

About the research result

Study type: Comparative space-for-time study of 59 restored, near-natural, and degraded stream stretches in central Jutland.

External funding: The project is supported by the Aage V. Jensen Charity Foundation and the EU's Horizon 2020 program through the MERLIN project.

Link to scientific publication:
https://onlinelibrary.wiley.com/doi/full/10.1111/rec.70386

Contact:
Annette Baattrup-Pedersen
Professor at the Department of Biology
E-mail: abp@bio.au.dk
Phone: (+45) 23 22 71 00