Habitat quality is more important than habitat length in river restoration projects
We live in an age of widespread ecological restoration, in which centuries of human impacts on the environment are being addressed, under guiding principles such as rewilding, biodiversity conservation and ecosystem service provision. Restoration projects are particularly common on rivers and streams, partly because these ecosystems have often been particularly impacted by pollution, fragmentation and so on; but also because restoration approaches such as weir removal, ‘daylighting’ and riparian zone planting have become widely adopted. However, despite the increasing number of river restoration projects implemented across the world, there is still sparse scientific evidence on the long-term effects of such projects, and the factors affecting their success or failure.
A new study by MARS project leader Daniel Hering and colleagues addresses this shortfall in knowledge by studying the effects of the restoration of river hydromorphology on aquatic habitats and biodiversity. Hydromorphology is the term used to bring together the interactions of hydrology (e.g. water flows), geomorphology (e.g. bedrock) and ecology (e.g. plant and animal communities) in river or stream. Hydromorphological processes include the formation of meanders, riffles and pools: all of which contribute to a diverse set of habitat niches for freshwater life.
As reports such as the 2012 European Environment Agency ‘European Waters: assessment of status and pressures‘ outline, there is increasing evidence that river hydromorphology often has a strong impact on the health and diversity of aquatic ecosystems. However, projects which restore river hydromorphology often have limited effects on freshwater biodiversity. One common explanation for this effect is that river restoration often takes place over short sections of river, which are insufficient to allow ecological communities to develop, and geomorphological processes to take place.
To investigate this phenomenon, Hering and colleagues undertook experiments on rivers in ten regions across Northern Europe, as part of the EU REFORM project. For each region, they studied the impacts of two restoration projects: one on a short section of river; one on a longer section. At each section of restored river, the team sampled habitat composition in the river and its floodplain, three aquatic organism groups (aquatic plants, insects and fish), two floodplain-inhabiting organism groups (floodplain vegetation, ground beetles), and food web composition and land–water interactions. These findings were then compared to samples taken upstream at non-restored sections of river of roughly equal length.
After accounting for regional variations in river size and restoration approaches, the team, publishing in the Journal of Applied Ecology, found that the length of the restored river made no significant difference to its ecological health and diversity, as sampled by the range of indicators above. Instead, they found that what mattered was the substrate composition of the river bed: in other words, the aquatic habitat it provides for organisms.
Where habitat had been created in restoration, populations of fish, aquatic insects, aquatic plants and floodplain vegetation increased, regardless of the length of the restored river section. Substrate habitat restoration might include measures such as increasing the number of boulders or presence of wood (for example tree trunks) to diversify water flows and depths, and habitat niches for different aquatic organisms.
Whilst in-stream habitat restoration was important for aquatic biodiversity, restoration projects also had pronounced impacts on floodplain ecology. The authors suggest that this is because hydromorphological restoration, even at relatively small scales, tends to create habitat types close to the river bank (such as gravel and sand bars), which are often completely lacking in degraded sites. Such habitat types are rapidly colonized by ground beetles and, to a lesser degree, by specialised floodplain vegetation. Both organism groups have comparatively high dispersal abilities and are less affected by barriers such as weirs when compared to aquatic organism groups.
Overall, the study suggests that the ecological success of river restoration for aquatic organisms doesn’t depend on the length of restored river, but rather the quality and diversity of habitats on the river’s bed. However, for floodplain organisms, relatively small-scale restoration projects may yield significant positive effects.