River restoration is returning a section of a river to as close to an undisturbed state as possible. River restoration restores the health of a river, but how well does it restore the health of the plants and animals in and around the river?
Numbers vs. Habits
In most studies that evaluate river restoration effectiveness, cross-ecosystem effects are rarely studied. Either the aquatic or terrestrial groups are considered – not both. The focus is typically on taxonomic richness and composition, that is, the variety within species. The traits of the species, or how they act within the system, are rarely observed. Would these river restoration projects still be called successful if the aquatic and terrestrial groups were evaluated at the same time? How successful would they be if a trait-based analysis were used in addition to a taxonomic richness and composition analysis?
A study released in 2019 sought to answer some of those questions. The hypothesis: river restoration affects trait diversity and composition more than taxonomic richness and composition; terrestrial groups are impacted more by restoration than aquatic groups. The study analyzed 43 German river restoration projects completed between 1988 and 2012. The restoration projects consisted of improving hydromorphological structures, flood protection, river continuity, or combinations of all three. Nine different restoration methods were used.
Five taxonomic groups were sampled – fish, macroinvertebrate, macrophyte, carabid beetle, and floodplain vegetation. Increases in the number of subspecies in each group indicates the restoration had a positive impact. Traits recorded included habitat preferences, dispersal and mobility, organism size/form, life history and reproduction, and feeding group. Increases in these traits indicates the restoration had a positive impact.
Is River Restoration a Success?
Restoration made the river more complex and that, in turn, increased the habitat variety and created more niches. No method restored river environments to undisturbed conditions — restoring to a 100% undisturbed condition is impossible. The length of time since restoration and the length of the reach had no significant effect on how the plants and animals responded to the restoration.
In eight out of ten cases, the taxonomic and trait analysis showed similar responses. In the other two cases, taxonomic richness increased slightly more than trait diversity in response to restoration. For the macrophytes and the carabid beetles, an increase in the taxonomic diversity was not followed with an increase in trait diversity. The study speculated that the pattern for macrophytes could be due to functional redundancy, or the ability of species to share similar roles in an environment. The carabid beetle is from only one family. The resulting limited trait diversity could account for the difference in taxonomic and trait diversity responses.
The study found that the terrestrial groups had a stronger response to restoration than the aquatic groups. In many of the studies, the water quality of the reach was not improved. Upstream sources of contamination were not addressed during restoration. The lingering contamination has a stronger impact on aquatic groups than terrestrial groups. The separation from the stream would act as a filter for contaminants before reaching the terrestrial groups.
Restoration often doesn’t include important physical features like wood logs. This exclusion decreases channel roughness — reducing the river’s ability to retain drifting plants and animals. Decreasing this retention capacity impacts taxonomic and trait diversity. River restoration is often great for restoring the connection between the floodplain and the stream– this connection and the resulting nutrient influx to the floodplain would account for much of the improvement to the terrestrial groups. River longitudinal continuity, or the connection between reaches of the river, is often neglected in reach-scale restoration. Terrestrial groups are less dependent on longitudinal connectivity of the stream, while this is often vital to aquatic groups.
Rivers are complex systems and small changes can have unintended impacts. There is lots of debate on the effectiveness of river restoration — is it worth the cost? To get a better idea of how well these methods are working, a more complex and cross-ecosystem approach to monitoring is needed. We need to know not only how many and what types of species are present, but also how these species act in response to the restoration. River restoration would be more holistically successful if we took a larger catchment scale perspective during restoration.