What influences the ecological success of river restoration?
In the last twenty years or so, environmental managers on many rivers and streams around the world have undertaken restoration schemes in an attempt to rectify the ecological damage caused by decades – if not centuries – of human modifications and pollution.
Just last week, the Environment Agency in the UK announced that populations of lampreys – eel-like ‘living fossils’ which were around 200 million years before the dinosaurs – have started to return to rivers in Northern England – heavily polluted and fragmented in the Industrial Revolution – for the first time in decades. Populations of migratory river and sea lampreys are beginning to return to the Derwent, Ouse and Trent as a result of EA restoration work which has improved water quality and removed barriers to migration through innovative ‘lamprey tiles’ that allow the fish to use their suckers to navigate over obstructions such as weirs.
River restoration schemes take a range of different approaches. Many restoration projects attempt to recreate ‘natural’ river processes and features such as flow amount and speed, stream depth and width, meanders and riffles. Another common restoration approach is to remove human barriers such as weirs and dams to improve continuity and connectivity between different habitats along a river’s course.
Other restoration approaches focus on the areas of land around rivers, planting strips of riparian vegetation along the river, to buffer pollutants and sediment from reaching the river, or using environmental policy to reduce groundwater abstraction from agriculture and industry. And finally, some restoration schemes focus on reintroducing plants and animals that have been lost over time – for example beavers or juvenile salmon. Most river restoration schemes use a combination of these approaches, depending on the individual river to be restored, its ecological and social histories, and the various priorities for restoration outcomes.
But as yet, there is little synthesised information on the factors that influence the success of river restoration initiatives across the world. However, a new study bringing together all the available global scientific literature and data on the ecological effects of river restoration, led by Jochem Kail from the University of Duisberg-Essen in Germany and published in Ecological Indicators, may help shed new light on this shortfall, and help guide environmental managers in designing restoration work.
Kail and colleagues from Masaryk University in the Czech Republic and BOKU in Austria, compiled river restoration monitoring results and scientific literature and databases to quantify the effects of restoration measures on three organism groups: fish, aquatic insects (macroinvertebrates) and aquatic plants (macrophytes). The team then looked to identify the factors that most strongly influence the effects of river restoration.
Kail explains the rationale for this research, “There is currently a controversial discussion about whether river restoration “works” – i.e. has a significant effect on biota – and scientific studies show contrasting results of restoration. Existing river restoration studies have already been summarised in several narrative reviews but quantitative summaries – so called “meta-analyses” – are rare and missing at a global scale. Our meta-analysis of studies from around the world on different organism groups might fill this gap: providing another – hopefully helpful – piece of the puzzle to inform environmental managers and policy makers.”
Funded by the EU REFORM project, the team’s results show that river restoration has significant, but varied, effects on all three organism groups. In general, restoration projects had a positive ecological effect, but around one-third showed negligible or negative effects. The responses of aquatic plant richness and diversity to restoration were higher than those for fish and insects. Aquatic plant richness and diversity was most significantly increased by river widening and rebraiding projects. This is because such initiatives reduce flow velocities and often cerate sparsely shaded pioneer habitats such as bare riparian areas and gravel bars that encourage the spread of pioneer plants, both in and around the river.
Fish and aquatic insect populations benefited from instream restoration measures, such as river margin enhancement, riffle creation and boulder placement. For all organism groups, abundance and biomass was more frequently increased than richness and diversity. Kail and colleagues suggest that this is because it is generally easier to increase population numbers of existing organisms in a restored river ecosystem than it is to establish new species.
River restoration effects were most strongly affected by agricultural land use around the river, river width and restoration project age. Agriculture around the restored river generally inhibited the positive ecological effects of restoration. However, drawing out large-scale trends from from complex and locally-specific land use patterns is difficult.
Project age was the most important factor influencing the effects of river restoration, but the effects of age were found be unpredictable and even negative on the health of the ‘restored’ ecosystem. This means that the positive effects of restoration may vanish over time, requiring long-term monitoring and adaptive management of restoration initiatives.
As an example, Kail and colleagues found that the response of aquatic plant abundance to restoration was reduced in older projects, suggesting that this can be due to the initial restoration processes of river widening and meandering not persisting in the restored ecosystem, and as such, the niches for many plants being lost over time. However, the results from the surveyed scientific literature were variable and showed no clear trend, both across organisms and ecosystems.
This finding raises the question of how, and perhaps more specifically when, to assess the success of river restoration initiatives. Typically, restoration projects require many years to mature and to impact the diverse and complex components of an ecosystem. However, Kail and colleagues’ study suggests that some river restoration projects may have positive ecological effects within a few years, that then diminish over time.
Kail explains, “The time since project implementation was the most important factor in influencing the effects of restoration, but these effects did not simply increase over time as one might expect. Instead, they showed different and non-linear relationships and effects even vanished over time in some studies. So we might ask: what about the long-term effects of restoration? Does it really help to establish more natural communities besides simply increasing the pure number of species, which might include mainly common or even non-native species? These are just some of the debates and frontiers in river restoration ecology.”
This innovative meta-analysis suggests that whilst river managers can generally expect positive effects from river restoration work on the three organism groups – fish, aquatic insects and aquatic plants – surveyed here, such initiatives may not be successful if initial work is not followed up with long-term ecological monitoring of the restored river, and adaptive management to intervene where restoration measures have altered or diminished effects.
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