Writing in response to ongoing global biodiversity loss, the biologist EO Wilson optimistically stated in his 1992 book The Diversity of Life that “Here is the means to end the great extinction spasm. The next century will, I believe, be the era of restoration in ecology.” His prediction has broadly come true: in the subsequent 25 years, ecological restoration has become a central goal for many environmental managers and policy makers across the world seeking to alleviate the impacts of decades, if not centuries, of human activity.
The forms that ecological restoration takes are increasingly informed and challenged by debates in ecology (e.g. non-equilibrium ecologies, state shifts and trophic cascades), social and cultural considerations (e.g. the appropriateness of ‘natural’ historical baselines, perceptual shifts to ‘living with’ environmental change), and ongoing climate changes (e.g. the imperative to ‘future-proof’ restored ecosystems with resilience to future climate shifts).
In freshwater systems, there are – as yet – few long-term studies of the ecological impacts of river restoration on aquatic biodiversity. Emerging research on multiple stressors (such as in the MARS project) demonstrates that the choosing restoration strategies is not a straightforward approach, as the removal of one stressor (e.g. fragmented flows) may cause alterations to the impact of others, either positively or negatively. New river restoration strategies are being implemented globally each year, and long-term studies on the successes and failures of existing restoration projects are thus extremely valuable resources.
In this context, a new study by researchers based in Germany and the USA examined the responses of fish communities to the restoration of the Lippe River in Germany over a 21 year period. The Lippe has been heavily modified by human activity since the early 1800s, with a largely reinforced and straightened channel and bed, highly fragmented flows as a result of numerous weirs, and the widespread destruction of its riparian and floodplain landscapes.
The research team analysed data collected for 4 years before, and 17 years after, restoration at Klostermersch, where two stretches totaling over 3km in length were restored in 1996 and 1997. Restoration involved reconnecting the river’s floodplain with the river, removing bank fixations, widening the river from 18 to 45 metres in width, building a series of small islands, introducing full trees as deadwood, and reintroducing ‘natural’ floodplain drainage systems. In essence, the Lippe River ecosystem was encouraged to become more diverse in structure and dynamic in process.
Writing in a newly published study in Conservation Biology, the research team’s results show that this restoration management has led to a doubling of both fish species richness (i.e. the number of species) and abundance (i.e. the total species population sizes). On restored sections the Lippe, fish abundance exhibited an initial overshoot, where population numbers increased rapidly, then declined at a similar rate, as their numbers exceeded the ecosystem’s carrying capacity.
Both fish abundance and richness broadly stabilised around seven years after restoration at more than double the pre-restoration levels. However, there was (and is) significant variation in both abundance and richness from one year to the next. There are two environmental management insights here: first, that fish communities may take at least seven years to recover post-restoration and; second, that ongoing monitoring strategies are necessary to assess the impact of restoration strategies, as single sampling efforts may be skewed by annual variations.
The research team examined different traits of the fish species in the study, to understand if they played a role in how fish responded to restoration. They found that life history and reproduction related traits were influential. Opportunistic, short-lived fish species whose females reached maturity quickly and had multiple spawning runs each year most strongly increased in abundance, as a result of their ability to rapidly colonise new habitats. Such small-bodied fishes typically live in the dynamic in-stream and floodplain habitats that the Lippe restoration measures addressed.
As a result, the research team argue that focusing on fish species traits may help aid our understanding of how their populations are likely to respond to restoration measures. By relating restoration outcomes to such species traits, there is the potential to draw broader insights for restoration measures in other biogeographic regions, they suggest.