Biodiversity and the water cycle: can rewilding mitigate the impacts of hydrological extremes?

Managing and responding to floods and droughts in the UK demands vast amounts of public spending, and the frequency and severity of extreme high and low flows will increase under future climate change. Many parts of our river catchments have been heavily modified over long timescales, but there is now increasing evidence that working with natural ecosystem processes can deliver multiple environmental, social and economic benefits. The rewilding of degraded landscapes is increasing in popularity globally, and represents a vital tool in reversing biodiversity decline. The changes in biodiversity arising from rewilding also have the potential to drive wider benefits, but many of these remain largely unexplored.

Rewilding is the practice of returning modified landscapes to a wilder state and may include the reintroduction of missing animal species. Approaches to rewilding are wide-ranging, but the types of biodiversity change include natural regeneration of grassland, woodland and scrubland, increased biodiversity of soil-dwelling organisms, and reintroduction of large herbivores (e.g. cattle, deer, pigs) and keystone species that have disproportionately large effects on ecosystems relative to their population size (e.g. beavers, wolves). Many of the animals and plants that recolonise rewilding projects are effective 'ecosystem engineers': organisms that are capable of modifying habitats and flows of resources in ecosystems. The presence of ecosystem engineers has been linked with increased success of environmental restoration projects.

Importantly, the fundamental principles that underpin rewilding offer potential for much wider-ranging benefits to support freshwater and terrestrial natural capital assets and ecosystem services but understanding of these wider effects is limited. A key knowledge gap is understanding the capacity for rewilded landscapes to modify the water cycle and mitigate hydrological extremes. This would contribute to the evidence base for the economics of biodiversity by delivering more effective water resource and water hazard management.

Rewilding, by nature and scale, will alter key elements of the water cycle. Changes to soils, vegetation and river-floodplain systems arising from rewilding will alter the movement and storage of water within landscapes. If rewilding outcomes mean that water takes slower routes through the landscape, it has the potential to make important contributions to flood and drought mitigation. Our project will explore the role that biodiversity gains from landscape rewilding play in modifying the water cycle and mitigating the impacts of hydrological extremes. We will use a review of the existing evidence from rewilding projects, other forms of landscape restoration and sustainable land management, and semi-natural ecosystems, to assess the state of the evidence for key types of rewilding-driven biodiversity change, and identify knowledge gaps and future priorities. In particular, we will answer the questions:

1. How does rewilding alter hydrological processes?
2. How do these effects differ across rewilding scenarios and environmental settings?
3. To what extent have these changes delivered measurable effects on floods and low flows?
4. Where are the gaps in knowledge across questions 1 to 3?

Answering these questions will determine the ways in which rewilding influences hydrology and hydrological extremes, support decision-making in rewilding practice across public and private sectors and inform future research priorities and improved monitoring of rewilding programmes.