This preliminary study examines how beaver engineering affects river condition in England and can be measured in relation to Biodiversity Net Gain (BNG) using the River Condition Assessment (RCA) methodology. Paired river reaches with (impact) and without (control) beaver engineering were compared at eight sites across southern England. At two sites, beavers were enclosed; at six, they were free-living. Two additional sites supported free-living beavers but without beaver engineering, so impact and control reaches could not be distinguished. Three hypotheses were tested: (i) in lowland English rivers, beaver engineering improves river condition as measured by the RCA; (ii) the magnitude of improvement depends on baseline river condition; and (iii) improvements in river condition are linked to increases in the presence and abundance of specific physical habitats. Findings from paired sites indicated that beaver-engineered reaches were generally assessed by the RCA to be in better condition than control reaches, supporting hypothesis (i), especially where control reaches were assessed to be overdeep (due to past management). Furthermore, the degree to which condition improved across paired reaches was inversely related to the condition of the control reach, supporting hypothesis (ii). Ten physical features showed significantly higher abundances in beaver-impacted reaches, while other physical features showed similar abundances in control and impact reaches, supporting hypothesis (iii). Although conducted in lowland, predominantly pastoral landscapes, the sites captured a typical range of environmental settings for beaver engineering and this preliminary study demonstrates that the RCA can effectively measure the influence of beaver engineering on river condition. These results provide a foundation for future research and offer practical guidance for considering beaver reintroduction as a nature-based tool for achieving Biodiversity Net Gain in development planning.
Gurnell et al. (Fri,) studied this question.