Dippers (Cinclus cinclus) as bioindicators of health and resilience of freshwater ecosystems in the face of complex ecological change

Freshwater ecosystems are of critical importance because they integrate effects of environmental pressures across landscapes and catchments and are more than twice as important as other broad habitat types in terms of ecosystem service delivery per unit area (Natural Capital Asset Index). Monitoring freshwaters effectively and understanding their response to multiple environmental stressors is, however, challenging.

Dippers are the quintessential river bird of Britain and, as a predator, are likely to provide a convenient and effective sentinel of the health of our riverine ecosystems and their response to various forms of environmental management. Dippers are proven sensitive indicators of anthropogenic pressures such as acidification from air pollution (e.g. Vickery, 1992). However, in the past 30 years, acid deposition has declined greatly following controls on emissions, while more environmentally benign forestry practices may have helped to mitigate impacts of conifer plantations on upland stream acidification. Water pollution from industrial and urban point sources has also been strictly regulated leading to improved water quality. The declines in dipper populations, and those of other riparian birds (e.g. grey wagtail, common sandpiper, goosander) revealed by Bird Atlas 2007-11, are therefore a conundrum.

Dippers are predicted to be highly responsive to the changes in temperature and flow regimes that will arise from climate change over the 21st century (Saether et al., 2000). While Dippers are adversely affected by severe winters these have been eclipsed by mild wet winters in recent decades which are reflected in overall increases in river flow and variability, most notably in the west of the country. Water colour is generally increasing ('brownification') in upland freshwaters as the constraining effects of acidification on decomposition rates and solubility of organic carbon are lifted, alongside increasing precipitation and temperature (Battarbee et al., 2014). There is a growing case that effects of more variable river flow regimes and increased water colour on aquatic invertebrate prey and Dipper foraging efficiency may be affecting Dipper demographic rates (e.g. Taylor & O'Halloran, 2001), and thus contributing to declines in this and other river birds. Finally, while there is good evidence for chemical improvement of acidified upland freshwaters, most assessments suggest that biological recovery of acid-sensitive invertebrates and fish is lagging some way behind (Battarbee et al., 2014). The current status of Dippers may be an extension of this trend upon which other stressors have gradually been superimposed.

Scotland has a rich archive of detailed climate projections, land cover maps, recent management histories, and high quality historical and current data on river flow, chemistry and invertebrate populations. Combined with the multiple detailed historical studies of Dipper ecology and demography from different parts of the country this provides a unique opportunity to test how the changing balance of environmental pressures on freshwaters is integrated in its impact on a charismatic and accessible bio-indicator species. It also presents an opportunity to explore if various local and catchment scale management interventions, ultimately intended to benefit biodiversity and ecosystem service delivery, are captured in improved Dipper performance. Native riparian tree planting and peatland restoration, for example, are measures that might be expected, via a range of mechanisms, to benefit Dippers.