Factors Driving Localized Extinctions of the Rare and Threatened Natterjack Toad (Epidalea calamita) in Scotland (UK)

Amphibians are declining on a global scale, and such large-scale declines are often driven by localized extinctions. Across the UK, the rarest amphibian species is the Natterjack Toad (Epidalea calamita). This species has experienced severe declines, with only one viable population remaining in Scotland (Mersehead RSPB). In this study, we set out to investigate the potential contribution of sub-optimal water quality to declines in its previous stronghold, in South West Scotland. First (2018), water quality parameters were recorded from Mersehead and compared with water quality in pools (n = 3) from which Natterjack Toads had been previously recorded but were no longer found (since ∼2013, anecdotal evidence). This was followed by a field-to-laboratory experimental design (2019, 2021), whereby toadspawn strings/embryos were reared in water collected from Mersehead (‘control’) and from potentially suitable pools for Natterjack Toads (n = 7, ‘test sites’). Water quality parameters (temperature, pH, total dissolved solids, salinity) were recorded across test sites, as well as biological endpoints in reared tadpoles. Mersehead consistently presented favorable water quality compared to sites not currently inhabited by Natterjack Toads (e.g., higher temperature, neutral pH, lower total dissolved solids, lower salinity). In 2019, embryos did not hatch successfully when reared in water collected from two of the test sites, likely due to the excessively high salinity recorded (13–20 PSU). For the remaining test sites (2021, n = 5), hatch success did not differ compared to Mersehead. However, at completion of metamorphosis, smaller body mass and snout–vent length were recorded in individuals reared in water collected from all of the uninhabited test sites compared to those reared in water collected from Mersehead. In addition, masculinization and/or delayed gonadal development was observed for two of the test sites, compared to Mersehead. Both these features have been reported in response to pollution in wild anuran populations previously. The findings from this project provide indications for a negative impact of water quality on survival, growth, and development of Natterjack Toads and indicate sub-optimal water quality could be contributing to localized extinctions and subsequent population declines for Natterjack Toads in Scotland.