Salty Water Prevents Natterjack Toad Recovery in Scotland, Threatening Species Survival
High levels of salinity in water are preventing the recovery of one of the UK's rarest amphibians, rendering former breeding sites unsuitable for natterjack toad survival, a new study has concluded.
The natterjack toad, a species found in only a few locations across the UK, has its only remaining Scottish populations along the Solway Coast, including the Royal Society for the Protection of Birds' Mersehead Reserve near Southerness.
Scientists discovered a direct link between salt concentrations in water from former breeding sites in south-west Scotland and failed hatching, reduced growth, and altered development in natterjack toad spawn. Dr Frances Orton, an environmental biologist at Edinburgh's Heriot-Watt University, led the research, published in the journal Ichthyology and Herpetology.
Dr Orton explained, "Natterjack toads have declined across the UK, not just in Scotland. We aimed to understand why these tiny toads were surviving in the nature reserve in Dumfries and Galloway, but had vanished from other sites along that same coast."
The research team analysed water samples from Mersehead, where the toads persist, and from identified former breeding ponds in Caerlaverock, Southerness, and several farms. They measured temperature, pH, and salinity, exposing natterjack spawn to water from each location.
"Some of the former breeding sites had such a high level of salinity that no embryos survived to hatching," Dr Orton stated. "Others, while not lethally salty, resulted in significantly smaller toads. Size is crucial for survival; 95% of tadpoles succumb to predators, so the 5% that reach the next developmental stage need to be as large as possible to stand a chance."
These findings are crucial for improving conservation efforts. "Previously, much natterjack toad restoration focused on terrestrial habitat improvements, like clearing scrub. While still important, we now understand that unless water salinity is addressed, these tiny toads face insurmountable survival challenges," Dr Orton added. Urgent action is necessary, as amphibians represent the fastest-declining vertebrate group globally, with species like the natterjack toad rapidly nearing extinction.
