UK’s rarest toad can’t survive in salty water, study shows
Salty water could be preventing the recovery of the Natterjack Toad, one of the UK’s rarest amphibians, by making former breeding sites unsuitable for their survival.
The Natterjack Toad is found in a handful of locations across the UK.
In Scotland, there is just one remaining population in the RSPB Mersehead Nature Reserve on the Solway Coast.
Scientists have found that water from former breeding sites in south west Scotland was linked to failed hatching, smaller growth and altered development.
The research is published in the academic journal Ichthyology & Herpetology.
Dr Frances Orton, an environmental biologist at Heriot-Watt University in Edinburgh, led the research project.
Dr Orton, from Heriot-Watt's School of Energy, Geoscience, Infrastructure and Society said: “Natterjack Toads have declined across the UK, not just in Scotland.
“We wanted to find out why these tiny toads were surviving in the nature reserve in Dumfries and Galloway, but had disappeared from sites along that coast.
“We used anecdotal reports from farmers and local wildlife groups to identify former breeding ponds in Caerlaverock, Southerness and several farms.”
The team analysed water samples from Mersehead, where the Natterjack Toad survives, and six other sites.
They measured temperature, pH and salinity and exposed Natterjack spawn to water from each of the sites.
Dr Orton said “Some of the former breeding sites had such a high level of salinity that no embryos survived to hatching.
“Some weren’t as lethally salty, but what we saw there was that the toads were much smaller.
“That doesn’t sound like a big deal, but when you’re a frog, size really does matter. 95% of tadpoles are eaten by predators. For the five percent that make it to the next stage of development, they need to be as big as possible for a chance at survival.”
Orton says the findings could help more focused restoration efforts.
“Until now, a lot of Natterjack Toad restoration efforts have focused on improving terrestrial habitat, like clearing scrub or controlling vegetation.
“That’s still important, but now we know that unless the salinity of the water is tackled, the tiny toads will have no chance of survival.
“Amphibians are the fastest-declining vertebrate group globally. They’ve been around for 350 million years, but now species like the Natterjack Toad are disappearing, quickly.
“They play a huge ecological role as both predators and prey - they feed lots of animal species and, as gardeners will tell you, they eat lots of slugs and midges.
“Natterjack Toads are on the verge of extinction and it’s vital we understand ways to protect and boost the populations that remain.”
Dr Orton and her team conducted the research in 2019-2021 across seven sites in Dumfries and Galloway. The research was supported by the Carnegie Trust and Nature Scot.