ARC's Pool Frog Recovery Project Manager, John Baker explains why an extremely dry summer may have had negative impacts on the UK's rarest frogs, but it's not all bad news

In spite of what seems like an awful lot of rain over the last month or so, the water tables at the pool frog sites are still low.  At the beginning of November some of ponds were still completely dry. Apparently the ponds at Thompson Common have been this dry before – and bounced back – but it is something I have not seen before. Pond drying is likely to have harmed the northern pool frog populations in several ways.  Some breeding ponds dried before the tadpoles could complete metamorphosis, which usually occurs throughout August. Other breeding ponds held some water, but the levels dropped to expose bare mud around the margins, with no vegetation cover for metamorphosing frogs to hide from predators. The dry conditions are also likely to have been problematic even after the remaining young frogs had left their ponds because the terrestrial habitat was also exceptionally dry, reducing foraging opportunities and, ultimately, their chances of survival.

On a more positive note, the low water levels have given an unusual view of the pond basins. The ponds inhabited by northern pool frogs in Norfolk are pingos, which were naturally created by repeated freezing and thawing of ice. Although some of the pingos are relatively large, it turns out that they are quite shallow in the centre. Shallower than you might expect for such large ponds.  Perhaps it is the combination of large, but relatively shallow ponds that don’t dry in most years that makes pingos suitable breeding sites for northern pool frogs. In general, large ponds tend to be deep and hence relatively cold.  Northern pool frogs need warm aquatic habitat.

Pond drying may not be entirely harmful. Northern pool frogs prefer the early stages of succession in ponds. Over time ponds tend to become more heavily vegetated and shaded and populations of predators, including fish and large predatory insects, may build up. Pond drying may ‘re-set’ the pond to an earlier successional state, removing predators and improving conditions for the frogs. It will take some time for the impacts of this year’s drought and heatwaves to filter through to survey results. The frogs reintroduced to Thompson Common (2015-2021) had just become firmly established this year - immediately prior to this summer. So hopefully the population will be sufficiently robust to withstand this year’s weather extremes. Time (and population monitoring) will tell.

Ponds at the other site, where northern pool frog were originally introduced from Sweden, also dried this summer.  Most of these had become too overgrown and shaded for the frogs. Pond drying won’t knock back the shading vegetation, which is mostly tall emergent plants such as reed. Fortunately ARC has engaged an experienced pond management contractor to remove vegetation mechanically. The benefits of this pond restoration will probably not kick in immediately, because the frogs generally do not move into restored ponds until the second year after restoration. Again – time will tell.

If you would like to find out more about ARC’s Pool Frog Recovery Project please visit the project page.

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This is a long-term project to restore England’s rarest amphibian.  ARC’s pool frog recovery work is currently funded by The Beveridge Herpetological Trust, The Douglas and Joanne Chapman Chapman Animal Trust and The William Dean Countryside and Educational Trust.