Archives for 2022

Written by Roger Downie, Froglife and University of Glasgow

Froglife’s vision is of a world in which reptile and amphibian populations are flourishing as part of healthy ecosystems. One of the ways in which we work towards this overall aim is by transforming landscapes: increasing the availability of freshwater and terrestrial habitat suitable for amphibians and reptiles. Our ‘living water’ series of projects has addressed this by creating and repairing ponds around the UK. Producing habitats is the first part of the task, but we also need to assess how well these habitats perform in helping populations to grow. Our strategy commits us to evaluating project sites for up to 10 years after completion, with some remedial work possible if damage, such as vandalism is found.

What does the scientific literature tell us about the success of created ponds? Smith et al. (2020) reviewed 28 published studies from around the world, and concluded that pond creation is an effective, beneficial conservation action. Some of the studies concerned small numbers of ponds over a fairly short time, but others were much larger in scope e.g. a Danish study of 3446 ponds over 11 years. Now comes a major study from the Swiss canton of Aargau (Moor et al., 2022) published in the high-profile journal Proceedings of the National Academy of Sciences, USA (PNAS).

Aargau, in the north of the country on the border with Germany, is one of the most densely populated and least hilly parts of Switzerland. Although about one-third wooded, it includes many small towns and an extensive road network with much agricultural land. Following concern about 30 years ago over the decline in amphibian populations (around the time of the creation of IUCN’s Declining Amphibian Populations Taskforce- DAPTF), a pond creation programme began in 1991 and continues. Moor et al. have analysed up to 20 years of data from 856 ponds, 422 of them constructed as part of the conservation programme. The canton is home to 12 species of pond-breeding amphibians (eight anurans and four newts). The authors note that these species differ in their characteristics: life history, habitat preference, and dispersal ability.

Across the whole canton, the number of ponds occupied by amphibians increased over the period 1999-2019 for 10 of the 12 species: the number remained pretty constant for the midwife toad and declined significantly only for the natterjack toad. Results differed between regions: essentially, the more ponds constructed in a region, the more metapopulations increased and the fewer declined (‘metapopulation’ here meaning the number of ponds occupied per species). The authors detected three mechanisms at play in the effects of new ponds on metapopulation sizes. First, the increased number of ponds increases breeding habitat, particularly benefitting mobile generalist species like the common toad. Second, the constant addition of new early succession stage ponds benefits pioneer species such as natterjack toads: they prefer shallow ponds with short hydroperiods, lacking vegetation and predators; it is likely that most of the constructed ponds lacked these features, and that older ponds were losing the features suitable for natterjacks, hence this species was the only one to show a decline across the canton. Third, increased pond density enhances connectivity, a particular benefit for species with low dispersal ability, like newts.

Although new ponds were rapidly colonised by several species, populations did not always persist. This could be the result of several factors, such as eventual colonisation by predators not initially present, or successional changes that made the new ponds less suitable for early colonisers. The only species where persistence probability was higher in new than in old ponds was the tree-frog, Hyla arborea: the reasons for this are not clear, but the result was a particularly sharp increase in metapopulation size across the study from <25 to over 75 sites. Several factors mattered for the success of new ponds: surface area and shape (most often, large pond size led to more species, but one species, the yellow-bellied toad Bombina variegata preferred smaller ponds; the type of surrounding habitat (some species had positive associations with surrounding forest, but natterjack toads and smooth newts preferred less forested surroundings); roads generally had negative effects on dispersal and colonisation.

The authors made the ‘encouraging conclusion’ that ‘no effort in pond construction is really wasted’. The continuous creation of new ponds helped amphibian populations to recover despite the persistence of threats like increasing urbanisation and associated roads, the presence of non-native fish and pathogens, and the effects of agrichemicals. However, there are caveats related to the particular circumstances of this study. The canton of Aargau is 37% forested, providing abundant habitat for amphibian dispersal and shelter; contrast this with the UK at 13% (England even less at 10%). The range of species in Aargau is quite large, with their range of needs being diverse enough that different kinds of ponds can be suitable for some if not all. The authors comment particularly that pond creation in the tropics may have limited value, since so many species are not pond breeders.

Finally, the authors note that it was crucial to the success reported in their study that the authorities in Aargau responded quickly to the evidence of amphibian declines by enabling a long-term pond creation programme with associated monitoring. Froglife has often been able to find funding for pond creation projects in particular areas, but funding the monitoring has been more difficult, and persuading authorities (and funders) to think of pond creation as long-term commitment is more difficult still.

References

Moor, H. et al. (2022). Bending the curve: simple but massive conservation action leads to landscape-scale recovery of amphibians. PNAS 119, e2123070119.

Smith, R.K. et al. (2020). Amphibian Conservation. Ps 9-64, in: Sutherland, W.J. et al. (eds.) What works in conservation 2020. Open Book Publishers, Cambridge, UK.