Population genetic structure of the Natterjack toad (Epidalea calamita) in Ireland: implications for conservation management

Abstract

Molecular methods can play a crucial role in species management and conservation. Despite the usefulness of genetic approaches, they are often not explicitly included as part of species recovery plans and conservation practises. The Natterjack toad (Epidalea calamita) is regionally Red-Listed as Endangered in Ireland. The species is declining and is now present at just seven sites within a highly restricted range. This study used 13 highly polymorphic microsatellite markers to analyse the population genetic diversity and structure. Genetic diversity was high with expected heterozygosity between 0.55 and 0.61 and allelic richness between 4.77 and 5.92. Effective population sizes were small (Ne < 100 individuals), but not abnormal for pond breeding amphibians. However, there was no evidence of historical or contemporary genetic bottlenecks or high levels of inbreeding. We identified a positive relationship between Ne and breeding pond surface area, suggesting that environmental factors are a key determinant of population size. Significant genetic structuring was detected throughout the species’ range, and we identified four genetic entities that should be considered in the species’ conservation strategies. Management should focus on preventing further population declines and future loss of genetic diversity overall and within genetic entities while maintaining adequate local effective population size through site-specific protection, human-mediated translocations and head-start programs. The apparent high levels of genetic variation give hope for the conservation of Ireland’s rarest amphibian if appropriately protected and managed.

Landscape genetics identifies barriers to Natterjack toad metapopulation dispersal

“…Other demographic parameters such as low local breeding success may also contribute to lower recruitment and dispersal. Certainly, Natterjack toadsat Killeen and Roscullen have exhibited substantial census size declines over the past 15 years (Reyne et al 2019(Reyne et al , 2021b. We propose including Killeen and Roscullen in the Natterjack toad NPWS 'Head-start and Reintroduction Programme' with occupied breeding sites used as a source population, with assisted migration and translocation to adjacent artificial ponds.…”

“…The main habitats in this area are sand dunes and grasslands both identified in the present study as facilitating dispersal and gene flow. Cox et al (2017) demonstrated that dispersal of the Natterjack toad is facilitated by sand dunes and beaches, and the largest Natterjack toad population in Ireland is in the Magharees sand dune system, but breeding depends on the formation of ephemeral breeding pools in sand dune slacks (Reyne et al 2019(Reyne et al , 2021b. Access to permanent ponds at the nearby Castlegregory Golf Course (CGC) and suitable sites along the lakeshore of Lough Gill provide a valuable alternative in dry years, thus maintaining connectivity between the breeding sites is essential.…”

“…Grasslands and proximity to the coast facilitated Natterjack toad dispersal and gene flow with the species known to inhabit and forage even on intensively managed agricultural lands most notably during summer (Miaud et al 2000;Miaud and Sanuy 2005;Schweizer 2014). A further 20 artificial ponds that have been created in proximity (within 4 km) to Killeen and Roscullen have not yet been colonised naturally up to 10 years after their creation (Reyne et al 2019(Reyne et al , 2021b. Most adult toads have high breeding-site fidelity with dispersal usually by juveniles (Stevens et al 2004).…”

“…DNA extraction and genotyping followed Reyne et al (2022). Genomic DNA was extracted following a high salt extraction protocol.…”

“…To halt the Natterjack toad decline, the National Parks and Wildlife Service (NPWS) initiated a 'Pond Creation Scheme' in 2008 installing over 100 artificial ponds on agricultural grasslands throughout the Natterjack toad range. Only artificial ponds in close proximity to natural breeding pools had been colonised by 2018 (Reyne et al 2019(Reyne et al , 2021a. Consequently, the NPWS initiated a Natterjack toad 'Head-start and Reintroduction Programme' in 2016 by collecting egg strings/tadpoles annually to raise toadlets in captivity before release back into the wild as part of assisted migration and translocation to newly created ponds.…”