Natterjack Toad (Epidalea calamita) Monitoring and Conservation Status 2016-2018

Abstract

The current report represents the outcome of a three-year survey from 2016-18 of the Natterjack Toad (Epidalea calamita) population in Ireland. The goal was to provide information for the assessment of the conservation status of the species and provide recommendations for future management. A total of 169 water bodies, both traditional (natural) sites and artificially constructed ponds created as part of the National Parks and Wildlife Service (NPWS) Pond Creation Scheme, were surveyed for presence of Natterjack Toad breeding activity (egg strings, tadpoles, toadlets or adults) during each breeding season (April-July) 2016-18. There was substantial variation in fecundity between years with 3,222 egg strings ± 2% [95%CI 3,156 - 3,288] laid during 2016, 1,449 ± 1.6% [1,431 - 1,467] egg strings in 2017 and 2,681 ± 1.2% [2,639 - 2,723] egg strings in 2018. Low productivity during 2017 was likely due to fewer ephemeral ponds forming in sand dune slacks, most notably at the Maharees and Inch, due to low overwinter rainfall reducing the number of sites available for breeding. Translating egg string counts into total population estimates (as has been attempted previously) resulted in extremely wide estimates of potential error. Egg string counts allow the number of females that bred each year to be estimated but without data on the proportion of females that breed and the sex ratio in operation in Ireland synchronous with egg string counts, we strongly recommend that no attempt is made at estimating the total population. Toadlet abundance (the estimated number of individuals per pond) was highly correlated with egg string production (r2=0.90) making it redundant as a second measure of population productivity i.e. it shows the same pattern despite requiring considerable additional survey effort. Thus, we recommend that egg string production is used as the sole measure of temporal trends in population productivity. Traditional (naturally occurring) breeding sites accounted for the vast majority (>90%) of egg string production with the single most productive area being the Maharees, north Dingle Peninsula. Variation in egg string counts was related to the preceding overwinter rainfall and the number of ephemeral ponds that formed by the beginning of the breeding season i.e. the availability of suitable sites, but appeared unaffected by spring temperatures. Natterjack Toad egg string production declined by 23% over the period 2004 to 2018 (using data from previous studies). After statistical correction for the effects of overwinter rainfall and the number of ephemeral ponds that formed each year, this decline was estimated to be as much as 66%. Ecological analyses suggested that the Natterjack Toad is more likely to breed in ponds with a large surface area, those in sand dune habitats, with high conductivity, and a high percentage cover of aquatic vegetation at the substrate with short terrestrial vegetation in the surrounding vicinity. Natterjack Toads colonised 22 artificial ponds (not all occupied every year) with breeding in 16 ponds during 2016, 10 during 2017 and 13 during 2018. Twenty new breeding sites (all adjacent to established breeding sites) were discovered resulting in the current species range increasing by 3 x 2km cells (+19%) compared to the previous survey during 2011-12 (Sweeney et al. 2013); representing better information and not reflective of actual range extension. The main threats and pressures that the Natterjack Toad faces in Ireland include land abandonment and lack of grazing (ponds must be surrounded by short swards); climate change altering overwinter conditions and the onset of spring, for example, influencing the formation of ephemeral ponds and the risk of early drying out; as well as water pollution i.e. toxic, or deoxygenating, algal blooms. We make a series of recommendations for consideration in future studies to reduce survey effort making surveys more efficient whilst providing suggestions for priority actions to maintain or restore specific sites to good conservation status.