The eastern boundary of the geographic range of the Pallas’ spadefoot Pelobates vespertinus (Anura, Amphibia) is limited by overwintering temperatures

Abstract

We studied the relationship between the geographic distribution of the Pallas’ spadefoot (Pelobates vespertinus) and the soil temperature regime at the eastern boundary of this species’ range (south-western West Siberia and northwest Kazakhstan). This species overwinters underground, burrowing down to 1.5 m or deeper and has poor frost tolerance – it is unable to withstand temperatures below 0 °C, therefore, the temperature at its overwintering depth has to be above zero. A cartographic approach to identifying the distribution of zero isotherms was used. Winter soil temperatures at depths of 80 cm, 120 cm, 160 cm and 240 cm were plotted, based on deep soil thermometer data and the outline of the spadefoot’s range was compared with close-to-zero isotherms in deep soil. Within the range of this species, the depth of the position of zero isotherms increases eastwards: from 80 cm in European Russia, to more than 160 cm in the south-west of Western Siberia. The eastern boundary of the species’ range lies in the forest-steppe, steppe and semi-desert zones in the Tobol-Ishim interfluve and further south, on the left bank area of the Turgai River. This boundary clearly coincides with the zero isotherm, which lies here at a depth of 160 cm, forming an impermeable barrier to the spadefoot’s distribution.