Phenological Changes in the Wintering of Pelobates fuscus (Pelobatidae, Amphibia) in the Climate Transformation Conditions in the Northern Lower Volga Region

The patterns of change in the calendar dates of the start, end, and duration of wintering of the common spadefoot toad (Pelobates fuscus) in the Medveditsa River valley (Lysogorskii district, Saratov oblast) in the period from 1892 until 2014 are considered. A reduction in the duration of wintering by seven days due to a shift in the start of wintering to a later date and that of the end of wintering to an earlier date (on average) over the last 120 years is shown. The changes in the end and start of wintering mainly began to emerge in the 1950s and 1980s, respectively, and significantly accelerated in 2002–2014. The periodicity of changes in the dates of the start and end of wintering occur mainly owing to the winter severity cycles (8–9 years) and the humidity–aridity ones (Bruckner cycle of 36–38 years). Our analysis of the temperature regime across the soil profile shows that the shortening of wintering is mainly caused by the earlier start of the spawning migration of mature individuals of this species in the spring. The depth of location of P. fuscus individuals in the soil profile has decreased steadily from the early 1980s to 2014. Predictive models have been developed allowing us to forecast the termination of movement of P. fuscus individuals in the soil profile from the horizons where they can be found in the period of activity by 2050, as well as a significant reduction in the wintering duration or even its complete disappearance as a phase of the annual cycle in the course of 315 years provided that the modern climatic trend will continue.