The range of marsh frogs (complex Pelophylax ridibundus, Amphibia, Ranidae) in Kazakhstan: Progressive dispersal or cyclic fluctuations?

According to 2005 data, during the second half of the 20th century, the range of marsh frogs (Pelophylax ridibundus complex) in Kazakhstan almost doubled, which was facilitated by the unintentional introduction of these amphibians in the central and eastern regions of the country against the backdrop of favorable climate change. This paper analyzes the results of the next monitoring of the distribution of the marsh frogs in Kazakhstan in the light of the hypothesis of the ongoing dispersal of amphibians throughout the country. During the revision of literature, museum and archival materials over the past 15 years and the analysis of the authors’ field data for 2021, about 500 amphibian sighting points were collected, which is almost 2 times higher than previously known information. It has been established that the modern range of the complex occupies the territory of all major hydrographic basins of Kazakhstan: The Ural-Emba, Aral-Syrdarya, Nura-Tengiz, Balkhash-Alakol, Tobol-Ishim and Irtysh basins, of which only the last two belong to the area of oceanic runoff, the rest are the drainless inland. A chronological analysis of the data obtained for each basin made it possible to conclude that over the historical period the area of the marsh frogs’ range has changed, but mainly due to periodic reductions or expansions within the drainless inland basins, the level and mineralization of water bodies of which are determined by cyclic climate fluctuations. In a broad sense, it is proposed to talk about the constancy of the autochthonous range of the marsh frogs in the west, south and southeast of Kazakhstan. The phenomenon of “settlement” includes the movements of lake frogs within the Nura-Tengiz and Irtysh basins, where they did not live in the historical past. In geological retrospect, this process probably restores the boundaries of the Neogene distribution of representatives of the P. ridibundus complex. From the point of view of the genetic composition of the complex, one can speak of the dispersal or even expansion of the Anatolian P. cf. bedriagae, which has successfully advanced to all regions of Kazakhstan from the eastern borders of its autochthonous range in the Caspian Plain and the coasts of the Mangyshlak Peninsula.