Invasion of the North: The Eastern Tree Frog (Hyla orientalis) Range Shift in the East Baltic Sea Region

Peculiarities of distribution of eastern tree frog in Kaliningrad Oblast’ of Russia were studied. Until recently, only rare records of tree frogs were observed here, which are probably associated with introductions or penetration of the species from border regions of Lithuania and Poland. However, in the period 2007 – 2022 we revealed here 41 new localities of Hyla orientalis throughout the southern part of Kaliningrad Oblast’. In all studied localities, where we found mating tree frogs, shallow fish-free puddles surrounding by agricultural fields were used as breeding habitats. Local populations of the species are quite abundant. The northern boundary of native range of H. orientalis in 1940 was on the adjacent territory of north-eastern Poland. Thus, we recorded a shift to the north of the range boundary of the species by approximately 30 – 40 km over the past 82 years. This is most likely is related to global warming. In the region, the average temperature and the amount of precipitation are increasing, which lead to favorable conditions for the long-term existence of warm puddles in the fields, in which tadpoles can successfully and more rapidly develop. The appearance of a large number of such biotopes allowed this species to successfully populate this territory.