Occurrence of the amphibians in the Volga, Don River basins and adjacent territories (Russia): research in 1996-2020

Abstract Background Knowledge about the distribution of living organisms on Earth is very important for many areas of biological science and understanding of the surrounding world. However, much of the existing distributional data are scattered throughout a multitude of sources, such as taxonomic publications, checklists and natural history collections and often, bringing them together is difficult. A very successful attempt to solve this problem is the GBIF project, which allows a huge number of researchers to publish data in one place in a single standard. Our dataset represents a significant addition to the occurrences of amphibians in the Volga, Don riverine basins and adjacent territories. The dataset contains up-to-date information on amphibian occurrences in the Volga river basin and adjacent territories, located for the most part on the Russian plain of European Russia. The dataset is based on our own studies that were conducted in the years 1996-2020. The dataset consists of 5,030 incident records, all linked to geographical coordinates. A total of 13 amphibian species belonging to nine genera and six families have been registered within the studied territory, although the distribution of amphibian species in this region of Russia has not yet been fully studied. This is especially relevant with the spread of cryptic species that can only be identified using molecular genetic research methods. The main purpose of publishing a database is to make our data available in the global biodiversity system to a wide range of users. The data can be used by researchers, as well as helping the authorities to manage their territory more efficiently. New information All occurrences are published in GBIF for the first time. Most of the data are stored in field diaries and we would like to make it available to everyone by adding it in the global biodiversity database (GBIF).

the existing distributional data are scattered throughout a multitude of sources, such as taxonomic publications, checklists and natural history collections and often, bringing them together is difficult. A very successful attempt to solve this problem is the GBIF project, which allows a huge number of researchers to publish data in one place in a single standard. Our dataset represents a significant addition to the occurrences of amphibians in the Volga, Don riverine basins and adjacent territories.
The dataset contains up-to-date information on amphibian occurrences in the Volga river basin and adjacent territories, located for the most part on the Russian plain of European Russia. The dataset is based on our own studies that were conducted in the years 1996-2020. The dataset consists of 5,030 incident records, all linked to geographical coordinates. A total of 13 amphibian species belonging to nine genera and six families have been registered within the studied territory, although the distribution of amphibian species in this region of Russia has not yet been fully studied. This is especially relevant with the spread of cryptic species that can only be identified using molecular genetic research methods.
The main purpose of publishing a database is to make our data available in the global biodiversity system to a wide range of users. The data can be used by researchers, as well as helping the authorities to manage their territory more efficiently.

Introduction
Amphibians are an important group of ectothermal animals that are particularly sensitive to global climate change and environmental conditions (Trochet et al. 2016, Kestemont 2019, Frishkoff et al. 2019, Préau et al. 2019, Chikhlyaev et al. 2020. Climate change will change the geographical ranges of species and have an effect on dispersal capacity (Duan et al. 2016, Johovic et al. 2020. Changes in climate conditions in recent years have also affected growth, reproduction, phenology, survival, dispersal, distribution, parasitic relationships, competitive interactions and food availability for individuals (Blaustein et al. 2002, Askenderov et al. 2018, Bosch et al. 2018, Lebedinskii et al. 2019, Vedernikov et al. 2020. The introduction of invasive species into reservoirs is also important for reproduction, as it influences the ability of amphibian populations to reproduce (Ruchin et al. 2019, Polo-Cavia et al. 2020. Climate change contributed to the expansion of the range of Pelophylax ridibundus, which is gradually spreading to the north and begins to occupy all habitats suitable for it. On the other hand, the abundance and range of Salamandrella keyserlingii in the study area is gradually decreasing due to climate warming. Tailed amphibians are also significantly influenced by the appearance of the fish Perccottus glenii in spawning reservoirs as this is capable of destroying their larvae (Reshetnikov and Karyagina 2015). Over the past 10 years, the population of Bombina bombina has decreased significantly. It can be assumed that this decrease is associated with weak floods, which do not fill the spawning reservoirs with water.
The Volga is the longest river in Europe and the 16th largest in the world. Its length is 3690 km. The area of the Volga basin is about 1.36 million km , which is 33% of the territory of European Russia. There are different types of biomes in the Volga basin, such as taiga in the north and semi-desert in the south (Tockner et al. 2009). The Volga basin includes all or part of the territory of 37 regions of Russia. The Don is the third largest river in the European part of Russia with a length of 1870 km and area of 0.43 million km . The main part of the Don basin is forest-steppe and steppe biomes, which are very heavily exposed to agricultural activities. This basin includes all or part of the territory of 14 regions of Russia.
This work can be considered the next stage in the study of distribution and abundance of amphibians for this territory after publication of "Materials for inventory of amphibians and reptiles of the Middle Volga" (Pestov 2002). In addition to new data on distribution and abundance of species, it has been established that species, previously considered Pelobates fuscus, consists of two cryptic species -Pelobates fuscus and Pelobates vespertinus. The boundaries of their areas became clear, passing within the study area (Dufresnes et al. 2019b). The distribution of population systems of the Pelophylax esculentus group in the Volga basin also became clearer. A specific feature is the reduced occurrence (lower abundance) of Pelophylax esculentus here and relatively frequent occurrence of the REL-type population systems (Litvinchuk et al. 2020).
This study aims to describe a dataset consisting of up-to-date data on the occurrence of amphibians in the Volga and Don river basins (European Russia), which we have recently published in GBIF as the Darwin Core Archive (Ruchin 2020). This article was prepared as a "data paper" (Penev et al. 2017).

Project description
Title: Occurrence of the amphibians in the Volga and Don River basins (Russia): research in 1996-2020 Study area description: Brief description of the Volga and Don River basins 2 2 Sampling methods Quality control: Each observation contained fundamental information, such as location (coordinates), date, name of observer and name of identifier. A large part of the coordinates was determined directly on site with the help of a GPS device. In other cases, Google Maps (2020) were used. Species were identified according to Dunaev and Orlova (Dunaev and Orlova 2017). The main part of the species was determined at the site by external signs, as well as by acoustic methods, without killing the animal.
Geographical reference was made by fixing the coordinates of the meeting point of the amphibians using a GPS Navigator or using Google maps. The margin of error in the measurement of coordinates is 50 m. The accuracy of determining coordinates is up to the fourth digit. In all cases, the WGS-84 coordinate system is used.

Geographic coverage
Description: The dataset contains information about the occurrence of amphibians in 27 regions of Russia: the Chuvash Republic, the Republic of Mari-El, the Republic of Tatarstan, the Republic of Mordovia, the Republic of Kalmykia, the Republic of Udmurtia, the Republic of Komi, Perm Kray, Kaluga, Vladimir, Ryazan, Ivanovo, Tambov, Penza, Moscow, Voronezh, Kursk, Saratov, Samara, Astrakhan, Rostov, Lipetsk, Tula, Kirov, Orenburg, Ulyanovsk and Nizhny Novgorod regions.
The study area is located within the Eastern European plain (Fig. 1). In the eEast, there is the Volga upland with maximum heights up to 350 m above sea level, in the West is the Central Russian upland (up to 300 m above sea level). Between them, there is the Oka-Don plain (up to 180 m above sea level). The territory is located in a temperate climate zone. The total duration of the period with an average daily air temperature below freezing is 140-150 days per year. The study area is divided into two different basins -the Black Sea basin (the Don river watershed) and the Caspian Sea basin (the Volga river watershed). All the rivers in the region are typically low-lying and belong to the Eastern European type. Its main characteristic is seasonal run-off. Spring floods occur in spring, water flow is minimal in summer and winter and river flow increases in autumn. Rivers have a mixed feed, which is made up of melting snow, precipitation and groundwater The study area is crossed by the boundaries of the ranges of 11 species. In the east, the Kama basin passes the western part of the border area Salamandrella keyserlingii. The southern border of the distribution of many species (Lissotriton vulgaris, Triturus cristatus, Bombina bombina, Bufo bufo, Rana arvalis, Rana temporaria) coincides with the border of the middle and lower Volga (Kuzmin 2012Faizulin et al. 2018Zaks et al. 2011). The range of Pelobates vespertinus almost completely coincides with the boundaries of the study area, with the exception of the Bryansk and Kaluga regions in the west and Ural Mountains in the east (Bulakhova et al. 2020). In a small part of the eastern area, Pelobates fuscus is included in boundaries of the study area (Dufresnes et al. 2019b). The study area includes the western branch of the Pelophylax lessonae range, which also largely excludes the Lower Volga. The range of Pelophylax esculentus practically coincides with the range of Pelophylax lessonae (Fayzulin et al. 2018Ivanov et al. 2019.

Taxonomic coverage
Description: All amphibian individuals were identified to species. The taxonomic diversity of the studied area is represented by 13 species belonging to six families from two orders. Given the scale of targeted studies of fauna, this is an almost exhaustive list of species that form natural self-reproducing populations.  Collecting sites in the Volga and Don regions.
Occurrence of the amphibians in the Volga, Don River basins and adjacent ...