Taxonomic diversity and biogeography of the snakes in the aralo-caspian basin

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The Aralo-Caspian region is a lowland depression in the Middle and partly Western Asia and surrounded by mountain ridges of the Great and Lesser Caucasus, Alborz, Turkmeno-Khorasan, Paropamisus, northern Hindu Kush, Tien-Shan and Pamiro-Alay. The Aralo-Caspian Depression is around of Aral and Caspian Sea and includes two unequal in size, Caspian and Turan depressions. The Caspian Depression encompassing the northern part of the Caspian Sea and bordered by the Caucasus in the west and Ustyurt Plateau in the east. The Turan Depression bordered in the north by Turgay Plateau and Mugodzhar Hills, by Kopet Dagh and Paropamisus in the south, by Tien Shan and Pamiro-Alay in the south-east, and by Chu-Ily Mountains and Betpak-Dala in the east. In general, the snake fauna is composed by species of northern deserts and mountains of the Central Asia and the Eastern Mediterranea. The herpetofauna of these vast territory is relatively well studied, with the exception of its southernmost part. Further study of the biodiversity of mountain systems of the Paropamisus and Hindu Kush is required, which still remain underexplored and the species list is incomplete. The species diversity and biogeographic relations of ophidiofauna of the Aralo-Caspian basin within the Caspian and Turanian lowlands and adjacent mountainous regions will be considered here.

MATERIAL AND METODS

The species diversity on of the snake fauna and their distribution have been described in numerous monographs on the herpetofauna of the USSR, Caucasus, Iran, Middle East and Western Palaearctic (Terentyev, Chernov, 1949; Chernov, 1959; Bannikov et al., 1977; Latifi, 1991; Tuniev et al., 2009, 2019; Sindaco et al., 2013; Rajabizadeh, 2018; Egan, 2022). Physiogeographic maps of the Caucasus, northern Iran, and Middle Asia were used to clarify the distribution and species list of each region. The previously proposed schemes of zoogeographic and herpetofaunistic zoning of the Northern Eurasia, territory of the USSR, Middle Asia with adjacent territories were also used to describe the ophidiofaunas of different zoogeographic provinces (Kryzhanovsky, 1965; Szczerbak, 1981, 1982, 2003; Bobrov, Aleshchenko, 2001, 2001a; Ravkin et al., 2010; Sindaco et al., 2013). Arealography was based on the analysis of the current distribution of each species, analyzing its entire range and presence/absence in certain regions of the Aralo-Caspian basin. Chorotypes were classified according to previously proposed (Gorodkov, 1984; Sindaco et al., 2000, 2013; Taglianti et al., 1999), or new combinations were used. The classification of biogeographic groups for this region was created based on those previously proposed for various groups of animals with modifications (Darevsky, 1957, 1959, 1981; Anderson, 1968; Spitzenberger, Bauer, 1979; Tchernov, 1992; Ataev et al., 1994; Fet, 1994; Mikhailov, Fet, 1994; Tuniyev, 1995; Disi, Boehme, 1996; Borisov, 2009; Mazanayeva, Tuniyev, 2011). Comparison of species diversity from different units is estimated using a standard similarity index. Sørensen’s Quotient of Similarity: 2 × number of taxa common to both areas/sum of totals of taxa from both areas, expressed as a percentage (Sørensen, 1948).

RESULTS AND DISSCUSSION

Taxonomic diversity. Species diversity of snakes (Serpentes) of Aralo-Caspian basin are composed by 61 species from 8 families. Two species from two families of blind snakes (Scolecophidia): Leptotyphlopidae и Typhlopidae opposite to all others snakes (Alethinophidia) and number 59 species. Primitive recent snakes (Henophidia) represented only by 5 species from one family (Boidae), while the vast majority belong to advanced snakes (Caenophidia) who are 54 species from 5 families. The true colubrids (Colubridae) and vipers (Viperidae) are most diverse groups and represented by 31 and 16 species respectively. Three species of the sand snakes (Psammophiidae), two species of water snakes (Natricidae) and two species of elapids (Elapidae) rounding out the list.

Local faunas and gamma diversity. Eleven geographical units belonging to the Aralo-Caspian region and possessing characteristic ophidiofauna were identified. The snake fauna of the Aral-Caspian Basin is composed of the faunas of the North Caspian Lowland (11 species), Great Caucasus and Ciscaucasia (23 species), Lesser Caucasus and Transcaucasia (29 species), Alborz (39 species), Turkmeno-Khorasanian Mountains (29 species), Paropamisus and Hindu Kush (23 species), Karakum Desert (19 species), Kyzylkum Desert (16 species), Aralo-Caspian Isthmus (13 species), North-East Aral Lowland (10 species), Tien-Shan and Pamiro-Alay (18 species). The most species-rich regions are the Alborz with piedmont plain, Turkmeno-Khorasanian Mountains and the Transcaucasia (Table 1).

Table 1. Number of the snake species in subregions of Aralo-Caspian basin

Richness of species and genera. The higher richness of snakes in Aralo-Caspian basin was found primarily at the species level. Generic richness in three regions was identical (14 genera). In North Caspian Lowland and North-East Aral Lowland, there were fewer species. In Kyzylkum Desert were a lower average number of species per genera (1.14 species genus vs. 1.71 in Lesser Caucasus and Transcaucasia). The maximal richness is accounted for by two genera, Eirenis and Vipera, wich contain 6 of the 8 total species in the Alborz and 5 of the 7 total species in the Lesser Caucasia and Transcaucasia, respectively. 16 genera had single species per genus represented. Colubrids were the most speciose group in all subregions, whereas leptotyphlopids were most rare component in snake fauna. The North Caspian Lowland, Aralo-Caspian Isthmus and North-East Aral Lowland lacked leptotyphlopids, typhlopids and elapids, of which the others sites usually had 1–2 species (Table 2).

Table 2. Comparison of generis and species richness in the 11 subregions of study area

Zoogeographic zoning. Wide territory of Aralo-Caspian basin traditionally divided into seven zoogeographic provinces (after Kryzhanovsky, 1965): Caspian (20 snake species), Caucasian (24 species), Hyrcanian (20 species), Irano-Azerbaijanian mountane (42 species), Turanian deserted (19 species), Kazakh (13 species) and Afghano-Turkestanian (28 species). The Irano-Azerbaijanian mountane province has the highest number of species in Aralo-Caspian basin. The diversity of the Afghano-Turkestanian province remains underestimated. The whole territory of Aralo-Caspian region, according to herpeto-geographical zoning data, belongs to the Arid Mediterrano-Central Asian subregion (Szczerbak, 1981), or is divided into four sub-regions (Bobrov, Aleshchenko, 2001), with the majority of the territory falling into the Saharo-Gobian subregion (39 species), the significant part – on the Eurasian steppe (13 species), and, to a lesser extent, on the Mediterranean mountain-forest (22 species), Central Asian desert (43 species) and Central Asian mountain (11 species). Nevertheless, the most significant contribution to diversity is made precisely by the West Asian desert subregion.

Chorotype classification. Five major groups of chorotypes emerged from this study: 1. species widely distributed in Palaearctic

2. species from Mediterranean region
3. species widely or strictly distributed in West Asia
4. species distributed or extending Central Asia
5. species distributed in Paleotropics and extending to Palaearctic

Distribution patterns of species with Paleotropic extension and mainly distributed in Paleotropics reffered to same major group. Species widely distributed in the West Asia and locally distributed in Armenian Upland and Iranian Plateau merged to one group. Species distributed in countries of Eastern Mediterranea, Caucasus and Transcaucasia reffered to one big Mediterranean group. Small portion of species distributed mainly in regions of Central and Middle Asia provisionally referred to Central Asian group.

Palaearctic chorotypes

1. West Palaearctic. Chorotype of species wide distributed in the Europe, North Africa and South-West Asia.
2. East Palaearctic. Chorotype of species widespread in Middle and Central Asia and Siberia with penetration into Eastern Europe.

Mediterranean chorotypes

1. East Mediterranean. Chorotype of species distributed in the eastern part of Mediterranea including Anatolia, Armenian highland, Levant, Balkans and Caucasus.
2. Caucasian. Chorotype of species with Caucasian and Transcaucasian distribution.

West Asian chorotypes

1. Saharo-Sindian. Chorotype of species widespread from Sahara to the Sind through Arabia.
2. West Asian. Chorotype of species occuring in the Middle East including Levant, Hyrcania, Mesopotamia, Armenian and Iranian highlands with penetration into Turan and Afghanistan.

Central Asian chorotypes

1. Middle Asian. Chorotype of species distributed in the Middle Asia with extension to Iran and Central Asia.
2. Central Asian. Chorotype of species occuring in the Central Asia with extension to the Aralo-Caspian Depression.

Paleotropic chorotypes

1. South Asian. Chorotype of species distributed in the Indian subcontinent, Pakistan, Afghanistan with extension to the Aralo-Caspian Depression.
2. Afroasiatic. Chorotype of species widespread in East and North Africa, Middle East, Turan, Afghanistan and Pakistan.

Chorology and biogeographic groups. Snake diversity is unevenly distributed in the study area. The maximum diversity is recorded for Transcaucasia, Northern Iran and Afghanistan. The largest number of biogeographic groups of species is represented here. All diversity of distributional patterns is described by 10 main chorotype groups and 36 basic chorotypes (Table 3).

Table 3. Chorotypes and biogeographic affinities of snake species in Aralo-Caspian basin

Which, in turn, are formed into 25 faunistic elements, or biogeographic groups (Western Palaearctic, Eastern Palaearctic, Afro-Sindian, Saharo-Sindian, Irano-Sindian, Turano-Sindian, Mediterranean, Transcaucasian, Caucasian, West Asian, Mesopotamian, Armenian, Armeno-Iranian, Iranian, Hyrcanian, Hyrcano-Khorasanian, Turkmeno-Khorasanian, Irano-Afghanian, Turanian, Turano-Turkestanian, Hissaro-Zeravshanian, Hissaro-Alaian, Indo-Turanian, Indo-Oriental). Of these, 11 are West Asian chorotypes (27 species); 3 East Mediterranean (15 species); 3 South Asian (6 species); 3 Central Asian (3 species); 2 Middle Asian (2 species); 2 Palaearctic (6 species) and 1 Afroasiatic (2 species). The Mediterranean group included species that have a significant part of the range in the Eastern Mediterranean and a probable mediterranean origin. A new Armeno-Iranian group is also proposed for species that have a compact distribution in the Armenian and Iranian Highlands. Species widely distributed in the Middle Asia, Afghanistan and Pakistan are assigned to the West Asian, or South Palaearctic group. Wide distributed species of the deserted areas from Sahara to Southern Pakistan traditionally refer to Saharo-Sindian group. The most common element is Mediterranean (12 species), as well as Armeno-Iranian (7 species). The faunistic core is formed by West Asian and Mediterranean species. Wide distributed Palaearctic species are 6; species with South Asian distributional type are 6; the number of Caucasian (including Transcaucasian) species is 5, while the number of Central Asian species is insignificant.

Delta diversity. The similarity of local faunas is determined by the presence of common species in each unit. Zoogeographical similarity is determined by the presence of common faunal elements. The Caucasus, Transcaucasia and Alborz characterized by predominance of Mediterranien faunal element. Northern regions such as North Caspian Lowland, Aralo-Caspian Isthmus, North-East Aral Lowland and Caucasus have maximal percent of species with palaearctic distribution. Transcaucasia region and Alborz mountain system are characterized by higher number of Armeno-Iranian species. Hyrcano-Khorasanian element presents respectively in Alborz and Turkmeno-Khorasanian mountains. The real diversity of Paropamisus and northern Hindu Kush mountains still unknown, only 23 snake species currently registered there. Snakes conform to the prediction that latitudinal position and attendant higher mean annual temperature, precipitation and primary productivity correlates with increasing species richness. Δ-diversity grows in the latitudinal direction, from the plains of the Aral Sea region and the North Caspian Lowland to the mountain systems of the Iranian Plateau. The broad latitudinal gradient is however disrupted by high percentage of species restricted to the Alborz and Turkmeno-Khorasanian Mountains. Furthermore, high levels of biodiversity are also recorded in the Eastern Transcaucasia.

Endemism. The study area has a moderate level of species endemism, which is 25%. Fifteen species from three families are endemic (Eryx elegans, Eryx vittatus, Eirenis medus, Eirenis walteri, Oligodon transcaspicus, Platyceps atayevi, Zamenis persicus, Montivipera latifii, Montivipera wagneri, Vipera darevskii, Vipera dinniki, Vipera kaznakovi, Vipera pontica, Gloydius caucasicus, Gloydius rickmersi). Eight groups of endemics identified from this study (Transcaucasian, Caucasian, Armenian, Hyrcanian, Hyrcano-Khorasanian, Turkmeno-Khorasanian, Hissaro-Zeravshanian and Hissaro-Alaian). The Alborz Mountains had substantionally more endemic species than others sites. The Alborz Mountains, distinguished by the highest γ-diversity, are at the same time the center of endemism. Six of the fourteen endemics occur here, with two (Zamenis persicus, Montivipera latifii) being strictly distributed in Alborz. In general, the endemism rate is not high, with only the southernmost regions having significant species diversity and endemics. The endemic species are mainly distributed along side three main mountain ranges. The Lesser Caucasus with Armenian Upland, Alborz and Turkmeno-Khorasanian Mountains located almost in the subtropical zone and climatically different from the cold deserts and mountains of the Middle and Central Asia. The importance of the mountain systems of Paropamisus and northern Hindu Kush cannot be assessed due to the poor knowledge. It is very likely, these two mountain ridges are also centers of species richness and endemism. At present their biodiversity is composed of widespread species of West and Middle Asia. In contrast, vast areas of the subboreal zone are characterized by noticeably less diversity and their importance is secondary. The proportion of narrow-ranged species is small (11 species), most of them relate to vipers (6 species) and colubrids (4 species). The previously proposed schemes of more fractional division into provinces are not always ensured by the originality of ophidiofauna and the presence of endemism.

Gamma diversity. The faunas are most similar: Karakum Desert and Kyzylkum Desert (86%); Paropamisus + Hundu Kush and Karakum Desert (82%); Tien Shan + Pamiro-Alay and Kyzylkum Desert (82%); Kyzylkum Desert and Paropamisus + Hundu Kush (82%); Aralo-Caspian Isthmus and North-East Aral Lowland (78%); Tien Shan + Pamiro-Alay and Karakum Desert (76%); Tien Shan +Pamiro-Alay and Paropamisus + Hindu Kush (73%); Great Caucasus + Ciscaucasia and Lesser Caucasus + Transcaucasia (73%); Turkmeno-Khorasan Mountains and Paropamisus and Hundu Kush (69%); North-East Aral Lowland and North Caspian Lowland (67%); Lesser Caucasus + Transcaucasia and Alborz (65%); Turkmeno-Khorasan Mountains and Alborz (65%). Neighboring regions usually have the greatest similarity, while at the same time there is a low percentage of similarity between Lesser Caucasus + Transcaucasia and Alborz and Turkmeno-Khorasan Mountains and Alborz. The lowest similarity indices are characterized by remote regions, such as Great Caucasus and Tien Shan or Aralo-Caspian Isthmus and Lesser Caucasus (Table 4). The similarity of faunas in such cases is formed by wide-distributed and palaearctic species.

Table 4. Quotiens (%) of similarity obtained from comparing total Aralo-Caspian subregion assemblages between eleven units, gamma diversity in each case

The highest values are in bold.

CONCLUSION

The Aralo-Caspian snake fauna is more likely to be West Asian-Mediterranean than Middle Asian. The contribution of Central Asian and South Asian biogeographic elements is insignificant. The proportion of Palaearctic species is also low, and the proportion of Turanian species is minimal. Endemism of the species level is associated exclusively with the mountainous regions bordering the Aralo-Caspian Depression to the south. There are no endemic or even sub-endemic species of the vast plains of the Ciscaucasia, North Caspian and Aral Sea depressions, Ustyurt Plateau, Karakum and Kyzylkum deserts. The lack of endemics and the high level of similarity between the faunas of the northern deserts suggest that the above characteristics reflect the fauna’s youthfulness, which would limit the amount of time available for adaptive diversification. Considering the high level of biodiversity and a significant percentage of endemic species in the Alborz-Turkmeno-Khorasanian Mountains, it is possible to assume a West Asian origin of the Turanian ophidiofauna. The contribution and importance of the fauna of the vast mountain systems of Paropamisus and northern Hindu Kush are still unclear, further faunistic study of these regions will lead to the discovery of new biodiversity hotspots.

FUNDING

The study was carried out in the framework of the State Theme of the Zoological Institute, Russian Academy of Sciences (№ 122031100282-2).

CONFLICT OF INTEREST

The author declares that he has no conflict of interest.

About the authors

K. D. Milto

Author for correspondence.
Email: coluber@zin.ru
Russian Federation, Universitetskaya emb., 1, St. Petersburg, 199034

References

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