Based on molecular evidence, Vipera nikolskii represents a subspecific lineage of V. berus, with some evidence of hybridization between the two taxa in natural populations (Milto and Zinenko 2005, Zinenko et al. 2010), and in this assessment Vipera nikolskii is retained within Vipera berus, in contrast with the current approach of The Reptile Database (Uetz 2023) which considers V. nikolskii a valid species.

Vipera sachalinensis, another taxon sometimes considered a full species, is included within the V. berus complex by Kalyabina-Hauf et al. (2004) and Joger et al. (2007), and this too is followed here.

Freitas et al. (2020) proposed synonymising V. barani within V. berus based on their integrative evaluation, and it was found to fall within V. berus by Dufresnes et al. (in press). Further data is however required to validate this arrangement and the Taxonomic Committee of the Viper Specialist Group has so far not reached a consensus decision on the appropriate status for this taxon (W. Wüster pers. comm. 2023). Vipera barani is however an evolutionarily distinct lineage, and to maintain taxonomic stability it is treated as a distinct species for consistency with past Red List assessments of both V. barani and V. berus.

Finally, subpopulations in the western Alps in Italy were described as a new species, Vipera walser, by Ghielmi et al. (2016). The validity of this species is contested, and it was not accepted by Speybroeck et al. (2020). Phylogenomic research (Dufresnes et al. in press) which included additional nuclear genes confirmed that V. walser falls within Vipera berus in a nuclear phylogeny. These authors proposed that this taxon should be considered subspecies of V. berus, a position endorsed by the Taxonomic Commitee of the Viper Specialist Group (W. Wüster pers. comm. 2024) and adopted here.

European regional assessment: Least Concern (LC)
EU 27 regional assessment: Least Concern (LC)

The Adder is widespread across the European region, although it now persists in parts of its range as fragmented subpopulations. The species is assessed as Least Concern for Europe in view of its wide distribution, which encompasses the majority of Europe, tolerance of a broad range of habitats, and presumed large population size.

As it is declining across much of its range within the European Union it is assessed as Least Concern, on the basis that the evidence for range-wide rates of decline in this region is not presently sufficient to justify an inference that the rate of decline approaches 30% over a three-generation period. Despite difficulties in quantifying rates of decline in all range states for which data are available and the variable quality and spatial scale of available data, in at least some well-studied parts of this long-lived snake's range it appears to have approached or exceeded 30% - and in some areas 60% - in an assumed three generation period of at least 24 year. A future assessment may indicate a more threatened category could be warranted. Global warming will have a very strong impact on the southernmost subpopulations, which are already isolated in insular habitats within landscapes and at mid- to high elevations (e.g. in the Balkans).

This widespread species has the largest native range of any terrestrial snake. In Europe, it ranges southwards from 70°N in Scandinavia (where the species has a fragmented distribution in the northern part of the range) to the Baltic region through to France and Great Britain. It also occurs eastwards through Europe (excluding Iberia, peninsular Italy and most of Greece) to European Russia (where it occurs in all regions of the country other than Southern European Russia). In the northwest of European Russia, the range extends in to the Arctic Circle (where it is known from the Lapland Reserve and from the coast of the Barents Sea; I. Doronin pers. comm. January 2023). On the Balkan Peninsula, it is largely restricted to lowland riparian zones of large rivers in the northernmost part and to montane areas in most of the region and has a very fragmented distribution there. In Slovenia, it is predominantly distributed above 1,000 m asl (M. Vogrin pers. comm. November 2022). In Bulgaria, it is mainly distributed in the mid- to alpine mountains (up to 2,700 m asl.), with a few extant lowland subpopulations in the eastern part (Buresch and Zonkov 1934, Stojanov et al. 2011). In Greece, it occurs only in regional units of Florina, Pella and Rodopi (northern Greece) (M. Dimaki pers. comm. November 2022). In the Alps, it has been recorded from sea level up to 3,100 m above sea level (S. Ursenbacher pers. obs. 2022). In the European part of Russia, the southern border runs along the Volga region (Republic of Bashkortostan, Saratov, Ulyanovsk regions; I. Doronin pers. comm. January 2023).

Beyond the European region, the species ranges from European Russia as far east as eastern Siberia and the Lena River basin in Asian Russia, northern Mongolia, and northwestern China (Zhao and Adler 1993). In China, this species is known from Altai, Fuhai (Zhao 2006) and Kanas (Wang and Zhai 2005) in Xinjiang, and from the Changbai Mountains of Jilin (Bannikov et al. 1977, Ananjeva et al. 1998, Bakiyev et al. 2015).

The Adder can be common in suitable habitats. Across its range, it is often restricted to fragmentary subpopulations and isolated montane relicts (Sillero et al. 2014). In some montane parts of its range, such as Bulgaria and Greece, it is reported to be a rare species (A. Westerström pers. comm.). Conversely, in some Bulgarian subpopulations (e.g. Vitosha Mountain) it can be very abundant at present, although many are isolated and may be at risk from climate change and habitat modification (Y. Kornilev and A. Dyugmedzhiev pers. comm. November 2022). Based on field research in Belarus conducted between 2011 and 2017 a national population size of 500-550,000 individuals has been estimated (Drobiankov 2018), with densities varying from 0.5-150 individuals/ha on average (Drobiankov 2018).

In the European portion of Russia, it is still found across a vast territory and exhibits a fairly high density of subpopulations in a number of regions of Central European Russia, the Volga Region, and in the Novgorod and Leningrad regions (B. Tuniyev pers. comm. January 2023). In areas where it remains common, animals may gather in aggregations of several hundred in preparation for entering winter dormancy, and may locally occur at densities of up to several tens per hectare in summer (I. Doronin pers. comm. 2022). It does however occur in patchily, where in natural habitats its density rarely exceeds at 1-2 ind./ha even in comparatively high-density "snake pockets" (I. Doronin pers. comm. 2022). In European Russia, densities approach or reach 10 ind./ha only in moderately disturbed anthropogenic habitats, such as abandoned villages and dumps of industrial timber waste (I. Doronin pers. comm. 2022). In Germany, there have been population declines and some local extinctions, with an overall decline of 50-70% over the preceding century reported by Völkl and Thiesmeier (2002) and no evidence that efforts at conservation over the subsequent 20 years have yet succeeded in halting declines (T. Kirschey pers. comm. 2022). It is considered to be rare and restricted to cooler areas in this country, and stable subpopulations exist only at higher elevations and parts of the Baltic Sea coast (Julian and Hodges 2019). It suffered from strong declines in the early part of the 20th Century, and due to ongoing pressures in the country it is assumed to have undergone a sharp decline in the last two decades which is presumably ongoing (Rote-Liste-Gremium Amphibien und Reptilien 2020). Significant declines were reported in 2002 and have not been reversed despite 20 years of conservation efforts in Germany (Otte et al. 2020). In Brandenburg, only one of the three healthiest subpopulations exhibited a positive population trend (from 15 to 35 vipers recorded) between 2006 and 2019, as an incidental consequence of deforestation (Otte et al. 2020). One other exhibited an 80% decline between 2008 and 2019. The final one recovered until 2015 before declining, the latter attributed to drainage and increased wild boar populations. In Saxony, subpopulations are in gradual but significant decline, inferred from the proportion of subpopulations lost in three time periods: 1960-1990 (54.1%), 1990-2001 (35.5%), and 2002-2018 (31.7%) (Teufert et al. 2022). Efforts at data collection in Saxony have been intensified over the past decade, allowing confidence that the absence of records from formerly occupied cells represents the genuine loss of those subpopulations (T. Kirschey pers. comm. 2022).

It is considered the most threatened of Britain's widespread reptiles, due to observed declines combined with naturally low population densities. In Britain, only small, fragmented subpopulations remain in many places and these are in steep decline, whereas larger subpopulations appear to be maintaining themselves (Gardner et al. 2019). The British population is estimated to have undergone an island-wide decline of as much as 39% between the 1980s and the early 2010s (Gleed-Owen and Langham 2012). A conference on the current status of the species in Britain concluded that the country may be left with "a few large, flourishing populations" but that most smaller subpopulations will become extinct if declines continue at the current rate, a pattern already observed in Belgium, France, Germany and the Netherlands (Julian and Hodges 2019). Garder et al. (2019) project extinction for all smaller subpopulations, corresponding to 90% of the British subpopulations (119 of 129 subpopulations), by 2032 (approximately or less than two generations; S. Ursenbacher pers. comm. 2022).

A large subpopulation in Belgium preserved on a military training site contains several thousand individuals at any time and is "exceptionally stable" based on a 20-year study (Julian and Hodges 2019). This subpopulation reaches "high" densities of 6-14 ind./ha, and based on a capture-mark-recapture survey this subpopulation may contain as many as 3,000-6,000 individuals (Graitson et al. 2022). Habitat suitability modelling suggests that adders are present across only about 10% of the available habitat and that it is, therefore, possible that this subpopulation could expand in future (Graitson et al. 2022). But more generally, in Wallonia, the local red list suggested a generally strong reduction in the whole country (Jacob et al. 2007).

The population in Switzerland decreased by 78% during the last century (Monney and Meyer 2005). A subsequent update to the Swiss Red List has estimated a 32.4% decline over 14 years, corresponding to a decline of approximately 61.9% over three generations (OFEV and Info Fauna 2023). Its range in this country is projected to contract by 59% between 2010 and the period 2020-2049 (OFEV and Info Fauna 2023).

In France, the main subpopulations are observed to have noticeably decreased during the last decade (UICN France, MNHN and SHF 2015). The number of 250 x 250 m cells occupied by this species in one area of western France declined by more than 90% between two survey periods, one from 1994-1997 and 2012-2015 (Guiller et al. in press). A mark-recapture study at two sites in the same area between 1999 and 2011 exhibited a decline in the number of captures, from a maximum of 51-54 individuals to zero or nearly zero over eight to nine years (Guiller et al. in press). These authors estimated annual rates of decline of 12 and 35.8%. Conversely, a study investigating population trends based on citizen science data collected between 1980 and 2018 found no evidence of decline in the Languedoc-Roussillon region of southern France (Santos et al. 2022).

Strong declines were also observed in the Netherlands, especially in peripheral populations (Van Delft and Janssen 2015). In Slovenia, the adder is considered to have declined by more than 30% over the last 20 years (M. Vogrin pers. comm. November 2022). Based on the index devised by Jelić et al. (2013) - which incorporated available data on exploitation, life history traits, habitat breadth and adaptability to modified habitats - this species was deemed at possible risk of decline in the Balkans. Subpopulations have scattered distributions in the Balkans (Jelić et al. 2022), particularly in North Croatia, Serbia (Tomović et al. 2015) and North Macedonia (Sterijovski and Arsovski 2020). Severe drought can be observed in most of its range in Hungary, where peripheral populations show declining trends. Subpopulations are fragmented and often narrowly connected to suitable habitats such as patches of forests and river valleys in Ukraine, but there are still numerous observations from different parts of the country (O. Zinenko pers. comm. 2022). Lowland populations from the Po Plain in northern Italy became extirpated in the 20^th Century.

This species is present in a wide variety of habitats including open woodland and shrubland, hedgerows, field edges, heathland, moors, grasslands, alpine meadows, dunes and marshes. It is frequently encountered along hedgerows (Guiller et al. in press). In Hautes Fagnes in Belgium, the probability of occurrence was found to be negatively associated with the presence of even small numbers of trees, and positively with ericaceous plant and sphagnum moss (the latter suggesting a possible association with acidic soils) (Graitson et al. 2022). Conversely, in Russia, it has been found to overwinter in sphagnum pine forests (I. Doronin pers. comm. 2022). Its occurrence is limited by the availability of suitable overwintering sites, in which temperatures do not fall below 2-4 °C (I. Doronin pers. comm. 2022). It generally requires humid habitats and is more mesophylic in the southern parts of the range (populations in the Balkan Peninsula, also Vipera berus nikolskii, and populations previously assigned to V. barani). Water is a key limiting factor for the distribution of the species, especially in the southern part of its distribution (Guillon 2012). Research on subpopulations in Belgium and the Netherlands indicates that the species undertakes seasonal migrations between moist summer foraging habitats and nutrient-poor heathland used for overwintering (Julian and Hodges 2019). Adders are most often observed in heathland wintering areas, but summer foraging habitat appears to be important for immature and non-reproductive individuals (Julian and Hodges 2019). The maximum recorded distance between overwintering and summer foraging grounds is 1.5 km, and animals are typically sedentary within 100-150 m of a shelter site (I. Doronin pers. comm. 2022).

Animals may overwinter singly or in groups of up to 200-300 individuals (I. Doronin pers. comm. 2022). Emergence from winter dormancy occurs at different times across its vast range, sometimes as early as February in parts of Europe and as late as April to early May in central Russia (I. Doronin pers. comm. 2022). The female produces 3–18 live young (Arnold and Ovenden 2002). Females mostly reproduce at four years of age or older, and give birth at intervals of two to three years (Bauwens and Claus 2019). In the Belgian study, population females produce an average of 1.3 litters over their entire reproductive lifetime (Bauwens and Claus 2019). Prey includes rodents, frogs, lizards and rarely fledgling birds. The age at maturity can vary between localities but is generally between three and five years (S. Ursenbacher pers. comm. February 2023). Most females mature at four years of age and reproduce every two or sometimes three years. Maximum longevity of up to 15 years has been recorded, and up to 30 years in wild animals has been inferred from the results of tagging studies (Bannikov et al. 1977, Ananjeva et al. 1998, Bakiyev et al. 2015). The total number of offspring produced by a female depends on her adult body size and longevity (Madsen and Shine 1992). They usually give birth during August or early September (Spellerberg 2002). Average adult mortality varies between 10% and 39% per annum, depending mainly on the location and their reproductive status (Ursenbacher 1997, Bauwens and Claus 2019a, 2019b). Research in England estimated that about 45% of animals survive to the end of their first year, while 20% and 10% of the initial number survive their second and third year respectively (Prestt 1971). However, Bauwens and Claus (2018) demonstrated no difference in survival rate between juveniles and adults, only a slight difference between sexes, but a strong variability between sampled years in the Netherlands.

The generation length is poorly-known, and it is supposed to vary locally (S. Ursenbacher pers. comm. February 2023). Based on data from the UK and Switzerland, it is likely to range between six and eight years (R. Griffiths and S. Ursenbacher pers. comm. 2022), probably closer to six years in lowland France, up to eight or even more in the Swiss Alps. An average of eight to nine years should be viewed as conservative (S. Ursenbacher pers. comm. February 2023).

In large parts of Europe, subpopulations of this species have been significantly fragmented by the intensification of agricultural methods, fragmentation, and destruction of riparian forests (e.g. Serbia; Crnobrnja-Isailović et al. 2015) and by development. Habitat fragmentation has led to inbreeding depression in some subpopulations (Madsen et al. 1999). Subpopulations of this species are susceptible to persecution and collection for the pet trade (Ursenbacher 1997).

In Croatia, the lowland subpopulations occupying the riparian zone of large rivers have been recovering over the 30 years since the Croatian war of independence due to large-scale land abandonment and the resulting encroachment of meadows and riparian forest (habitats particularly favoured by the subspecies Vipera berus bosniensis) into former agricultural land (D. Jelić pers. comm. 2022). Following Croatia joining the European Union, a large amount of funding has focused on agricultural land reclamation, a process which is expected to result in population declines in impacted areas (D. Jelić pers. comm. 2022).

Poor habitat management (including inappropriate use of fire) and increased levels of predation from invasive predators (Wild Boar, pheasants) and native birds were implicated in major declines in the UK over the 50 years prior to 2016, and land use change associated with large-scale mechanised forestry is likely to have contributed to the species' extinction in Warwickshire, Oxfordshire and Nottinghamshire (Julian and Hodges 2019). "Massive" pheasant releases have been correlated with the absence of lizards and snakes from release sites in Belgium (Graitson and Taymans 2022). Adders were not detected at one of these authors' study sites a decade after release, and Graitson and Taymans (2022) concluded that this subpopulation, which was isolated and to which no recolonisation is possible, is probably now extinct as a result of pheasant predation. As pheasant releases are concentrated in agricultural areas, where reptile subpopulations are often already small and isolated, even temporary predation by the invasive birds during annual releases has a high likelihood of driving local subpopulations to extinction (Graitson and Taymans 2022).

Increasing Wild Boar (Sus scrofa) populations are driving extinctions in parts of Western Europe (Graitson et al. 2019). "Drastic" declines have been observed in areas of Belgium impacted by wild boars in research focused on sites with no known confounding pressures from habitat loss, invasive species or climate change, implicating boar predation as the major cause (Graitson et al. 2018). This work found that even the largest study subpopulations were almost extinct within five to six years of a Wild Boars irruption, and sites with moderate Wild Boar impacts retained only "very small" Adder populations at the end of the 12-year (2005-2016) study. The authors concluded that strong declines or extinctions occurred over most of southern Belgium - including those in protected areas - in less than 10 years as a result of the boar irruption. Unquantified observations suggest that Wild Boars negatively impact the populations on Vitosha Mountain in Bulgaria as well; the high number of hikers visiting this natural park might also be a threat since vipers often use the open footpaths to bask and come into contact with humans (A. Stojanov and A. Dyugmedzhiev, unpublished data).

In some parts of the range, such as France, Adders are accorded low priority and are often impacted by inappropriate land management such as overgrazing and scrub clearance (Julian and Hodges 2019). In France, 57% of the country's total length of hedgerows were removed in the second half of the 20^th Century (Guiller et al. in press); extensive hedgerow clearance has also occurred elsewhere in Western Europe (e.g. Great Britain; Robinson and Sutherland 2002). Modelling by Guiller et al. (in press) found that hedgerow loss (averaging 62% across the study area) best explained these authors' finding that the number of cells occupied in the study area declined by 92% over a 15-year period. These authors found that degradation (defined as the combined impacts of structural simplification of hedgerows and habitat closure of abandoned fields) was correlated with both population declines and changes in demographic parameters (lower survival, lower recruitment and lower growth rates) recorded during their mark-recapture study.

Habitat loss due to peatland extraction, destruction of moorland and heathland habitats, land reclamation, agriculture and drought are driving declines in Germany (Julian and Hodges 2019). It is threatened in Romania by the illegal collection of animals for venom extraction (CoE 2003). In some parts of its range reforestation, and succession as a result of the abandonment of traditional agricultural practices are also threats. From the beginning of the 20th century up to the 1960s, people killed large numbers of this species (Prestt 1971), which is the only or most common venomous snake in parts of its range.

In mountainous parts of the Balkan Peninsula expansion of run-of-river small hydropower plants in the mountainous parts of the Balkan peninsula could indirectly, through the change of microhabitats (an overall decrease of humidity), negatively impact local subpopulations of this species (Crnobrnja-Isailović et al. 2021).

Observations from Germany suggest that climate change is exacerbating desiccation of suitable moist habitats, and a study in Sweden found that numbers of males in two sites declined by 50 and 80% following the "hot, dry" year 2018, corresponding to a fall in both surface moisture and dew, and that survivors lost a fifth of their body mass (Julian and Hodges 2019). The results of a 40 year-long monitoring study of Madsen et al. (2023) point on the harsh impact of ongoing climate change on isolated adder subpopulations. Experiments conducted on Adders taken from a subpopulation in western France found that removing access to drinking water for a period of two weeks reduced the body condition of pregnant females and increased the physiological costs of reproduction, with potentially serious impacts on female and hatchling survivorship (Dezetter et al. 2021). Modelling based on observations and temperature trends over 34 years in Cornwall found that Adders are emerging from hibernation earlier in response to increased surface temperatures (as previously reported across the UK as a whole - Gardner et al. 2019), increasing their exposure to ground frosts at coastal sites (Turner and Maclean 2022). The consequences of increased frost exposure on individual snakes are unclear, but are likely to result in reduced body condition and, in cases of prolonged freezing post-emergence, direct mortality (Turner and Maclean 2022). An evaluation of the potential distribution area in Switzerland in the future (2020-2049 compared to 2010) suggested a strong reduction (59%) (OFEV and info fauna 2023). The impacts of exotic snake releases, such as through transmission of novel pathogens, is unstudied (Julian and Hodges 2019).

This very widespread species is subject to limited collection for venom extraction and for the pet trade, and these may represent localised threats (e.g. in Romania). Hobbyists often seek out individuals from Balkan populations of this species (based on communications from online forums and groups).

This species is listed in Annex III of the Bern Convention. This species is regarded as threatened in many European Countries (among them France: VU; Belgium: EN; Switzerland: EN; Austria: VU; Czech Republic: VU; Slovenia: VU; Serbia: VU, North Macedonia: EN, Romania: EN). In Germany, it is listed as regionally threatened in all 13 of the country's 16 states where it still occurs (Critically Endangered in four states, Endangered in nine) and as Extinct in Berlin. In Great Britain, Hungary and Croatia, it is considered Near Threatened. In Bulgaria, no national assessment has been conducted and the species remains unprotected (Y. Kornilev and A. Dyugmedzhiev pers. comm. November 2022). It is not protected in Montenegro (Gvozdenović and Iković 2022), but it is protected by national legislation in parts of its European range (e.g. France, Belgium, Netherlands, Serbia). The subspecies Vipera berus nikolskii is listed in the Red Data Book of Ukraine and is threatened by habitat loss and fragmentation (Kotenko et al. 2009). It is The adder occurs in many protected areas throughout its range.

Habitat management throughout the species' European range needs to accommodate the needs of this species, and "special care" is required for the preservation of habitats used for both summer foraging and winter refugia (with conservation efforts in Britain traditionally having neglected summer foraging areas), and when conducted appropriately pond creation may be important to support prey populations (Julian and Hodges 2019). Graitson et al. (2022) highlight the importance of establishing and maintaining areas of extensive open habitat for the benefit of this species. Julian and Hughes (2019) propose the creation of species-specific management areas ('ARKs') to protect key habitats with important subpopulations, and work is underway to develop these in some areas of the UK. Measures to increase moisture retention, especially in foraging habitats, is vital to buffer this snake against the impacts of climate change in areas such as northern Europe where levels of aridity are increasing (Julian and Hodges 2019). "Better-defined" legislation is recommended to regulate the use of planned burning (discussed in more detail by Julian and Hodges 2019). Conservation translocation of males to restore genetic diversity may be useful for some subpopulations, and the scenarios in which this is appropriate require "urgent consideration" (Madsen et al. 1999, Julian and Hodges 2019). Madsen et al. (2020) report the results of a genetic rescue attempt on a Swedish subpopulation that was "severely inbred" when 20 males were introduced in 1992. These authors found that both genetic diversity and levels of recruitment had increased in the subsequent 26 years. Regional action plans are underway or proposed in much of western and northern Europe between 2012 and 2028, with the aim of involving all relevant stakeholders in discussions to improve conservation outcomes for this species (Julian and Hodges 2019). The supported activities include training volunteers in population monitoring, developing more sympathetic land management practices and to increase public awareness in order to reduce levels of persecution - the latter including formal education and measures to reduce the exposure of humans and their pets to conflict with snakes that can generate negative media attention (Julian and Hodges 2019). There is a need to conserve separate evolutionary lineages of this species, in order to maintain the highest genetic diversity. Active control of wild boar populations is recommended in areas where hunting pressure is insufficient to manage boar at levels that the snakes can sustain (Graitson et al. 2018).