The Sporades Islands have had a fairly long history of herpetological field exploration, predominantly by western European scientists. Indeed, up until the very end of the 20^th century, herpetological investigations on the islands were almost completely dominated by German-speaking biologists. Commencing in the 1920s and `30s, and through the 1950s, short visits by F. Werner, O. Cyrén, and O.v. Wettstein provided important but limited information on the species communities inhabiting the islands, as well as their basic taxonomic affiliations (Werner 1930, 1938; Cyrén 1935, 1941; Wettstein 1953, 1957a, b, c).

Following these early expeditionary investigations and motivated by the lack of herpetological knowledge in the area, T. Schultze-Westrum and W. Weigand spent extended periods of time on the islands and obtained more fine-grained information on the resident reptiles and amphibians. Their fieldwork, culminating in a substantial series of specimens now residing at the Collection of the Zoological Museum A. Koenig in Bonn, resulted in several foundational publications (Buchholz and Schultze-Westrum 1964; Gruber and Schultze-Westrum 1971; Gruber 1974; Beutler and Gruber 1977; Mayer and Tiedemann 1980) and provided the backbone of herpetological understanding of the Sporades Islands. This basic information was subsequently incorporated into broader-scope review works such as Ondrias (1964), Chondropoulos (1986, 1989), Foufopoulos and Ives (1999), Legakis (2004), Roca et al. (2009), Foufopoulos et al. (2011), etc.

In the last 30 years, however, additional, brief visits by predominantly non-academic herpetologists have resulted in several additional publications, mostly descriptive in nature, that have filled out many knowledge gaps (Bergman 1995; Cattaneo 1997, 1998, 2010; Broggi 2010, 2020; Grano et al. 2013; Passarge 2019; Kalogiannis 2020, 2021). Last but not least, regional-scope laboratory studies utilizing molecular approaches have shed light on the taxonomic affiliations of many of the regional reptile taxa (e.g., Poulakakis et al. 2005; Roussos 2015; Kornilios et al. 2020), though only Poulakakis et al. (2005) explicitly included Sporades samples in the analyses. Consequently, additional molecular studies from the islands are needed, as a fine-grained understanding of the genetic structure and diversity among the reptile populations in the Sporades remains unknown.

This expanding, but relatively disorganized information has created the need for a comprehensive review of the herpetological literature of the archipelago. In this work, we: 1) report on a plethora of new records of species we documented during numerous field trips to the Sporades over the last seven years; 2) review, summarize, and integrate this new information with the existing literature records on the herpetofauna of the region, with the aim of creating a body of information that can be used as a baseline for future studies; 3) provide new field ecology information, as well as conservation and management recommendations for the archipelago’s reptile and amphibian communities.

The Sporades Islands consist of a diversity of geological substrates, including sedimentary deposits and volcanic rocks. Most of the islands are composed of various forms of limestone, with the exception of Psathoura and Mikropsathoura, which are volcanic in origin (Jacobshagen and Wallbrecher 1984). These fissured and well-draining substrates result in a comparative paucity of surface wetlands with unfavorable effects on amphibians and hydrophilic reptiles. The climate is Mediterranean, characterized by wet winters and warm, dry summers (Iliadou et al. 2020). Though summers in the Sporades can be long and dry, mean annual precipitation is significantly higher and mean annual temperatures are significantly lower than in island groups further south, such as the Cyclades, with important ramifications for the species occurring there (Harris et al. 2020).

From a paleogeographic perspective, most of the western Sporades (west of approximately the 24°E longitude line) are land-bridge islands that were connected to the adjacent Thessalian mainland during the Last Glacial Maximum ~20 kya, when sea levels were about 120 m lower than today (Bintanja et al. 2005). However, even within this continental group, substantial variation exists in the extent of connectivity and insularity throughout the Pliocene and Pleistocene glacial and interglacial periods (Rohling et al. 2014). While Skiathos and its satellite islets have had only a relatively short period of isolation from Thessaly, being connected until 8.5 kya, the rest of the major chain islands (Skopelos, Alonnisos, Mikros Adelphos, and Megalos Adelphos) became separated from the mainland and each other approximately 14–15 kya (Kalb 2021). In contrast, the eastern islands of the main chain (Skantzoura, Kyra Panagia, Gioura, Psathoura, and Piperi) have land bridges submerged below 120 m, and have been isolated for much longer periods of time (> 200 kya) (Kalb 2021).

The natural habitat on the Sporades Islands has been shaped extensively by millennia of human activities. Undisturbed vegetation cover can be relatively dense compared to other, more arid, Aegean islands. The original woodland, likely dominated by Quercus sp., was largely reduced and degraded by wood cutting, agricultural activities, and livestock husbandry (Ginalis 2018). As a result, large areas of the main three islands have been converted to a matrix of terraced agricultural slopes, olive groves, and seasonal pasturelands, creating a heterogeneous landscape that is conducive to many typical lowland Mediterranean reptiles and amphibians (Papanastasis et al. 2009; Zakak et al. 2015; Băncilă et al. 2023). Today, in the western Sporades and especially on Skopelos and Skiathos, large areas are also dominated by pine forest (Pinus sp.), perhaps due to past forest fires and the abandonment of agricultural areas. In contrast, while vegetation in the eastern Sporades tends to be lower, any relict forest patches consist of Kermes Oaks (Quercus coccifera) and Holm Oaks (Q. ilex), as pines are absent (with the exception of Piperi). Medium-sized islands support at most tall maquis (dominated by Juniperus turbinata on the Skantzoura cluster, Q. coccifera everywhere else), while smaller islets, where soils are shallower, support generally only coastal heaths (non-spinose phrygana).

We surveyed the herpetofauna of the islands during multiple field expeditions in the period between 2016 and 2024. In addition, one of the authors (SK) lives on the islands and has been conducting impromptu herpetological searches for several years. We used both diurnal and nocturnal surveys, covering all major vegetation types, as well as a variety of distinct microhabitats across the whole archipelago. Whenever possible, we document new records with specimens or photographs, which we have deposited in official museum collections. Beyond ecological and presence data, we also collected information on local reptile names (see Appendix 1: Table A2). All islands of the archipelago were visited, often numerous times, with only 5 islets not visited to date (Gaidaronisi, Manolas, Mikroskandili, Lachanou, and Kassidis). All visited islands harbored at least one species of reptile, with the exception of Kastronisia, a small islet near Skiathos where no reptiles were detected. See Appendix 1: Table A3 for a list of both official and alternative island names.

We summarize our findings as well as past findings on species occurrences in Tables 1, 2. Species richness decreases across the archipelago (Fig. 2), from the youngest to the oldest islands with regards to insularity, and from the largest to the smallest islands in terms of area. Since the archipelago is a chain of islands, and submerged land bridges get deeper towards the more terminal end of the chain in the east, decreasing richness generally follows a west-to-east geographic pattern, as the oldest islands to the east have probably experienced more species extinctions over their longer period of isolation. Subspecific taxa of lizards (e.g., P. erhardii ruthveni, M. kotschyi fuchsi) have been described from this archipelago based on morphology, and these taxa are more frequently represented on the smallest Sporades islands, resulting in higher percent endemism on the smallest islets (Fig. 3).

Squamates are the most speciose group in the Sporades, with eight lizard and eight snake species. The most widespread lizard species are Kotschyi’s gecko (M. kotschyi – 30 islands) and the Aegean wall lizard (P. erhardii – 40 islands). Lizards with the most limited distributions in the Sporades are the Skyros wall lizard (P. gaigeae – 1 island) and the European glass lizard (P. apodus – 2 islands). The most widely spread snake species are the Caspian whipsnake (D. caspius – 5 islands), the nose-horned viper (V. ammodytes – 5 islands), and the four-lined rat snake (E. quatuorlineata – 5 islands). The snake species with the most limited distributions in the Sporades are the cat-eyed snake (T. fallax – recorded only from Alonnisos, Skopelos, and Peristera), the Balkan whipsnake (H. gemonensis – only recorded from Tsougria and Aspronisi), the Eastern Montpellier snake (M. insignitus – recorded from Skiathos and Skopelos), and Dahl’s whipsnake (P. najadum – only recorded from Skiathos). Overall, there appear to be some checkerboard distributions of snakes across the Sporades suggestive of competitive intraspecific relationships. Amphibians and turtles are the least common groups of herpetofauna in the Sporades and are most at risk of local extinction due to habitat loss across the archipelago. Amphibians occur only on the young, westernmost large islands of Skiathos and Skopelos, where they are relatively abundant near surface waters. Though anapsids occur on the four largest islands, they are the rarest group of herpetofauna in the Sporades, and terrapins are declining as crucial wetland habitat disappears.

Amphibians

Amphibians have a marginal presence on the islands, with only three species from three families (Bufonidae, Hylidae, and Ranidae) historically recorded from the three largest Sporades (Skiathos, Skopelos, and Alonnisos). However, all amphibians are now likely extinct from Alonnisos, and their future on Skiathos and Skopelos is uncertain. Despite years of searching, we have no new records to report for amphibians.

Bufotes viridis , green toad (Laurenti, 1768)

The green toad is known only from Skopelos, where it was first reported by Werner (1930) and subsequently Cattaneo (1998). We made several observations of this species in the eastern part of Skopelos, often somewhat close to inhabited places (e.g., near the main town and around the meadows of Staphylos), where there is standing or flowing water in the form of wells, flooded fields, and canals. Adults were very active on warm, humid nights during the breeding period. The largest count of individuals was made on 25^th March 2022, at 19:40 h, with an air temperature of 16 °C, when about 100 calling males were observed in a flooded field near Staphylos. During a visit to the same spot in April 2023, the fields were completely dry, and only a few calls were heard in the distance. Roadkilled individuals were also frequent on the main road crossing the agricultural plain of Staphylos, south of Skopelos town, and a few adults were found under stones in the same area. Despite the seemingly healthy population on Skopelos, toads are largely dependent on the few suitable breeding spots scattered in a handful of degraded locations that are heavily influenced by human activity and are therefore under the threat of decline.

Hyla arborea , European tree-frog (Linnaeus, 1758)

We confirm the presence of the European tree-frog, Hyla arborea, on Skiathos. Until now, this species was only reported by Cattaneo (1997), who discovered a single roadkilled individual. We found the European tree-frog to be common on Skiathos, where it is widespread across the island and mainly inhabits humid, lowland habitats such as slow-flowing streams and their estuaries, flooded fields, vernal pools, and marshes. Particularly dense populations also breed in man-made structures such as abandoned swimming pools, fountains, and canals around the airport area and in the settlement of Troulos. We heard large choruses in Megalos Aselinos and Vromolimnos during the breeding season, especially after rainfalls when males occupied and called from any form of stagnant water. Reproduction takes place between February and April. It seems that the presence of several swimming pools in inhabited areas has contributed to the large population of this species on Skiathos, since tree-frogs possess adhesive toe pads and can easily climb smooth, vertical human-made walls. Despite extensive searching, we cannot confirm a doubtful report with no photo or specimen of H. arborea from Alonnisos (Broggi 2010).

Pelophylax kurtmuelleri , Balkan marsh frog (Gayda, 1940)

We confirm the presence of the Balkan marsh frog on Skopelos and Skiathos, where it has also been recorded in the past (Cattaneo 1997, 1998). The marsh frog tends to have a patchy distribution, but can be abundant in suitable habitats. On Skopelos, some individuals were observed in a small seasonal pond located in the Ditropo area and in the coastal marsh of Milia. In Milia, we heard large choruses during spring, and good numbers of the species seem to exist. In the Ditropo pond, which is mostly dry during the warmer months, we observed young froglets during the summer of 2021, indicating successful reproduction. Furthermore, a single adult was seen in a small artificial ditch at the water dam near Panormos. Marsh frogs are more widespread on Skiathos, where we have found them in good numbers in the marsh of Vromolimnos and in the streams in Troulos, Megalos Aselinos, and Lechouni. The stream of Lechouni hosted the largest population recorded so far, with hundreds of individuals of various life stages seen in a single visit. Marsh frogs also used to occur in good numbers on Alonnisos, at least up until the 1980’s, but have died out since the late 2000’s, most likely due to the loss of breeding habitat. This is evident in several reports by locals who say that frogs were widespread across numerous freshwater springs on the island (Grillitsch and Tiedemann 1984; Broggi 2010). In recent times, the intense exploitation of flowing water used for irrigation purposes on Alonnisos has dried out most of the island’s streams. Additionally, the traditional wells and ditches used for agriculture have been replaced by modern ground pumps that can pull significantly more water (Broggi 2010). We detected no evidence of amphibians surviving on Alonnisos despite systematic surveying over several years.

Lizards

Lizards are the most abundant group of herpetofauna in the Sporades. To date, seven lizard species spanning four families have been confirmed in the Sporades: Ablepharus kitaibelii (Scincidae), Pseudopus apodus (Anguidae), Hemidactylus turcicus, and Mediodactylus kotschyi fuchsi (Gekkonidae), as well as Lacerta trilineata, Podarcis erhardii ruthveni, and Podarcis gaigeae (Lacertidae). We report here on 16 new island records for M. kotschyi (12), H. turcicus (3), and P. apodus (1). Additionally, we report on one ambiguous museum record for Chalcides ocellatus.

Ablepharus kitaibelii , snake-eyed skink (Bibron and Bory, 1833)

The snake-eyed skink, A. kitaibelii, has a wide distribution across most of the Aegean Sea islands (Chondropoulos 1986). In the Sporades, this species has only been observed on the three larger islands of Skiathos, Skopelos, and Alonnisos (Cyren 1935; Cattaneo 1997; Broggi 2010), and we confirm the presence of A. kitaibelii on these islands. During our expeditions, we often found this small, delicate species on the ground in leaf litter, maquis, olive groves, meadows, and especially pine woodlands. We usually encounter A. kitaibelii during early morning hours, with overcast conditions, and during the winter, given that its habitat is apt to heat up rapidly during the warmer months. Due to its small size and secrecy, A. kitaibelii may seem rare but is actually quite common on Skiathos, Skopelos, and Alonnisos.

Chalcides ocellatus , ocellated skink (Forskål, 1775)

We report here on an earlier, unpublished record of C. ocellatus from Skopelos. An adult specimen from the island was deposited in the Natural History Museum of Vienna by a collector (O. Reiser in the 1890s, Voucher Natural History Museum of Vienna 19419/1). However, the species was not mentioned in the travel account, and Reiser’s Sporades visit was bookended by visits to other areas of Greece harboring C. ocellatus, so doubt exists as to the validity of the record (Reiser 1905). Much of the ocellated skink’s contemporary range is the result of historical introductions, generating a discontinuous and patchy distribution (Speybroeck et al. 2016). This species has been found on other Aegean islands, but was likely transported there by humans and successfully established. Despite searching, we have never encountered C. ocellatus in the Sporades. Assuming that Reiser’s record is not erroneous, and given the long gap since it was observed and the lack of any other specimens or recorded observations of this species in this region, we expect that C. ocellatus is not a native species and does not currently occur in the Sporades.

Specimen: Skopelos (NHMW 19419/1).

Pseudopus apodus , European glass lizard (Pallas, 1775)

The only species from the slow -worm family Anguidae found in the Sporades is the European glass lizard. It was first reported by Bergman (1995) on the island of Skiathos, and we report here on a new record of the species from a photograph taken by a citizen on Skopelos (July 2009; Theodora Tsimpo). We can also confirm P. apodus’ presence on Skiathos, where we observed a few individuals of the species on slopes with dense shrubs in the eastern part of the island. Pseudopus apodus is usually found in dense brambles and in grassy areas with some canopy cover on Skiathos (Cattaneo 1997), though this taxon is known to also occupy fairly dry, stony habitats if some vegetation cover is available (Speybroeck et al. 2016). Due to its unique morphology as a legless lizard, this species is difficult to confuse with other herpetofauna in this region.

Photographic voucher: Skopelos (NHMC 80.3.20.52).

Hemidactylus turcicus , Mediterranean house gecko (Linnaeus, 1758)

We report three new island records of the Mediterranean house gecko from Kyra Panagia, Peristera (Fig. 4H), and Arkos. In addition, the species was also recently documented on Piperi (Daftsios et al. 2024). Previously, this species was only known from five islands in the Sporades: the three main islands of Skiathos (Cattaneo 1997), Skopelos (Gruber 1974), and Alonnisos (Grillitsch and Tiedemann 1984), and the small recently uninhabited islands of Tsougria (Grano et al. 2013) off the southeast coast of Skiathos, as well as Pappous near Kyra Panagia (Gruber 1974). We found H. turcicus in a variety of warm, rocky habitats, hiding under rocks during the day. It is especially common near areas of human activity and settlements such as buildings, agricultural areas, dry stone walls, piles of rubbish, and ruins. The Mediterranean house gecko originated in the Near East and is considered to have been introduced widely across the warmer areas of the Mediterranean Basin (Moravec et al. 2011). Its distribution in the Sporades maps closely to human habitation, and even among the smaller islets, it occurs only on those with regular human presence. Pappous Islet, for example, was a skete, and Tsougria was settled until the mid -1900s. Even our new record from Arkos is based on an individual found near building materials brought to the island for the construction of a beach bar.

Photographic voucher: Kyra Panagia (NHMC 80.3.87.303), Peristera (NHMC 80.3.87.304).

Mediodactylus kotschyi , Kotschy’s gecko (Boettger, 1888)

We report here 12 new island records for Kotschy’s gecko, M. kotschyi, from the islets of Tsougriaki, Arkos, Daskalonisi, Paximadi, Agios Georgios, Mikronisi, Polemika, Kyrgiagos, Prasso, Skandili, Korakas, Pelerissa, and Piperi (Table 2). Although reliable records of this species are still missing from some islands, this is probably the vertebrate with the widest distribution in the archipelago, being able to survive on small islets with scarcely any vegetation that is otherwise reptile-free (e.g., Daskalonisi by Skiathos or Kassidis and Paximadi by Skopelos). Interestingly, M. kotschyi has not yet been found on Skiathos itself, perhaps because of the dense vegetation and plethora of water, or some other ecological or historical factor. This species has an irregular distribution across the Aegean and is relatively rare on the mainland, and Skiathos is more mainland-like than the rest of the Sporades.

Kotschy’s gecko is a small Palearctic “naked-toed” gecko (Beutler 1981). It is easily recognized by its gray coloration, dark chevron dorsal patterning, and thin, kinked toes that lack adhesive toe pads (see Fig. 4E). The populations of most Sporades islands have been taxonomically assigned to M. kotschyi fuchsi, which is morphologically distinct from the mainland M. k. bibroni. This taxon can be distinguished by the general lack of pre-anal pores (an average of 0.6 in M. k. fuchsi versus 3.8 in M. k. bibroni); the larger number of ventral scale rows (an average of 35.8 in M. k. fuchsi versus 24 in M. k. bibroni); and the larger number of separate tubercles on the hind thigh (an average of 6 in M. k. fuchsi versus 3.4 in M. k. bibroni) (see Beutler and Gruber 1977; Beutler 1981 for further morphological details). Mediodactylus k. fuchsi is an island endemic that is restricted to the eastern Sporades islands up to, and including, the Alonnisos cluster. The remaining populations, Skopelos and westward, are considered by Beutler and Gruber (1977) to be transitional to the mainland M. k. bibroni. Beyond these lineages, the populations from the extralimital Skyros archipelago have been assigned to M. k. schultzewestrumi (Beutler and Gruber 1977). The taxonomic affiliation of the intermediate Piperi population remains to be determined. While these subspecies have been described based primarily on morphology, it is not clear whether they will hold up in a more holistic species delimitation framework that incorporates molecular data. Prior studies that have inferred evolutionary relationships with one or a few genes have synonymized many subspecies in the Aegean islands to the species M. kotschyi from the European continental shelf (Kasapidis et al. 2005; Kotsakiozi et al. 2018).

While Kotschy’s gecko is primarily crepuscular, we commonly encountered it basking on rocks, stone walls, or trees in the first hours of the morning sun and last hours of daylight, even during the hot summer months. This species favors dry, rocky habitats and can reach high densities on dry stone walls, though we also observed M. kotschyi on trees and wooden telephone poles. On smaller, uninhabited, predator-free islets, it is more typically encountered low to the ground under rocks or debris. Our observations suggest that the density of this species is inversely proportional to the size of the island, achieving the highest densities on small rocky islets. This may be both the result of island competitive release, as well as susceptibility to predation on larger islands (Itescu et al. 2017).

Specimens: Tsougriaki (NHMC 80.3.85.1970), Mikros Adelphos (NHMC 80.3.85.1971-1972), Skantzoura (NHMC 80.3.85.1973-1975), Prasso (NHMC 80.3.85.1976-1979), Kyra Panagia (NHMC 80.3.85.1980-1981), Gioura (NHMC 80.3.85.1982-1984), Grammeza (NHMC 80.3.85.1985)

Photographic voucher: Skandili (NHMC 80.3.85.2012), Korakas (NHMC 80.3.85.2011).

Lacerta trilineata, Balkan green lizard (Bedriaga, 1886)

The Balkan green lizard occurs on the three largest and westernmost islands of the Sporades, as well as on two uninhabited Skiathos satellite islets: Tsougria and Arkos. In 1938, Werner claimed that he observed both Lacerta trilineata and L. viridis on Skiathos (Werner 1938). However, no observations of L. viridis have been reported since, and at present it is considered an erroneous observation. We have no new records of Lacerta to report from our expeditions, and we confirm the presence of L. trilineata on Skiathos, Skopelos, Alonnisos, Tsougria, and Arkos.

Adult L. trilineata from the Sporades has a vivid, bright green coloration that can be speckled with black or bright blue scales. Individuals from Skiathos are noticeably smaller and seem to have a slenderer head than adults from the mainland, Skopelos, and Alonnisos. The typical throat color we observed on the islands is bright yellow, but in some males, a small area of the throat is light blue. Additionally, adult L. trilineata have dramatically different coloration than juveniles, the latter of which have light grey/brown dorsal coloration often with three bright pale-yellow lines running in parallel down the dorsum, though about 4/5 of juveniles in the Sporades have the uniform morph, as based on 100s of field observations by us and noted by others (Cattaneo 1997). Adults can be difficult to find, as they generally escape quickly into dense vegetation when approached by humans. Typically, we found this species basking or climbing high up on shrubs, reeds, and stonewalls. Lacerta trilineata is exceptionally common on Skiathos, where it is the most abundant lizard species and is found in virtually all habitat types. Fewer observations were made on Skopelos and Alonnisos, mostly from agricultural areas and olive groves.

Podarcis erhardii, Aegean wall lizard (Bedriaga, 1882)

The Aegean wall lizard is, together with Kotschy’s gecko, the most abundant reptile in the Sporades (Kalogiannis 2020) and occurs on at least 40 islands in the region. We have no new records to report for P. erhardii, despite searching islands like Tsougria and Kastronisia, where we would expect to find wall lizards despite the previous lack of records. The only islands where P. erhardii has not been observed, and is presumed absent, are Tsougria, the Kastronisia islets, Daskalonisi, Paximadi, Kassidis, and Piperi. Furthermore, we were not able to confirm the species on Skiathos, despite a past record by Bergman (1995), likely of a marginal population of introduced individuals. Notably, this species is absent from Piperi, where it is replaced by an endemic subspecies of the Skyros wall lizard, P. gaigeae weigandi (Gruber and Schultze-Westrum 1971). Though many subspecies of P. erhardii have been described from the Aegean Sea, only P. e. ruthveni (Werner 1930) is presently recognized in the Sporades (see Gruber 1986 for discussion). Podarcis e. ruthveni can be distinguished from mainland P. e. riveti based on dorsal scalation, as P. e. ruthveni has more dorsal scales than P. e. riveti. In the Sporades, P. erhardii is phenotypically variable, both within and between islands (Cyren 1941).

Few studies have quantitatively assessed morphological variability between island populations of wall lizards in the Sporades, though we confirm here some patterns that have been noted by others prior (Cyrén 1941; Gruber and Schultze-Westrum 1971). Morphologically, wall lizards from Skopelos were relatively small and had the longest legs, while lizards from the easternmost islands were larger and had shorter, more stout legs. This species also varies considerably in dorsal coloration from island to island. Lizards from the western islands tend to be brown, while lizards from the eastern islands are dusky gray-green (Fig. 5). Thus, with progressive distance from the mainland, lizards get larger, darker, more stout, and have fewer scales, especially east of the 24°E longitude line, marking the "old" islands located off the continental shelf. Cyrén (1941) also noted a similar pattern and found a morphological distinction between the older island populations east of the line and the western populations encapsulated under the presently defunct subspecies psathurensis vrs. skopelensis. Gruber and Schultze Westrum (1971) note that P. erhardii has on-average larger body sizes on smaller rather than on larger islands, presumably because of marine subsidies (Stadler et al. 2023). Only the very smallest rock islets without substantial seabirds are exceptions to this rule (e.g., Mikroskandli). Colorwise, we noticed a tendency for P. e. ruthveni to follow the general substrate pattern, with lighter -colored animals on limestone and relatively darker ones on dark, volcanic substrate, like on the easternmost islands of Psathoura and Mikropsathoura. We observed larger blue and aquamarine dorsolateral color patches on small island lizards. The venter of individuals is typically one-colored throughout the archipelago, ranging from dirty white to yellowish, orange, and reddish on Kokkinonisi (a small island across from the Kokkinokastro peninsula of Alonnisos). This lizard species has polymorphic throat coloration, and individuals can be either orange, yellow, white, or a mosaic combination of two of these colors (Brock et al. 2020). Based on our observations, white -throated morphs are by far the most common on every island in the Sporades. With the exception of Alonnisos, we only found orange, white, and mosaic orange-white individuals during our expeditions and note that these are the only throat color morphs currently documented in georeferenced photographs on iNaturalist (www.inaturalist.org), suggesting that the yellow throated morph is largely absent from this region.

Like other species of Podarcis, P. e. ruthveni is an ecological generalist and can occur in almost any habitat in the Sporades, including agricultural and urban areas. As its common name suggests, we frequently find this lizard in high densities on dry stone walls characteristic of the region, and it can thrive around humans unless feral cats are present (Krawczyk et al. 2019).

Schultze-Westrum and others note that all islands in this region that sustain the shrub Pistacia lentiscus are inhabited by P. erhardii (Buchholz and Schultze-Westrum 1964; Gruber and Schultze-Westrum 1971), and even refer to it as a ‘true bush lizard’ (‘richtiggehende Buscheidechse’). We agree that it achieves the highest densities in relatively open habitat with P. lentiscus, which provides cover without completely shading out the ground as Pinus forest does. Distributional data also suggest that in the absence of anthropogenic refugia, the species can be susceptible to the presence of diurnal snake predators. For example, P. erhardii is absent from Tsougria while occurring on neighboring Tsougriaki (aka Mikrotsougria), perhaps because of the presence of H. gemonensis, which is an effective Podarcis predator (Speybroeck et al. 2021). This effect may be exacerbated by competition by the syntopic Lacerta, which, by virtue of its size, is not as susceptible to H. gemonensis predation.

On small, uninhabited islands, we usually find this species basking on rocks or under larger, evergreen shrubs and other vegetation during the heat of the day. Notably, this species was extremely common on every island it was found on, even on the smallest uninhabited islands with very few resources. As small-bodied secondary consumers, these lizards play an important ecological role as they eat and are eaten by a variety of animals. On the Sporades, the species harbors relatively simple generalist parasitic helminth communities, the complexity of which declines with island area (Roca et al. 2009). Though wall lizards are primarily insectivorous, they can thrive on tiny islands due to their flexible diets that include plant material, large venomous arthropods, and other conspecifics (Brock et al. 2014; Madden and Brock 2018; Patharkar et al. 2022). On some islands like Strongyllo (or Kyriagos [near Skantzoura]), these lizards are associated with Falco eleonorae nests and appear to feed on falcon prey leftovers (Schultze-Westrum 1961). Lizards from small islands appear to be more inquisitive, have shorter flight initiation distances, and appear also to be slower runners than on the larger islands, similar to P. erhardii in the Cyclades (Brock et al. 2015; Semegen 2018). Additionally, a recent study found that average clutch size varies considerably between island populations proportionally to the number of predators, ranging from a minimum of 1.6 eggs on Mikropsathoura to a maximum of 2.6 eggs on Skopelos (Foufopoulos et al. 2023).

Specimens: Tsougriaki (NHMC 80.3.51.2981-2982), Agios Petros (NHMC 80.3.51.2983-2984), Grammeza (NHMC 80.3.51.2985-2986), Aspronisi (NHMC 80.3.51.2987-2988), Mikros Adelphos (NHMC 80.3.51.2989), Kyra Panagia (NHMC 80.3.51.2990-2991), Gioura (NHMC 80.3.51.2992-2993), Skatzoura (NHMC 80.3.51.3037).

Podarcis gaigeae, Skyros wall lizard (Werner, 1930)

The Skyros wall lizard is a narrow-range endemic species with a distribution centered on the Skyros island cluster, southeast of the focal area. In our study area, the species occurs only on the island of Piperi, which is inhabited by the endemic subspecies P. g. weigandi (see Gruber and Schultze-Westrum 1971). The individuals we observed on Piperi were normally sized for a Podarcis and did not exhibit gigantism of P. gaigeae populations from some Skyros satellite islets (Pafilis et al. 2009; Runemark et al. 2015). Thus, individuals from Piperi were similar in size to P. e. ruthveni, but had generally darker body coloration, often bright green backs, and dark dorsal reticulation. Gruber and Schultze-Westrum (1971) noted five criteria that distinguish P. gaigeae from P. erhardii in the Sporades: 1. Coloration: P. gaigeae has a strong green component to their dorsal coloration compared to P. e. ruthveni, which is more grey-brown to olive-brown. 2. Patterning: P. gaigeae has dark occipital spots, whereas P. e. ruthveni lacks occipital spots. 3. Femoral pores: P. gaigeae has more femoral pores, on average, compared to P. e. ruthveni. 4. Scalation: In P. gaigeae, the scutum masseteric abuts directly on the scuta supratemporalia more often than in P. e. ruthveni, and 5. Morphology: P. gaigeae has a relatively larger masseteric diameter than P. e. ruthveni.

Similar to other Podarcis species, P. gaigeae is a generalist that can be found in almost every habitat type. On Piperi, this species was most abundant in open areas near the settlement and at forest margins near the rocky coast, and less abundant in closed forested areas.

Snakes

The Sporades harbor eight snake species from three families: Dolichophis caspius, Elaphe quatuorlineata, Hierophis gemonensis, Platyceps najadum, Telescopus fallax, and Zamenis situla (all Colubridae), Malpolon insignitus (Psammophiidae), and Vipera ammodytes (Viperidae). We report here on nine new island records for: D. caspius (2), E. quatuorlineata (2), T. fallax (2), V. ammodytes (1), Z. situla (1), and H. gemonensis (1). We also observed a non-viperid snake species on the islet of Agios Georgios (between Skopelos and Alonnisos), but the species could not be identified.

Dolichophis caspius, Caspian whipsnake (Gmelin, 1789)

We recorded the species for the first time on the island of Skatzoura (Fig. 4F) and the nearby islet of Prasso, which likely confirms the unspecified snake observation by Buchholz and Schultze-Westrum (1964) from there. Four large individuals were found on Skatzoura near stonewalls close to abandoned buildings, and an additional subadult was seen moving along the ground in the same area. We observed traces of rodents in and around these buildings, suggesting a good prey resource for this species. On the islet of Prasso, we found a large D. caspius basking in the sun, and later we collected a shed skin of the same species from elsewhere on the island. This species tends to be the only diurnal large-bodied colubrid on the islands where it is present. The Caspian whipsnake was previously recorded on the small islet of Lechousa, north of Peristera (Buchholz and Schultze-Westrum 1964), then on Alonnisos (Cattaneo 1998, 2010), and then Peristera itself (Kalogiannis 2021). The species has not been reported from Skiathos, Skopelos, Kyra Panagia, or Gioura, where E. quatuorlineata and/or another large colubrid species exist.

On the Sporades, the Caspian whipsnake reaches up to 160 cm in total length, thus approaching the largest lengths for its species known across Greece (the length-record being 208 cm from Samos; Cattaneo 2003). The species is common in the archipelago and is encountered in a diversity of habitats ranging from agricultural land (wheat fields, olive groves, and vineyards) and areas near human habitation, to pine and juniper forests, as well as maquis and phrygana vegetation. Notably, it even occurs on relatively small islands like Lechousa and Prasso, where extensive exposed bedrock limits the amount of evergreen woody vegetation. The species feeds primarily on rodents but is opportunistic and will pursue birds, invertebrates, and reptiles, including other snakes (Speybroeck et al. 2016; Plettenberg-Laing and Mee 2020). On Alonnisos, we found individuals preying on lizards (e.g., A. kitaibelii, L. trilineata), snakes (V. ammodytes and other D. caspius), and large rodents, while Cattaneo (1998) also reports finding rabbit remains in ingesta content. Thus, it is a significant, generalist predator of a broad range of species, including its own, and a strong competitor with other snake species. Melanism is prominent among the populations of Alonnisos and Peristera, with several melanistic individuals encountered by the authors and others (Cattaneo 1998; Broggi 2010; Kalogiannis 2021).

Specimens: Skantzoura (NHMC 80.3.117.58).

Photographic voucher: Prasso (NHMC 80.3.117.70).

Elaphe quatuorlineata, four-lined rat snake (Lacépède, 1789)

We report here on two newly documented populations of this species on the island of Gioura (Fig. 4C) and the nearby islet of Grammeza. One adult male (SVL 126 cm; total length 155 cm) and a sub-adult male (SVL 71 cm; total length 89 cm) were found on Gioura, and another large adult male (SVL approximately 115 cm with a damaged tail) was found on Grammeza. In addition, we also report here an earlier, unpublished discovery of a sub-adult photographed near the Bay of Agios Petros on Kyra Panagia (Genevieve Leaper 2020 on iNaturalist).

Early publications had previously reported this species only from Skiathos (Buchholz and Schultze-Westrum 1964; Cattaneo 1997) and Skopelos (Buchholz and Schultze-Westrum 1964; Cattaneo 1998). We confirm the presence of the four-lined snake on Skiathos, where we found four juveniles and one sub-adult. We encountered E. quatuorlineata in a variety of habitats on Skiathos, such as olive groves, riparian forests, shrubland, and often near settlements, where it seems to be common. On the eastern islands, we found this species in rocky habitats with phrygana and maquis. This is the largest rat snake in Greece and can reach total lengths up to 200 cm on the mainland, though island populations usually tend to be smaller. On the Cyclades, the species reaches sexual maturity at substantially shorter total lengths (60–80 cm) (Speybroeck et al. 2016). For the Sporades, Cattaneo (1997, 1998) reported specimens with total lengths up to 180 cm on Skiathos and Skopelos. The four-lined snake primarily feeds on rodents and birds, though juveniles feed on lizards as well. On Grammeza, large adults likely also prey on rabbits that have been released and are now numerous on the islet. This was evident in the single adult we found, who exhibited several healed injuries throughout its body, a phenomenon usually attributed to theriophagy (Cattaneo 1998). All adult individuals from the Sporades exhibited the typical phenotype with four dark dorsal lines on a light, beige-yellow background (Fig. 4C). Juveniles from Skiathos showed the typical phenotype for their developmental stage, with dark blotches on a light background, similar to mainland populations, and a pink-red ventral area.

Photographic vouchers: Gioura (NHMC 80.3.31.37) and Grammeza (NHMC 80.3.31.38).

Hierophis gemonensis, Balkan whipsnake (Laurenti, 1768)

We confirm the presence of this species on the small island of Tsougria (Fig. 4J), near Skiathos. A juvenile individual was found in the rock ruins of a building in the vicinity of the coastal wetland area. This species was first recorded in the Sporades from Aspronisi (Buchholz and Schultze-Westrum 1964) and then from Tsougria (Grano et al. 2013), but has not been recorded from Skiathos itself. We also report on a previously unpublished record from Skopelos, confirmed by a specimen deposited in the Alexander Koenig Museum of Natural History in Bonn by T. Schuze-Westrum and W. Weigand on August 25^th, 1957. However, we were not able to confirm this species during our own field work, neither on Skopelos nor on Aspronisi, perhaps due to inappropriate timing or low densities. Ultimately, H. gemonensis could be present on Skiathos, as the conditions seem suitable, and it is present on a nearby smaller island, though this remains speculative at present. The H. gemonensis documented in the Sporades does not exhibit markedly different phenotypes from the mainland populations.

Specimens: Tsougria (NHMC 80.3.25.134), Skopelos (ZMFK 003606)

Malpolon insignitus, Eastern Montpellier snake (Geoffroy Saint-Hillaire, 1827)

Eastern Montpellier snakes have a broad distribution in the lowlands of the Aegean-Anatolian region but do not survive well in island environments. In the Aegean archipelago, their distribution is restricted to the largest, young, near-shore landbridge islands. This conspicuous snake is common and well documented, both from Skiathos (Buchholz and Schultze-Westrum 1964; Cattaneo 1997) and Skopelos (Werner 1930, 1938a; Buresch and Zonkow 1934; Wettstein 1953; Mertens and Wermuth 1960; Buchholz and Schultze-Westrum 1964; Cattaneo 1998). We confirm this species on Skiathos (n=4), as well as Skopelos (n=20), but on no other islands, despite past speculation about its existence on Gioura and Skantzoura (Legakis 2004). Furthermore, a report of the species from Alonnisos (Crucitti and Tringali 1987; Legakis 2004) almost certainly refers to a misidentified D. caspius (Cattaneo 1998).

The Eastern Montpellier snake is a large-bodied species (record length in Greece is 160 cm; Kalogiannis and Stefanopoulos 2023) and a very active, diurnal hunter. We often encountered this snake in human-modified environments, including agricultural fields, olive groves, and near settlements, as well as in forested areas and maquis. It is a significant terrestrial predator with a catholic diet and a likely competitor to a number of other snake taxa. We found individuals preying on P. erhardii, while Cattaneo (1998) reports mammal and bird remains in stomach content, thus indicating that the species is largely euryphagous on the islands. Adults on Skopelos ranged from 80 cm to 140 cm in total length.

Platyceps najadum, Dahl’s whipsnake (Eichwald, 1831)

This species is only documented in the Sporades on the island of Skiathos, where it is common (Bergman 1985, 1995; Cattaneo 1997). We found 13 individuals throughout the island and across a diversity of habitats, including agricultural land, settlements, mesic forested areas, and xeric sites such as phrygana and maquis. The largest individual had a total length of 125 cm. This is a very thin, quick, and agile snake that is also common on mainland Greece. It primarily feeds on small lizards and invertebrates (Speybroeck et al. 2016). Specimens from Skiathos exhibited the same typical phenotype as those from the nearby mainland, with about 25–30 ocelli across the front half of the body.

Telescopus fallax, cat-eyed snake (Fleischmann, 1831)

While this is a relatively widespread species on the Aegean islands, in the Sporades it has previously only been reported from Alonnisos (Broggi 2010). We report here for the first time two new photographic records from Peristera (Vasiliko bay area, August 2020; Vassilis Malamatenios) and the eastern part of Skopelos (August 2018; Luca di Cianni) (Appendix 1: Table A1). We also confirm the Broggi (2010) record (based on an unpublished photograph) with 12 individuals found in various locations around Alonnisos (Fig. 4G). This species is thinly distributed on Alonnisos and is more common at higher elevations in the central and eastern parts of the island, where the habitat is more xeric and rockier. Cat-eyed snakes are nocturnal, and we found most of them on Alonnisos while they were actively crossing roads or moving across rocky terrain at night (n=9), as well as under metal lids that cover concrete wells (n=3). This taxon is rear-fanged and venomous, though it is not considered a threat to humans as it rarely injects venom (Kochva 1965). The snake feeds mainly on geckos and lacertid lizards, which are abundant in the arid, sparsely vegetated areas this snake prefers.

Photographic voucher: Peristera (NHMC 80.3.38.147), Skopelos (NHMC 80.3.38.148).

Zamenis situla, leopard snake (Linnaeus, 1758)

We report here the presence of this species on Alonnisos, as supported by a photograph deposited in the NHMC (November 2020; Konstantina Malamateniou, see Appendix 1: Table A1). Thus, we confirm Broggi (2010), who mentions having seen pictures of the species taken by a local on Alonnisos but did not include them in his publication. We did not encounter any Z. situla during our field observations. The species has previously been recorded on Skiathos (Bergman 1995; Cattaneo 1997) and Skopelos multiple times, where both the striped and blotched phenotypes occur (Werner 1929, 1930, 1938a; Buresch and Zonkow 1934; Wettstein 1953; Buchholz and Schultz-Westrum 1964; Bruno 1969; Sigg 1984; Cattaneo 1998).

Zamenis situla is a colorful, slender, medium-sized snake that occurs in a blotched and striped form across its range (Speybroeck et al. 2016). On the Sporades, most individuals have red dorsal blotches outlined in black on a light-grey background with lateral spots. Additionally, there exists a striped form with a dorsal pattern of red stripes, or where the dorsal pattern is lacking any red and has more of a ladder-like black/dark gray bars. The species is typically more common near human habitations, giving it its Greek name, ‘Spitofido’ (house snake). It appears to be uncommon on the Sporades.

Photographic voucher: Alonnisos (NHMC 80.3.30.82).

Vipera ammodytes , nose-horned viper (Linnaeus, 1758)

We report this viper for the first time from the island of Peristera (Fig. 4B). The specimen was a female about 40 cm in total length and had a light brown dorsal zig-zag band on a light beige background, resembling the phenotype of females from adjacent Alonnisos. The animal was found in one of the few open areas of the island, at the margins of a wet meadow and abandoned agricultural habitat invaded by patchy maquis.

This species had previously been recorded from Skiathos, Alonnisos, Megalos Adelphos, and Mikros Adelphos (Buchholz and Schultze-Westrum 1964; Cattaneo 1997, 1998). It is absent from all of the islands to the north-east of Alonnisos (e.g., Gioura, Legakis 2004). It also has not been documented yet in Skopelos, where the locals, however, claim it is present. On Skiathos, it is uncommon, and we did not observe any there during our surveys. Others have reported that individuals on Skiathos are relatively large (e.g., a male specimen of 53.2 cm reported by Buchholz and Schultze-Westrum 1964). Female V. ammodytes from Skiathos have the same phenotype as conspecifics from the adjacent mainland, with a dark brown dorsal zigzag pattern on a light brown background (Cattaneo 2021). In contrast, females from the islands of Alonnisos and Peristera exhibit a distinct phenotype that is very “washed out” or “blonde,” which blends in well with the light-grey limestone bedrock of the island (Fig. 4B and see fig. 8 in Cattaneo 2021). This female phenotype appears to be unique to Alonnisos and Peristera, according to extensive comparisons across numerous Aegean Sea islands (Roussos 2015; Cattaneo 2021). Males from Alonnisos have faint (or lack) lateral spots along the body, and the tail tip is black instead of green or yellow as found on the mainland. Vipera from Skiathos and Alonnisos also differ in scalation, with snakes from Skiathos possessing more dorsal scale rows (Cattaneo 2021). In contrast to Skiathos, the species is common on Alonnisos and can be found in a variety of habitats and at all elevations. A total of 25 observations have been made by the authors across the island near stone walls, settlements, phrygana, olive groves, maquis, and pine forest. Several individuals we found were victims of road mortality, especially during the mating period (May–June), as snakes tend to move during post-copulation dispersion. The largest individual we found on Alonnisos was a male with a total length of 75 cm, and others have recently reported individuals > 50 cm SVL (Cattaneo 2021).

Vipers are typically not able to survive on smaller islands. In the Sporades, the two small islands of Mikros and Megalos Adelphos are exceptional in that they retain viper populations. Vipers on the Adelphia cluster exhibit dwarfism (total adult lengths of 20–30 cm), similar to some Cycladic islands (e.g., Koufonissi, Roussos 2015; Itescu et al. 2018). The only size-appropriate prey items available on these islands are geckos and Podarcis lizards, Scolopendra centipedes, and, according to Buchholz and Schultze-Westrum (1964), Orthoptera, especially during the summer months. The islands are covered by a mixture of phrygana with some maquis bushes, as well as a few olive and juniper trees. Goats are also present and keep the shrubs heavily manicured. Vipers were found active during the day and near these bushes. The viper populations on these two small islands appear to be relatively dense, and this is the only snake species occurring there. While previous researchers (Buchholz and Schultze-Westrum 1964) have suggested that the vipers may prey on migratory birds, this is, in our opinion, unlikely, as the extremely small viper body size all but precludes the consumption of birds, except perhaps small songbirds by the largest individuals (>30 cm). What is more likely is that snakes benefit indirectly from the marine nutrient subsidies delivered by the strong gull colonies. Indeed, data from other islands in the Aegean Sea suggests that the presence of significant marine subsidies allows for the existence of predators on islands that would normally be considered too small to support them.

Photographic voucher: Peristera (NHMC 80.3.40.61).

Testudines

Only two Testudine species, each from a different family (Testudinidae and Geoemydidae), are present in the Sporades. We report here on one new island record for Testudo marginata. Testudines are the rarest group of native herpetofauna on these islands and appear to be continually declining. A variety of threats to their habitat make the long-term future of Testudines uncertain outside of protected areas (Hailey and Willemsen 2003).

Testudo marginata, marginated tortoise (Schoepff, 1789)

The marginated tortoise is known from several islands in the Northern Sporades archipelago, where it has been documented on Skiathos (Cattaneo 1997), Alonnisos (Broggi 2010), and Kyra Panagia (Kock and Storch 1979). This species is also known from the neighboring Skyros cluster, including Skyros proper (Watson 1962; Arnold and Burton 1978; Kock and Storch 1979; Bringsoe 1985; Broggi 2006), and offshore Valaxa island (Gruber and Fuchs 1977). The marginated tortoise appears to be rare on the Sporades, with only occasional recorded encounters.

Here we document for the first time the presence of the marginated tortoise on Skopelos, therefore completing the known occurrence of the species across all major islands of the Sporades chain. On Skopelos, we encountered one adult male (carapace length 25 cm) crossing a dirt road in an agricultural area in the eastern part of the island. We also confirm the presence of T. marginata on Kyra Panagia, where we found two alive and two deceased adult specimens. All animals were encountered either in areas with maquis or near agricultural land. All four individuals found on Kyra Panagia were large, full-grown adults, and one of the live specimens had a healed injury to its carapace.

Photographic voucher: Skopelos (NHMC 80.3.22.18).

Mauremys rivulata, Balkan terrapin (Valenciennes, 1833)

The Balkan terrapin has been reported from 31 Aegean and Ionian islands (Broggi 2023), where many of their populations have significantly declined over the last decades and, on some islands, are expected to go extinct (Broggi 2012). This species seems to only be doing well on larger islands with a substantial number of wetlands, such as on Evia (Euboea), Lesvos (Mytilene), Limnos, Ikaria, and Naxos (Broggi 2012; Strachinis and Roussos 2016). On the Sporades, the Balkan terrapin was first reported by Cattaneo (1997) from Skiathos, where small populations currently occur in the stream estuaries of Troulos and Agia Paraskevi. We observed 10 individuals in the streams of Troulos during the summer, where locals have put up signs to dissuade the taking of turtles. Here, the terrapins are regularly fed by tourists and are quite tame. This species is also very likely present in Vromolimnos marsh, but the dense vegetation has made observing them difficult. The Balkan terrapin is also present on Skopelos, where Cattaneo (1998) found a few specimens in the small pond of Ditropo. Furthermore, single individuals of this species have been observed in the coastal marsh of Milia (Broggi pers. comm.). Therefore, similarly to other regions of the Aegean Sea, terrapins in the Sporades survive only in a few isolated, relict wetland sites, and are likely threatened with extinction with the removal of these habitats (Cattaneo 1997; Broggi 2023).

Despite the relatively small extent of the Sporades archipelago, the islands vary significantly in species richness and herptile community structure (Figs 2, 3, Tables 1, 2). At the most basic level, the species assemblages of the Sporades are derivative of those from the Thessaly mainland. Island species richness declines with decreasing island size but also with increasing island age (age of insularity), with the oldest islands in the east having much more depauperate herptile species communities relative to the younger, larger islands in the west (Fig. 2). Thus, Lechousa, a small, young island with a recent landbridge connection to Alonnisos and the Thessallian mainland, has as many herptile species (3) as the almost 20× larger Gioura located off the mainland shelf. Given that the landbridge islands most likely shared the same species assemblages during the last ice age when they were connected (Foufopoulos et al. 2011), present-day differences in species richness are best interpreted as the outcome of differential extinctions.

Recent research has shown that Holocene reptile extinctions in the Aegean are highly predictable and are determined by both species identity and island characteristics (Kalb 2021). Thus, larger-bodied species (e.g., lizards like P. apodus or snakes like M. insignitus), as well as habitat specialists (such as the moisture-loving species L. trilineata and M. rivulata), tend to have smaller populations and are more likely to go extinct first following isolation by rising sea levels (Foufopoulos and Ives 1999). Because the particular species’ life history traits determining extinction sensitivity are the same across an archipelago, species tend to go extinct in a predictable sequence across all islands, reflecting each taxon’s relative susceptibility to underlying extinction drivers. What varies is the extent to which island characteristics cause the original species community to progress down the general sequence of species loss. Thus, islands that are smaller, older, more rugged, and have less water, (e.g., Skantzoura, Piperi, and Gioura) tend to lose a higher fraction of their original species communities (Kalb 2021). Ultimately, this results in a distinct pattern of nested species occurrences where smaller and older islands support only species found on larger ones. The only exception to this pattern is species that likely inhabit similar ecological niches, such as E. quatuorlineata and D. caspius, which do not co-occur on the same island in the Sporades (though we note these species do co-occur in the Cyclades islands), but instead show characteristic ‘checkerboard’ distributions (Diamond 1975). Furthermore, some species, such as Mediodactylus and Podarcis, display a phenomenon called density compensation, occurring in more dense populations and having broader habitat utilization on smaller, species-poor islands relative to large islands (Rodda and Dean-Bradley 2002). This means that they are sometimes rare and very hard to detect on larger islands, such as Mediodactylus on the larger Sporades.

Skiathos is the island closest to the species-rich Thessaly mainland and was last connected to it until fairly recently (~8.5 kya). Because extinction, which is a time-dependent process, has had less time to act on the resident species communities, the island retains a greater fraction of the original mainland species communities, being the Sporades island with the most snake species and the only island that retains both P. najadum and H. arborea. Skiathos has more significant wetland areas compared to other larger islands to the east, with 1% of its total surface covered by freshwater (Catsadorakis and Paragamian 2007). Interestingly, M. kotschyi, one of the most widely distributed species in the archipelago, has not yet been recorded from the island, even though it occurs on the surrounding satellite islets (Tsougkriaki, Arkos, and Daskalonisi). Similarly, H. gemonensis has been recorded from two islets off Skiathos (Tsougria and Aspronisi) but not yet from Skiathos itself.

The island pair of Skopelos and Alonnisos have been isolated almost twice as long as Skiathos from the mainland (15 kya, see Fig. 1, 3) yet harbor similar numbers of taxa. Because Skopelos is more densely forested and retains more wetland areas than Alonnisos, it still sustains several hydrophilic species (P. kurtmuelleri, B. viridis, and M. rivulata), the latter of which is likely lost to extinction. Snake communities between the two islands also differ somewhat, with only Skopelos harboring M. insignitus, H. gemonensis, and E. quatuorlineata, while Alonnisos harbors V. ammodytes and D. caspius instead. This pattern is likely caused by a combination of inter-island differences in habitat in conjunction with interspecific competition between the various snake species.

Skantzoura, Kyra Panagia, and Gioura have all been isolated for longer periods of time, are smaller by land area, and have lower habitat diversity than the main three islands to the west. These smaller islands and their satellite islets are surrounded by very deep waters (> 120 m), suggesting a very long period of isolation (> 200 kya and possibly up to 5.3 million years coinciding with the Zanclean flood; Rohling et al. 2014, assuming depths have not changed significantly due to tectonic movement).

Relatively few exotic reptile introductions appear to have occurred to date on the Sporades, and most species appear to be native to the islands. Three possible exceptions to this are the tortoise T. marginata, the gecko H. turcicus, and the skink C. ocellatus (assuming the Skopelos record is not erroneous). The former is often kept as a pet (Valakos et al. 2008) and may have been introduced in this manner onto the larger, human-inhabited islands of the archipelago. The latter two lizards are often transported by virtue of their small body size, peri-domestic occurrence, and their tendency to hide in building materials and livestock feed. Genetic analyses indicate that for both species, lizard populations across the Mediterranean lack genetic structure, consistent with recent anthropogenic transportation (Kornilios et al. 2010; Rato et al. 2011; Ergül Kalayci et al. 2023). In accordance, we encountered H. turcicus only on islands regularly visited by humans.

While reptiles occupy most natural habitats in the Sporades, some habitats are clearly more important than others and require particular management attention (see Table 3). In general, both amphibian and reptile taxa prefer wetter and more productive habitats, especially if they provide enough insolation for thermoregulation. Wetland areas are the most important habitats because, at least during the summer season, they serve as oases, allowing species to persist in an otherwise hostile habitat.

While conservation emphasis often centers on the actual water bodies, the surrounding habitats can be equally important. We found that the wet meadows surrounding many coastal wetlands were not only used by most of the local reptile and amphibian species, but that they were, when not degraded, extremely species-rich for many other organisms, including plants and invertebrates. We identified such wet meadows in Koukounaries, Aselinos, and Platanias (Skiathos), but also in Tsougria, in Livadakia (Peristera), and in Agios Petros (Kyra Panagia). Wet meadows face immense pressure not only from touristic development but also from overgrazing and undergrazing, which can result in the encroachment of shrubs and exotic invasives.

On the other hand, extensively shaded areas like pine forests are avoided by most taxa, presumably because they lack opportunities for thermoregulation. Our field observations also suggest that structurally and thermally diverse habitats, such as traditional agricultural landscapes, are disproportionately significant because they provide abundant thermoregulatory opportunities for resident species (Zakkak et al. 2015). For example, dry stone wall terraces, a common feature of agriculture fields on sloped island landscapes, not only create a varied set of thermal environments, but also offer valuable refugia from predation since they are challenging for snakes to ascend (Nossan 2023). Overall, most species of reptiles on the Sporades, with the exception of aquatic taxa, appear to be behaviorally quite flexible and can be found in numerous native and human-modified habitats.

Over the last century, human communities on the Sporades have experienced significant demographic and economic shifts with important ramifications for the ecology and herpetofauna of the islands. Extensive agricultural areas were abandoned, first because of a lack of economic incentives and emigration to urban centers, and again more recently with the emergence of tourism as a more lucrative income alternative. As a result of these changes, all of the smaller island settlements were completely abandoned in the last 50 years (Tsougria, Megalos Adelphos, Skantzoura, Grammeza, Pappous, Gioura, Psathoura, and Piperi) (Wikipedia n.d., National Statistical Service of Greece 2011). On the larger islands, and over the last 80 years, abandonment of traditional agricultural activities has progressively led to the incursion of tall maquis and pine forests. Vegetation modulates microclimates (Osorio et al. 2024), and changes to land cover seriously alter thermal landscapes that ectotherms like herpetofauna use to behaviorally maintain homeostasis. In general, shifts to a more closed vegetation structure are associated with a narrower, cooler thermal gradient that results in a greater mismatch to body temperatures preferred by herpetofauna and therefore decreased thermoregulatory efficiency (Muñoz and Losos 2018). Thus, the widespread shift to closed-canopy pine forest in the Sporades has likely decreased opportunities for efficient herptile thermoregulation, as evidenced by their general dearth in such habitats (Table 3).

Livestock husbandry and the associated vegetation changes appear to have a complex relationship with the resident herpetofauna. In the relatively humid Sporades climate, browsing by goats is an important process for keeping vegetation open and preventing the establishment of a closed canopy. Research in other parts of the Aegean Sea region has demonstrated that the impacts on vegetation cover rise with an increasing stocking rate (Cheng 2015). In the Sporades, similar to other parts of the region (Gizicki et al. 2018), island size determines both the type of livestock husbandry, as well as the ultimate impacts on vegetation structure and the resident reptile community. On the larger islands with permanent human populations, individual families have traditionally kept small numbers of goats and sheep for personal use. In addition, a limited number of larger flocks of livestock were typically kept by shepherds who carefully managed grazing patterns to avoid crop damage. In these open and carefully managed agropastoral systems, our personal observations indicate a particularly high reptile presence. Thus, on the main islands, most species of reptiles prefer habitats moderately disturbed by grazing, and avoid (with the apparent exceptions of D. caspius and A. kitaibelii) undisturbed, closed-canopy pine forests. Even on medium-sized islands like Peristera and Skantzoura, reptiles are concentrated in open field areas maintained by goats and appear to avoid the expanding dense maquis.

In contrast to the controlled animal husbandry of large islands, on small islands and islets, goats are typically released unguarded for extended periods of time and left to forage freely. Given the limited resources there, goat releases on islets typically result in severe soil erosion and large-scale vegetation degradation (Gizicki et al. 2018). A formal study from the nearby Skyros cluster demonstrated that goat grazing on small islands has serious negative impacts on both plant cover and the resident herpetofauna (Pafilis et al. 2013). Our own field observations confirm that on islets smaller than a few hectares, livestock grazing results in negative outcomes for local species communities. Based on our field observations, many, if not most, of the smaller islands in the Sporades appear to have suffered from past overgrazing, which has shifted plant communities and severely damaged the vegetation cover. Pelerissa Isl., near Kyra Panagia, still bears the scars of past overgrazing, with the evident trunks of many kermes oaks killed by goats, and the ground covered by a degraded graminoid monoculture that is inhabited only by Mediodactylus and Podarcis (Fig. 6A).

While this practice has nowadays been largely discontinued on the smallest islets, several of the somewhat bigger, yet uninhabited islands (Gioura, Piperi, Grammeza, Peristera, Lechousa, Skantzoura, Megalos Adelphos, Mikros Adelphos, and Agios Georgios) still harbor populations of goats in different stages of abandonment and only thin Podarcis and Mediodactylus populations. Other islands (Repi, Grammeza) suffer similarly, but from released rabbits that dig out top soil, creating another form of erosion detrimental to the sustainability of vegetation (Kossoff 2023).

Wetland ecosystems, both in the Aegean generally and on the Sporades specifically, are typically scarce, vulnerable, and disproportionately important for reptiles and amphibians (Catsadorakis and Paragamian 2007). From the whole archipelago, Skiathos has the most notable wetlands and streams, which are nonetheless few and under human pressure. The marsh of Vromolimnos on Skiathos is in good condition, although some hotels and beach bars exist nearby. The wetland of Koukounaries, although modified, is also in relatively good condition and is now strictly protected, despite the presence of hotels constructed in the surrounding area before the lake’s declaration as a nature reserve. The area of Koukounaries, including the lake, coastline, and Pinus pinea forest, is currently a Natura 2000 site (GR1430003). Skopelos possesses only three wetlands of significance, two of which are seasonal. The permanent lagoon of Milia is in seemingly good condition and hosts populations of frogs and terrapins. On Alonnisos, there is a single seasonal, coastal lagoon located at Agios Dimitrios. Additional small, typically seasonal wetlands exist on Tsougria, Peristera, Kyra Panagia, and Psathoura. Unfortunately, some of the natural wetlands of the archipelago have been reduced or completely destroyed by human activities during the last decades. On Skiathos, Skopelos, and Alonnisos, many wetlands, lagoons, and streams have been drained and built over for touristic development (e.g., hotels and airport infrastructure) or are heavily exploited for irrigation.

From the sites mentioned above, only the wetlands of Skiathos and Skopelos currently host hydrophilic reptiles and amphibians (M. rivulata, B. viridis, P. kurtmuelleri, and H. arborea). Green toads, B. viridis (Laurenti, 1768), are widespread throughout insular Greece, often inhabiting surprisingly small islets (e.g., Stille and Stille 2015; Strachinis 2022), where they commonly reproduce in artificial water bodies. Despite this, they can be severely affected by human activities and are known to have had significant population declines on some islands (e.g., Ithaki, Strachinis and Artavanis 2017). Similarly, marsh frogs of the genus Pelophylax are also well distributed across insular Greece (Valakos et al. 2008), occupying streams, marshes, lagoons, and artificial water bodies such as reservoirs and wells, where they greatly depend on these open water resources (Broggi 2021). In the Sporades, marsh frogs have vanished from Alonnisos, where, according to the locals, they used to thrive until the 1980’s. Specific changes in irrigation, along with heavy exploitation and destruction of streams, caused frogs on Alonnisos to steadily decline, eventually dying out during the 2000’s. Tree-frogs in the genus Hyla are less widespread in the Greek islands, occurring on Thassos, Skiathos, Evia, Corfu, Lefkada, Kefalonia, Zakynthos, Lesvos, Chios, Samos, Kos, Rhodes, and Crete (Valakos et al. 2008), islands known to have high freshwater availability (Catsadorakis and Paragamian 2007). On Skiathos, they seem to occur in abundance around settlements where they breed in anthropogenic structures such as swimming pools. Although they exist in seemingly good populations on the island, the aforementioned breeding sites are unreliable in the long term and readily impacted even by small changes in human activity. For the Balkan terrapins, both in the Sporades and in the rest of the Aegean, the main threats concerning the viability of island populations are habitat destruction and, in the long term, climate change (Broggi 2012). The populations of M. rivulata on Skopelos have significantly declined and have been limited to isolated waterbodies that are particularly susceptible to human activities and climate change.

Overall, habitat loss constitutes the main risk to amphibian and terrapin populations on the islands, since the remaining wetland habitats are now in small and isolated, relict sites. In addition to natural habitats, artificial water bodies (e.g., wells and canals) used for agriculture serve as important breeding sites for green toads and marsh frogs on Aegean islands. Nonetheless, these breeding sites are disappearing as traditional cisterns are being replaced by the modern irrigation system. Consequently, the conservation of wetland habitats, as well as of traditional agriculture, is crucial for the survival of amphibians and terrapins in the Sporades archipelago.

The maintenance of healthy reptile and amphibian communities is directly tied to appropriate habitat management. On the western, more developed islands, the main threats to the native communities are road construction, touristic development, and surface water diversion. On the smaller, eastern islands, the main problems center on the abandonment of the traditional agricultural activities, as well as the overgrazing of small islets. As a result, management recommendations differ among the islands. Given that many of the eastern islands are within the MPA and least threatened by human development, the terrestrial aspect of the reserve could be one of promising restoration and management. This would not only promote suitable habitat for the herptile community but also for all biota native to these islands. Another significant threat to reptile populations on human-inhabited Aegean islands are feral cats (Case and Bolger 1911). While spaying and neutering cats can help control their population numbers, it will not stop their killing of birds, small mammals, and reptiles. Outdoor cats are significant predators of snakes and lizards and have caused island extinctions the world over (Medina et al. 2011). We recommend that cats be kept indoors for their own health and safety and the preservation of biodiversity. Fortunately, smaller islands that do not have cats are refuges for snakes and lizards from this invasive predator.

Our field research suggests that the continuation of traditional agricultural activities, including some goat grazing, which create a mosaic of thermally variable microhabitats, is important for the persistence of native reptiles and amphibians (Papanastasis et al. 2009). This includes the preservation of the diverse stone structures such as drystone walls and terraces, stables, and cisterns, all of which provide cover and refugia for resident reptiles. In addition, maintenance of any sources of surface water, whether in the form of springs, seasonal streams, or cisterns, is essential for many species. The practice of illegally drawing water from springs and streams with small pumps for irrigation desiccates the landscape and is particularly damaging, and its prohibition should be enforced. On Skopelos and Skiathos, exurban development for tourism and associated roads is increasingly leading to the fragmentation of the natural habitats and represents a growing problem for native reptiles and amphibians.

Wetlands and wet meadows harbor many unique species and require urgent priority protection and management. On Skiathos and Skopelos, perhaps small ponds could be constructed in order to enhance the reproductive success of amphibian species. Wet meadows, such as the wetlands of Koukounaries and the mouths of the Aselinos stream (Skiathos), are tremendously important and require special management. This requires the absence of water extraction to maintain high water tables, the absence of plowing, and the removal of encroaching bushes, ideally through activities such as scything. Grazing, if it occurs, must happen only briefly, perhaps late in the summer, with a focus on controlling encroaching bushes.

We observed significant seasonal road mortality in certain areas, suggesting that the creation of under road tunnels would be an excellent way to allow for safer passages. These can be used as general wildlife crossings for other species as well. Road signage is a cost-efficient and educational strategy to warn drivers to slow down and be alert to species’ crossings.

Overgrazing by unmanaged, feral goats constitutes the biggest problem for the resident herpetofauna on medium-sized, uninhabited islands. Overstocking on such islands can lead to dramatic habitat degradation through vegetation loss and subsequent soil erosion, as seen on Kyra Panagia and Pelerissa islands (Fig. 6A). Nonetheless, lower levels of grazing on some larger uninhabited islands can open up forest and dense maquis thickets and provide basking opportunities for local reptiles. On some islands where grazing pressure has declined significantly in the last few years (esp. on Peristera and Skantzoura), habitat quality for resident reptiles may be declining, as has been the case in other parts of Greece (Zakak et al. 2015).

Some of the medium-sized islands of the archipelago also include key wet meadows that exist today in severely overgrazed and degraded form: Livadakia (on Peristera) and Agios Petros (on Kyra Panagia). Given the importance of these habitats for reptiles, it would be important to eliminate grazing and plowing on these specific sites, something that on Agios Petros is particularly easy given that the property is already fenced in. In the medium term, additional restoration measures may be needed, including the re-introduction of some sensitive plant species as well as potential scything to prevent invasion of the bushes.

While small islets harbor only a few reptiles and no amphibians, they are often disproportionately important for conservation because they tend to be free of human interference, act as important stop-over points for migrating birds, and often showcase unusual reptile populations with often extreme phenotypes. Lizards from seabird islands occur in extremely high population densities, possess unusually large body sizes, and often display exceptional territorial, cannibalism, or fearlessness behaviors (Brock et al. 2014, 2015; Donihue et al. 2016). Islet species communities are primarily dependent on the existence of large seabird colonies to provide marine nutrient subsidies (Stadler et al. 2023) and tend to be severely impacted by invasive goats, rabbits, and rats (Weber 2014; Gizicki et al. 2018). As a result, the removal of invasive species and the promotion of seabird colonies are the most important management activities for small island reptile populations .