The Sporades are a biogeographically coherent group whose herpetofauna derives primarily from the nearby Greek mainland, but the islands differ dramatically in age and isolation. During the Last Glacial Maximum roughly 20,000 years ago, when sea levels stood about 120 meters lower than today, the western islands were connected to Thessaly by land bridges. Skiathos remained attached to the mainland until approximately 8,500 years ago, while Skopelos and Alonnisos were cut off around 14,000 to 15,000 years ago. The eastern islands of the chain, including Skantzoura, Kyra Panagia, Gioura, Psathoura, and Piperi, sit beyond the 120-meter isobath and have been isolated for far longer, likely exceeding 200,000 years and possibly stretching back millions of years.

That gradient of isolation leaves a clear signature in the distribution of species. Species richness declines from west to east and from large islands to small ones, a pattern the authors interpret as the outcome of differential extinctions rather than differences in colonization. Because the land-bridge islands presumably shared the same mainland species pool when connected, present-day differences in community composition reflect how many species each island has lost since rising seas severed the connection. Lechousa, a small, young island with a recent land bridge to Alonnisos, supports as many herptile species as Gioura, an island nearly twenty times larger but isolated far longer. Larger-bodied species and habitat specialists, such as moisture-loving terrapins and green lizards, tend to disappear first, producing a nested pattern in which depauperate communities on old, small islands contain only species also found on the larger western islands.

Squamates dominate the archipelago, with eight lizard and eight snake species recorded. The Aegean wall lizard (Podarcis erhardii) is the most widespread reptile, present on at least 40 islands, followed by Kotschy’s gecko (Mediodactylus kotschyi) on 30 islands. Among snakes, the Caspian whipsnake, nose-horned viper, and four-lined rat snake each occur on five islands, while others such as Dahl’s whipsnake are confined to Skiathos alone. Notably, several large snake species show checkerboard distributions, in which ecologically similar species such as the four-lined rat snake and the Caspian whipsnake never co-occur on the same island, a pattern suggestive of competitive exclusion.

The survey yielded a series of notable firsts. The marginated tortoise was documented for the first time on Skopelos, completing its known occurrence across all major islands of the chain. The cat-eyed snake was recorded for the first time on Peristera and Skopelos, the leopard snake was confirmed on Alonnisos from a photograph, and the nose-horned viper was documented for the first time on Peristera. The Caspian whipsnake was newly recorded on Skantzoura and the islet of Prasso, and the four-lined rat snake was found on Gioura and Grammeza. Sixteen new island records were added for Kotschy’s gecko alone, along with three for the Mediterranean house gecko and one for the legless European glass lizard on Skopelos.

Amphibians tell a more troubling story. Only three species occur in the archipelago, all confined to the younger western islands, and all are now likely extinct on Alonnisos despite systematic searching. Marsh frogs thrived there until the 1980s, according to local reports, but the intensification of irrigation, the replacement of traditional wells with powerful modern pumps, and the desiccation of streams eliminated their breeding habitat, and the frogs died out during the 2000s. On Skiathos, the European tree-frog has found an unexpected ally in abandoned swimming pools and fountains, where dense breeding populations now persist, though such anthropogenic breeding sites are unreliable over the long term. Green toads on Skopelos depend on a handful of degraded wet spots near agricultural land, and one count recorded roughly 100 calling males in a flooded field near Staphylos in March 2022.

The islands’ endemic element adds to their conservation significance. Piperi, despite belonging to the separate Skyros shelf, hosts the endemic subspecies Podarcis gaigeae weigandi, the only representative of the Skyros wall lizard within the study area. The gecko subspecies Mediodactylus kotschyi fuchsi and the wall lizard subspecies Podarcis erhardii ruthveni are distributed across much of the archipelago, though the authors caution that these morphologically defined taxa have not yet been tested with molecular data, and prior genetic studies have synonymized many Aegean subspecies. Intriguingly, Kotschy’s gecko has never been found on Skiathos itself, even though it occurs on the island’s satellite islets, and the Aegean wall lizard is absent from Skiathos despite a marginal historical record.

Human activity has shaped these communities for millennia, and the study documents how recent demographic shifts continue to transform reptile habitat. Rural exodus emptied the smaller island settlements within the last 50 years, while the abandonment of traditional agriculture on the larger islands has allowed dense maquis and closed-canopy pine forest to expand. Because ectotherms rely on environmental heat to regulate body temperature, these closed, shaded habitats offer a narrower and cooler thermal landscape that most reptiles avoid. Conversely, traditional agricultural mosaics with dry stone wall terraces, olive groves, and lightly grazed fields provide both thermal diversity and refugia from snake predators, and the authors found reptile presence highest in these moderately disturbed landscapes. On small islets, however, unmanaged goats have devastated vegetation and soil, leaving degraded landscapes that support little beyond geckos and wall lizards.

Wetlands emerge as the archipelago’s most vulnerable and most important habitats. Amphibians and Balkan terrapins now survive only at a few relict sites on Skiathos and Skopelos, such as the Vromolimnos marsh, the Koukounaries wetland, and the Milia lagoon, all threatened by touristic development, water extraction, and climate change. The authors recommend urgent protection of wetlands and wet meadows, enforcement against illegal water pumping, construction of small ponds and road-crossing tunnels, and controls on feral cats, which are significant predators of lizards and snakes on inhabited islands. For the uninhabited eastern islands within the National Marine Park of Alonnisos and Northern Sporades, they propose terrestrial restoration, including removal of invasive goats and rabbits and promotion of seabird colonies, whose marine nutrient subsidies sustain unusually dense and large lizard populations on tiny islets.

Overall, the study concludes that the Sporades herpetofauna remains rich and largely native, with relatively few introduced taxa, and that none of the non-aquatic populations appear outright endangered. Yet the trajectory is uneven: while lizards and most snakes persist across the archipelago, the moisture-dependent species, terrapins and amphibians alike, are in decline and depend on the survival of a few small, isolated wetlands. The authors frame their synthesis as a baseline for future research, noting that molecular studies of the islands’ reptile populations are still largely lacking, and they hope the work will guide the terrestrial dimension of conservation within the marine park and across the wider archipelago.

Subject of Research: Herpetofaunal survey and biogeographic review of the Sporades Islands, Greece

Article Title: The herpetofauna of the Sporades Islands (Aegean Sea, Greece): New discoveries and a review of a century of research

Article References: Foufopoulos, J., Roussos, S., Kalogiannis, S., Kalb, S., Strachinis, I., & Brock, K. M. (2024). The herpetofauna of the Sporades Islands (Aegean Sea, Greece): New discoveries and a review of a century of research. Herpetozoa, 37, 231-256. https://doi.org/10.3897/herpetozoa.37.e125965

Image Credits: AI Generated

DOI: 10.3897/herpetozoa.37.e125965

Keywords: Sporades Islands, herpetology, reptiles, amphibians, island biogeography, Aegean Sea, Greece, endemism, wetland conservation, lizards, snakes, biodiversity survey