Land snails and the Aegean Archipelago offer an intriguing combination for studying biodiversity, biogeography and ecology. A region with high environmental and temporal heterogeneity and a tri-continental biotic influence and a group of organisms with low active dispersal abilities, high endemism, as well as the particularity to leave shells as traces of past presence, set an ideal stage for testing biodiversity patterns and exploring multisource threats, especially in the era of the ongoing biodiversity crisis. In this study, we examine Helix godetiana, a large-sized, threatened and endemic land snail of the central and south Aegean Islands. The species has been extirpated from 22 of the 32 islands where it was historically present. We identify potential drivers of its extinction, as Helix godetiana faces several threats across its current range, including competitive exclusion by Cornu aspersum, a species with continuing expansion in the Aegean and climate change disrupting its unusual breeding cycle, which occurs in late spring. Our findings shed light on potentially major, yet previously unexplored, threats on endemic molluscs of the Aegean Islands, a European biodiversity hotspot.

Aegean Archipelago, biodiversity, extinction risks, islands, molluscs

Islands contain a disproportionately high percentage of the world’s biodiversity relative to their size, yet they also serve as hotspots for past, present and predicted future biodiversity loss (Russell and Kueffer 2019; Fernández-Palacios et al. 2021). Threatened species on islands usually have to face multiple threats simultaneously with biological invasions, wildlife exploitation and cultivation responsible for most insular extinctions (Leclerc et al. 2018). Moreover, over 60% of recorded terrestrial species extinctions since 1500 AD have been island endemics (Whittaker et al. 2017; Matthews and Triantis 2021).

Land snails are amongst the most studied and well-known invertebrate taxa on islands (Cameron et al. 2013), characterised by high species richness and notable rates of endemism (Proios et al. 2021). They are also one of the most threatened groups of terrestrial animals (Lydeard et al. 2004; Cowie et al. 2017), comprising the majority of currently-known extinctions amongst all terrestrial plant and animal taxa (191 species out of 984, IUCN 2022), with over 70% occurring on islands (Régnier et al. 2009). Significant factors contributing to land snail extinctions include habitat loss, degradation and fragmentation, climate change, pollution and the introduction of invasive species (e.g. snails, rats, flatworms) (Holland 2009; Chiba and Cowie 2016). Particularly for European land snail species, the most significant threats are natural ecosystem modifications, residential and commercial development and agriculture (Neubert et al. 2019).

Obtaining detailed and accurate population data for snails, especially on remote islands, presents significant challenges. Consequently, distributional data serve as a major — and arguably the most useful — tool for assessing threat status and inferring extinction risk under the provisions of the IUCN Red List (see Criterion B, IUCN 2012). The 495 Aegean land snail species offer an exemplary case of particularly well-detailed knowledge of their island-specific distribution (Mylonas and Vardinoyannis 2022). Furthermore, land snails are amongst the most amenable invertebrates in this context, as their presence can be traced back in space and time due to their shell remains, thus providing a largely complete documentation of past extinction events (Cameron and Pokryszko 2005).

Given that biodiversity hotspots, areas of endemism and species with restricted distributions are pivotal concepts in biodiversity studies (Reid 1998), a deeper understanding of the mechanisms and processes driving the endangerment of land snails across the Aegean islands — a European biodiversity hotspot — is essential. Such insights could greatly enhance efforts to address the current extinction crisis, not only for snails, but for global biodiversity overall (Cowie et al. 2022).

In this study, we focus on a large-sized endemic species of the central and south-eastern Aegean, Helix godetiana Kobelt, 1878 (Fig. 1), which has been assessed as Endangered (EN) (IUCN 2022). This species is primarily found in maquis shrublands with a calcareous substrate and exhibits relatively small population sizes (Mylonas 1985). Numerous studies have attempted to clarify the phylogenetic position of H. godetiana and recent analyses, based on mitochondrial and nuclear markers, place it within the genus Helix as a distinct, monotypic subgenus (Korábek et al. 2021; Korábek and Hausdorf 2023). Based on extensive field data collected over the last 45 years and additional historical records, we document the past and present distribution of the species and explore the potential drivers of its decline.

Figure 1.

Helix godetiana from Naxos Island.

We compiled historical data on the distribution of H. godetiana, based on the existing extensive literature regarding Aegean islands’ malacofauna (Fuchs and Käufel 1936; Mylonas 1982, 1985; Vardinoyannis 1994; Frank 1997; Triantis et al. 2008; Neubert 2014; Mylonas and Vardinoyannis 2022). We also reviewed the rich material held at the Natural History Museum of Crete (NHMC), collected over the past 45 years during numerous field trips organised to study the malacofauna of the Aegean Islands. Using this holistic approach, we provide an updated compilation of data on the distribution of H. godetiana.

We characterised the population as extant when museum material or bibliographic data confirmed the presence of living specimens or fresh shells. Conversely, populations were considered extinct if only very old, subfossil shells were recorded (following Moody 1986).

It has been proposed that the relatively recent expansion of the more euryoecious Helicid species Cornu aspersum, Cantareus apertus and Eobania vermiculata may have acted as a restrictive factor for H. godetiana (Mylonas 1985). To examine the potential competitive effect of these three large helicid species and to determine whether the other indigenous Helix species of the region have similar effects, we employed a co-occurrence approach, testing the co-occurrence between extant H. godetiana and several Helicidae with presumably similar preferred habitats (i.e. C. aspersum, C. aperta, E. vermiculata, H. nucula, H. figulina and H. cincta). Using the material housed in the NHMC, we analysed all sampling localities (n = 766 from 40 islands) within the distributional range of H. godetiana (Fig. 2) where at least one of the above species was sampled extant, excluding subfossil and fossil shells. To quantify species co-occurrence, we applied the probabilistic model proposed by Veech (2013), which quantifies co-occurrence in a simple and straightforward way by counting the number of sampling sites where two species are found together. Observed co-occurrence is compared to expected co-occurrence, which is calculated as the product of the probabilities of occurrence for the two species multiplied by the number of sampling sites. The model uses combinatorics to assess whether the observed frequency of co-occurrence is significantly higher than expected (indicating a positive association), significantly lower than expected (indicating a negative association) or not significantly different from expected (indicating a random association) (Veech 2013; Griffith et al. 2016). In cases where negative correlation was found (i.e. P_lt < 0.05), we considered that the species is highly likely to be negatively influenced by the presence of another species (but see also Blanchet et al. 2020). The analysis was conducted in R (R Core Team 2024) using the package “cooccur” (Griffith et al. 2016).

Figure 2.

The distribution of H. godetiana. Pink shows where the species is currently extant, while black depicts the islands where only old shells are encountered.

Herein, we present an updated profile of the past and present distribution of H. godetiana (Fig. 2), which has historically inhabited 32 islands, but is currently found extant on only 10. This represents a reduction to 31% of its original distribution (Table 1, Fig. 2)

Additionally, we present the species co-occurrence probabilities with other helicid snails, using it as a proxy to assess potential competitive interactions. We note its negative association with C. aspersum, with which H. godetiana does not co-exist in any locality (Table 2). Apart from C. aspersum, its association with every other helicid species is not different from the null expectations.

Overall, our study demonstrates a notable decrease in the distribution and population size of H. godetiana, one of the largest endemic snails of the Aegean. Based on extensive field data spanning more than four decades, H. godetiana has become extinct on the majority of the islands where it was once found and is now restricted to the central islands of its former range (Fig. 2). Moreover, recent comprehensive sampling expeditions to Naxos, Anafi, Astypalaia and Syrna islands yielded only sparse shells in sites where the species was previously abundant, suggesting declining populations on islands where the species is still extant and indicating that the species is on the path to extinction.

Undoubtedly, contemporary threats such as intensive grazing, fire and habitat destruction and degradation, continue to pose significant risks to invertebrate species, with these impacts likely amplified for large, slow-moving organisms like helicid snails. Could these factors, coupled with the species’ intrinsic traits, such as large body size, be responsible for the observed range contractions? Large body size has been correlated with several extinction-promoting traits (McKinney 1997; Purvis et al. 2000) and is linked to higher extinction risk across various animal taxa (Chiba and Roy 2011; Seibold et al. 2015; Terzopoulou et al. 2015; Verde Arregoitia 2016). However, considering that H. godetiana is the only threatened species within the iconic, large-sized genus Helix in Greece, the reasons for its predicament likely extend beyond just shell size. Several synergistic factors appear to be driving the species toward extinction.

Firstly, a potentially major drawback of H. godetiana is its reproductive behaviour, as it has been observed to copulate and lay eggs at the end of spring, specifically in May on Naxos Island (Mylonas, personal observation). With late spring temperatures rising and precipitation decreasing on several Aegean Islands, shifting them towards semi-arid conditions (Myronidis and Nikolaos 2021), hatchlings must endure the hot and dry summer months, aestivating in the soil and ground, thereby facing increased risks of desiccation and predation. This reproductive strategy deviates strikingly from that of most land snails in the Aegean, as nearly all species copulate and lay eggs after the first autumnal rains or rarely in early spring (Vardinoyannis et al. 2018). There is a considerable possibility that the reproductive strategy of H. godetiana was well-adapted to the climatic conditions of several millennia ago, when the area was less arid and had higher levels of precipitation. However, this strategy may no longer be viable due to modern climate change, making H. godetiana one of the first documented land snail species severely threatened by climate change. Interestingly, species of the genus Codringtonia in the Peloponnese, which exhibit a similar — peculiar and rare in the area — breeding cycle, also show declining populations, as reported by Hadjicharalambous (1996).

Secondly, based on co-occurrence probabilities, we tested whether the expansion of the large non-native snails Cantareus apertus, Cornu aspersum and Eobania vermiculata across several Aegean Islands had a possible influence on the diminishing distribution of H. godetiana. This hypothesis is partially supported, as only C. aspersum has a negative and statistically significant association with H. godetiana (Table 2). On the other hand, sympatric occurrences of E. vermiculata and C. apertus with H. godetiana do not differ from random expectations (Table 2), despite their relatively close preferred habitats and perceived ecological niches. Therefore, C. aspersum is a possible restricting factor for the survival of H. godetiana.

Finally, H. godetiana is an edible snail and human exploitation as a food source (Mylonas and Vardinoyannis 2022) has likely contributed to its decreased occurrence across Aegean Islands. While not the primary driver of its decline, this factor could compound the effects of other threats, further exacerbating the species’ vulnerability.

Molluscs, constituting the second-largest animal phylum, are a reasonably well-known group of invertebrates and identifying the reasons for the decline of particular species holds strong potential for altering the Earth’s trajectory towards a Sixth Mass Extinction (Cowie et al. 2022). Helix godetiana presents a characteristic case study as an endemic island snail facing cascading effects from competition with a non-indigenous (yet well-established and expanding) snail (C. aspersum), climate change disrupting its breeding cycle and human exploitation as a food source, coupled with habitat degradation and land use changes on islands that pose a significant threat to numerous species.

We would like to thank Ruud Bank and Bernhard Hausdorf for their valuable comments on a previous version of the manuscript, which have notably improved this paper.