This study combines ENMs for various amphibian species in the Mediterranean with a broader examination of the region's biodiversity to project and explore amphibian diversity patterns under past, present, and future climate scenarios. The Mediterranean Basin, widely recognized as a biodiversity hotspot, plays a critical role in harboring endemic and threatened species across various taxa (Maiorano et al., 2013; Numa et al., 2020). ENM, a method considered highly useful for interpreting species' ecological and historical biogeography (Perktaş et al., 2015a), was chosen for its potential to deliver meaningful, geographically explicit conservation outcomes for this region, which is under significant threat from climate change and human activities.
The Mediterranean Basin’s status as a global biodiversity focus is further emphasized by its qualification as one of the original 36 Global Biodiversity Hotspots, a distinction it earns by hosting > 1500 endemic species vascular plants and experiencing > 30% loss of its original natural vegetation (Myers et al., 2000; Hrdina and Romportl, 2017). This study focuses on endemic amphibian species, the significant number of which underscores the critical nature of the region in terms of its unique and threatened biodiversity.
The geographic distributions of species in the Mediterranean have been influenced by alternating glacial and interglacial periods, as evidenced by historical climate dynamics. The expansion-contraction model, especially relevant during colder periods, is supported by the findings from Hewitt (2004) and Schmitt (2007). This model suggests species sought refuge in southern, warmer climates during glaciations (Stojak and Jędrzejewska, 2022). Analyses across various taxa, including mammals, plants, and invertebrates, underline the genetic impacts of these climatic shifts on historical biogeography (Gür, 2013; Taberlet et al., 1998). Notably, the LIG models show a westward distribution shift in amphibians, specifically in Portugal and France, attributed to regional climatic stability (Rioual et al., 2001), which is congruent with individual species' suitabilities across various taxa (Perktaş et al., 2017; Ülker et al., 2018). These patterns confirm the relevance of historical climate refugia suggested by Médail and Diadema (2009), although our LGM outputs suggest a slight misalignment, they still match current species suitability. The similarity of Mid-Holocene distributions to contemporary conditions reflects the findings by Badis et al. (2024), Wu et al. (2021), and Segatto et al. (2017), highlighting the ongoing influence of historical climates on present biodiversity patterns and emphasizing the need for targeted conservation strategies.
Biodiversity hotspots have been instrumental in informing global conservation priorities, and have been used to focus conservation actions on particular regions with exceptional concentrations of endemic species undergoing significant habitat loss (Myers et al. 2000). As a contrasting concept, biodiversity coldspots are limited-diversity regions often explained by harsh ecological conditions (Schröter et al., 2017). Biodiversity hotspots have received significant attention and conservation efforts, yet coldspots remain relatively overlooked and poorly understood (Procheş, 2022), even though the scarce biodiversity in coldspots can be sensitive to environmental changes and human activities (Liu et al., 2023). These two concepts are relevant, given that current suitable areas may transition between the two states over time (REF), as has also been illustrated in this study. For example, the southern part of the Iberian Peninsula and central Anatolia appear to lose their significance, whereas northern Europe and Caucasia will have increasing significance under future climate scenarios.
In the Mediterranean Basin, invasive species such as the African clawed frog (Xenopus laevis) and various invasive crayfish species (e.g., Procambarus clarkii and Pacifastacus leniusculus) pose significant ecological threats. These invaders are particularly problematic because of their adaptability to Mediterranean climates and their aggressive competition with native amphibians for critical resources and habitats (Lillo et al., 2010; Mota-Ferreira & Beja, 2020). The presence of invasive species has been linked to reductions in the reproductive success of native species, fundamentally altering ecological interactions, and potentially leading to local extinctions even without direct habitat destruction (Clavero et al., 2010). Shifting distributions of species, driven by present climate change processes, is emerging as a threat to biodiversity, especially for amphibians in the Mediterranean.
Moreover, the phenomenon of introgression through hybridization can present a further challenge to conserving genetic diversity in the region. Introgressive hybridization, particularly observed between native European tree frogs (Hyla arborea) and introduced species like H. intermedia, can significantly diminish the viability and fertility of hybrid individuals, potentially reducing the overall fitness of local amphibian populations (Dufresnes et al., 2015). Such genetic intermixing may lead to homogenization and eventual replacement of native gene pools, further complicating conservation efforts and threatening the unique biodiversity of the Mediterranean Basin (Dufresnes et al., 2015).
As illustrated by Médail and Diadema (2009), the Mediterranean biodiversity hotspot is home to 52 putative or possible refugia based on studies of plant distributions. Our ENM-based analyses align these refugia with the current distributions of amphibians. Notably, influences of climate change suggest significant temporal transitions for these refugial areas, with several changing from hotspots to coldspots for our target species. These transitions are expected to affect climatic suitability of these areas by the 2050 and 2070 time horizons. Implications of different RCP scenarios are profound, with the best-case scenario RCP 2.6 showing less-dramatic shifts compared to the worst-case scenario RCP 8.5. This result indicates that some conservation priorities may need to shift towards areas that are currently regarded as coldspots, which have not traditionally been considered as areas of conservation focus (Cox and Underwood, 2011). For instance, whereas the Iberian Peninsula and Central Anatolia are projected to lose significant diversity, regions in central and western France are expected to gain diversity, suggesting a need to reevaluate conservation priorities and resource allocation in these emerging priority areas, especially considering the high human population density. In line with other studies, our findings confirm that the Mediterranean Basin is exceedingly vulnerable to climate change, which poses significant risks to biodiversity conservation (Almeida et al., 2022; Benítez-Benítez et al., 2022; Guiot and Cramer, 2016).
A notable limitation of this study lies in its focus solely on climatic variables to explore distributional shifts of amphibians in the Mediterranean Basin. While this approach is critical for understanding the impacts of climate change, it inherently overlooks other significant ecological factors that can also profoundly influence amphibian populations (Costa et al., 2021; Dvorsky et al., 2022; Youngquist and Boone, 2021). Non-climatic threats such as vegetation distributions, habitat destruction, urbanization, pollution, and introduction of invasive species, are also at least potentially important in shaping present and future distributions of these species but are not directly accounted for in our models (Harper et al., 2008; Underwood et al., 2009; Veron et al., 2016). In this sense, this study should be interpreted as exploring climate-based potential geographic distributions of species.
Emerging threats from diseases such as chytridiomycosis, caused by Batrachochytrium dendrobatidis, and other coinfections further exemplify the challenges that need to be integrated into full conservation planning applications of such studies (Pereira et al., 2013; Wuerthner et al., 2017). Such pathogenic factors are known to cause rapid declines in amphibian populations and could drastically alter the landscape of biodiversity in the Mediterranean, independent of or in conjunction with climate change. This study also not incorporate effects of multiple stressors beyond climatic factors, such as interactions between disease pathogens and environmental contaminants. For instance, effects of trematode infections are exacerbated by pesticide exposure, highlighting the complex interplay between biotic and abiotic factors which can influence amphibian health and survival (Kiesecker, 2002). These factors could significantly alter the outcomes of climate change impact predictions by either mitigating or intensifying the effects perceived solely through the lens of climatic variables.