Anthropogenically induced climate change is quickly reshaping the world’s ecosystems with major impacts on biodiversity [1]. Marine ecosystems have been particularly hit, with roughly 66% of the global oceans being affected by multiple human pressures [2]. The consequences for marine biodiversity are severe. The Ocean Living Planet Index indicates that global ocean fish stocks have been overexploited by 29%, ocean species have decreased by 39%, and world coral reefs have decreased by 50% [3]. Among all threats affecting marine ecosystems, climate change is currently considered one of the most important [4], with 14% of all marine species already heavily impacted by climatic factors [5] and with increasingly severe impacts over time. The extreme heat waves in 2019 hit 57% of the ocean surface waters, with significant impacts on marine ecosystems and their ability to provide resources to coastal communities [6].
Future projections return an overly grim outcast, even considering the near future, with the Mediterranean Sea representing one of the most vulnerable regions in the world when considering the impacts of global warming [7]. The Mediterranean has always been hugely exploited by humans, with important and potentially negative interactions between human activities and climate changes. The basin currently hosts more than 500 million inhabitants, a third of whom live along the coasts, and is the first tourist destination in the world with 360 million visitors per year [8, 9]. The combination of climate change and human impacts clearly generate the starting baseline for a biodiversity collapse [10], especially for species exploiting at the same time marine and terrestrial habitats as sea turtles.
Many species of vertebrates (birds, mammals, and reptiles) roam over both the marine and terrestrial realm and are potentially very sensitive to global changes. In fact, they exploit very different habitats during their life cycles, often migrating over long distances to reach nesting, feeding, and reproductive grounds [11, 12], being therefore exposed to multiple and often uncoupled threats. Many of these species (e.g., Mirounga leonina, Aptenodytes patagonicus) are responding to climate change by changing their phenology and/or distribution range [13, 14], while others (e.g., Fratercula arctica) seem unable to adapt fast enough [11].
Single species responses are expected to lead to changes also in the pattern of species richness [15], in community and ecosystem structure [16], and in ecosystem services [17]. Although negative impacts are predominant, climate changes could also have positive effects on marine biodiversity. For instance, increased temperatures and CO2 concentrations seem to accelerate primary biomass production [18], while milder winters might increase survival probabilities of many threatened species in temperate regions [19, 20].
Understanding the impact of global changes in marine systems is particularly challenging given their complexity and the huge number of factors that should be considered. Umbrella and/or keystone species can clearly represent a good proxy for global change effects, and marine turtles are particularly interesting in this context. In fact, during their life cycles, they exploit different habitats, nesting on the terrestrial realm and then using both the neritic and oceanic habitats, being potentially exposed to multiple, unrelated, and very different threats [11]. Being highly mobile and migratory species, they have the potential to adapt to changing conditions [21] and can respond to changes by shifting their spatiotemporal distribution and/or changing their nesting and foraging behaviour [22, 23]. However, sea turtles have a peculiar reproductive physiology, and therefore are particularly sensitive to climatic conditions during eggs incubation and development [24], when climate change can directly affect sex ratio in hatchlings [25]. In fact, turtles have temperature-dependent sex determination, with females associated with high incubation temperatures (roughly above 29°C) [26] and males associated with low temperatures. Some authors have proposed a positive effect of increasing temperatures on sea turtle populations, suggesting that more female hatchlings would automatically translate into more breeding females, more nests and, consequently, population growth [27, 28]. However, an excessive shortage of males can produce negative consequences (e.g., reductions in egg fertility, which may threaten population viability) [29] and extreme temperatures may even lead to embryonic mortality [30]. Climate changes can also impact adult turtles. Being ectothermic, seawater temperature can radically impact physiology and behaviour of turtles [31]. In fact, below a thermal threshold of around 10 − 15°C (depending on the species) their metabolic rates decrease, and turtles become less mobile [32, 33, 34]. On the other side, higher temperatures can induce indirect stress by promoting the growth of pathogens, with clear consequences on the biology of populations [35]. Overall, different analyses have predicted significant losses in nesting habitat for sea turtle populations in the Great Barrier Reef, Greece, and the Hawaiian Islands, even exceeding 65% of the areas suitable for nesting [36, 37, 38, 39]. However, these projections, catastrophic in some cases, are often focused exclusively on climate and do not consider other important factors which can contribute to an even worse outcome. For example, the flooding of beaches, associated with an increasing prevalence and intensity of storms, will further reduce both hatching success and availability of nesting habitats [40, 41].
Currently, most research efforts in relation to climate change impacts in the Mediterranean have been related to loggerhead sea turtles (Caretta caretta), focusing on the current and potential distribution [23], ecology [42], and conservation of the species [43]. Much less is known about green turtles (Chelonia mydas), for which only a general overview of the ecology and conservation status exists [44]. A recent analysis [45], considering only bioclimatic variables, found no appreciable change in the potential suitability for green turtle nesting grounds under different global change scenarios in the Mediterranean. As a species tightly linked to sea surface temperature, with an optimum temperature range for functioning > 25°C [46, 47, 48, 49, 50], these results are surprising. In fact, green turtles are strongly adapted to hot conditions with a distribution that is mostly tropical. In the Mediterranean, they are limited to the hottest areas, and particularly to the Levantine basin (Turkey, Syria, Cyprus, Lebanon, Israel, Egypt). Their nesting grounds are all in the easternmost part of the Mediterranean, with the vast majority of the nests located in Cyprus and Turkey [51]. Foraging areas extend to Greece and Libya [44], while a few wandering green turtles can be occasionally found in the Adriatic Sea and very rarely in the western basin.
Using a species distribution modelling approach fully focused on the species ecology, and therefore including both marine and terrestrial variables, anthropogenic and natural factors, we provide here a thorough investigation of the impact of future climate change scenarios on the distribution of nesting grounds for green turtles in the Mediterranean basin. Our main hypothesis, contrary to what is available in the literature for the same species [45], is that nesting ground availability for green turtles will potentially increase in the Mediterranean following hotter summer and winters. If our hypothesis is confirmed, our results can be extremely important for a proactive management approach in which the conservation of the species and the management of potential conflicts with human coastal activities is planned and enforced well before the species arrival.