Thesis title: SOME LIKE IT HOT (OR NOT?): GLOBAL CHANGES AND CONSERVATION OF CHELONIIDAE IN THE MEDITERRANEAN SEA
Extended abstract
Background
Global changes affect population dynamics and community interactions across terrestrial, freshwater, and marine ecosystems. These changes, characterized by rising temperatures, ocean acidification, and altered oceanic circulation, pose significant threats to biodiversity worldwide. Particularly vulnerable are migratory species, which face challenges in adapting to changing environmental conditions across vast spatial scales. Among these species, sea turtles stand out as ideal model for assessing the impacts of global changes due to their diverse habitats and life cycles. In the Mediterranean Sea, where species like loggerhead and green turtles reside, rising temperatures and changing ocean conditions present significant challenges. Understanding the potential impacts of these changes on sea turtle populations is crucial for effective conservation efforts.
The general scope of my Ph.D. is to study the distribution and biology of sea turtles in the Mediterranean Sea, focusing on key research objectives including examining nesting grounds, assessing potential habitat expansions, understanding feeding ecology, identifying breeding areas, and investigating nesting physiology in response to climate change. By addressing these objectives, the Ph.D. aims to provide valuable insights into the effects of global changes on sea turtle populations and inform conservation strategies in the Mediterranean Sea.
Chapter 1 - Going west: Range expansion for loggerhead sea turtles in the Mediterranean Sea under climate change
Global changes represent possibly the greatest threat to the future of biodiversity, and this is especially true for species using very different habitats during their life cycle. The problem is even greater when dealing with human dominated landscapes (e.g., the Mediterranean basin) where climate change and habitat destruction and degradation often interact synergistically. In Chapter 1, I explored this synergy focusing on the loggerhead sea turtle (Caretta caretta) in the Mediterranean Sea. Sea turtles depend on both marine and terrestrial environments and are influenced at the same time by changes occurring in both realms. To explore the changes of nesting grounds in the last decades, I first analysed the changes in the 10-year geographical centers of gravity for all nests from 1960 to 2020. By focusing only on the last 20 years, I incorporated both terrestrial and marine variables into a species distribution model (SDM), while accounting for temporal variability by using a multi-temporal calibration approach. The center of gravity for all nesting grounds shifted roughly by 1300 km to the northwest, and the climate suitability model (with the lowest AICc value and a mean AUC = 0.919 ± 0.047; p-value ≤ 0.001) highlighted a sharp increase over time in the northwest Mediterranean Sea. In the same time frame, the southeast Mediterranean showed a very limited increase in climate suitability for nesting. The most important variables were anthropogenic variables, which negatively influence nesting probability, and sea surface temperature, with an increase up to a maximum probability of nesting around 24–25 °C, but a rapid decrease at higher temperatures. The potential importance of the North-western Mediterranean beaches as possible nesting range for sea turtles highlights the relevance of proactive efforts to assist sea turtles’ conservation during their range expansion. More in general, my analyses demonstrate the importance of considering variables from multiple realms when modeling the distribution of species with complex life cycles.
Chapter 2 - Increase of nesting habitat suitability for green turtles in a warming Mediterranean Sea
Climate change is reshaping global ecosystems at an unprecedented rate, with major impacts on biodiversity. Therefore, understanding how organisms can withstand change is key to identify priority conservation objectives. Marine ectotherms are being extremely impacted because their biology and phenology are directly related to temperature. Among these species, sea turtles are particularly problematic because they roam over both marine and terrestrial habitats throughout their life cycles. Focusing on green turtles (Chelonia mydas) in the Mediterranean Sea, in Chapter 2, I investigated the future potential changes of nesting grounds through time, assuming that marine turtles would shift their nesting locations. I modelled the current distribution of nesting grounds including both terrestrial and marine variables, and I projected the potential nesting distribution across the Mediterranean basin under alternative future greenhouse gas emission scenario (2000–2100). My models show an increase in nesting probability in the Western Mediterranean Sea, irrespective of the climate scenario I consider. Contrary to what is found in most global change studies, the worse the climate change scenario, the more suitable areas for green turtles will potentially be available. The most important predictors were anthropogenic variables, which negatively affect nesting probability, and sea surface temperature, positively linked to nesting probability, up to a maximum of 24–25 °C. The importance of the Western Mediterranean beaches as potential nesting areas for green turtles in the near future clearly call for a proactive conservation and management effort, focusing on monitoring actions (to document the potential range expansion) and threat detection.
Chapter 3 - Mapping potential foraging areas for loggerhead turtles in the Mediterranean Sea: the role of climate change and biotic factors
The effects of global change on biodiversity become notably conspicuous when examining species engaged in extensive migrations. Their significant mobility allows these species to exploit various areas with suitable characteristics for different phases of their life cycle, including reproduction, migration, and foraging. While it is widely acknowledged that climate change has the potential to modify the distribution of sea turtles, there is a limited number of studies that investigate whether and how climate change projections might impact the distribution of potential foraging habitats on a broader scale. Traditionally, abiotic factors are typically regarded as the main drivers, while biotic factors receive limited attention. Moreover, the few studies that consider biotic factors often rely on proxies for trophic resources. In Chapter 3, I used a combination of state-space models and habitat suitability models to show how climate change will influence the distribution of foraging habitats for loggerhead turtles in the Mediterranean basin. I sampled a total of 80 adults from a collection of satellite telemetry data from 2000 to 2022. I calibrated and projected to the future scenarios three habitat suitability models (HSMs): one considering both biotic and abiotic variables (biomass-HSM), one with a proxy for trophic resources (sea chlorophyll concentration) (chl-HSM), and the last one with abiotic variables only (climate-HSM). All models gave AUC > 0.9, with higher values in biomass-HSM compared to the other ones. Turtles are likely to choose calm waters with lower pollution levels, lower salinity, and temperatures around 18-20 °C, along with a high benthic biomass. The neritic zone of the foraging grounds was found to cover 80 % of the suitability, increasing in future scenarios. I obtained significant spatial discrepancy among all models, mainly in the Adriatic Sea, Tunisian Plateau, Aegean Sea, and along the French and Spanish coasts. Prey availability is a major biotic factor determining the actual use of a potential climatically suitable area. I highlighted the importance of including both climatic factors and species-specific trophic ecology to assess the potential impacts of global change more thoroughly on feeding grounds and implement targeted conservation measures.
Chapter 4 - Loggerhead sea turtle breeding grounds in the Mediterranean Sea: key sites and hotspots of risk
A major challenge for conservation biology is defining priority sites for migratory marine megafauna, considering both geographical characteristics and threat exposure. Among the species inhabiting the Mediterranean basin, sea turtles represent an ideal model to assess the potential impacts of global changes, spanning different systems (marine and terrestrial), having a migratory life stage, and being exposed to multiple threats in different phases of their life cycle. Several studies have focused on nesting and foraging stages, while only a handful of studies investigated the reproduction period. In Chapter 4, I used a combination of state-space models and species distribution models to illustrate key breeding areas and risk hotspots for adult loggerhead sea turtles in the Mediterranean Sea. I found that breeding areas, characterized by a low current velocity, covered about 48% of suitability in the neritic zone. Turtles are likely to select less polluted water at temperatures above 18 °C, avoiding anthropic pressures (fishing effort and marine traffic). On the contrary, I identified important breeding sites never reported so far (e.g., North African coast). My results provide for the first time a description of the potentially suitable breeding areas of Caretta caretta in the Mediterranean Sea, suggesting the need to extend current conservation and management efforts to human impacted areas (e.g., westernmost region of the Mediterranean Sea).
Chapter 5 - Understanding the impact of climate change on nesting physiology: microclimate modelling of sand temperatures at loggerhead turtle nesting beaches in the Mediterranean
The fundamental aim of ecology is to understand the functional connections between organisms and their environments. Climate datasets are commonly used to study how shifts in distribution ranges and population viability are affected by human-induced climate change. However, these datasets usually focus on conditions above the soil surface, neglecting the underground environment experienced by organisms. Marine ectotherms are being extremely impacted, as they have to adapt to thermal tolerance limits, in particular species whose biology and phenology are related to temperature, such as sea turtles. Hotter sand (linked to increasing temperatures) can completely alter population biology for the species which have a temperature-dependent sex determination. In fact, extreme hot summers can drive to complete nest failure or at least to decreased hatching rates. Increased sand temperatures also affect hatchlings by altering natural sex ratios, with hotter temperatures producing more female offspring. In Chapter 5, I analysed sand temperature and loggerhead hatching success data to investigate the correlation between incubation temperature and hatching success, subsequently employing a general-purpose microclimate model to assess the predictive accuracy of a mechanistic model by comparing observed and predicted temperatures. I successfully modelled the correlation between incubation temperature and hatchling success, noting high success rates at mean temperatures ranging from 30°C to 31°C, with a minimum of 27.5°C and a maximum of 32°C. I found a generally good agreement between predicted and observed temperatures across various nesting depths, although microclimate models tended to overestimate temperatures by an average of 3°C during nesting seasons. To explore climate change effects on sea turtle nesting beach temperatures along Mediterranean coasts, I recommend employing mechanistic models for their ability to utilize diverse environmental variables, offering a more thorough examination of potential scenarios and their impacts on sea turtle nesting habitats amidst changing climate conditions.
Conclusions
My Ph.D. project emphasizes the importance of considering the full life cycle of species to identify management pitfalls, particularly in the face of climate change impacts. Protected areas alone may be insufficient for comprehensive conservation, necessitating collaborative efforts across agencies and countries. Sea turtles serve as an example of land-dependent marine species whose distribution could be impacted by climate change. While certain models suggest potential expansion of nesting grounds, assessing the suitability of habitats requires further investigation. Mechanistic models offer advantages over correlative models in this regard. This research in the Mediterranean Sea provides valuable insights into sea turtle ecology, emphasizing the necessity for proactive conservation measures. Future work aims to integrate species conservation with EU policies for effective spatial planning amidst shifting distributions.