Impacts of climate change on amphibians : past declines, predicted trends, and future perspectives

Abstract

Mounting evidences indicate that global climate is changing, that biological responses to warming are under way, and that current conservation strategies will need to be revised for being effective in the face of future climate change. Because temperature and moisture affect multiple aspects of amphibian biology, this class of vertebrates is especially sensitive to the alteration of global climate. Amphibians are particularly interesting because they decline more rapidly than either birds or mammals and are already threatened by a plethora of interacting human‐caused factors including pathogens, exotic species, pollution, habitat destruction, and ultraviolet radiation. In this context, the overall goal of my Ph.D. thesis was to give a contribution for clarifying the effects of climate change on amphibian diversity. I mainly focused on Italian fauna to address the following objectives: (i) studying the contributions of climate change to amphibian past declines; (ii) predicting the potential range shifts due to future changes in Italian climate, and (iii) evaluating whether the most important areas for amphibian long term conservation are included in the Italian reserve network. Additionally, this thesis aimed at (iv) examining the implications of within-taxon variation in niche occupancy for predictions of the effects of climate change. This thesis is structured along a number of independent studies that, taken together, provide a broad picture of current and potential effects of climate change on amphibian diversity. To assess the effects of past climatic variation on amphibians in Italy I conducted a multi-factorial study based on the spatial patterns of recent declines, testing contemporarily several potential culprits. Focusing firstly on the national extent, I utilized amphibian data with 10 Km resolution, and I demonstrated that complex influences of several factors are associated with amphibian population declines and vary across species and areas. At the same time, the study revealed that, while various factors have contributed to declines, climate change has been a major cause of population disappearances for multiple amphibian species in past decades. Furthermore, moving to a smaller extent and increasing the resolution of the study a general agreement was evidenced both in terms of factors interactions and of climate change importance. In fact, all species identified as influenced by climate variations at the regional level were also associated with at least one climatic variable at the national level. The climatic shifts were important even for those species with the lowest rates of decline, suggesting that these species may be quite vulnerable to ongoing climatic changes in the region. IPCC projections forecast changes in the global climate during the 21st century even larger than those observed during the 20th century. Biological responses will likely be more pronounced because climate is projected to change faster than in the recent past. Considering climatic projections for mid 21st century, I forecasted amphibian range shifts in Italy through an ensemble niche modelling exercise, utilizing two carbon emission scenarios (A1FI and B1) and two dispersal assumptions. Under all the future settings, range modifications are predicted for every amphibian species. Predictions under different emission scenarios and dispersal assumptions agree in indicating that the species most sensitive to climate change are Pelobates fuscus, Salamandra atra, and Triturus carnifex. As reduction of distributional area is consistently a good predictor of extinction risk, my forecasts suggest increasing danger for these species by the middle of the current century. As species range adjustments take place, protected areas may lose their importance for the protection of some habitats and populations in the future. Based on the predicted distributions under future alternative climatic and dispersal scenarios I analyzed the efficacy of the Italian reserve network for protecting multiple amphibian species, considering both nationally designated areas and Natura 2000 sites. Gap and irreplaceability analyses demonstrated that the Italian reserve network incompletely represents current amphibian diversity and its geographic pattern. This inadequacy will get even worse on the long-term, as climate change will decrease the amount of suitable range falling into reserves for many species, regardless of the assumptions about dispersal. By analyzing the spatial pattern of conservation value, I identified some currently unprotected areas that have high irreplaceability scores for species conservation and that maintain their importance under all the considered future scenarios. If predictive modeling of species distribution is to be useful for estimating extinction risk under climate change, we need to ensure that we use the best methods available. Species are made up of local populations that may be locally adapted to different portions of their range. Geographic variation in environmental tolerances suggests that regional populations should be modeled to capture these environmental dependencies. The analyses presented in this thesis explore the implications of within-taxon variation in niche occupancy for predictions of the effects of climate change. The subclade models capture the climate-distribution relationship for groups of populations with similar evolutionary histories and shared selective regimes. In comparison to models that ignore sub-taxon structure, models that incorporate this structure generally predict larger areas of suitable conditions, consistently perform better, and can reveal divergent potential effects of climatic variation on sub-taxa. This richness of additional detail in the potential response of taxa and sub-taxa to climate change can be relevant for conservation purposes. In sum, the results of this thesis reveal that shifting climatic conditions are already affecting amphibian communities. This represents important evidence of global warming‘s impacts on amphibians in a region where declines have nearly always been ascribed to habitat loss. Future threats can derive from the predicted changes in the global climate during the 21st century. The existing protected area network provides a precious service in shielding habitat from destructive use and hence in reducing biodiversity loss. However, this research clearly demonstrates that the network is still far from complete. The most valuable areas for amphibian conservation highlighted in this thesis can direct reserve designation policy to efficiently protect species both in current and future conditions. Such measures are essential in preventing a further loss of amphibian diversity. Finally, this thesis suggests that the incorporation of phylogeographic information into modeling exercises can contribute considerably to an improved theoretical understanding of responses to climate forcing, and to the forecasting of future changes. With regard to conservation, this can help preserving biodiversity both at specific and genetic levels in a rapidly changing world.