Dissertation committee:
1. Κωνσταντίνος Τριάντης, Αναπ. Καθ. ΕΚΠΑ (Επιβλέπων)
3. Albert Phillimore, Chancellor's Fellow & Reader, University of Edinburgh (Συμβουλευτική επιτροπή)
2. Αριστείδης Παρμακέλης, Αναπ. Καθ. Ε.Κ.Π.Α. (Συμβουλευτική επιτροπή)
4. Σπύρος Σφενδουράκης, Καθηγητής Πανεπιστημίου Κύπρου (Εξεταστική επιτροπή)
5. Αθανάσιος Καλλιμάνης, Καθηγητής Α.Π.Θ. (Εξεταστική επιτροπή)
6. Παναγιώτης Παφίλης, Αναπ. Καθ. Ε.Κ.Π.Α. (Εξεταστική επιτροπή)
7. Σίνος Γκιώκας, Καθηγητής Πανεπιστημίου Πατρών (Εξεταστική επιτροπή)
Summary:
Island biotas have been fascinating naturalists for centuries. From the age of Johann Reinhold Forster (1729–98) and Joseph Banks (1743–1820) – who embarked on James Cook’s circumnavigating voyages cataloging the diversity of the utmost isolated islands – through to Charles Darwin (1809–82) and Alfred Russel Wallace (1823–1913) – who independently conceived the theory of evolution by natural selection by observing the exceptional nature of insular species – the observation of island biodiversity has played a pivotal role in realizing that the distribution of species on the planet is spatially and temporally structured and that this structure is dictated by explicit geographic and environmental factors. The subsequent development of modern Island Biogeography – mainly through the formulation of MacArthur and Wilson’s Equilibrium Theory of Island Biogeography and its deep influence on mainstream ecology – has since contributed major advances in our understanding of the fundamental processes governing the emergence of large- to global-scale macroecological patterns. However, such patterns have been described and studied based mainly on vertebrates, and to a lesser extent on plants. Invertebrate species, which form the enormous majority of global biodiversity, have markedly been overlooked from global-scale macroecological studies, partly because of the absence of comprehensive datasets and partly due to a preferential research engagement on their arguably more attractive vertebrate and plant counterparts.
Here, I aim to inform this gap for land snails, i.e. a taxon which is commonly used as a model organism in island biogeographic studies. Insular land snails are an ideal group of organisms for the investigation of diversity patterns at a macroecological scale: i) they are numerous – with ca. 23,000 known species, ii) they exhibit extraordinary rates of endemism – e.g. 99% in the Hawaiian and 92% in the Galapagos Islands, iii) they are found in a variety of terrestrial ecosystems – from coastal sand dunes through to alpine moraines, and iv) they demonstrate ample morphological variability – with interspecific variation in size and weight varying by orders of magnitude even across short distances. At the same time, insular land snails are among the most threatened animals on Earth, already having suffered extensive human-caused extinctions. These characteristics make them excellent models for inferring processes determining community assembly and distribution from observed macro-scale diversity patterns as well as for elucidating drivers of extinction.
Therefore, the objective of the present thesis is two-fold. First, I endeavor to identify the main drivers of land snail species richness and endemism across the islands of the world. Second, I seek to address the key drivers of vulnerability of insular land snail faunas. To do these, I compiled – to my knowledge – the currently largest database on insular invertebrate diversity.
In Chapter 1, I develop the GlobaLSnails, a Global inventory of insular Land Snail occurrences across 727 continental and oceanic islands, scattered across 86 archipelagos around the globe. Taking advantage of various available bibliographic resources, I produce a presence-absence matrix which contains 11,139 species, that is ca. 48% of globally known land snail species richness. I find unparallel levels of endemism, with 75% of the retrieved species being single-island and 91% being archipelagic endemics. Considering the fact that the total area of these 727 islands represents ca. 2.9% of global landmass, the number of retrieved land snail species reveals remarkable levels of non-marine mollusc representation on islands and marks potential underestimations of the current estimates of global land snail richness (ca. 23,000 species).
In Chapter 2, I use generalized linear mixed models to assess the factors determining variation in species richness across these 727 islands. Using a set of explanatory variables relating to island geography (e.g. area, current and past isolation), environmental heterogeneity (e.g. average climatic conditions, productive energy, habitat diversity and topography) and human influence (e.g. presence of human and road network) and analyzing each chorotypic component of species richness – i.e. native endemic vs. native non-endemic – separately, I find that different chorotypes respond differently to the various environmental metrics, suggesting that different underlying processes might be responsible for the emergence of chorotype-specific observed patterns. These results corroborate previous studies on other better studied taxa – such as birds, mammals and plants – and give overall support to the prominent species-area-isolation and species-energy theories.
In Chapter 3 – which I co-author with colleagues – we quantify the influence of past archipelago configuration on present‐day insular biodiversity patterns for a subset of 53 oceanic islands, and compare the role of long‐lasting archipelago configurations over the Pleistocene to configurations of short duration such as at the Last Glacial Maximum (LGM) and the present-day. In order to assess whether such influence is taxon-dependent, we contrast land snails to angiosperm plants, i.e. a taxon with arguably higher dispersal ability. We find that long-lasting past geographic arrangements affect endemic richness (either single- or multiple-island) in a more profound way than non-endemic native richness, more so for land snails than for plants, and suggest that archipelago configurations at intermediate sea levels – which are representative of the Pleistocene – have left a stronger imprint on single‐island endemic richness patterns on volcanic oceanic islands than extreme archipelago configurations that persisted for only a few thousand years (such as the LGM).
In Chapter 4, I i) examine spatial patterns of vulnerability for >3,600 species with available risk status assessments and ii) attempt to quantify the influence of species’ intrinsic traits, such as shell morphology and geographic range size, on the probability of extinction, using cumulative link mixed effect models. Results show that insular land snails are indeed among the most imperiled faunas globally, with >10% of assessed species having gone extinct and another ~31% being threatened. I also find that species-specific traits do play a major role in defining extinction risk: larger and globular shell forms with small geographic ranges have a higher probability of getting extinct. These results suggest that – as with other taxa – the consideration of species-specific traits can substantially improve our understanding of the fundamental factors influencing the probability of human-driven extinctions.
Overall, the present thesis sheds light on the global biogeographic patterns of one of the most valuable non-vertebrate model organisms in macroecological studies. The adopted approach will – hopefully – provide a useful baseline for future research in predicting richness of yet poorly explored islands and extinction patterns of other insular biotas, which are essential both from theoretical and methodological as well as practical conservation standpoints.
Keywords:
Island biogeography, Macroecology, Invertebrates, Land snails, Global biodiversity, Global database, Species richness, Endemism, Taxonomic diversity, Morphological diversity, sea-level oscillations, Pleistocene, Past environmental change, Anthropocene, IUCN Red List, Extinction, Ecological modelling
