| Abstract |
Due to their apparent simplicity and discrete nature, scientists have long viewed islands as model systems for the investigation of questions related to biodiversity and its regulation. In this work, we try to investigate the biogeographical patterns occurring in three insular groups of the Aegean Sea (Skyros, Astypalaia and Kalymnos) comprising 38 islands and islets. The biogeographical patterns are being approached through the study of two animal groups, land snails and isopods. Both groups have successfully been used for the study of ecological and biogeographical issues since they are species-rich groups with a more or less applicable taxonomy, usually maintain dense populations, exhibit limited dispersal abilities and are usually found on various habitats. The insular groups studied have been selected so as to exhibit both long-term and relatively recent isolation from neighboring areas. The group of Astypalaia is one of the most isolated insular areas of the Aegean Sea. On the other hand, the group of Kalymnos has recently been isolated from the western coasts of Asia Minor and the neighboring islands. Finally, the group of Skyros although being very close to mainland Greece, is characterized by a long-term isolation. The simultaneous study of the biogeography of two animal groups takes place for the first time in Greece and is an exception in the biogeographic literature. The main advantage of this comparative study is that it can more effectively point towards the mechanisms that define a regions biodiversity, as well as the range that these mechanisms apply to. Regarding land snails, 89 species belonging to 52 genera were collected in total from the three insular complexes. The respective number for isopods was 40 species belonging to 24 genera. 42 land snail species were found in Skyros, 34 in Astypalaia and 47 in Kalymnos. 32 species of isopods were collected from Astypalaia and 26 from Kalymnos. For the 31 out of the 38 islands studied, both the land snail and isopod species reported are new records since none of these islands has been studied in the past. For the remaining islands, the faunal records were significantly improved. Ecology year after year is becoming a super-science covering a large number of topics, containing a large number of subdisciplines and a rapidly growing number of scientists. Thus, the need for a common language among ecologists and scientists of contextual sciences as biogeography, evolution and concervation biology is becoming a top priority need. Especially terms related with the environmental heterogeneity such as habitat, biotope and ecotope should be approached under a general scheme, with specific reference to the organization level they are referred to. Related to the above is the proposition that habitat types could be used as a measure of environmental heterogeneity in regard to the taxonomical level of class. Nevertheless, the theoretical approach of these terms along with the difficulties arising from their practical application should not in any case disencourage the study and quantification of the environmental heterogeneity. On the other hand we should avoid a scholastic approach of the ecological terms related to the environmental heterogeneity which will result in refraining from using them. The species-area relationship is considered as one of the few laws of Ecology. Almost always, an increase of species number will be observed as the area sampled is increased. Nevertheless, the causal patterns of this increase are not constant through space and time (Chapter 3). So far the most effective mathematical model expressing the increase of species number with area, is that proposed almost 90 years before by Arrhenius (1921), but it cannot account for all the cases. Although I consider the search for an effective mathematical model essential, my belief is that due to the interchange of processes establishing biodiversity throughout space and time, the implementation of different models will be needed in each case and in most instances only one will be effective. The low z-values for of the three island groups are the anticipated result of the recent formation of the island groups. The three island groups despite the different geological histories have both been formatted recently in the geological time. This recent formation results in small islands that still behave as parts of a continuous land mass, and consequently, the reduction of area has not yet led to a significant loss of species, with small islands exhibiting high number of species. Characteristic is the case of Mesa Diavatis, which holds both Helix cincta and H. aspersa. The presence of two Helix species in such a small island is reported here for the first time from the Aegean. Short distances between the islets and the long presence of man contribute to this, as the probability of new immigrants reaching the islands is high, despite the low dispersal ability of land snails. Hence, z values fall within the intra-provincial range, rather than the inter-island range. The studied island groups are typical cases of the majority of the island groups in the Aegean and Ionian Sea. They are recent formations, for which even taxa with low dispersal abilities, such as terrestrial isopods and snails, behave as being parts of larger landmasses and not isolated islands. The long term isolation of the landmasses now constituting the insular complexes, has not played a major role in defining the number of species present on the islands. Nevertheless, it has a profound effect both on the level of endemicity and the faunal composition of the islands. Thus, for animal groups with higher dispersal abilities and capable of overcoming the sea barrier, the effects of isolation will be even milder. Consequently, in cases like this the study and detection of the parameters that establish the composition of the flora and fauna should be considered of primary importance relative to those relating to species richness. Knowledge of how species interact with their environment is of obvious importance to understanding large-scale patterns of diversity as well as more local ecological patterns. Our results offer several lines of evidence that environmental heterogeneity, studied through the habitat types, is central to patterns of land snail and isopod species richness on all the three island groups (Chapter 4). The choros model has more explanatory power than area alone. While the increase in fit (is not great between area alone and choros, these results, together with previous findings show that the increased fit of choros over area alone is relatively consistent. The utility and novelty of the choros model are not necessarily due to the better fit to observed data, but arise from the relative simplicity of the technique, as well as the increased explanatory power of the model. Choros is a concept that holds not only quantitative but also qualitative information for the area under study, facilitating a more appropriate approach to mechanisms responsible for the establishment of species richness. I do not consider the proposed model as the ultimate solution of the query for the inclusion of environmental heterogeneity in mathematical models describing species richness. Howevre, the choros model can be considered a stepping-stone to the direction of a more effective model to both theoretical and applicational grounds. Although the classical speciesarea models have been used to estimate the upper limit of the small island effect (SIE), we propose that the detection of this phenomenon should be undertaken independently from the speciesarea relationship, so that the net effects of area are calculated excluding the surrogate action of area on other variables, such as environmental heterogeneity. The SIE appears when and where area ceases to influence species richness directly. There are two distinct SIE patterns: (1) the classical SIE where both the direct and indirect effects of area are eliminated and (2) the cryptic SIE where area affects species richness indirectly. Our approach offers the opportunity of studying the different factors influencing biodiversity on small scales more accurately. SIE was detected only in the case of land snails and terrestrial isopods of the island group of Kalymnos. That is a clear indication that the very recent formation of the particular island group has resulted in area having no direct effects on the species number of both isopods and land snails. The SIE cannot be considered a general pattern with fixed behaviour that can be described by the same model for different island groups and taxa. The SIE should be recognized as a genuine but idiosyncratic phenomenon. Keywords: Aegean Sea, biodiversity, environmental heterogeneity, species-area relationship, choros model, small island effect, land snails, terrestrial isopods.
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